Emission Control Technologies for Reduction of NOx and Particulate Matter from Automotive Diesel Engines
vijay tharad
Director Operations at Corporate Professional Academy for Technical Training & Career Development
Pollutant Emissions
In modern internal combustion engines, two primary systems are responsible for the formation and reduction of pollutants:
· the combustion system, and
· the emission aftertreatment system.
The combustion system includes the combustion chamber, its shape and characteristics such as charge composition, charge motion, and fuel distribution. This is where pollutants such as NOx, CO and PM are created as well as where incomplete oxidation of fuel occurs. What happens in the combustion system is greatly influenced by other engine systems such as the intake charge management system and the fuel injection system. In fact, the primary purpose of these secondary systems is to influence what happens during the combustion process. Numerous options are available to limit the formation of pollutants resulting from the combustion system. Once exhaust gas leaves the combustion system, its composition is essentially frozen until it reaches the emission aftertreatment system (ATS, also abbreviated EAT or EATS) where further reductions in pollutants can be realized and also where secondary emissions such as N2O, NO2 and NH3 can originate.
The aftertreatment system consists of catalytic reactors that attempt to further lower pollutants. In some cases, such as stoichiometric spark ignition (SI) engines, a single three-way catalyst (TWC) is sufficient to achieve very significant reductions in pollutants. In other cases such as lean burn diesel engines, a number of catalytic devices are required. Secondary systems are required to ensure the ATS works as intended. These include: control of exhaust gas composition through control of exhaust stoichiometry or supply of additional reactants not normally found in exhaust gas or not present in sufficient quantity (e.g., urea, additional HCs, additional air or O2), thermal management to ensure the catalysts operate within the required temperature window, systems to ensure contaminants and pollutants that might accumulate are removed (regeneration of filters, sulfur management, urea deposits,) and systems to minimize the formation of secondary pollutants such as the ammonia slip catalyst (ASC).
It would be a mistake to consider the combustion system and the ATS as separate systems. In order maximize their effectiveness, a high degree of integration is required. A classic example is air-to-fuel ratio (AFR) in SI engines where a very high level of control precision is required to ensure the TWC performance is maximized. Thermal management of the ATS can be carried out by adjustments within the engine to affect the temperature of the exhaust gas leaving the cylinder. In some cases, additional fuel required by the ATS (e.g., for thermal management) can be supplied by the engine’s fuel injectors.
It is important to realize that the objective of engine optimization is not to minimize the pollutant emissions from the combustion system or maximize the reduction of pollutants in the ATS. Rather the objective is to achieve a target level of emissions from the entire system. The target is generally sufficiently below the regulatory limit to allow for production variability. Doing so may require the emission of some pollutants from the combustion system to increase if ATS performance is sufficiently high to still allow design target to be met. For example, NOx emissions from engines equipped with a urea-SCR catalyst can be allowed to increase to minimize GHG emissions (due to the NOx-BSFC trade-off) if high NOx conversion in the SCR catalyst is achieved.
Fuels and lubricants are an important “partner” in the combined engine and aftertreatment system. Low emissions over the life of the engine would not be possible unless fuel contaminants such as sulfur and some inorganic minerals are controlled to very low levels.
Emission Control from in-Use Engines
The above technologies, discussed further in the following sections, are applicable to new (OEM) internal combustion engines. Some of these technologies may also be used to reduce emissions and/or improve efficiency of existing, in-use engines. There is also a group of technologies developed specifically for in-use applications, that are normally not used in new engines.
GHG Emissions and Fuel Economy
GHG emission limits and fuel efficiency standards have created opportunities for a wide range of technologies to be incorporated into engines and vehicles. The search for increased fuel efficiency is focusing attention on at least three key areas:
· powertrain efficiency,
· vehicle technology, and
· operational parameters.
As powertrain efficiency has a direct impact on fuel consumption, it is an obvious choice for improving fuel efficiency. Important approaches include improved engine efficiency, kinetic energy recovery (such as through regenerative braking), waste heat recovery, and reduction of parasitic losses from ancillary devices such as pumps. Among vehicle technologies, improved vehicle aerodynamics and reduced rolling friction are two obvious factors that affect fuel economy. Other factors include vehicle weight and power used by non-powertrain auxiliaries such as air-conditioning. Last but not least, vehicle operational parameters such as driving patterns and route selection can also be used to gain significant improvements in fuel economy . These technologies were discussed under Efficiency Technoiogies.
a. Fuel injection system modifications
b. Injection timing
c. High injection pressure
d. Electronic injection control
e. Injector nozzles (small holes, low sac volume)
f. Air intake improvements
g. Cold charge air (inter- and aftercoolers)
h. Progress in turbocharger technology (variable geometry turbochargers)
i. New intake manifolds
j. Swirl ratio match with fuel injection characteristics
k. Combustion chamber modifications
l. Reentrant bowls
m. Higher top piston rings
n. Better air utilization (central injection, four valves)
o. Higher compression ratio
p. Exhaust gas recirculation
q. Electronic engine control
Emission Control Technologies
Emission control options can be grouped into three categories: (1) engine design techniques, (2) fuel and lubricant related technologies, and (3) exhaust gas aftertreatment. Each of these approaches can be divided into sub-categories, as shown in the following tables. In addition, powertrain integration and control technologies play a very important role in reducing emissions and improving the engine and vehicle efficiency. Some of the methods discussed below are implemented in today’s engines, others—still under development—show promise for future applications.
Table 1 Engine design technologies for emission reduction
Emission Impact : Capabilities have evolved significantly. Significant improvements in injection technology started in the 1990s with widespread implementation of systems capable of variable injection timing through the use of electronic controls. Engines with EGR place the highest demand on fuel injection pressure. Light-duty vehicles use the most demanding multiple injection strategies.
As is apparent from the earlier sections on diesel fuel injection, diesel fuel injection systems have seen monumental changes starting in the later part of the 20th century. The P-L-N injection systems that characterised the diesel engine from the 1920s has all but disappeared from diesel engines intended for the most advanced markets. This evolution has been almost entirely driven by the need to reduce exhaust emissions to levels that were not though possible even as late as the 1990s. These advances in fuel injection system hardware have enabled such features as:
While these features have been fundamentally driven by the need to lower emissions, in many cases, they can also be utilized to reduce noise, increase specific power and manage exhaust temperatures for improving the performance of aftertreatment systems that can be used to achieve further reductions in exhaust emissions.
2. Technology : Injection timing
Emission Impact : Primarily used to limit NOx emissions
Significance : Injection timing affects combustion phasing; retarding the combustion phasing can be used to limit NOx emissions.
Injection timing is one of the most important factors influencing combustion and the resulting emissions. All important engine performance parameters, including specific fuel consumption, emissions of NOx, PM, HC, and peak cylinder pressure, are a strong function of injection timing. Mechanical fuel system have fairly limited injection timing capabilities. They usually require hardware changes in order to modify injection schedules. The timing schedule of mechanical systems has very few degrees of freedom, so that handling such anomalies as cold start advance or low coolant temperature advance requires additional mechanical elements. Furthermore, cam- driven fuel systems typically exhibit a relationship between injection timing and fuel injection rate, which can influence injection pressure and duration. In contrast, electronic control allows to command injection timing in response to engine load, engine speed, or ambient conditions. Exhaust emissions can be optimized over an emissions test cycle, which samples a wide range of engine speeds and loads, through implementing injection timings that optimize emissions and fuel consumption over local regions of the operating spectrum. The electronic injection control also allows for much better accuracy of the injection timing. Some engines, e.g. the AUDI HSDI engine, use an injector needle lift sensor to provide feedback. Injection timing accuracy within ±1 degree of crankshaft rotation was claimed. Advanced high pressure rotary fuel pumps typically have a rotor position sensor. The ECU can compensate for any angular errors between the pump and the engine by comparing the crank position sensor signal with the fuel pump position signal.
3. Technology : Injection Pressure
Emission Impact : Primarily used to limit soot (PM) emissions
Significance : Higher injection pressure can lower soot emissions; especially important when combined with NOx control technologies such as EGR that would otherwise increase soot emissions.
primarily used to limit soot (PM) emissions : Higher injection pressure can lower soot emissions; especially important when combined with NOx control technologies such as EGR that would otherwise increase soot emissions.
A fuel system on a common rail diesel engine consists of a low pressure system (up to 100 PSI) for supplying fuel from the tank to the engine and a high-pressure system (up to 32,000 PSI) for injecting the fuel into the cylinder.
The high-pressure system uses a secondary fuel pump to pressurize the fuel within a common fuel rail used to feed the injectors. Increasing the injection pressure allows for finer atomization of the fuel, improved air-fuel mixing, and greater control of injection timing. Diesel engines with mechanical fuel systems, such as the Mitsubishi used to power Kohler generators, are limited in their injection pressures and fuel injection control capability.
There are two main technical paths to deal with diesel emissions: first within the combustion chamber and second via a downstream exhaust after-treatment system.. Manipulation of fuel injection can be helpful by raising injection pressures to the 2,000-2,500 bar class (yielding finer atomization), by retarding the injection timing (which reduces NOx formation but raises soot output), by introducing multiple injection events per combustion cycle (3-7 injections: pre-injections, main injections, post-injections), and by adjusting spray patterns (via injector exit hole size/number - typically 4-12 holes).
4. Technology : Multiple Injections
Emission Impact : Various
Significance : Multiple injections strategies have been developed to lower NOx, soot, HC and CO emissions. Various : Multiple injections strategies have been developed to lower NOx, soot, HC and CO emissions.
There are two main technical paths to deal with diesel emissions: first within the combustion chamber and second via a downstream exhaust after-treatment system.. Manipulation of fuel injection can be helpful by raising injection pressures to the 2,000-2,500 bar class (yielding finer atomization), by retarding the injection timing (which reduces NOx formation but raises soot output), by introducing multiple injection events per combustion cycle (3-7 injections: pre-injections, main injections, post-injections), and by adjusting spray patterns (via injector exit hole size/number - typically 4-12 holes).
5. Fuel Injector Nozzles
The design of fuel injector nozzles has paramount importance for the performance of the engine and its emissions. The evolution of fuel injection nozzles involved reduced sac volumes, optimized injection hole numbers and diameters, optimized length of the nozzle hole and optimized spray cone.
6. Electronic Injection Systems
Electronically controlled fuel injection systems have replaced traditional mechanical systems in both heavy-duty and light-duty diesel engines. The benefits of electronic full authority fuel injection systems include better control of all injection parameters resulting in lower emissions and increased performance, integration with the engine ECU and other vehicle systems, self-diagnostics, and performance checks.
7. Electronically Controlled Injection Pumps. Mechanical fuel injection pumps vary the fueling and injection timing using governors and mechanical linkages. Electronic distributor type pumps were introduced by several manufacturers that are controlled using electrohydraulic devices. In the Lucas EPIC system the cam ring was rotated by a hydraulic actuator to vary injection timing [Hawley 1998]. Fueling was varied by moving the rotor mechanism axially using another actuator. The pressure of working fluid, which was diesel fuel, was regulated by the ECU through a solenoid valve. In newer distribution pumps by Bosch (VP30, VP44), a solenoid operated spill valve controls the injection quantity. The injection timing is still set by rotating a cam ring.
8. Electronic Unit Injector (EUI) System. Unit injector systems do not employ the central injection pump supplying individual injectors. Instead, both pump and injector are combined into a single unit for each cylinder. Each pumping plunger is driven directly by the camshaft. This allows for very high injection pressures, up to 150-200 MPa. In the EUI system, the fueling is controlled by a solenoid valve. The system has also some capacity for pilot injections to reduce NOx emissions and noise.
9. HEUI System. Caterpillar has developed a hydraulic electronic unit injector (HEUI) system, which is powered by hydraulic pressure and controlled electronically .The features of the HEUI system include no mechanical actuation or mechanical control devices, injection pressure independent from the engine speed and load, and flexible injection timing. The system is controlled by the ECU through a solenoid control valve that starts and ends the injection process. The unit injectors are powered by hydraulic oil delivered by an engine driven high pressure oil pump. The pump raises the system ’s oil pressure from typical engine operating levels to the actuation pressure required by the injector. The high pressure oil is distributed to the injectors through a manifold.
10. Common Rail System. In the common rail system, fuel is distributed to the injectors through a high pressure manifold, called the rail. Injectors are simply solenoid actuated nozzles, operated by the ECU. Common rail is a competitor of the EUI system. The advantages of common rail, relative to the traditional pump-line-nozzle system, include injection pressure independent from engine speed and load and a flexibility to control injection timing, duration, and shape. The system is capable of pilot injections to lower NOx and engine noise, as well as post-injections that can be utilized to increase combustion temperatures in order to regenerate diesel particulate filters.
11. Technology : Exhaust gas recirculation (EGR)
Emission Impact : In diesel engines, primary application is to control NOx emissions
Significance : Commonly used in many light- and heavy-duty diesel engines. High pressure EGR delivery can introduce a fuel consumption penalty through higher pumping losses. Low pressure EGR has lower pumping losses but is more difficult to control during transient operation. Other measures to limit potential increases in soot and possibly HC and CO can be required.
12. Technology: Intake boosting
Emission Impact : Primary emissions impact is to lower soot (PM) production. Also important for efficiency gains.
Significance : Higher intake pressure increases air/fuel ratio for given fuel injection amount and lowers soot production.
Practically all today’s automotive diesel engines are turbocharged. Modern turbochargers, including the waste gated turbocharger and variable geometry turbocharger technologies, allow to control the intake manifold (boost) pressure and, thus, the air flow rate to the engine cylinders. The waste gate by-passes some of the exhaust gas around the turbocharger turbine at high speeds to prevent excessive boost pressure and air flow. In many “mechanical” implementations, the waste gate is controlled by a Boost pressure feedback to a pneumatic diaphragm actuator that operates the waste gate valve through a linkage. This type of boost pressure control is only as accurate as allowed by the setup procedure during assembly.
There is no compensation for changes in the characteristics of the pneumatic actuator or the waste gate valve. Therefore, errors in the boost pressure can occur. An increasing proportion of engines is equipped with electronic waste gate control, where the boost can be controlled as part of an integrated engine control strategy. The air pressure fed to the pneumatic waste gate actuator is modulated by a pressure control valve. This allows for true closed-loop control of the boost pressure, compensated for manufacturing variability and changes during engine lifetime, as well as environmental conditions including altitude. Another advantage of the electronic control is the possibility to include several other factors, besides boost pressure, in the waste gate control algorithm. For example, the boost pressure can be higher during transients than it is at steady state to improve acceleration.
13. Air Intake Improvements Charge Air Cooling
Practically all modern automotive diesel engines are turbocharged. Turbochargers are devices incorporating a turbine and a compressor on tne rotor shaft. The exhaust gases expand in the turbine, which drives the compressor placed in the intake stream. Turbocharging is attractive thermodynamically, because it recovers exhaust gas energy that would be otherwise wasted. Increasingly more turbocharged engines use charge air cooling. The charge air coolers, called aftercoolers or intercoolers, are heat exchangers that cool the intake air that was heated during compression in the turbocharger Charge air cooling reduces the intake manifold temperature (IMT), thus increasing the density of charge air and, therefore, the engine specific power output. The benefits of charge air cooling include improved fuel economy and emissions. First generation of aftercooled engines in the late 1980’s utilized watercooled intercoolers, as shown above. In the 1990’s, the use of air-to-air coolers becomes increasingly popular for both heavy- and medium-duty diesel engines.
14. Variable Geometry Turbocharging
Due to a nonlinear flow versus pressure ratio characteristic, a conventional turbocharger is generally unable to provide adequate air flow at low engine speeds. This inability to provide sufficient air flow at low speeds results in reduced torque, reduced driveability, and smoke emissions. This problem is traditionally solved by matching the turbine at low engine speed (i.e. using a small turbine) and either (1) accepting the penalty of high boost pressure at high engine speed, or (2) using a turbine by-pass valve, called the wastegate, to reduce the turbine power. To overcome these inherent limitations of the fixed geometry turbine, several variable geometry concepts have been developed . In the most common vaned nozzle approach, the turbine nozzles are able to rotate along an axis parallel to the turbine rotor. The effective flow area of the nozzle can be increased or decreased, as dictated by the engine needs. Variable geometry turbochargers may be used to enhance emissions, notably transient smoke emissions, as well as a number of engine parameters, such as the low speed torque, without compromising the high speed fuel consumption.
15. Technology: Intake temperature management
Emission Impact : Most direct impact on NOx emissions. Can lower soot emissions as well.
Significance : Increased boosting and/or EGR can increase intake manifold temperature. Intake charge cooling capability improvements are required to limit intake charge temperature and minimize associated NOx emission increases, reductions in air-fuel ratio and losses in power density.
16. Technology : Combustion chamber design
Emission Impact : Important soot control measure
Significance : Combustion chamber design changes are commonly used to offset increases in soot emissions when measures are taken to limit NOx emissions. In many cases, improvements enhance mixing late in the combustion process to improve soot burn-out.
The objective in the combustion chamber design is to provide good mixing of fuel and air before the start of combustion. Turbulence in the air motion in the combustion bowl (crater) in the piston crown improves the mixing process. Proper design of the combustion bowl can enhance the air swirl created by the intake port and increase turbulence.
There is a general tendency to replace the traditional straight-sided bowl design with reentrant bowls. The location of the bowl, which used to be eccentric, is now moving to the center of the piston crown. The central bowl location is found in 4-valve light-duty engines. The position of the injector is central relative to the bowl, in both concentric and eccentric bowl design.
However, in eccentric bowls, the injector is positioned at an angle, while it is vertical in concentric bowls. Another feature of new combustion chambers is higher compression ratio, which reduces ignition delay, reduces the premixed flame, and allows more ignition timing retardation to reduce NOx.
There is a general tendency to replace the traditional straight-sided bowl design with reentrant bowls. The location of the bowl, which used to be eccentric, is now moving to the center of the piston crown. The central bowl location is found in 4-valve light-duty engines. The position of the injector is central relative to the bowl, in both concentric and eccentric bowl design.
However, in eccentric bowls, the injector is positioned at an angle, while it is vertical in concentric bowls. Another feature of new combustion chambers is higher compression ratio, which reduces ignition delay, reduces the premixed flame, and allows more ignition timing retardation to reduce NOx.
Engine design technologies for emission reduction Positive Ignition (SI) Engines
17. Technology : Fuel injection
Emission Impact : Fuel consumption and particulate emissions
Significance : The shift from port injection to gasoline direct injection (GDI) was driven by the use of engine downsizing to meet fuel consumption and CO2 requirements. GDI engines have a higher tendency to produce small particle emissions that can be partially offset by refinements in fuel injection system design.
Improving fuel injection technology is another approach to reducing emissions. For example, Bosch’s modular common-rail system for commercial vehicles (CRSN), which has been designed specifically for use in off-highway applications, features CRIN solenoid valve injectors that operate at up to 2,500 bar (36,259 psi). This atomizes fuel into microscopic droplets. The CRIN also rapidly switches, injecting “a specific quantity of fuel into the cylinders to match the driving situation,” according to the Bosch website. Both result in a cleaner burn, with lower CO2 and particulate emissions.
18. Technology : Intake boosting
Emission Impact :Fuel consumption
Significance : Enabler for engine downsizing and reduced fuel consumption and CO2 emissions. Primary emissions impact is to lower soot (PM) production. Also important for efficiency gains. Higher intake pressure increases air/fuel ratio for given fuel injection amount and lowers soot production.
Can be an important measure to offset unwanted decreases in performance and increased emissions with NOx control measures such as EGR. Often accompanied by improved intake charge cooling capabilities. Enables engine downsizing for efficiency gains. Introduces challenges such as turbocharger lag that can require complex solutions.
19. Technology : Variable valve actuation
Emission Impact : Various
Significance Some examples include: variable valve timing is an important measure to reduce cold start HCs. Variable valve lift enables throttleless operation and improved efficiency. Cylinder deactivation reduces part load pumping losses and improves efficiency. Variable valve timing enables Miller cycle operation for reduced pumping losses.
20. Technology : Lean burn
Emission Impact: Fuel consumption
Significance : Lean burn can reduce pumping losses, heat transfer and improve working fluid characteristics to provide higher efficiency. Introduces the need for expensive NOx aftertreatment technologies.
21. Technology : Combustion
Emission Impact : Fuel consumption
Significance : Advanced combustion concepts can improve efficiency through faster combustion and lower heat losses.
22. Technology EGR
Emission Impact :At one time used to limit NOx emissions. Modern approaches focus mainly on reducing fuel consumption.
Significance : In SI engines, EGR is an alternative to fuel enrichment at high loads to reduce knock propensity and lower exhaust temperature at high power. At part load conditions, it can reduce pumping losses.
Table 2 Fuel & lubricant technologies
23, Technology : Lubricating oil
Emission Impact : Important to reduce fuel consumption
Significance Low viscosity lubricants are important for fuel consumption/CO2 reductions but require other changes to ensure engine wear levels do not increase. Limiting the content of catalyst poisons (e.g., sulfur, inorganic ash, phosphorus) is a key enabler for ensuring durability and performance of catalytic exhaust emission control technologies.
24. Technology : Alternative fuels
Emission Impact : Primary impact is life-cycle CO2 emissions
Significance : Limited criteria emission reduction potential from modern engines with full range of aftertreatment for NOx and PM. Some effect on criteria pollutants (PM, NOx, SOx) is possible in applications without aftertreatment (e.g., marine). In some cases, lower operating cost is a major consideration (e.g., natural gas). Demand can often be driven by government incentives or mandates.
25. Technology : Fuel additives
Emission Impact : Various
Significance : Small direct emission effect with modern engines and high quality fuels. Important to maintain long term stable operation of emission control technologies. For examples, cetane additives help ensure consistent and reliable ignition quality of modern diesel fuels to ensure reliable and predictable performance; injector cleanliness additives and lubricity additives are intended to keep fuel injection system components clean and reduce wear to ensure long term durability and consistent performance of fuel injection systems; some diesel particulate filter systems use fuel additives to assist particulate filter regeneration.
THE IMPORTANCE OF FUEL QUALITY
Fuel and lubricant quality affects the performance of emissions control systems either by Preventing the use of a technology unless the fuel quality is improved (the improved fuel is ‘enabling’ the use of that technology) or by ‘enhancing’ the performance of emissions control systems. In this case both the existing fleet and new vehicle registrations benefit. The motor industry has published information on the effects of fuel quality, with recommendations, in the ‘Worldwide Fuel Charter’ .Examples of enabling fuels are unleaded petrol that allows three-way catalysts to be used and ultra-low sulfur fuels required so that NOx adsorbers can be used and which ease the use of catalyst-based Diesel Particulate Filters (DPF). Lead has long been recognized as a catalyst poison as well as having impacts on human health, and is no longer permitted in European fuels. The ban on the sale of leaded petrol in EU and elsewhere, provides an example to influence other regions.
Examples of enhancing fuels are the further reductions in the levels of lead, phosphorus and alkali metals that improve the performance and life of three-way catalysts and the introduction of ultra-low sulfur gasoline and diesel fuels. Reducing sulfur levels all the way down to near-zero delivers improved performance of catalysts.
The negative impact on catalyst performance of sulfur in gasoline and diesel fuels has been reported by AECC as part of the stakeholders input to the European Commission’s review on fuel quality. A technical summary on EU fuel quality is available on the AECC website.
The sulfate storage and release process was minimized by the introduction of the <10 ppm sulfur diesel fuel being progressively introduce across the EU since 2005. This fuel quality is necessary for the full potential of emissions control systems to be realized. Ultra-low sulfur fuels became mandatory in the EU in 2011 for passenger cars, heavy-duty applications and Non-Road Mobile Machinery. Also, there are concerns over the use of some metallic additives, with suggestions that their use in gasoline fuel may, under some driving conditions, lead to deposits on exhaust system components such as the oxygen sensor and catalyst. Metallic or other ash-forming materials in diesel fuel will also add to the amount of ash captured by particulate filters and may require the system to be designed so as to allow for the additional ash. Detergent additives, on the other hand, offer positive benefits. Their use helps keep the fuel injection system and combustion system clean, so helping to prolong optimum operating conditions for the emissions control technology.
Table 3 Exhaust aftertreatment technologies : Compression Ignition (Diesel) Engines
26. Technology : Diesel oxidation catalyst (DOC)
Emission Impact : High reduction of HC/CO emissions, small to moderate PM conversion. The oxidation of NO to NO2 enhances the performance of SCR/DPF systems.
Significance : Widely used on Euro 2/3 cars and on some US1994 and later heavy- and medium-duty diesel engines. In modern engines, used as an auxiliary catalyst in SCR/DPF aftertreatment systems (NO2 generation, ammonia slip control).
27. Technology : Particle oxidation catalysts
Emission Impact : Up to ~50% PM emission reduction
Significance : Limited commercial application in selected (EGR-equipped) Euro IV heavy-duty truck engines, as well as in some light-duty and nonroad engines.
28. Technology : Diesel particulate filters (DPF)
Emission Impact : 90%+ PM emission reduction
Significance : Mainstream technology used on all Euro 5 and US Tier 2 and later light-duty diesels; in all US2007 and Euro VI and later heavy-duty engines; in all Stage V nonroad engines; in retrofit programs worldwide.
29. Technology : Urea-SCR catalysts
Emission Impact : 90%+ NOx reduction
Significance : Mainstream technology used in US2010, Euro V and later heavy-duty engines; in US Tier 2 and Euro 5/6 and later light-duty diesel vehicles; in nonroad, marine, and stationary engines.
30. Technology : NOx adsorber catalysts
Emission Impact : Up to ~70-90% NOx reduction, depends on the drive cycle
Significance : Used as a stand-alone NOx reduction catalyst in some US Tier 2 and Euro 5/6 light-duty vehicles. Used as a cold-start NOx reduction catalyst on some Euro 6 vehicles with SCR.
31. Technology : Lean NOx catalysts (HC-SCR)
Emission Impact : NOx reduction potential of ~10-20% in passive systems, up to 50% in active systems
Significance : Limited OEM and retrofit commercial application, mostly in the 2000s.
Exhaust Aftertreatment Technologies : Positive Ignition (SI) Engines
32.Technology : Oxidation catalyst (OC)
Emission Impact : 90%+ HC and CO emission reduction
Significance : Used in older gasoline cars (circa 1980-1990).
33. Technology Three-way catalyst (TWC)
Emission Impact : 90%+ NOx, HC and CO emission reduction
Significance : The most important gasoline engine emission control technology. Widely used on stoichiometric SI engines worldwide.
Three-way catalysts (TWC)
Gasoline fuelled vehicles utilise a three-way catalyst that can reduce carbon monoxide (CO), hydrocarbon (HC) and oxides of nitrogen (NOx) emissions by over 99% – if the air-to-fuel ratio (AFR) is accurately controlled. Optimised conversion is achieved with an AFR of 14.7:1.
[CO] + O2 → CO2
[HC] + O2 → CO2 + H2O
[NOx] + H2 → N2 + H2O
Composition: Typically, precious metals (Pd, Pt or Rh) with alumina and rare earth oxide, coated on a flow-through monolith.
34. Technology : NOx adsorber catalysts
Emission Impact :~70-90% NOx reduction
Significance : Used in lean-burn (stratified charge) gasoline direct injection (GDI) light-duty vehicles that were common in Europe in the 2000s.
35. Technology: Gasoline particulate filters (GPF)
Emission Impact : ~90% PN emission reduction
Significance : Increasing use in Euro 6 light-duty GDI vehicles. Expected to be widely used in China 6 light-duty vehicles. Gasoline Direct Injection (GDI) is a key technology of gasoline engine development to reduce CO2 emissions while improving torque and power output. However the drawback of GDI engines is an increase in particle number (PN) emissions compared to conventional Port Fuel Injection (PFI) engines.
Most of the GDI particles are formed during the cold-start phase, catalyst heating mode and dynamic engine modes. Therefore, the injection system including injection operating programme (e.g. number of injections, timing, and amount of injection) has been further developed in order to improve air-fuel mixture in the cold-start phase. Furthermore, internal engine measures such as improved mixture homogenization and minimized amount of injected fuel striking the walls helps to avoid the formation of particles. Thus, latest GDI vehicles can achieve the PN limit of 6×1011/km on the regulatory test cycle (NEDC or WLTC). The RDE procedure however also includes particle counting in a wide range of engine map operation. The Gasoline Particulate Filter (GPF) technology has been derived from successful experience with DPF and is available. It ensures control of ultrafine particles from Gasoline Direct Injection engines under real-world driving conditions.
Table 4 Control, diagnostics & powertrain technologies
36. Technology: Hybridization
Emission Impact : Primarily to reduce fuel consumption
Significance : Hybridization with battery electric drive can enable the engine to operate longer in regions of higher thermal efficiency and less at low efficiency points such as idle and low load. Electric motor boost enables the use of efficiency technologies that might otherwise not be practical because of detrimental performance impacts.
37. Technology : Diagnostics
Emission Impact : OBD ensures long term emissions compliance.
Significance : Intended to detect malfunctions that would cause emissions over the certification test to increase beyond a defined threshold.
38.. Technology : Controls
Emission Impact : Electronic controls ensure accurate control of numerous emissions and powertrain control components can be maintained over the life of the vehicle. Variations in ambient conditions, system integration and system aging effects can be accommodated.
Significance : Diesel engine controls include: EGR control, intake boost pressure control, fuel injection timing control and combustion control. Aftertreatment system controls include: urea dosing, temperature management to ensure high emission reduction efficiency, regeneration control to ensure accumulated materials such as soot, sulfur and urea deposits are regularly removed. Integrated system controls: Some control functions require a strongly integrated approach to ensure the engine and aftertreatment system work together. Examples include the NOx adsorber catalyst that require the engine’s air/fuel ratio to be enriched regularly to remove accumulated NOx; adjustment of engine parameters such as fuel injection timing to raise exhaust temperature for keeping the aftertreatment system efficiency high; and DPF regeneration which may require the engine operation to be strictly controlled to avoid damaging the DPF.
SI engine controls include: Air/fuel ratio control, spark timing control, idle speed control. Aftertreatment system controls include: thermal management to ensure rapid warm-up and high emission reduction efficiency; and air/fuel ratio control to ensure maximum conversion of the TWC. Integrated system controls: The need to accurately control the air/fuel ratio is driven by the very narrow air/fuel ratio window where high conversion of NOx, HC and CO is possible in a TWC.
39. Exhaust gas recirculation (EGR)
EGR is a system developed to incorporate some of the exhaust gases in the cylinder into the combustion process with the intake air. The purpose here is to lower the combustion end temperature and thus the NOx emission values by deteriorating the combustion performance, because high temperature is the main influence in the formation of NOx emissions. The use of the EGR system reduces the amount of oxygen in the cylinder, and therefore resulting in a decrease in the combustion end pressure and temperature. The decrease in the amount of oxygen suppresses the formation of NOx. The exhaust gas recirculated in the cylinder and containing large amounts of CO2 and H2O increases the specific heat capacity of the intake charge, and this reduces the temperature values in the compression and combustion processes . Displacement of some of the oxygen content in the intake charge by the exiting exhaust gases reduces the air excess coefficient and increases the ignition delay by diluting the intake charge. This slows the mixture of oxygen with fuel and therefore the combustion rate.
The EGR system developed to reduce the combustion end temperature in the cylinder has been widely used by automobile manufacturers since the past. The circulation of the exhaust gas with the intake air can be achieved in two different ways; external and internal . In the external exhaust gas recirculation, the exhaust gas taken from the exhaust manifold is sent to the intake stream through a valve and a coolant. In the internal exhaust gas recirculation, unlike the external exhaust gas recirculation, some of the combustion exhaust gas is withdrawn to the combustion chamber before exiting the exhaust valve. This is accomplished by delaying the camshaft and closing the exhaust valves a little later than normal. The late closing of the exhaust valve allows the piston to draw a portion of the exhaust gas at the outlet of the exhaust valve into the cylinder while the piston is moving downward at intake stroke. The engines, which have the internal EGR systems, are run with variable valve timing. Compared with external EGR, the internal EGR system remains weak in controlling exhaust gas into the cylinder. In addition, since no cooling operation can be performed on the recirculated exhaust gas, desired reductions in NOx emission values are not achieved. The internal EGR system is generally preferred for gasoline engines, which have lower NOx emissions compared to diesel engines. On the other hand, external EGR systems are widely used in diesel engines.
Figure 4 shows the structure of a conventional EGR system used in diesel engines. The system simply consists of valve, control unit and coolant. The EGR valve mounted on the intake manifold is controlled by the control unit. The function of the EGR valve is to control the flow of exhaust gas to intake port depending on the engine load. The amount of exhaust gas sent to the intake port may constitute a maximum of 50% of the air taken into the combustion chamber . Because of the high exhaust gas content included in the combustion, the combustion performance is greatly affected; therefore the engine performance can decrease significantly. For this reason, the exhaust gas content mixed with the intake air does not exceed 20% in practice.
Figure 4. Exhaust gas recirculation (EGR).
The cooling of the exhaust gas included in the combustion process in the EGR system allows the higher amount of exhaust gas to be included in the combustion process and at the same time the combustion chamber temperature and hence NOx emissions can be further reduced. For this reason, in the EGR systems, the exhaust gas is passed through a cooler and sent to the intake stream. Cooling is carried out using engine coolant. In an electronically controlled cooling system, the cooling process is optimized depending on different engine loads, temperatures and conditions.
In turbocharged diesel engines, the use of EGR takes place in two different ways; high pressure and low pressure. In a high-pressure EGR system, the exhaust gas is recirculated to the intake channel before the exhaust gas goes to the turbine and in the low-pressure EGR system the exhaust gas is recirculated after passing through the turbine.
Thanks to the EGR system, the NOx emission in diesel engines can be reduced by up to 50% ]. However, in this method, combustion is worsened, engine performance decreases, and other pollutants, especially particulate emissions (PM) slightly increases. At the same time, the EGR system leads to an increase of about 2% in fuel consumption . Due to the flow of the exhaust gas, EGR system can affect the quality of lubrication oil and the engine durability negatively, and erosion on piston rings and cylinder liner can increase . These disadvantages and developed aftertreatment emission control technologies have overshadowed the EGR system .
Exhaust Gas Recirculation System (EGR) : In EGR a certain portion of exhaust gases are directed back to thenbsp;cylindernbsp;head, where they arenbsp;combinednbsp;with the fuel-air mixture and enter thenbsp;combustion chamber. The recirculated exhaust gases serve to lower the temperature of combustion, a condition that favours lower production of nitrogen oxides as combustion products (though at some loss of engine efficiency).Exhaust Gas Recirculation (EGR)
Exhaust gas recirculation is a vehicle emissions control system used in both diesel engines and petroleum engines.
The purpose of exhaust gas recirculation is to reduce the formation of nitrogen oxides.
It’s important to note that while the SCR system helps to remove already formed NOx gases, the EGR system helps to prevent them from forming in the first place.
The EGR system achieves this by recirculating a portion of the engine’s exhaust gases back to the combustion chamber.
The addition of exhaust gas into the combustion chamber reduces the amount of oxygen available in the combustion process. In effect, the reduced oxygen lowers the combustion temperature.
This reduced temperature is important because nitrogen oxide is formed in the combustion process when Nitrogen and Oxygen molecules react at high temperatures. In fact, NOx production increases exponentially as temperature increases, making temperature control a key factor in reducing emissions.
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In-cylinder control measures include slightly reducing compression ratios to bring down both combustion temperatures and NOx formation. This has been a recent trend. A key strategy for NOx control is external exhaust gas recirculation (EGR). A portion of the pressurized exhaust is diverted, cooled, and introduced to the intake manifold (provided that the intake air pressure is not too high), which dilutes and cools the combustion charge, albeit at the expense of power output (exhaust gas displaces the oxygen/air) and reduced fuel economy.
Internal EGR is also available to engine designers, which is attained through clever variable valve timing. This allows some exhaust gas to be retained in the combustion chamber and carried over to the intake stroke. Internal EGR is much more common in the gasoline engine community than in the CI engine world today because internal EGR is not cooled and, thus, much less effective than cooled external EGR for diesel NOx control.
In diesel engines, primary application is to control NOx emissions. Commonly used in many light- and heavy-duty diesel engines. High pressure EGR delivery can introduce a fuel consumption penalty through higher pumping losses. Low pressure EGR has lower pumping losses but is more difficult to control during transient operation. Other measures to limit potential increases in soot and possibly HC and CO can be required.
Exhaust gas recirculation is a vehicle emissions control system used in both diesel engines and petroleum engines. It is used in diesel engines to reduce the NOx content of emission. It works by recirculating a portion of exhaust gas back to engine cylinders, which replaces some of excess oxygen in the pre-combustion mixture. Because NOx forms when a mixture of nitrogen & oxygen is subjected to high temperature, lower combustion chamber temperatures caused by EGR reduces the amount of NOx generated. A sophisticated version of EGR was introduced by Chrysler in 1973.
The purpose of exhaust gas recirculation is to reduce the formation of nitrogen oxides.
It’s important to note that while the SCR system helps to remove already formed NOx gases, the EGR system helps to prevent them from forming in the first place.
The EGR system achieves this by recirculating a portion of the engine’s exhaust gases back to the combustion chamber.
The addition of exhaust gas into the combustion chamber reduces the amount of oxygen available in the combustion process. In effect, the reduced oxygen lowers the combustion temperature.
This reduced temperature is important because nitrogen oxide is formed in the combustion process when Nitrogen and Oxygen molecules react at high temperatures. In fact, NOx production increases exponentially as temperature increases, making temperature control a key factor in reducing emissions.
Commonly referred to as EGR, exhaust gas recirculation is a NOx-reduction technology widely used in on-highway and nonroad diesel engines, see Figure 5.
During operation, exhaust gases are recycled back into the combustion chamber. The exhaust gases are mixed with intake air to reduce oxygen content and combustion temperature. It is considered an in-cylinder control technology since it is reducing the creation of NOx.
EGR Disadvantages
2. By reducing the amount of oxygen available for combustion, the EGR system reduces the power and performance available to a vehicle.
3. By introducing exhaust gas into the combustion chamber, the EGR system may cause soot deposits on glow plugs, intake valves, cylinder heads, turbochargers etc.
4.Exhaust Gas Recirculation is very useful in lowering emissions and keeping the engine temperatures low as possible. EGR is mostly available with turbocharged petrol and diesel engines and petrol engines adopted this technology much earlier than diesel engines. Talking about the construction, the exhaust manifold channels some of the exhaust gases into the intake manifold and that helps to decrease the engine temperature and overall emissions. EGR is used in diesel engines to reduce NOx emissions whereas it comes in handy to increase efficiency in petrol engines.
Exhaust gases are already hot so you must be thinking about how something hot can decrease the temperature? Well, the exhaust gases are inert gases that mean they are already burnt. Hence, they decrease the combustion capacity of each of the cylinders and that decreases the heat produced and less heat means lower emissions. It is worth noting that EGR reduces the power to some extent and this system doesn’t work when absolute power is demanded. If the engine is not running hot, EGR also doesn’t work until the engine touches its optimum operating temperature.
40. Evaporative Emission Control
Evaporative emission control not only helps to reduce the emissions but also saves fuel and increases the overall efficiency of the vehicles. In technical terms, an evaporative emission control system eliminates the evaporation of hydrocarbons from the fuel tank and circulates them into the combustion chamber. The key mechanical component of this emission control system is the carbon canister that stores the hydrocarbons. The carbon canister absorbs the fuel vapours via loose chemical bonds and releases them via the purge solenoid that is controlled via the onboard computer module.
The fuel vapours are flammable and channelled into the combustion chamber for combustion. It saves fuel and also controls the emissions as the vapours evaporate through the fuel tank lid when opened for refuelling. The vapours are injected into the intake manifold of the engine via the PCV valve.
41. Air Injection System:
In a typical air-injection system, an engine-driven pump injects air into the exhaust manifold, where the air combines with unburned hydrocarbons and carbon monoxide at a high temperature and, in effect, continues the combustion process. In this way a large percentage of the pollutants that were formerly discharged through the exhaust system are burned (though with no additional generation of power).
42. NOx Adsorbers
NOx Adsorber Concept
The concept of NOx adsorbers has been developed based on acid-base washcoat chemistry. It involves storage of NOx on the catalyst washcoat during lean exhaust conditions and release during rich operation and/or increased temperatures. Depending on the NOx release strategy, NOx adsorber systems can be classified as:
In active NOx adsorbers, stored NOx is periodically released—with a typical frequency of about once per minute—during a short period of rich air-to-fuel ratio operation, called NOx adsorber regeneration. The released NOx is catalytically converted to nitrogen, in a process similar to that occurring over three-way catalysts (TWC) for stoichiometric gasoline engines. Normally, three-way catalysts are inactive in converting NOx under lean exhaust conditions, when oxygen is present in the exhaust gas. By alternating the lean storage and rich release-and-conversion phases, the applicability of the three-way catalyst has been extended to lean burn engines.
The technology was first commercialized on lean burn gasoline direct injected (GDI) engines, followed by light-duty diesel engines around 2007/2009 (US Tier 2, Euro 5). NOx adsorber systems have also been introduced for NOx control from stationary natural gas turbine applications . Due to their declining NOx reduction performance at higher exhaust temperatures, active NOx adsorbers found practically no application on heavy-duty truck engines.
In light-duty engines—with increasing focus on in-use emissions outside of laboratory test cycles and the real driving emissions (RDE) testing requirements that became effective in the EU in 2017—the use of active NOx adsorbers as the primary stand-alone aftertreatment technology for NOx control has declined considerably. However, “part-time” active NOx adsorbers (or multifunctional NOx adsorber/DOC catalysts) continue to be used to control cold start/low temperature NOx emissions in many light-duty diesels with urea-SCR systems. A close-coupled, actively regenerated NOx adsorber is used during cold start and once exhaust temperatures increase, NOx is reduced over the SCR catalyst using urea. This and other configurations of emission systems with NOx adsorbers are discussed under NOx Adsorber Applications.
Passive NOx adsorbers (PNA)—a simpler but less mature variant of the technology—adsorb NOx during vehicle cold start and release it when the exhaust temperature increases—without a rich regeneration—to be converted over a downstream NOx reduction catalyst. Hence, passive NOx adsorbers (or traps) are not a stand-alone NOx control technology—rather, they can be used with urea-SCR aftertreatment to improve the low temperature performance of the system. An early demonstration of PNA technology was conducted by Cummins on their 2.8 L US Tier 2 Bin 2 diesel engine developed under the US DOE ATLAS project . Passive NOx adsorbers were also considered for heavy-duty diesel engines meeting California and other low NOx standards on the order of 0.05-0.02 g/bhp-hr .
Other Concepts. A technique called Selective NOx Recirculation (SNR) was an early concept of a NOx adsorber system without catalytic reduction of NOx. In the SNR concept, two NOx adsorbers are installed in parallel in the exhaust system. Control valves allow to switch the gas flow, so each of the adsorbers alternates between adsorption and desorption modes. While in the desorption mode, the NOx carrying gas from the adsorber is recirculated to the engine intake air. This way, the desorbed NOx can be reduced through in-cylinder reactions during combustion. The regeneration strategy of SNR adsorber was not demonstrated. In experiments involving feeding NO/NO2 from bottles to the diesel engine air intake port—i.e., not accounting for the adsorber performance—a NOx reduction efficiency of 60% was achieved.
Terms & Definitions
Different authors use different terms when discussing (active) NOx adsorbers, such as:
All these names are synonyms describing the same emission control technology. The term lean NOx catalyst, on the other hand, refers to the selective catalytic reduction of NOx by hydrocarbons—an entirely different technology that should not be confused with NOx adsorbers.
The passive NOx adsorber has also been referred to as low temperature NOx adsorber (LTNA)—a term proposed by researchers from Ford .
The Technology of Emissions Reduction
NOx Reduction and the Chemistry of SCR…
NOx is a general term referring to Nitrogen Oxide (NO) gas and Nitrogen Dioxide (NO2 ) gas. It forms at the high temperatures of the engine combustion process.
NOx is successfully converted to Nitrogen gas (N2) using Selective Catalytic Reduction, or SCR, the most effective technology available today.
SCR works by promoting a chemical reaction between NOx and ammonia gas (NH3). The ammonia comes from a reductant fluid such as aqueous urea solution, that is injected into the exhaust stream ahead of the SCR catalyst. The heat of the exhaust transforms the reductant fluid to NH3. This chemically reacts with the NO and NO2 at the surfaces of the highly porous catalyst to form nitrogen gas (N2) and water vapor (H2O).
The resulting nitrogen gas and water vapor are harmless to health and the environment, and flow through the system and exit the exhaust stack.
Reducing HC & CO: The Oxidation Catalyst
Carbon Monoxide (CO) and Hydrocarbon (HC) emissions result from incomplete combustion of fuel. Oxidation catalysts, or “oxicats”, are highly effective devices that reduce CO and HC emissions by 90% or more from diesel and lean-burn natural gas engines.
Oxicats consist of a substrate made up of thousands of small channels. Each channel is coated with a highly porous layer containing precious metal catalysts such as platinum or palladium. As exhaust gas travels down the channel, HC and CO react with oxygen within the porous catalyst layer to form carbon dioxide (CO2) and water vapor (H2O). The catalyst can also reduce a small amount of Particulate Matter (PM) by converting it to CO2.
The resulting gases then exit the channels and flow through the rest of the exhaust system.
Eliminating Soot Emissions: Particulate Filter Mechanics…
Particulate emissions (PM) are also a consequence of the combustion process, primarily due to unburned fuel. PM, more commonly known as soot, is removed from the exhaust using a Diesel Particulate Filter, or DPF. The structure of the DPF is similar to that of an Oxicat, except every other channel is blocked with a dense plug.
To remove the soot, raw exhaust enters an open channel at the front of the DPF. The walls of the filter are engineered to be semipermeable, allowing gases to pass through but trapping the soot particles inside.
The DPF contains a thin layer of catalyst that chemically converts the soot particles to harmless carbon dioxide (CO2 ). This allows the filter to function continuously during engine operation. The catalyst also allows the DPF to function as an Oxicat, reducing HC and CO in addition to PM.
Once converted, the gases pass through the filter’s porous walls and into a much cleaner exhaust stream.
Putting it all together into a comprehensive emissions control system…achieve compliance with all regulated engine emissions: NOx up to 98%, PM more than 85%, CO up to 95%, HC up to 90%.
How it works:
The Reductant: Ammonia, Aqueous Ammonia or Aqueous Urea?SCR systems work by catalyzing the reduction of NOx with ammonia. The ammonia can be introduced from a variety of sources, including directly as a gas (anhydrous ammonia) or as an aqueous ammonia solution. However, it is more common to use aqueous urea solution as the reductant. A solution made from urea, a common component of agricultural fertilizer, is much safer and easier to handle, and is readily available in several different concentrations and storage capacities. There are advantages and disadvantages to both ammonia-based and urea-based reductants.
Methods for reducing NOx and PM emissions in compression ignition engine: A review
NOx reduction technology improvements are essential for the worldwide environment [1]. Three methods produce NOx during combustion: prompt, thermal and fuel [1]. The triple bond in between nitrogen molecules is vanishes with higher temperature of combustion (1300 °C) [2], As a result, atomic nitrogen becomes extremely reactive, interacting with oxygen to form thermal NOx. In flame front of HC flames, free radicals are produced [3] enhances fast NOx synthesis. Since, biodiesel has no fuel-bound nitrogen, the major processes in biodiesel-fuelled engines are thermal and fast NOx [4]. According to the NREL, the increase in NOx is due to chemistry of HC free radicals before combustion, rather than thermal NOx formation. [5]. As a result, the production of quick NOx would rise. The fast process is impacted by free radical concentrations in the reaction zone, but the thermal mechanism is mostly unaffected due to fuel chemistry. According to Brezinsky et al. [6], the quantity of acetylene produced by biodiesel's unsaturated components is the major contributor to increased NOx generation. The CH radical is produced by acetylene, and it is responsible for the fast generation of NOx. Biodiesel's impact on NOx emissions, according to some experts, is mostly due to increased combustion temperatures. According to Lin et al. [7], reasons of complete combustion are the high oxygen content of biodiesel and high temperatures produces more NOx. Lu et al. [8] It was found use of ethanol with biodiesel is effective to reduce NOx. Due to advance injection deeper fuel penetration occurred which minimized the soot formation.
The NOx family consists of seven different chemicals [2]. This includes NO, NO2, N2O, N2O2, N2O3, N2O4, N2O5. Source of N2 is air(major) and it is also available in fuel (minor). As N2 is stable and less reactive therefor N2O, N2O2, N2O3, N2O4, N2O5 are less reactive. NO2 is a significant air pollutant, which interacts with different gases in the atmosphere and produces O3 and acid rain. Ozone which we want to eliminate is tropospheric ozone, which is available in air we inhale [9]. The most common type of NOx emissions from combustion is NO. Pathways of NO formation are at low temperature nitrogen is available as diatomic form, it is stable and not reactive but at high temperature around 1800 K, N2 is dissociated into monoatomic, which is unstable and more reactive. Hence to avoid NO formation Low Combustion Temperature Technology plays an important role. High temperatures are available when equivalence ratio is equal to 1 and NOx is more at equivalence ratio equal to 0.95. According to the Zeldovich equations, NO is created at higher temperature above 1300 °C, below 760 °C NO not created [9].
There are six processes namely, nucleation, pyrolysis, coalescences, surface growth, oxidation and agglomeration, that convert liquid or vapor-phase HC to solid soot particles, then back to gaseous products. Fig. 1 illustrates the soot formation process with the help of above-mentioned methods [10].
· 1. Modification of fuel or Pre-combustion Method
Addition of Additive
Using Blend/Alternative Fuel
Modification in Air Technology
In-Cylinder Method/On-Cylinder Method
Using Low Combustion Temperature Concept/ PCCI, HCCI
Reducing Compression Ratio
Split Injection
Modification in Combustion Chamber
Modification in Nozzle Diameter
Varying Injection Timings/Retardation of Injection Timing
Varying Injection Pressure
Water Steam Injection[4]
Exhaust Gas Recirculation
After-Treatment Method
SCR
DOC
DPF
Lean NOx Trap
Selective Non-Catalytic Reduction
Hybrid Systems
SCR + DPF
LNT + DPF
EGR + DPF
EGR + SCR
SCR + EGR + DPF
Additive + Blend + SCR
Additive + EGR
SCR/DPF + LNT/DPF
Pre-Combustion + In-Cylinder + After-Treatment (Proposed)
Pre-combustion or fuel alteration is one technique, in-cylinder method is another, and post treatment approach is the third. To minimize exhaust emissions, the Pre-Combustion technique involves making changes to the fuel, such as adding additives[5], [6], [7], [8], [9], [10], [11], [12], [13]. PPDA, EDA, BHT, Ethanol, Methanol, Diethyl Ether, and other additives are added to gasoline in this system to reduce emissions. NOx levels fall in this system, while CO and HC levels rise. To minimize NOx and PM emissions, various mixes of diesel and biodiesel are used in varying proportions in some situations. PPDA is an effective addition for lowering both NOx, PM. The reduction in NOx is mostly due to the generation of free radicals in the PPDA addition. PPDA may have antioxidant properties that effectively capture free radicals in the burning process, reducing NOx significantly [8]. Technologies, such as high-pressure injection and combustion cylinder modification, are used in in-cylinder combustion[6], Low Combustion Temperature (LTC) technique[3], [14], [15], [16], EGR, iEGR. LTC regulates combustion temperatures to keep NOx and soot production to a minimum. LTC techniques include HCCI and PCCI. In the combustion chamber, LTC emission creation is achieved. Fuel efficiency is greater in BS IV technology than BS-III technology due to improvement in exhaust and combustion system. As opposed to SCR systems, iEGR does not require Ad-Blue for emission control, resulting in no payload loss and cost savings. It also has no additional sensors or complicated electronics, and many of the same parts as BS III engines. If anybody want to make the transfer from BS-IV to BS-VI as painless as possible, iEGR is the best BS-VI technology.
After-treatment techniques such as SCR, on the other hand, are used to decrease NOx emissions from diesel and gasoline engines[17], [18], [19], [20], [21]. SCR is widely used after treatment technique. LNT technique for NOx removal, DPF for Particulate matter removal[22], [23], [24], [25]. A diesel exhaust burner is a choice for quick heat-up at the expense of fuel economy. VVA system with early exhaust valve opening, intake valve closure modulation, and cylinder deactivation is excellent for after-treatment temperature management.
Because the above-mentioned approaches are insufficient to meet forthcoming severe requirements, hybrid technologies are also employed to minimize these two emissions. Because connected LNT-SCR systems have shown that by adding a downstream SCR catalyst, the NOx reduction performance of LNT catalysts may be considerably improved [25], there has been a lot of study done on paired LNT-SCR systems. Many academics have looked at using a combination of LNT/DPF and SCR/DPF to reduce NOx and PM at the same time[24]. The transformation of NOx is rapidly reducing due to difficulties that need to be rectified in order to run the LNT/DPF system smoothly. It is critical to preserve the lifespan of the SCR/DPF since the temperature must be maintained during DPF regeneration. As a result, in certain circumstances, a combination of LNT/DPF + SCR/DPF is examined [30]. It is the most recent one aimed at lowering PM and Emissions of NOx. At 450 °C, the LNT/DPF has a 25 % NOx conversion rate, while the hybrid system's NOx transformation is 40 %. The PM's LOT50 in the hybrid system was 598 °C, while it was 627 °C in the LNT/DPF system. Furthermore, compared to the LNT/DPF system, this hybrid model oxidized PMs at a faster pace. As a result, hybrid system > LNT/DPF > DPF > SCR/DPF was the order of good PM oxidation properties [30].
44. THE NEW EXHAUST AFTERTREATMENT SYSTEM FOR REDUCING NOX
EMISSIONS OF DIESEL ENGINES: LEAN NOX TRAP (LNT).
Lean-burn engines provide more efficient fuel combustion and lower CO2 emissions compared with traditional stoichiometric engines. However, the effective removal of NOx from lean exhaust represents a challenge to the automotive industry. In this context,lean NOx traps (LNTs), also known as NOx storage-reduction (NSR) catalysts, represent a promising technology, particularly for light duty diesel and gasoline lean-burn applications. Moreover, recent studies have shown that the performance of LNTs can be significantly improved by adding a selective catalytic reduction (SCR) catalyst in series downstream .
The lean NOx trap (LNT) technology is considered as one of the aftertreatment solutions to reduce NOx emissions from lean burn or diesel engines, those that operate under highly oxidizing conditions. Typically, LNT catalysts usually consisting of precious metals (e.g. Pt, Pd, Rh), a storage element (BaO) and a high surface area support material (e.g. Al2O3, CeO2, ZrO2), operate under transient conditions that include lean and rich phases. Pt material properties, including dispersion and particle size, are known to be important factors in determining NOx uptake performance, since Pt provides mactive sites for NO oxidation to NO2 necessary for storing NOx as nitrates, and for the reduction of nitrates to N2 . LNT catalysts are typically composed of at least one precious metal component and one alkali or alkaline-earth component which are supported on a high surface area refractory oxide. These catalysts operate in a cyclic manner, whereby the catalyst stores or "traps" NOx as nitrate species during lean period of operation. Periodically a short rich pulse is introduced so that the trapped NOx is released and reduced to N2, thereby regenerating the trapping capacity of the catalyst . The LNT operates by storing NOx during normal lean operation (when excess oxygen in the exhaust hinders the chemical reduction of NOx). The LNT must be regenerated periodically by a rich excursion, a brief event in which the exhaust air/fuel ratio (AFR) is driven rich to achieve overall reducing conditions. The excess-fuel derived reductants (HCs, CO, H2) cause the release and subsequent reduction of the stored NOx .
Reducing emissions of the nitrogen oxide (NOx) in diesel engines become a main goal for the future, because it's needed to maintain the diesel engine as a propulsion source with highest fuel economy. Due to strict legislation, the automotive manufacturers are forced to adjust to the new requirements on exhaust emissions.
This paper presents the necessity and importance of the LNT catalyst for reducing the NOx emissions of the new generation of diesel engines. Also, this study wants to show the benefits of this new technology in combination with other catalytic systems.
2. Operating characteristics and performance
Under lean conditions, NO is oxidized to NO2 in the gas phase over platinum. The resulting NO2 is adsorbed on an oxide surface as barium nitrate. Typical adsorbents include oxides of potassium, calcium, cerium, zirconium, lanthanum, calcium and barium. The sequence of steps is :
Step I: NO + ½ O2 --> NO2
Step II: BaCO3 + 2NO2 --> Ba(NO3)2
At rich air fuel ratios, the adsorbed barium nitrate is released from the trap as barium oxide. In the presence of reducing agents such as CO, HC and H2 and Pt/Rh catalyst, the NOx is converted to nitrogen and the trapping constituent, barium carbonate is restored.
The sequence of steps is :
Step III: Ba(NO3)2 -->BaO + 2NO2
Step IV: 2 NO2+ 2 CO/HC ==Pt/Rh=> N2 + 2CO2
Step V: BaO + CO2 => BaCO3
Sulfur present in the fuel acts as a poisoning agent. In the combustion process, the sulfur is oxidized to sulfur dioxide (SO2). The sulfur dioxide is oxidized to sulfur trioxide in the presence of platinum. The sulfur trioxide is trapped as barium sulfate at the trap operating conditions .
NOx adsorber technology removes NOx in a lean (i.e. oxygen rich) exhaust environment for diesel engines. The mechanism involves (see figure 1 and figure 2) :
- Catalytically oxidizing NO to NO2 over a precious metal catalyst;- Storing NO2 in an adjacent alkaline earth oxide trapping site as a nitrate;
Fig. 2 The lean NOx trap running under period regeneration (rich) conditions [6,7].
Fig. 1 The lean NOx trap running under lean conditions [6,7].
The stored NOx is then periodically removed in a two-step regeneration step by temporarily inducing a rich exhaust condition followed by reduction to nitrogen by a conventional three-way catalyst reaction.
In order to reduce the trapped NOx to nitrogen, called the NOx regeneration cycle, the engine must be operated rich periodically for a short period of time (a few seconds). This cycling is also referred to as a lean/rich modulation. The rich running portion can be accomplished in a number of ways including :
• Intake air throttling;
• Exhaust gas recirculation;
• Post combustion fuel injection in the cylinder;
• In-exhaust fuel injection;
It is likely to dominate for small diesel vehicles, such as passenger cars, at least in the near term, as it is a more cost effective solution for these vehicles than SCR. In a NOx trap, a NOx storage component, usually an alkali or alkaline earth metal oxide, e.g. barium oxide, is added to the platinum and rhodium catalyst. Under normal lean diesel conditions this stores NOx as nitrate, but every 60-120 seconds or so the nitrate regenerates by running the engine with more fuel for a few seconds, so that some carbon monoxide and hydrocarbon can reduce the nitrate to harmless nitrogen .
In the scientific work, are presented the typical application, estimated cost per vehicle, the advantages and limitations of LNT system. As typical application are the light-duty vehicles with engine displacements below 2.0 liters. Cost 320 $ for engines < 2.0 l and 509 $ for displacement of engines > 2.0 l.
Advantages :
- 70-90% efficiency at low loads;
- Good durability and NOx reduction performance;
- More economical for engines less than 2.0 l;
- No additional reductant tank is needed (lower packaging constraints);
- Reductant fluid not required (no refills needed).
Limitations :
- NOx storage capacity is limited by physical size of LNT; Highway and uphill driving can overwhelm the capacity of LNT, leading to high NOx emission events;
- For engines > 2.0 l, more frequent trap regeneration events are required, leading to additional fuel penalties (around 2%);
- Precious metal usage is high (approximately 10 to 12 g for a 2.0 l engine);
- NOx adsorbers also adsorb sulfur oxides resulting from the fuel sulfur content, and thus require fuels with a very low sulfur content (< 10 ppm). Sulfur compounds are more difficult to desorb, so the system has to periodically run a short “desulfation” cycle.
Application examples: VW Polo, VW Golf, BMW 2-Series .
The Lean NOx Trap (Fig. 3) is also now known as a NOx Storage Catalyst or NOx Adsorber Catalyst . It collects NOx using compounds that form nitrates under stable conditions in lean operation.
The LNT was originally used on gasoline direct injection (GDI) engines which could switch between normal gasoline operation (at or around stoichiometric air/ fuel ratios) and lean mixtures. Any sulphur build-up is exhausted by running at an elevated temperatureof between 600°C and 700°C. This is rather more easily achieved on gasoline engines which are able to run up to 900°C compared to the diesel engine’s 700°C.
In a diesel engine use may be made of the very flexible “Common Rail” fuel system to create a “post” injection to effect the required temperature rise. An unwanted emission from the LNT is that of ammonia. This requires an oxidation catalyst to keep within the European limit of 10 ppm at tailpipe. The reducing chemical equations are :
Ba(NO2)2 => Ba + N2 + 2O2 (1)
Ba(NO3)2 => Ba + N2 + 3O2 (2)
For diesel LNTs the future challenge is to maximize NOx conversion at low speed driving conditions as well as providing high NOx conversion during high speed driving . The Selective Catalytic Reduction (SCR) system is proposed as first choice for large vehicles which require high NOx conversion efficiencies over high vehicle mileage (such as SUVs for US Tier 2 Bin 5 emission standards). The LNT technology is considered as an attractive alternative for smaller vehicles with lower NOx reduction efficiency demand (e.g. for EU5 and post EU5 legislation) .
A major challenge in the future for LNT technology is desulfurization and thermal aging and thus the long-term stability. Conversely, system packaging in the vehicle including the required SCR catalyst, tank volume and the low temperature activity will be important issues to be solved for SCR technology . A second considerable challenge remains, which is the issue of the infrastructure for the urea distribution, especially in the U.S. The concerns of the EPA regarding this technology remain and have to be addressed by each manufacturer that attempts to launch a diesel vehicle in the U.S. using SCR exhaust aftertreatment .
3. System combination between LNT and SCR
The removal of NOx and particulate emissions in light-duty diesel vehicles will require the use of aftertreatment methods like Diesel Particulate Filters (DPF) and Selective Catalytic Reduction (SCR) with urea and Lean NOx Trap (LNT). A new combination is between LNT and SCR, which enables on-board synthesis of ammonia (NH3), which reacts with NOx on the SCR catalyst .
The SCR may utilize any NH3 emanating from the Lean NOx system to eliminate further NOx from the tailpipe. This has been used, for instance, on a Mercedes-Benz “Bluetec” vehicle system and may become a much more general approach as the diesel engine OEMs are faced with ever more stringent NOx legislation . In the case of the LNT/SCR dual bed, again the amounts of NOx removed are lower at any temperature because of the inhibition of CO2 on the storage of NOx. The NOx removal efficiency is always higher for the hybrid LNT–SCR systems, both dual bed and physical mixture compared to single LNT, in the absence and in the presence of CO2 and H2O. This is due to the contribution of NOx stored onto the LNT catalyst and of N2 produced by the SCR reactions over the Fe-ZSM-5 catalyst during the lean phase. The presence of CO2 and H2O reduces the NOx removal efficiency over all the investigated systems .
J. Wang et al. have studied the effect of simulated road aging on the NOx reduction performance of coupled Pt/Rh LNT and CuCHA SCR catalysts using H2, CO and C3H6 as NOx reductants ..Figure 4 shows the product selectivity for the LNT catalyst and LNT-SCR systems using 2.5% CO as the reductant. As was the case for H2, the selectivity of NOx reduction to NH3 over the LNT is increased after aging. It is also noteworthy that after aging a decrease in the selectivity of NOx reduction to N2O over the LNT Fig. 3 The lean NOx trap .
Fig. 4 Comparison of product selectivity over LNT catalyst and
LNT–SCR system using 2.5% H2 as reductant; top: 192°C; middle:274°C; bottom: 413°C [14].
The selectivity to N2O is high at 192°C for both the fresh and aged LNT when CO is used as the reductant, although the low NOx conversion at this temperature limits the N2O emission from the LNT in absolute terms. In general, the factors controlling the selectivity of NOx reduction to N2O are poorly understood, although catalyst composition appears to play a major role. The nature of the reductant has also been highlighted; however, published data are conflicting on the subject of which reductant affords the highest selectivity to N2O. This is presumably a consequence of differences in the composition of the catalysts used in these studies, as well as the use of different reaction conditions . In figure 5 [15] is presented a LNT-SCR system for NOx reduction.
The LNT-SCR system has several significant benefits in comparison with NOx reduction technologies. Most importantly, the LNT-SCR system requires only fuel as the reductant and therefore eliminates the need and associated cost for the urea infrastructure. The LNT-SCR system also has several advantages over an LNT only system. First, the SCR catalyst eliminates NH3 slip from the LNT by storing it and subsequently catalyzing its reaction with unreacted NOx from the LNT. Second, the presence of the SCR catalyst relaxes the NOx conversion requirements of the LNT. Consequently, the LNT catalyst volume in the LNT-SCR system can be lower than for an LNT only system, reducing the precious metal costs for the system. Third, the durability of the LNT-SCR should be superior since the system requires both less frequent and shorter desulfations than an LNT-only system owing to its higher overall efficiency and mitigation of H2S emissions, respectively . Figure 6 shows an SCR deNOx system in its most extensive layout. SCR systems for Euro 4 (requiring about 60% NOx reduction) will generally not have the NO to NO2 catalysts. Also then hydrolysis catalyst is optional, since the SCR catalyst itself is very effective for hydrolysis. An NH3 clean-up catalyst may be applied as a safeguard measure. The urea dosage control will be open-loop with look-up tables for NOx or urea quantity as a function of engine speed and load and catalyst temperature. Primary advantage of a closed-loop control strategy with a NOx sensor is that urea dosage can be adapted to engine-out NOx variations due to variations in ambient conditions and fuel quality
.
Some key differences between EU and US NOx technology control choices (e.g., the prevalence of LNT in Europe, and the emergence of combined SCR+LNT solutions in the US, likely because this type of solution is ultimately required for compliance with the low-emission bins of US Tier 2 regulations) seem to indicate that the different regulatory frameworks (the US has lower nominal emission limits, more demanding test cycles, and a robust enforcement and compliance program that the EU lacks) have a direct influence upon the technological choices made by diesel passenger car manufacturers .
4. System Combination Between LNT, SCR and DPF
Additionally a DPF (Diesel particle filter) may be added to the LNT+SCR system for treatment of particulates. DPFs will become necessary for Euro 6 and beyond as partical number legislation has been introduced for Diesel and DISI Gasoline types .
The combination system between Lean NOx Trap (LNT), Selective Catalytic Reduction (SCR) and Diesel Particulate Filter (DPF) catalysts, is shown in figure 7. Engine NOx is reduced by the LNT and SCR catalysts. The LNT stores NOx and undergoes controlled periodic regeneration, releasing the NOx as nitrogen and ammonia. The SCR collects the released ammonia and uses it to continuously treat the remaining NOx. A Diesel Particulate Filter (DPF) traps Particulate Matter (PM) and undergoes periodic regeneration .
5. Conclusions
The lean NOx trap (LNT) technology is considered as one of the aftertreatment solutions to reduce NOx emissions from lean burn or diesel engines, those that operate under highly oxidizing conditions. LNT catalysts are typically composed of at least one precious metal component and one alkali or alkaline-earth component which are supported on a high surface area refractory oxide. LNT catalysts usually consisting of precious metals (e.g. Pt, Pd, Rh), a storage element (BaO) and a high surface area support material (e.g. Al2O3, CeO2, ZrO2), operate under transient conditions that include lean and rich phases.
NOx adsorber technology removes NOx in a lean (i.e. oxygen rich) exhaust environment for diesel engines. For diesel LNTs the future challenge is to maximize NOx conversion at low speed driving conditions as well as providing high NOx conversion during high speed driving. The LNT technology is considered as an attractive alternative for smaller vehicles with lower NOx reduction efficiency demand (e.g. for EU5 and post EU5 legislation).
A major challenge in the future for LNT technology is desulfurization and thermal aging and thus the long-term stability. The SCR catalyst eliminates NH3 slip from the LNT by storing it and subsequently catalyzing its reaction with unreacted NOx from the LNT.
The presence of the SCR catalyst relaxes the NOx conversion requirements of the LNT. Consequently, the LNT catalyst volume in the LNT-SCR system can be lower than for an LNT only system, reducing the precious metal costs for the system. The durability of the LNT-SCR should be superior since the system requires both less frequent and shorter desulfations than an LNT-only system owing to its higher overall efficiency and mitigation of H2S emissions, respectively. Further it’s recommended more tests and experiments using catalyst systems combined LNT-SCR-DPF and LNT-SCR-DPF with Diesel Oxidation Catalyst (DOC), and the diesel engines tested to be fueled with alternative fuels, such as simple mixtures bodiesel-diesel (with different concentrations of biodiesel),
Fig. 5 Coupled LNT-SCR System for NOx reduction .
Fig. 6 Layout of urea SCR deNOx system .
Fig. 7 Combination system between LNT, SCR and DPF .
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