Boosting Hybrid Appeal

Watching the news recently an interesting story came to light about US based company Wrightspeed. Ian Wright, the founder being one of the original founders of Tesla. Wright exited the Tesla business early on and set up his own powertrain development company Wrightspeed, based in California. 

 Wrightspeed were in the press recently as they have won a $30Million USD contract to retrofit “hundreds” of buses in New Zealand with their series hybrid system. This project is interesting for several reasons.

As well as introducing new vehicles with low emission powertrain, it’s a good call to re-power existing buses, this is re-cycling on a large scale, upgrading and improving existing vehicles to extend their life is something that AVID has been involved in for many years with its Micro Hybrid Systems and supporting our customers with their full hybrid re-power programs providing our electrified ancillary systems and advanced thermal management solutions.

What is interesting about this system is that Wrightspeed will use a turbine to drive a high speed generator,

 and then an electric powertrain to drive the bus. The benefit of this approach are multiple:

A high speed turbine generates much lower NOx and PM emissions than an equivalent compression ignition Diesel engine. It is possible to achieve Euro 5 emissions levels with no aftertreatment and better than Euro 6 with a much much smaller aftertreatment package than required for a Diesel. With a good thermal recuperation system and two stage compression as used by Wrightspeed a well managed turbine should be able to achieve peak levels of thermal efficiency approaching that of a good diesel engine.

The turbine and generator can be significantly reduced in size and weight due to the high-speed operation of the turbine. Higher rpm operations allows for a significantly reduced package compared to a conventional diesel engine, a turbogenerator such as this would be typically 25% of the mass of a conventional diesel genset on a power for power basis. Taking hundreds of kilos out of the weight of the powertrain helps overall system efficiency.

Turbines are typically air cooled so require no jacket water cooling circuit, although the Wrightspeed unit does feature an air to air intercooler between the 2 inlet compressor stages. Looking at the layout of the Wrightspeed unit they have integrated the power electronics and high speed electrical machine into the turbine unit, and have an elegant solution whereby the intake air (which will be nice and clean as it will have been through a filter) is being drawn in over the electronics and electrical machine to cool them. There is a relatively small low temperature liquid cooling circuit required for the traction electronics and electrical machines and the ancillary devices.

In theory turbines are relatively simple devices, with few moving parts and a smooth operation compared to a “lumpy” piston engine with hundreds of moving parts and lots of noise and vibration. There are some challenges with making a high-speed turbine robust enough to cope with the rigours of operation in a mobile machine, especially the highly transient and demanding operations of a transit bus!

In the Series hybrid configuration used by Wrightspeed the turbine can be run in a steady state condition to recharge batteries and then be shut down, avoiding part load operation at which a tubine engine is not efficient. Turbines don’t respond well to highly transient operating conditions so are well suited to a series hybrid rather than trying to use them to directly drive the vehicle as was tried by several car manufacturers in the 60’s and 70’s. Plus no gearbox is needed to down speed the output speed of the turbine to the road speed it will be buffered and isolated from the traction system by a battery pack.

Having a turbine on board allows for a radically smaller battery pack, coming in at only 40kWh compared to over 300kWh for a fully electric bus, that’s a massive cost and weight saving. Plus reduces the charging infrastructure demands. The Wrightspeed system is a plug in hybrid meaning the vehicles can be charged and pre-conditioned (heated or cooled) from a shore power supply, but because the battery is smaller sorting out the infrastructure is simpler to charge a number of vehicles in a reasonably short time. The downside is reduced fully electric range.

Using a turbine in a series hybrid like this is not new, and in fact a notable New Zealand company Designline was quite famously producing buses like this more than 10 years ago. Using Capstone turbines as the thermal engine. There were several Designline buses operating on a contract with the Tyne and Wear PTA Nexus in the area where AVID is based, until they were scrapped off due to reliability issues around 3 years ago. The Designline buses suffered reliability problems with their turbines, and although they used the best batteries that were available at the time these also proved not to be very reliable according to people familiar with the vehicles. Designline were perhaps ahead of their time, but these challenges and a lack of market demand caused them a number of severe financial issues. Interestingly Wrightspeed also started out using a Capstone unit and presumably decided they could do it better themselves. A quick online review of Capstone finds that they are still in business but struggling to commercialise their technology and turn a profit as a business, with almost $1 Billion USD of loses carried forwards. Thats an order of magnitude more than the reported $17Million of investment that Wrightspeed has raised.

More recently the Williams Engineering developed Jaguar CX-75 concept hyper car used a pair of Bladon Jets micro turbines as its power source. The principle with using 2 micro turbines rather than a single larger unit is that it is easier to keep the turbine in its efficiency sweet spot at part load charging conditions. Bladon launched with a micro axial flow turbine and are now also producing radial flow units for a range of applications.

Another interesting comment in the press surrounding the Wrightspeed system is the use of automotive derived turbo charger components to reduce costs. Having seen some decent cut away pictures it is not immediately obvious which parts these would be as but it is likely to be the compressor stages of the machine rather than the turbine itself as an expert in the field has told me that without significant modification the turbine wheel of an exhaust driven turbo would deliver a very inefficient turbine.

A further advantage of the turbine is its ability to run on almost any fuel, they are not as sensitive to fuel grade or quality as a Euro 6 diesel, making 100% biodiesel operation feasible and buses that were initially specced for diesel could easily be converted to another liquid or gaseous fuel. An interesting concept is the gasification of waste material or biomass to run the machine which always makes me think of Mr Fusion from Back to the Future!

On the powertrain side of things, Wrightspeed have also made an interesting move with their Geared Traction Drives - GTD. To reduce the size and mass of their electric motors, they have used a variable ratio gearbox, rather than direct drive. This has allowed them to use compact high speed electric motors but still achieve high levels of torque at the wheels but significantly reduce the weight of the drive, neatly demonstrated by this Wrightspeed press picture.

Being able to alter the gear ratio between the motor and the wheels also enables the electric motor to be kept closer to its peak efficiency than a direct drive system. One of the purchase justifications cited by the Infratil was the ability of the Wrightspeed system to climb much steeper gradients than the competition. Which is a function of using these variable speed gearboxes. It will be interesting to see if Wrightspeed have chosen to use 2 GTD's per vehicle, eliminating the mechanical diff, they have used this approach on other demonstration projects but its not clear if these buses will feature the same approach. AVID has also used this approach on past projects, going to two smaller electrical machines rather than a single large unit can significantly improve the overall system efficiency, especially in a highly transient operating condition such as a city bus. 

The turbine engine remains promising way to improve urban air quality, and if Wrightspeed have cracked the reliability and cost this will be an exciting project to follow, with hopefully many more to come, well done Mr Wright, keep up the good work!

About the author; Ryan Maughan is the Managing Director of the AVID Technology Group Ltd. AVID is based in the North East of England and is a leader in the design and manufacture of electrified systems for electric, hybrid and ICE heavy duty and high performance vehicles.