THE STRUGGLE FOR POWER FROM IC ENGINES
Reproduced tram Engineering (London) - Vol. 199, No. 5168 (7th May 1965) with kind permission of the publishers – The Certificated Engineer April 1966.
Higher bmep seems to be the common aim of all diesel engine makers at CIMAC 65. Of the severity of the associated problems of stress and working capacity of components, opinions appear to vary between European and U.S. manufacturers.
As was not difficult to anticipate, the majority of papers in the diesel section of the recent Internal Combustion Engine Congress (CIMAC), held at London's Festival Hall, were concerned with the attainment of higher specific output and the diverse problems encountered along the way. Mr Dingle and Mr Stent, of Davey Paxman, discussed the difficulties they met in developing the Ventura engine to give 200 lb per sq. in bmep. Mr J. Radford and two Mirrlees colleagues described work along similar lines; 200 lb per sq. in has also been the goal of Mr E. K. Brock and his staff at Ruston Hornsby, whose paper stressed the importance of advanced research and development techniques in eliminating uncertainties and reducing the empirical nature of the process of introducing new or uprated designs into service.
With the growing complexity of modern engines, and the importance of adequate resistance to the necessarily much higher levels of mechanical and thermal stress associated with increasing turbocharger pressures, the amount of work required prior to the construction of the first prototype is now much greater than previously. Brock's team had predicted realistic temperatures and heat flows and simulated conditions in a multi-cylinder engine using a single-cylinder unit.
This trend towards making design of highly rated engines a more precise, reliable, and scientific process, will become even more marked in the future, said Mr Brock, with the whole design and development programme accelerated and less prone to error. Thorough predictions of performance, both mechanical and thermodynamic, will be made by calculation before manufacture of even a single-cylinder prototype, which will then be used to verify the theoretical results. Model and rig tests will be used to prove crankcase, connecting rod and cylinder head designs at a very early stage of development, while piston shapes, at present derived largely from trial and error, will be determined to a considerable extent from values of piston temperature and deflection obtained from a digital computer.
American Aspirations
American contributors talked of even higher-pressure ratings than had concerned their European colleagues. Paul Vaughan, a senior engineer with Alco Products, New York, described the development of a turbocharged four stroke engine of 228 mm bore to a bmep of 242 lb per sq. in. He believes that engines with commercial ratings of 275 bmep will be in common use within the next three to four years. In his opinion, there are no appreciable difficulties resulting from the cylinder pressures now in use, but the need for more power must inevitably result in higher pressures and temperatures. Nevertheless, large increases in pressure must be accepted only after all means of holding down pressure have been exhausted. Increased bmep could be obtained, without a significant increase in firing pressure and maintenance of acceptable thermal efficiency, by some reduction in compression ratio. And Vaughan opines that artificial air intake heating should be sufficient to overcome the problem of difficult cold weather starting which would inevitably result. Biceri Limited, whose variable ratio piston was described in these columns recently, are far from certain that any appreciable drop in compression ratio can be tolerated in the interests of reducing peak cylinder pressures without deterioration in performance.
Vaughan also stated that, without question, the biggest obstacle to higher ratings is the development of a turbocharger giving satisfactory operation at higher pressure ratios and temperatures. Temperatures and centrifugal stresses of even higher magnitude can be met by improved materials, but it is also necessary to achieve higher efficiencies.
More ambitious, Mr P. R. Robinson and Mr J. E.
Mitchell, of the Caterpillar Tractor Company, described a programme to develop several sizes of engine with two-stage turbocharging and charge air cooling to maximum bmep's above 300 lb per sq. in. They claimed that, though high output operation increases mechanical and thermal loading, no obvious fixed limit on output had been met.
Hydraulic Fuel Injection
Biceri Limited, who are responsible for some of the advanced IC engine research conducted in the U.K., tend to tread more warily in the realms of such elevated cylinder pressures. In an excellent paper, their research director, Dr W. P. Mansfield, and two colleagues discussed the attainment of a 300 lb per sq. in bmep rating without high mechanical or thermal loading. This high figure was obtained using their variable ratio piston and Dr Mansfield recalled that with fixed pistons 230 lb per sq. in was obtained only at the expense of a ratio too low to afford satisfactory starting and light load running.
Doubts were also expressed as to the ability of conventional fuel injection equipment to meet the requirements of increased speed range likely to be met in the future. Biceri have had available for several years a hydraulically operated system which has been found suitable for use over a wide speed range as well as a wide load range. It has satisfactorily completed some 5 000 hours of field testing and a licence has been granted to Bryce Berger, but the system has yet to be adopted commercially. It would appear that manufacturers are reluctant to adopt radical changes in the basic design of such traditional components the fuel injection pump until such changes are Virtually forced upon them by the inability of the existing components to support further engine development:
Any increase in speed or mep aggravates the problem of injecting the fuel at the high rates required. If the conventional jerk pump is used, increased pressures raise still further the very heavy impulsive forces to which it is subjected, and it must be made extremely robust to withstand them. In addition, high injection rates are inevitably accompanied by some stubborn hydraulic problems in any system where injectors are remote from their pumps and connected by length of apparently rigid (but in practice very flexible) steel pipe.
Biceri (at the time they were known as Bicera), working on high pressure turbocharging, were confronted by the problem of pumps which had practically reached their limit. And consideration was given to the fact that the jerk pump utilizes only about one-third of the plunger stroke, the first and last thirds being required to accelerate and decelerate the plunger. They concluded that if a means could be devised to substantially increase the rates of plunger acceleration, some two-thirds of the stroke could be performed at the high speed essential to injection and usable output of the pump doubled.
There was no way of obtaining this increased acceleration mechanically and it was decided to replace the normal cam follower by a hydraulic piston. Energy was derived continuously from the engine crankshaft through a chain-driven pump and stored in a hydro-pneumatic accumulator. At the appropriate moment, the energy was released by a light cam-operated valve which, being pressure-balanced, was actuated by a very small force. A dashpot was used to arrest the piston, with the pump plunger attached to it, at the end of the stroke.
In this form the system was scarcely tested since it quickly progressed to an all-hydraulic injection system with integral pumps and injectors little bigger than conventional injectors. Two factors contributed to this extreme compactness: the full plunger stroke was available for injection, so the pump could be relatively small; and the loading of the injector needle was assisted by servo fluid with only a very small spring in addition so that the relatively small pump could fit into the space normally occupied by the heavy injector spring. For convenience, fuel was used for the servo fluid and the controlling valve was operated from a small distributor instead of mechanically by a cam, as in the earlier system. This arrangement simplified placing the pump in the cylinder with the injector, so producing a single pump-injector with no tubing subjected to injection pressures.
The scheme described has been produced in two forms, known as the Biceri Mk 1 and Mk 2 systems. The Mk 1 was reported to give excellent injection characteristics over an exceptionally wide range of engine speeds and outputs. It is intended for large and medium-sized engines and possesses several features of particular importance to large marine engines. Where direct reversing is employed, retiming the distributor for astern running would be a very simple matter and the very straightforward procedure for altering the servo pressure provides ready adjustment for the injection pressure.
Similar in principle to the Mk 1, the Mk 2 system was designed for small engines and shares the benefits claimed for the Mk 1 scheme, but close grouping of the various components on a small engine has facilitated several simplifications.
Diesel v. Turbine
An interesting resume on the position of the large 2-stroke marine diesel engine was made by contributors from the Diesel Engine Department of Stork Brothers, Holland, led by Professor Ir. J. J. Broeze of the Technical University of Delft and engineering adviser to Stork. In their opinion, at the present stage in the development of marine propulsion, the direct-coupled super-charged single-acting 2-stroke engine remains the favourite. Its only serious competitor at present is the steam turbine, but it is a matter of speculation as to how long this situation will continue and what will be the form of any alternative dominant propulsive medium.
Fuel supply is certainly the pre-eminent factor in determining future configurations since engineering is now so flexible that the industry will very quickly develop the correct engine to suit the fuel must abundantly available. Thus, the whole of marine engineering today is dominated by the fact that heavy fuel is the prime source of energy available throughout the world at the most attractive prices. Coal was ousted with the ending of colonialism and cheap labour, and the more expensive, lighter petroleum fuels by the introduction of the highly alkaline cylinder lubricant, leaving steam and diesel power to contest superiority on an equalized fuel basis.
Of course, there are other factors besides straightforward fuel' economy, but the diesel engine has a clear lead in this important aspect which it is likely to retain for a long time. To compete, steam plant would require refinement and intricate complication in design, likely to make the result unsuitable for the rigours of operation at sea. On the other hand, the steam turbine has several attractions associated With secondary costs, such as lubricants and maintenance, and the diesel engine will require every possible attention from its sponsors on these scores if its lead is to be maintained.