Better pistons for better diesels
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by Paul Brockington, MIMechE
THE FUTURE of the piston engine is all-important to the future of the vehicle industry and papers read at the technical symposium of the Associated Engineering Group, held on Wednesday and Thursday of last week at Dunchurch Industrial Staff College, near Rugby, give promise that current and future research and development projects of member companies will accelerate the progress of the diesel engine in competition with other forms of power unit.
Pertinently, Mr R. Munro, director of engineering, Wellworthy Ltd, and Mr G. H. Hughes, technical liaison manager of the company, observed in a paper on piston ring applications in diesel engines: "The pressure of commercial competition both from within the industry and from other potential power units has continued to push up diesel engine ratings at a pace which shows no signs, at the moment, of diminishing."
Comments by speakers during the discussion revealed that the design of piston rings, particularly with regard to scuffing and running-in properties, is one of the most controversial aspects of engine development. Mr M. G. Aue, of Sulzer Brothers, said he had been concerned with piston-ring development for more than 30 years and the piston ring was a miracle, the operation of which was beyond his comprehension. Wear of a ring prevented scuffing. Modern lubricating oils reduced wear to a minimum and thus promoted scuffing.
In the Munro/Hughes paper the authors said that piston rings of the most modern type in production could operate satisfactorily at a bmep of over 300psi in acceptable conditions. These included adequate cooling round each cylinder bore, top groove temperatures of less than 225deg C, a clean cool charge of air, complete burning of the fuel injected and adequate lubrication.
Running-in cycles had a strong influence on break-in scuff and subsequent performance of the piston ring and liner equipment, said the authors. It was importantthat the initial wear process should take place in a controlled manner which would produce the correct surface characteristics before high loads were applied. In a paper on the design of pistons for diesel engines, Mr G. Longfoot, chief designer of Wellworthy, said that there had been a "spectacular increase" over the past 40 years in the power /weight ratio of pistons and the specific power they could accommodate. There appeared to be no immediate limitation to the development of diesel pistons to much higher ratings. If aluminium had insufficient strength, use could be made of SG iron or even steel pistons sprayed with aluminium alloy, a steel piston with a bearing bronze coating being a possibility. Although the variable-compression-ratio piston was not an attractive proposition for bmep below 300psi and as yet there had appeared to be little need for higher bmep, the VCR piston had been shown to be capable of accommodating a bmep of over 400psi and there was no apparent reason why it should not be used to obtain a bmep of 600psi or even higher pressure.
Explosion pressures Earlier in his paper, Mr Longfoot referred to detonation as a cause of piston destruction and said that it was, seemingly, unrelated to temperature effects. It appeared that unburnt fuel could penetrate into small spaces, such as the clearance between the piston and cylinder bore or between the top ring and its groove and be detonated by the pressure rise in the combustion space.
The explosion pressures generated were capable of eroding large portions of the piston. Examination of failures had indicated that the trouble was not necessarily attributable to overheating and was thought to result from defective fuel injection equipment that gave poor atomization.
In a detailed review of oil-cooled pistons, Mr Longfoot made particular mention of the efficacy of oil cooling a piston with the aid of oil-cooling galleries in the crown section and machined grooves in the main body of the piston, the ring belt being shrunk onto the crown and welded. The galleries are, therefore, located on the inside of the ring belt. They are supplied with oil by passages in the crown. A forecast that composite construction would be more extensively used for diesel pistons was made by Mr R. A. Day, chief metallurgist of Wellworthy, Mr R. F. Smart, head of the materials technology department of Associated Engineering Developments Ltd, and Mr N. Tonunis, chief metallurgist, Hepworth and Grandage Ltd, in a paper on piston materials technology. '
Such construction would be made possible by improved joining techniques which would also facilitate the more extensive use of inserts. The application of composite crowns was well established and the further development of this practice could be expected. In a response to a demand for higher fatigue resistance in gudgeon-pin bosses, steel-wire-reinforced inserts had been incorporated in the boss to prevent the spread of fatigue cracks and a study had been made of the use of carbon fibres for this purpose.
Cast-iron would continue to be used for the pistons of very highly rated diesels. An increase in the application of cast-iron pistons depended on the production of thin-wall castings having a weight comparable to that of aluminium alloy.
Speed control A growing need for road speed control of commercial vehicles as a means of reducing fuel and maintenance costs was mentioned by Mr P. LightoWlers, chief project engineer, speed controls, Hepworth and Grandage, in a paper on vehicle cruise controls. Mr Lightowlers cited the A. E. Road Speed Control system, an electro-mechanical device comprising a centrifugal governor assembly driven by a speedometer-type cable from the speedometer drive connector. A spring-loaded linkage between the accelerator pedal and the fuel control arm and a positive linkage connected the spring-loaded linkage to the control. Maximum engine power was obtainable up to the pre-set speed.
A reference to tests of engine bearings of overlay-plated aluminium silicon in the UK, America and Russia was made by Mr G. C. Pratt, manager, materials research department of the Glacier Metal Co Ltd, who said that results had been encouraging.. Although there was little doubt that within the foreseeable future a plastics material would be available with characteristics that were suitable for engine bearing applications, the likelihood of such a material competing on cost with that of a steel-backed strip type was remote.
Grooving in crankshaft bearings should be restricted to a circumferential groove, according to Mr C. A. Perkins, manager, automotive design and development department, Glacier Metal, and Dr J. M. Conway-Jones, manager, current products design and development department of the company. This could be a partial groove if used in conjunction with a cross-drilling in the crankshaft.
Grooving of a bearing was a necessary evil. Oil film pressures developed on the bearing surface that could be more than 50 times the oil-supply pressure and any interruption of the surface interfered with the development of the desired oil film pressure pattern and threw undesirable loads on other parts of the bearing. Oil films in these areas were exceedingly thin, resulting in high operating temperatures and in failures from wiping or seizure or in fatigue damage.
Petrol injection A claim that an improvement in fuel economy of 10/20 per cent had been obtained with petrol injection was made by Mr A. E. Baxend ale, section head, engine and vehicle development and applications, Brico Engineering Ltd, in a paper on exhaust emission control with particular reference to the A.E-Brico electronic petrol-injection system. Control of carbon monoxide emission was obtainable by virtue of the small variation in air/fuel ratio between engine cylinders. Hydrocarbon emission control was provided by the system because it maintained the optimum mean air/fuel ratio under all operating conditions.
In a summary of and commentary on the four papers on pistons, Mr J. E. Robinson, director of marketing, Hepworth and Grandage, appealed for greater co-operation between engine makers and piston manufacturers in the design stage; piston failure often resulted from an engine deficiency such as a faulty gasket or valve. The ability of the piston to cope with peak toads was seldom the limiting factor in engine development.
In the discussion on pistons, Mr C. J. Hind, of the Perkins Engine Group, pointed out that the designer of an automotive diesel was often bedevilled (in contrast to the designer of an industrial engine) by dimensional limitations dictated by installation requirements, notably with regard to water passages between the cylinders and the height of the crown relative to the gudgeon pin. It was necessary to "balance the cost of the piston against the cost of other component parts". A more costly piston could be justified if it enabled the cost of other parts to be reduced.
In reply, Mr Robinson said that if cylinder centres were too close the pistons tended to scuff and oil consumption might be excessive. In dealing with a later question on noise from Mr D. K. Roberts, of BLMC, Mr D. A. Law, Wel'worthy assistant managing director, who chaired the session, said that modifying the structure of the diesel was preferable to reducing piston clearances as the means of reducing noise. Pistons were not the only cause of mechanical noise.
Air cooling Although a paper on air-to-air inter-cooling by Mr M. Bush, assistant sales manager, industrial cooling division, Coventry Radiator and Presswork Ltd, related to large industrial engines, it was claimed by Mr D. A. Winspear, of this company, Mr Law, and other speakers taking part in the discussion, that air-to-air charge coolers had advantages over the water-to-air type for applications to turbocharged automotive diesels. After agreeing with this view, Mr Hind mentioned that Perkins would be equipping a turbocharged diesel with an air-to-air intercooler and that it would be introduced in September of this year. It would develop 55 bhp at 2800 rpm.
Scuffing Whether a piston was scuffed or not could in some cases be attributed to variations in the production of cylinder blocks, Mr G. E. Cayless told delegates during discussions on the second day. Asked if Mr Pratt had agreed with Dr 0. R. Lang, of Daimler-Benz, that the thin overlay of aluminium-silicon alloy automotive bearings became fatigued at a lower loading than the base metal, he said that the cracks took the form of fine crazing and that the overlay, continued to operate satisfactorily. In effect, crazing increased the fatigue strength of the overlay.
Some 115 delegates attended the symposium and included visitors from Italy, Germany, Poland, Holland, Switzerland, Hungary, Norway, Sweden, United States, India and Spain. All the delegates had the opportunity of investigating the research and development work of Associated Engineering Developments Ltd, Cawston
House, where current developments include all-aluminium lightweight radiators and a steam engine. A.E. technicians are confident that it will be possible to produce the radiators at a cost comparable to or lower than that of the conventional type, and that they will have a comparable if not better service life. An experimental single-cylinder steam engine of 175 cc has an output of 20 bhp. The fuel consumption is said to vary between 0.7Ib bhp /h and 1.01b bhp/h. Contaminants in the exhaust are negligible.