The diesel today an )morrow
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Rolls-Royce engineer reviews current practices in diesel development and some of the problems that the future may have in store. By Paul Brockington, MIMechE
IMPROVING a basic diesel engine package detail by detail with due regard to cost effectiveness is a continuing process that is essential to progress. The power of a basic package can be increased by up to three times the normal rating by turbocharging and other means. But a large-capacity diesel has advantages over a smaller, more highly rated engine and full allowance has to be made for neglect in service by the typical operator.
Will powers of 300 bhp, 400 bhp or higher output be required (after all) some time in the next five to 10 years for trunking vehicles in this country apart from special applications? And what will be the effect of anticipated anti-pollution laws in America, and later in other countries, on diesel-engine design, notably regulations that impose a limit, on the emission of oxides of nitrogen?
Mr R. H. Whiteside, chief engineer of Rolls-Royce Motors, Diesel Division, is an enthusiastic realist. When he poses such general concepts and questions he does so with a far-reaching knowledge of operator preferences, prejudices and shortcomings and with an appreciation of the potential of other forms of power unit. If his views have a moral it is that improving performance by painstaking attention to detail is greatly preferable to achieving the higher performance by complication. He emphasizes that obtaining additional horsepower from a turbocharged engine is rarely difficult, but it does not follow that the result will necessarily be a good truck engine.
It is in obtaining high output at modest combustion pressures and temperatures while retaining good cold starting and freedom from smoke and above all without complication of the engine design that timeconsuming detailed research pays off..There are at Rolls-Royce, Shrewsbury, very-highoutput engines operating in the laboratory giving proof of this philosophy.
Complication and simplification can be confused. For example (Mr Whiteside points out) increasing rpm is a ready means of raising output, but this reduces the time available for the combustion process and may thus reduce combustion efficiency. The wider the engine's operating speed range the greater the compromise which has to be made in valve timing, swirl levels and fuel-injection characteristics; increasing the rpm may well reduce torque back-up and necessitate •the use of a more complicated multi-ratio gearbox. However, a medium-speed engine with good torque back-up can have a flexibility that enables it to be successfully employed with a gearbox having a reduced number of gear ratios.
Turbocharging a long-stroke mediumspeed engine to increase its output may be regarded as a complication that simplifies combustion problems when compared with providing the same horsepower by reducing the stroke and increasing the bore and rpm. Mr Whiteside considers that the extra time available for combustion in a long-stroke medium-speed engine is of even more importance than the reduced surface /volume ratio of the combustion chamber. The problems of controlling crankshaft torsional vibrations generally increase as the engine's operating speed range is extended.
So turbocharging is not necessarily a complicated means of raising power output but is rarely a satisfactory substitute for lack of swept volume in an engine. A small, highly ' turbocharged engine lacks torque when the vehicle is starting from rest, and excessive clutch slip is the result. And this can be a decisive factor in favour of the larger engine in many applications.
In all cases, continued improvement of the basic package in detail is fundamental to evolving an engine design which caters for the increasingly demanding requirements of the customer, and some examples of recent research and development work at Rolls-Royce give point to this dictum. Normally it is necessary that the cylinder heads of a Rolls-Royce diesel be serviced after, say, 150,000 miles to remove deposits from the inlet valves and ports if a high standard of efficiency is to be maintained. Inlet port and valve deposits are produced by pressure lock-up at the end of the exhaust stroke.
The design of an experimental camshaft was called for to extend this servicing period. And the research engineeers were given the additional target of providing a significant improvement in volumetric efficiency and intake noise. Objectives also included the development of a camshaft that gave a similar improvement when the engine was turbocharged. The team considered that they needed at least nine to 12 months; they were given three and completed the job in six. All targets were met and together with other design advantages the team were able to mark up a creditable 8 per cent improvement in volumetric efficiency.
This is typical of the kind of work that makes a good engine better without complication, but it has to be cost-effective and discussions regarding value analysis can be controversial, as exemplified by discussions on the continued use of sodium-cooled exhaust valves. Nonsodium-cooled valves can have sufficient heat resistance to cope with quite arduous service between overhauls given that servicing recommendations are observed. But in default of regular servicing, the exhaust valve temperature will be increased by incorrect tappet clearances, faulty injection, a dirty air cleaner or cyinder-head port deposits.
Mr Whiteside maintains that cost effectiveness must take into account these irregularities in servicing and the more expensive sodium-cooled exhaust valves continue to be specified for Rolls-Royce engines.
Whether or not to employ an aluminium cylinder block is a "yes" or "no" question, the answer to which depends on the willingness of a sufficient number of customers to pay a premium price for a reduction in weight of 400 /5001b and on the acceptability of the higher noise level of an engine with an aluminium crankcase compared with a cast-iron counterpart. Currently the answer is no, and weight reduction by other means is a continuing process (as it would have been in any case) with a more modest target. Employing aluminium for timing gear covers and sumps and for any application in which a large surface area of metal is used tends to increase the noise level and puts greater emphasis on minimizing noise at the source.
Development work has included building an engine block with cast-in cylinders and thin dry-liners, which eliminates the need for bulkheads and provides a lighter and more rigid structure. As has been shown by full-power tests, it also reduces the noise level, which is a fortuitous but valuable advantage_ Torsional-vibration problems are endemic to any engine design exercise and due account must be taken of overspeeding in service. Balance weights are fitted to the crankshaft to reduce main bearing loads, but a fully-counterbalanced crank vibrates torsionally at lower speeds than a partially or non-counterbalanced shaft.
Rolls-Royce compromised by fitting a crankshaft in its Eagle engines which is approximately 70 per cent counterbalanced, due regard having been given to out-of-balance forces that can be produced which tend to distort the block structure. The cause of premature cylinder-head gasket failure due to frettage was in a past application attributed to prolonged high-speed operation of the noncounterbalanced crankshaft.
Some overspeeding of an engine cannot be prevented when a fully laden vehicle is driven down gradients and gross overspeeding can cause engine damage. When the flywheels of engines employed in ore-carrying trucks in the Middle East repeatedly loosened the attachments bolts there could only be one explanation. The operators were warned that gross overspeeding was increasing the crankshaft rpm to a critical speed at which torsional vibrations were produced. They were recommended to fit tachographs to enable checks of maximum rpm to be recorded, and were told that the trouble would not recur providing normal overspeed limits were not exceeded. There were no more reports of loose flywheels.
Crankshaft configuration was an important aspect of the design of the V8 automotive diesel prototypes of 8and I6-litre capacities. By rearranging the crank throws (and firing order) and other modifications, the overall length and weight of the engines was considerably reduced. These engines are approximately square in their bole/stroke ratio. Rolls-Royce has no immediate plans to introduce further V-form engines; Mr Whiteside is of the opinion that the straight six has currently a great deal to offer as a truck engine and that it is likely to prove the best work-horse in future.
The piston is described by Mr Whiteside as the Achilles heel of the diesel. While a deep bowl type of combustion chamber provides better efficiency and reduces combustion noise, it tends to increase the heat flow through the ring pack when compared with shallow bowl types. Experiments with heat reflecting ceramic coatings applied to piston crowns have shown that they can reduce ring-belt temperatures. Although the gain is worthwhile it does not significantly increase the efficiency of the cycle.
Cracking of the coating on sharp bends is a problem. It would be impossible to eliminate sharp bends in a combustion chamber without some loss of efficiency. Steps taken to lower ring-belt temperatures by modifying under-crown profiles and by the use of oil cooling (in various forms) have been much more rewarding.
Research projects
Reducing piston friction and pumping tosses is high on the list of Rolls-Royce research projects, and significant gains have been achieved by reducing the number of rings and relieving the non-thrust areas of piston/cylinder contact. Improving combustion by modification to the chamber shape, swirl levels and injection characteristics is also a continuing research exercise in the Rolls-Royce laboratories. The use of pilot injection and/or fumigation is not ruled out as a future possibility.
' A number of engines have been experimentally equipped with. tuned induction systems to promote pulsecharging of the cylinders. Because of its bulk, a tuned manifold presents installation difficulties and industrial engines are likely to be the first to benefit from the application of such systems.
However, Mr Whiteside is confident that the difficulties of applying a system to a vehicle engine can be overcome. It could be used to provide a useful increase in output over range of rpm or to increase the margin for neglect of air cleaners and so on before exhaust smoke became a problem. Design exercises also include "tidying up" the Eagle engine to give a cleaner appearance and
facilitate maintenance. It may be possible to produce a tuned inlet manifold in a more compact form that would match this tidied-up version.
Mr Whiteside is an advocate of the road-speed governor. Many operators who acknowledge that the use of a higher-powered engine can reduce fuel consumption and maintenance as well as journey times, opt for a smaller-powered engine in practice, he observes, because drivers over-exploit a high-speed capability in favourable conditions with a consequent decrease in fuel economy and a general increase in wear and tear. A road-speed governor adjusted to allow a sensible maximum speed would aid economy and promote safe driving. Moreover, it would probably be approved by the unions.
Dealing with the threat of future anti-pollution laws, Mr Whiteside does not discount the possibility of bmep being severely restricted to reduce the emission of oxides of nitrogen (NO„) and considers that this would favour the development of high-capacity naturally aspirated engines of lightweight construction. He points out that the knowledge being accumulated by research chemists may in due course enable an exhaust purifier to be developed for diesel engines which would be a less restrictive solution to the problem.
In the past few years the formation of NO x in petrol engines has been reduced by 90 per cent without significant effect upon performance. To comply with prospective legislation in America, a further reduction of 90 per cent will be necessary. At this point in time many engine designers are saying that it cannot be done, but it will.
Although the diesel engine has until now been considered a "clean" engine when compared with the petrol engines, the control of harmful emissions from a diesel engine may inhibit engine design to a greater extent than controlling the emissions of petrol engines; but Mr Whiteside regards the prospect as an exciting challenge to diesel engine designers. It might even encourage the reintroduction of the pre-combustion-chamber cylinder head.
Gas turbine solution
Could the gas turbine offer a solution to the NO problem? Mr Whiteside considers that the gas turbine with a regenerative heat exchanger has yet to prove that it is a viable. proposition, mainly because of technical problems with the heat exchanger. Established types of gas turbine operate at relatively low pressure ratios and turbine entry temperatures, and consequently their emission levels tend to be quite low.
To achieve competitive fuel economy it is probable that the gas turbine will have to operate at higher pressure ratios and turbine entry temperatures. It would seem doubtful whether this can be done without a major increase in the production of oxides of nitrogen.
Very high power outputs are readily obtainable but will they be wanted? An experimental turbocharged/charge-cooled engine based on the current Eagle has been running for some time now in the Rolls-Royce laboratories at bmep up to 300 psi developing 600 hp at the rated speed of 2100 rpm and at peak cylinder pressures little higher than that of standard production turbocharged Eagles.
Running engines at these outputs is a routine research exercise that enables theoretical desk work to be checked in practice and is necessary for continued improvement of the breed. For a given rating a charge-cooler enables the thermal stresses of a turbocharged engine to be reduced, but Mr Whiteside is doubtful whether any vehicle manufacturer will be willing to pay the extra for the cooler on this score alone.
Mr Whiteside says it would be possible to build a low-compression-ratio engine of this type, equipped with variable-compression ratio pistons, which would have a power/weight ratio of 41lb/bhp. A unit could be produced without the complication of VCR pistons that weighed 5 /61b /bhp. Experimental engines equipped with VCR pistons have been tested in Rolls-Royce laboratories.
On the subject of constant-horsepower engines, Mr Whiteside points to the failure of established units having a nearconstant-horsepower characteristic at the upper end of the power curve to gain acceptance on a worthwhile scale. The constant-horsepower engine tends to gain output in the mid-speed range at the expense of maximum horsepower at the upper end.
Truckers tend to buy horsepower not torque, and future construction-and-use legislation will encourage this attitude. Vehicle manufacturers and many operators are aware of its advantages; they know that it enables a gearbox with a smaller number of ratios to be used and that it could have a longer life and give a better fuel consumption than conventional engines. But it suffers on a cost-per-horsepower basis.