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Developing Small High Speed Oil Engines

8th April 1955, Page 45
8th April 1955
Page 45
Page 45, 8th April 1955 — Developing Small High Speed Oil Engines
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Five Experts Describe Problems of Design and Operation, and Suggest the Lines That Future Development Will Take

pAPERS read at a general meeting of the Automobile Division of the Institution of Mechanical Engineers in London on Tuesday indicate that much attention is being devoted to the development of the small high-speed oil engine.

The problem of attaining a balanced output between all cylinders over the wide range of speed required was referred to by Mr, G. M. Barrett, M.B.E., T.D., B.Sc., and Mr. H. G. Frceston, both of the Thornton Research Centre of the Shell Petroleum Co., Ltd. It was difficult, they said, to satisfy all operating conditions with

one fixed ignition timing. Reducing ignition delay by the use of suitable fuels might be a palliative, but the real answer was to abandon the orthodox in-line jerk pump. On the design side, Mr. B. W. Millington, B.Sc.(Eng.), and Mr. M. H. Howarth, MA., of Ricardo and Co., Engineers (1927), Ltd., said that volumetric efficiency was dependent upon good breathing. There was a problem with the oil engine, because, on account of the lack of clearance between the piston and valves at top dead centre, it was difficult to use a large-diameter inlet valve having a high lift.

Mr. N. M. F. Vulliamy, M.A., chief experimental engineer, F. Perkins, Ltd., said that in considering an oil-engine replacement unit for a petrol engine, it should be compared with the smaller of the alternative petrol engines, which appealed because of its lower running costs and relative simplicity.

High Standards in Atomization Needed

MR. BARRETT and Mr. Freeston alsaid that for high thermal efficiency it was essential that the major part of the combustion should take place close to top dead centre. This requirement called for an operating cycle closer to the Otto cycle than the Diesel cycle. Assuming that 300 crankshaft travel was allowed for injection and atomization of fuel, ignition delay and the major portion of combustion, there was approximately one millisecond available at 4,000 r.p.m. to complete these processes. It was clear, therefore, that the cycle must approximate to that of the petrol engine, but with the advantage of the high compression ratio.

The output of the oil engine was limited by acceptable exhaust conditions, and at these speeds the necessity of achieving efficient mixing of

fuel and air would undoubtedly call for high standards of fuel atomization, probably combined with fast air movement.

The authors thought that successful designs would be of the separatechamber type. Such engines, they said, would be expected to have higher airutilization factors, and be more flexible and possibly quieter, over the loadspeed range. Additionally, the better organized air movement was less affected by crankshaft speed, thus permitting the use of a single-hole or pintle nozzle.

In place of the in-line jerk pump, it was said that distributor-type pumps. as well as the B.I.C.E.R.A. servo-operated pump, lent themselves better to the incorporation of advance and retard mechanisms responsive to engine speed, and possibly engine load as well. Distributor-type pumps possessed the further advantage of improving the output balance, whilst the servooperated pump provided a flexible control of injection rates.

Scaled-down Versions Work Well

IN dealing with some aspects of the I design and performance of small high-speed oil engines. Mr. Millington and Mr. Howarth said that scaled-down versions of large engines could be made to perform adequately. A weakness of the oil engine was its restricted speed /range, for it was necessary to limit the maximum speed to a figure at which the volumetric efficiency had only begun to fall because of the diverging characteristics of the engine and fuel pump.

These speakers also agreed that successful designs would be based on a separate-chamber combustion system.

particular attention being devoted to a high degree of turbulence. This system, they said, did not require the exact matching of air and fuel movement that was so necessary in an open combustion chamber and which was difficult to maintain over a wide speed range. By careful selection of the fuelinjection-system components, the degree of over and under advance might be minimized, For instance, by the use of a fuel-injection pipe of larger bore than had been customary, a reduction of the change of fuel-injection delay over the speed range might be obtained. Similarly, by employing an unloading valve of such size that the residual line pressure rose to several hundred p.s.i. at high speed, a further reduction in delay change might be achieved.

Noise, they said, was a sore point, but these small units, if properly mounted, were really troublesome only at very low speeds. The reduction of noise without loss of performance was an elusive problem. Perhaps the most fruitful approach at the present time was to reduce the radiation of noise from inadequately stiff surfaces—the rocker box, timing cover or sump, for instance.

Essentials in Small Oilers

NAR. VULLIAMY considered that the almost essential features of the small high-speed oil engine that was to replace the petrol unit were a road performance equivalent in acceleration and speed, with no smoke and quick cold starting. These features were to be coupled with a low price.

Although there were difficulties, it was thought that before long reasonably acceptable results would be obtainable making engines of this type available for small vans and utility vehicles.

For a given maximum torque, he said, the maximum power obtainable might need limiting in the interests of economy. The road performance of the oil engine was not to be judged by the high-powered petrol unit.

Mr. Vulliamy was of the opinion that much had to be done to the engine, both internally and externally, to reduce noise.


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