Sudden Advance in Oil Engine Design Unlikely
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C.A.V. Chief Design Engineer Describes Steps Taken to Improve the Performance and Longevity of Oil Engines: Maintenance Work on Injectors Greatly Reduced
ANY prospect of outstanding improvement in the efficiency of oil engines was extremely remote and improvements to be made would certainly result from the aggregation of a number of small gains, rather than from one revolutionary discovery, said Mr. W. E. Nicolls, chief design engineer, C.A.V., Ltd., at a conference held by that company at Harrogate last week.
His company had been engaged for several years in the study of the combustion process in direct-injection engines, and the way in which it was influenced by injection characteristics, such as fuel-droplet size, spray distribution and penetration. It was too early to forecast the full extent of the improvements which might result, the initial aim being to increase understanding of what went on inside the combustion chamber, concerning which there had been a great deal of guesswork and little fact.
Oil Engine Efficiency It would be misleading to think that the efficiency of an oil engine could be expressed by any single figure. The specific consumption on a brake-horsepower basis varied with different speeds and different loads, and the requirements for optimum efficiency under one operating condition might differ appreciably from those at a different combination of speed and load.
• Another influence on economy, said Mr. Nicolls, was the shape of the engine torque curve, which was largely a function of the pump-delivery characteristic. The normal characteristic of fuel-pump delivery on a directinjection engine was nearly flat. If the output was set to give the maximum torque which the engine could develop at the speed corresponding to maximum efficiency, commonly around 1,100 r.p.m., efficiency at maximum speed might fall off appreciably, frequently accompanied by exhaust smoke. Were it not for the reverence accorded to maximum brake horse-power, the fuelpump curve could fairly readily be modified to give a falling pump delivery VIM.
"1 have yet to hear a valid argument put forward as to why maximum brake horse-power should matter at all on a commercial vehicle operated in this
country," he observed. "A torque curve which rises with falling speed should, on the other hand, enable the engine to 'hang on' in a higher gear and so operate at a more efficient speed." lvfaximum b.h.p. implied full throttle and peak engine r.p.m„ which in top gear meant excessive speed and in indirect gear was to be discouraged if fuel economy, to say nothing of long engine life, was the operator's aim. Engine efficiency, as measured on the test bed, and fuel economy on the road did not necessarily go together.
"In passenger service, the engine spends an appreciable time doing nothing," continued Mr. Nicolls. "Observations made by our laboratory have shown that on a fair average 20 per cent, of the total time is spent with the engine idling at bus stops or traffic
hold-ups. A modern 10-litre directinjection engine, idling in neutral at a speed of 350 r.p.m., consumes approximately one-fifth of a gallon per hour. In the case of a double-decker vehicle averaging, say, 12 m.p.h., and returning a total fuel consumption of 10 rn.p.g., the proportion of the fuel consumed accounted for by idling is approximately 3i per cent."
Idling on three cylinders had sometimes been proposed to save fuel. Checks on a typical engine had in fact shown that for a given idling speed, this system required more fuel than idling on all six cylinders, as a result of greater frictional losses in the cylinders which were not firing.
Slow Idling
C.A.V., Ltd., had been convinced of the desirability of low idling speeds in the interests of fuel consumption, also because of the marked reduction of noise. Although the hydraulic governor was capable of controlling idling speed below 200 engine r.p.m., such speeds were not usable in most cases, because of limitations imposed by flexible engine mountings. Until mountings could be devised with a lower natural frequency, the potentialities of the hydraulic governor could not be exploited to the full.
The trend of engine development dur ing the past 15 years had steadily increased the severity of the duty imposed on the fuel-injection equip
ment. Early experience with multihole injectors had led many operators to accept the costly practice of examining them at intervals of 5.000 miles or
less. By careful research into the factors affecting hole blockage and seat life, and the incorporation of improved technique, there was no longer any justification for this frequent attention.
Observation a few years ago showed that the service interval could be increased at least to 20,000 miles without loss of operating efficiency. Whilst such a figure might not yet have received universal acceptance, the interval had in many cases been pulled up to 15,000 miles with corresponding savings in injector maintenance costs.
Case for Hydraulic Governor The one component which had suffered more than any from the general trend in engine development was the mechanical governor. The realization of this was one of the primary reasons for the development " of a hydraulic governor which, having no heavy rotating masses, was virtually unaffected by the conditions of the drive. The company had ample justification for the soundness of its arguments in the considerable number of units which had been examined after, 100,000 miles with no measurable wear on any working part. The life of the governor, so far as replacement of parts was concerned, seemed to be infinite.
In the discussion that followed, Mr. V. H. F. Hopkins observed that fuel economy was small meat in the agricultural and earth-moving world, as compared with the continuance of work without breakdown. Further thought towards making the hydraulic governor operate on a closed circuit would be profitable, especially for overseas.
To meet conditions in export markets, said Mr. St. John Brown, manufacturers ought to provide equipment that could be dealt with by the Asiatic or African. mechanic. Designers had to seek the lowest common denominator and the aim of the oil-engine manufacturer should be to simplify equipment and maintenance work.