20 Years of
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ii Engine Development
HE new chairman of the Auto
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mobile Division of the Institution of Mechanical Engineers, Mr. CB. Dicksee, M.I.Mech.E., in his address, given last Tuesday, recounted certain of his experiences during 20 years of oilengine development. .
It surprised many that he gave eredi: for originating the development of the automobile oil engine -in Great Britain to the late Lord Ashfield, chairman of what was then the London General Omnibus Co., Ltd., and of its subsidiary, the Associated Equipment Co. Ltd. Lord Ashtield, he said, had realized the enormous saving in fuel cost possible, ' and gave orders in October, 1929, that every possible effort was to be expended in that direction.
Conversion from Petrol
Experiments by A.E.C. in 1928 with a converted petrol engine. on the Acro system, gave encouraging results, and a licence was secured. Conversion from petrol was, however, unsatisfactory, and the author was called upon to design what he claimed to be the first highspeed oil engine intended specifically for road work to reach the production stage here.
After an experimental six units, Lord Ashfieid, in May, 1930, ordered 100, to be distributed widely over the road transport fleet as an "extended experi ment." Unfortunately, the last two words were forgotten and the engine was expected to behave as If it had undergone prolonged development. This, plus users' inexperience, caused much disappointment.
With nothing to go upon, the design was based upon current commercialvehicle petrol-engine practice, and although stiffened up and more generously proportioned, it was found anything but adequate for oil-engine conditions. It had only an idling governor. and was based on a maximum of 2,750 r.p.m., which was even exceeded under service conditions. To' avoid bearing and other troubles, the speed was soon limited to 2,000 r.p.na.
Spurred on by Hope
In the early period, the mechanical troubles were numerous, hut the saving in fuel cost was so great that operators persevered, hoping that, ultimately.' the engine would prove as reliable as its rival. The first real troubles were with both connecting-rod bearings. A fully floating gudgeon pin proved a cure at one end, but, at the other, white metal did not adhere well to nickel-chrome steel, about 16,000 miles being the maximum life obtained. Separate bearing shells of mild steel, white-metalled, gave some improvement, but their life never exceeded some 24,000 miles, failure being mainly through cracking of the white metal, followed by its separation.
Experience proved that , shock was responsible. Finally, copper-lead bear ings provided a solution, giving excellent service on normal, heat-treated, nickel-chrome-steel shafts of 275-300 Brinell, and service mileages .up to 300,000 were not unknown. • Trouble with D-headed bolts for connecting rods were overcome by using a circular head, of relatively small diameter, fitted into a counter-bore in the rod. The head was given a coarse knurl and driven into the counter-bore. This was still in use.
Aluminium caps for main bearings, held by steel straps, were inadequate. Moderate success was attained with white metal run directly into steel caps. The use of copper-lead improved matters, but did not give the same measure of success as with the rod bearings. Now, white metal is retained for the upper halves and copper-lead for the lower.
Bearing Troubles
About 1933, a new and smaller sixcylindered oil engine was produced with a different light alloy • for the crankcase, the previous material sometimes cracking, but with this change mainbearing troubles increased considerably and there were many crankshaft breakages, whilst the lead-bronze bearings used at that time fired and damaged the crankshafts.
Then came difficulties with the pronounced difference in expansion between steel through-bolts and the crankcase material. Normal tightening of the nuts gave a load averaging 20.000 lb. per bolt. The expansion and various modifications in .the crankcase raised the stress in the bolts to 87,500 lb. per sq. in.
As the' yield point of the crankcase alloy was• only 15,000 lb. per sq. in., serious deformation of the material was to be expected, the load on the abutment area of the cap icaching the alarming figure of 37,000 lb. per sq,
Fatigue .Failures
Another result of this was that the main bearings were thrown out of line, intermediate ones as much as 0.01 in. This probably increased the crankshaft bending moment sufficiently to produce fatigue failures. After experiments with alloys used in aircraft engines, which proved a little better, a change was made to cast iron, with scantlings reduced to the minimum. The vveight increase was not prohibitive and the design proved cheaper. The trouble was thus overcome, and to-day cast iron was used exclusively for oil engines of this make.
Breakages with crankshafts were also found to result from stamping shafts with the cranks in one plane and twisting these while hot; theg must now be stamped with the throws at the correct angle. Cold straightening was also ruled out.
Where cylinder feet had been bolted to light-alloy crankcases, excessive loading could occur through the top decks of the block being normally hotter than the lower, so that the block tended to bow in the opposite direction. This trouble disappeared with the use of the same material for both block and crankcase.
Earlier, there were many tracked cylinder heads. These covered three bores and the crack occurred mainly over the centre one as the result of compression stress in the lower face, arising from its inability to expand freely on account of being bolted to the block.
The material probably took a permanent set and, on cooling, experieneed tensile stress: A modification to the combustion chamber improved temperature distribution. Cracking now seldom Occurred.
In the case. of aluminium heads fractures showed marked lifting of the material at the edges of the crack, indicating severe cramping. The solo.
tion found was to provide room for expansion by cutting a slot 3 in. wide right through the lower face and between adjacent bores.
The use of suitable oil additives and of solvent-extracted oils removed many lubrication difficulties.
To-day, only the open combustion chamber was used for the heaviest classes of vehicle made here.
Only a reduction in friction figures would give definite improvement in fuel consumption, and much more than appears possible from any improvement in cycle efficiency
The author stated that some of the difficulties were, no doubt, peculiar to the particular design. whilst certain of them were experienced iecommon with other manufacturers