High Compression Engines Promise Greater Fuel Economy r\. SIX-CYLINDER automobile engine built and tested by the General Motors R e search Laboratories operates a t a compression ratio almost double t h a t of present motors and gives fuel economies up t o 40%, declared C. F . Kettering, General Motors vice president in charge of r e search, in a paper delivered before t h e summer meeting of the Society of Automotive Engineers in French Lick, Ind., recently. Almost 30 year3 ago Mr. K e t t e r ing delivered a paper before the same group on t h e same topic, " M o r e Efficient Utilization of Fuels," and in the current paper he relates the developments in fuels and engines which h a v e taken place in the intervening period. I n t h e past 30 years the petroleum chemist has developed new fuels, new refining methods, and tetraethyllead. These developments h a v e raised t h e octane n u m b e r of gasoline available at t h e corner gas station from about 50 to 80 and in turn enabled t h e automotive engineer to push compression ratios up from about 4.5 t o I to 6 or 7 to 1. Their combined efforts have resulted in a one-third reduction in the fuel consumption of the modern motor car, and he stated t h a t a saving of another one-third can be made through additional cooperative effort between the petroleum chemist and t h e automotive engineer. " T h e key to higher efficiencies in internal combustion engines is in t h e increase of t h e compression r a t i o , " he declared, a n d pointed out t h a t the greater efficiencies of Diesel engines was due in large p a r t to t h e high compression ratios under which they operate. Diesel engines are a t present operating a t compression ratios of between L4 a n d 17 to 1 with brake thermal efficiencies as high as 3 8 % as compared to the 20 to 2 5 % efficiencies of the automobile engine with its compression ratio of about 6.5 to 1. Spark ignition engines with compression ratios similar to those of Diesel engines should have comparable efficiency, Mr. Kettering stated, b u t for such engines to be successful we m u s t have the proper fuels. Since the discovery more than 30 years ago t h a t there is a definite relation between knock and molecular structure, the petroleum industry has been producing fuels of consistently higher octane rating. One such fuel, t r i p t a n e (2,2,3-trimethyl butane), has an unknown knock rating because it is beyond the octane scale, b u t he said t h a t there was evidence t h a t its performance number with tetraethyllead added m a y be as much as 500 under some engine conditions as compared with 100 for iso-octane. With triptane as a fuel it would be possible to build an engine with a compression ratio as high as 15 to 1 t h e speaker declared, and this General Motors proceeded to do. T h e first step in the program of designing a high compression engine was the conVOLUME
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struction of a single cylinder engine in order to obtain necessary data. Mr. Kettering described this engine as being of 30 cubic inch displacement, bore and stroke were 3 3 /s X 3 3 A, and it was designed to give the required strength and rigidity for compression ratios up to 15 to 1. Overhead valves were used, and the compression ratio was varied from 6.2 to 1 to 15 to 1 in five steps by changes in pistons. From results obtained in tests with the single cylinder engine a six-cylinder, high compression engine was designed so as to be interchangeable with a standard engine for installation in a standard 1946 production model car. Mr. Kettering stated t h a t it was designed according to conventional procedures except t h a t it was made rigid enough to carry the higher loads imposed for they had found t h a t problems connected with engine roughness, ignition, high friction, idling, and others had been the result of carrying low compression designs beyond their range. H e claimed t h a t the new engine encountered no special difficulties from roughness and high friction and t h a t it weighed no more per horsepower t h a n stock engines although no weight restrictions had been placed on its design. H e also stated that experience gained in building Diesel engines was of valuable assistance. In order to get a good basis for comparison, the displacement of t h e high compression engine was selected so as to give approximately t h e same horsepower as the standard engine throughout comparable speed ranges. A compression ratio of 12.5 to 1 was chosen because single engine data had shown t h a t most of the gains in efficiency could be obtained a t this r a t i o . Comparison of dynamometer performance of the two engines indicated the overall superiority of the high compression engine. T h e speaker stated that a measure of the gain in specific o u t p u t is the increase in maximum brake mean effective pressure which was 109 p.s.i. a t 1,250 r.p.m. for the stock engine and 130 p.s.i. a t 1,750 r.p.m. for t h e high compression engine. A comparison of the full throttle performance of the two engines shows t h a t the friction m e a n effective pressure and mechanical efficiency are both improved in the high compression engine, indicating t h a t it is possible to design such an engine without excessive friction, he added. I n fuel consumption comparisons, the high compression engine showed a specific minimum fuel consumption of 0.40 lb. of fuel per brake horsepower in comparison with 0.54 for t h e stock engine, a gain of about 3 5 % . Mr. Kettering also pointed out t h a t t h e figures for t h e high compression engine are comparable with full throttle Diesel fuel consumption indicating t h a t a t equivalent high compression ratios it makes little difference whether there is »
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s p a r k ignition or compression ignition. H e also revealed t h a t the high compression engine loses less h e a t per horsepower through the exhaust a n d the cooling water and utilizes more in producing power. H e a t rejection for the- high compression engine is almost 3 0 % lower thian for stock engines. T h e final test for arty engine is how it operates under actual driving conditions. For these tests the high compression engine was installed in one of two of t h e sa .ie model stock automobiles both equipped with Hydra-Matic transmissions. According to Mr. Kettering t h e two cars handled about the sairie, and it was difficult t o tell them apart except a t high speed where the high compression c a r was somew h a t faster. Under conditions of constant speed, level road driving t h e high compression car gave from 35 to 4 0 % bettor economy iD miles per gallon. At 4 0 rap. hi. the standard car had a fuel economy of 18.5 m.p.g. while t h e test car used 26.5 m.p.g. Under widely varying road conditions the average gain in economy wis about 3 3 % , and trips made in city traffic show gains of over 4 0 % . He state
