ENGINE CHARACTERISTICS UNDER HIGH COMPRESSION 230007
This Annual Meeting paper is a report of a series of tests conducted during the summer of 1922 by the authors at the Engineering Experiment Station of Purdue University. The work consisted of research into the operation of internal-combustion engines under comparatively high compression on ordinary gasoline without detonation. The compression-ratio of the engine was 6.75 and the compression pressure was 122 lb. per sq. in., gage. The ingoing charge was passed through a hot-spot vaporizer and thence through a cooler between the carbureter and the valves. Jacket-water temperatures between 150 and 170 deg. fahr. were carried at the outlet port of the jacket.
The theory held by the authors as to the causes of detonation of the combustible charge is presented briefly. The source of the two phases of detonation encountered in this work is believed to be overheated areas in the combustion-chamber. The methods of combating these hot spots are given in detail, and the special equipment applied to the engine to accomplish the desired result is described. The effects of load, speed, compression-ratio and mixture-ratio are studied, and curves showing the variation in the engine characteristics due to each factor are submitted. All tests were run without a trace of detonation.
The results of the work point to the following conclusions:
Automobile engines designed to give a uniform cooling of the combustion-chamber walls will permit the use of much higher compression-ratios than those employed at present, with a consequent gain in engine power and economy
The fuel-air ratio for maximum economy borders upon the lean limit for reliable combustion
For maximum power, the mixture-ratio need not exceed 0.075 lb. of gasoline per lb. of dry air
With concentrated heating of the mixture at the carbureter outlet, a local temperature of 125 deg. fahr. is sufficient to give good operation at full throttle. Cooling to 100 deg. fahr. at the valves is desirable if the compression pressure of the engine is high. At part loads, a hot-spot temperature of 175 deg. fahr., with or without intermediate cooling, will give good results with lean mixtures