Comparative Studies of Methane and Propane as Engine Fuels

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Abstract
The performance of a single-cylinder, high-speed, spark-ignition engine has been studied with gaseous methane and propane as fuels. The power output and thermal efficiencies have been determined at two compression ratios, 5·5/1 and 10/1. A comparison for the same engine using liquid fuels is given in Appendix I. Both fuels will knock at high compression ratios; the limits have been determined for all ignitable mixture ratios, and also the effect of inlet temperatures has been studied. Both fuels are ‘temperature sensitive’, the effect being most marked with methane. An experimental comparison has been made of the auto-ignition limits of both fuels by the motored-engine method. Both fuels have been shown to give a stable region of visible cool-flame-type pre-reaction below the temperature for complete ignition. Despite the differing ignition temperature and pressure relationship determined by static experiments, a close similarity is revealed by auto-ignition experiments in the engine. Possible explanations of the differing effects are considered. Infra-red absorption gas analysis has been used to detect the small quantities of methane in the exhaust gas surviving combustion. A very low and constant concentration of methane is present with fast-burning mixtures, owing, it is believed, to valve overlap and scavenge effects. As the rich and weak mixture limits for steady running are approached unburnt methane increases in the exhaust, the effect being most marked near the weak limit. These effects are relevant to problems of atmospheric pollution from engine exhausts, and also assist in assigning the fall in engine efficiency at weak mixtures primarily to combustion inefficiency. Both methane and propane are shown to be suitable fuels for high-compression-ratio gas engines. Consideration of the combustion properties of these two pure hydrocarbons of simple structure is also of interest in extending the understanding of the general problem of engine knock.
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