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There has been concern that the use of natural gas in engines designed for gasoline fuelling can give rise to excessive temperatures. This is based on experience of exhaust valve recession, high piston temperatures and hot exhaust systems with conversions in New Zealand. In order to establish some data this work has measured exhaust gas temperatures, piston, exhaust valve and cylinder head temperatures in a single cylinder research engine. The data show that at wide open throttle and with the engine tuned to similar operating conditions (ie. the same fuel-air ratio and MBT spark timing), the natural gas fuelled engine generally has lower surface temperatures within the combustion chamber and lower exhaust gas temperatures than the gasoline fuelled engine. When the engine is operated at the same load on the two fuels the combustion chamber temperatures were similar for the two fuels.

There is a strong interest around the world in natural gas as an alternative fuel. This paper is concerned with the option of converting diesel engines to spark ignition operation. Although this may appear to be an outrageous thermodynamic action, it is preferable to using natural gas in a low compression gasoline engine conversion. An investigation is described in which engine maps were produced for a 5.6 litre direct injection diesel engine converted to CNG. The diesel operating characteristics have been compared with those of the spark ignition conversion at compression ratios of 18:1 (the original diesel value), 15:1 and 13:1. Detailed data are presented for the 15:1 compression ratio. These test results are supplemented by results for other diesel conversions. The use of these engines in bus fleet operations is also discussed.