This paper reports work which is part of continuing research into the use of natural gas as an alternative fuel for automotive engines. Much work has previously been reported on the use of natural gas in small spark ignition engines and the optimisation of these engines. Since a large potential use of natural gas is in heavy transport engines and since few large spark ignition engines suitable for gaseous fuel are available from manufacturers, there is a need to investigate the conversion of existing diesel engines to spark ignition operation.Two naturally aspirated diesel engines were characterised in the diesel mode to determine power, fuel consumption, peak combustion pressure, exhaust gas temperature and selected emissions characteristics over the engine speed and load range. After conversion to spark ignition operation these characteristics were again determined. The effects of various engine and tune parameters on performance were evaluated. The principal parameters considered were compression ratio, spark timing and air-fuel ratio.Compression ratios of 18:1, 15:1 and 13:1 were used. This series of compression ratios permitted the effect on peak combustion pressure, exhaust gas temperature and fuel consumption to be investigated. At each compression ratio the effects of spark timing and air-fuel ratio were determined. The results show no unexpected trends compared with normal spark ignition engine operation. It is significant, however, that with appropriate tuning the natural gas engine can supply higher power than the original diesel engine. In addition, at wide open throttle, the spark ignition engines give efficiency results which are approximately equal to those from the diesel engines at similar load and speed conditions.One of the converted engines has been optimised and installed in an urban bus which has been in operation for a year (58,000 km) with a compression ratio of 15:1. Operating experience and on-road fuel consumption data for this bus, compared with equivalent diesel powered buses are presented.