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The use of compressed natural gas (CNG) as a fuel for heavy transport engines is still in the development stage, however, experience has shown promising results in certain applications. While most of the recent work done in New Zealand on CNG use has been centered around non-turbocharged, high speed, spark ignition (SI) conversions, there is still likely to be a role for dual fuel engines in some applications. This paper presents a review of New Zealand experiences with the use of CNG as a fuel in heavy transport diesel engines. Both dual fuel and 100% CNG fuelling (dedicated spark ignition conversions) are discussed. The advantages and disadvantages of each are reviewed in terms of vehicle performance, fuel costs and operational experiences. Brief reviews of experimental results and discussions of technical aspects of conversions, experience in fleet use, economic and operational aspects, current research and computer modelling of fleet performance are also included.

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.

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.