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In the last decades, emission legislation on pollutant emissions generated by road transportation sector has become the main driving force for internal combustion engine development. Approximately 20% of worldwide emissions of carbon dioxide from fuel combustion come from the transportation sector, and road vehicles contribute up to 80% of those emissions [1]. Light-duty methane gas engines are usually spark-ignited due to similar combustion characteristics for methane gas and gasoline. Since spark ignition requires a low compression ratio to avoid knock problems, gas engines have lower efficiency than diesel engines. A combustion concept that has been successfully applied on large stationary engines and to some extent on heavy-duty engines is dual-fuel combustion, where a compression-ignited diesel pilot injection is used to ignite a homogeneous charge of methane gas and air.

Investigations have been made into control of knock in spark-ignited dedicated gas engines. Knock is a limiting factor determining the realisable rating of a spark ignited engine, and constraining the selection of fuelling and ignition characteristics for optimum efficiency and emissions. The knock phenomenon was studied using four multi-cylinder engines widely used in heavy transport applications, and adapted to high compression spark ignition with natural gas fuelling. Appropriate knock control strategies were developed and refined to accommodate the demands of field application in multi-cylinder engines. A knock control unit is operative for particular engines on which it has been characterised. Continuing development will include use of adaptive control technology to increase the system's versatility for a variety of engine applications.

A first retrofit conversion of a Cummins NTC335 engine to spark ignition was carried out in New Zealand in 1984. The conversion used widely available technology for stoichiometric control of natural gas fuel - air mixtures. Experience from the on-road application in a 40,000 kg GVW truck contributed much to the later development of a 400 hp gas-fuelled variant of the same engine family, using lean mixture carburettion control. A second engine entered service in a logging application in February 1989. The following paper summarises results arising from laboratory testing of the second engine, and from in-field monitoring. Also presented are preliminary results from testing of a third generation engine, using timed multi-point injection of gas fuel.