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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.