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As a result of the excavation of unconventional sources of natural gas, which has rich reserves, has attracted attention as a fuel for use in natural gas engines for power generation. From the viewpoints of efficient resource utilization and environmental protection, lean burn is an attractive technique for realizing a higher thermal efficiency with lower NOx emissions. However, ignition systems have to be improved for lean-burn operations. Laser ignition, which is expected to serve as an alternative to spark plug ignition, can decrease the heat loss and has no restriction on the ignition location because of the absence of an electrode. Consequently, an extension of the lean-burn limit by laser ignition has been demonstrated. In this study, we investigated the effects of the location and number of laser ignition points on engine performance and exhaust emissions. Laser ignition was also compared with conventional spark plug ignition.

The authors tried to use LP gas, mainly butane, as the main fuel of diesel engines to reduce soot and to maintain high thermal efficiency. LP gas was injected in the direction of the intake valve directly as a spray to prevent knocking and to preserve high charging efficiency. The newly developed electronic fuel injection provided accurate fuel control and injection timing. As a result, the dual-fuel operation produced high thermal efficiency almost identical to that of diesel engines. Soot in engine exhaust was almost negligible. Three quarters of maximum output was obtained with butane, and only small amount of gas oil for idling, in spite of an high compression ratio of 17 for gas engines. Increasing the proportion of gas oil resulted in maximum output from a diesel engine and almost no soot output.