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In high-swirl engines, such as gas engine, multi-spark discharge causes uneven wear of spark plugs because intense swirl breaks the spark discharge path. The ignition system with variable discharge energy solved this problem. Although the reduced discharge energy may weaken the ignitability, the variable discharge energy keeps high ignitability under any engine operating conditions. Gas engines require higher discharge voltage than gasoline engines. The high required voltage has hindered the development of long-life spark plugs. Attaching sub-electrodes strengthened the electric field intensity and succeeded in lowering the required voltage. The combination of the above-mentioned ignition system and spark plug has extended the plug life span.

We have attempted to reduce emissions and improve fuel economy of a small utility engine running with a lean A/F ratio under partial load conditions in the most frequent operating mode. However this caused undesirable hunching and many difficulties for running the engine at a lean A/F ratio. We derived the necessary amount of engine speed changes from the relationship between the spark timing and torque to control the hunching by adjusting the spark timing at various engine speeds. The method improved emissions and fuel economy, and made it possible to run the engine smoothly from low engine speed to higher speeds at a high load rating.