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.
