Agricultural irrigation pumps are frequently powered by common automobile or tractor spark-ignition (SI) engines burning natural gas. Typically, these engines operate at part-throttle and at constant speed and load. Investigated was the fuel consumption reduction that occurs with application of the more-complete-expansion cycle. Using a Chevrolet 5.74 liter (350 cubic inch) displacement engine for calculation purposes, fuel consumption rates are predicted as 12 to 16% less than a conventional engine (reference case). Economic analysis reveals that with irrigation pumps operating from 1000 to 2000 hr/year, the cost of modification is recovered by fuel savings (SO.97 per kg of natural gas) in one to two years.