Conversion of Two Small Utility Engines to LPG Fuel

Southwest Research Institute (SwRI) converted two small air-cooled, gasoline engines to operate on LPG (sometimes called propane since propane is LPG's major constituent). Typical two- and four-cycle engines were chosen for this investigation. The two-cycle engine used was a McCulloch string trimmer engine with 28 cc displacement. The four-cycle engine used was an L-head, Tecumseh TVS90 with 148 cc displacement. These are typical of engines found on lower cost lawn mowers and string trimmers. The engines were baseline tested on gasoline, converted to LPG, and tested to determine equivalence ratios at which the engines could be operated without exceeding manufacturers' recommended spark plug seat or exhaust temperatures. Engine startability and throttle response was maintained with the LPG conversion.

The emissions of the four-cycle engine were measured following the CARB 6-mode emissions test procedure. Test conditions include 85 percent of rated speed with 100, 75, 50, 25 and 0 percent load and idle. The emissions of the two-cycle engine were measured using the CARB 2-mode emissions test sequence consisting of a rated speed 100 percent load point and an idle condition.