The Effects of Exhaust Gas Recirculation in Utility Engines

The effects of residual gas mixing were studied in a single-cylinder, air-cooled utility engine using both external exhaust gas recirculation (EGR) and internal residual retention. EGR was introduced far upstream of the throttle to ensure proper mixing. Internal residual was changed by varying the length of the valve overlap period. EGR was measured in the intake system; the total in-cylinder diluent was directly measured using a skip-fire, cylinder dumping technique. A sweep of diluent fraction was performed at different engine speeds, engine loads, fuel mixture preparation systems, and ignition timings. An optimum level of diluent, where the combined hydrocarbon and NOx emissions were minimal, was found to exist for each operating condition. Higher levels of diluent, either through internal retention or external recirculation, caused the combined emissions to increase. The transition to higher emissions levels was found to correspond to conditions where the heat release rate extends to the point of exhaust valve opening. Combustion with a high level of variability, but heat release completing prior to exhaust valve opening, did not adversely affect the hydrocarbon emissions as seen by direct analysis of individual-cycle hydrocarbon emissions and combustion performance.