Among other requirements, small air-cooled utility engines must maximize power-to-weight ratio. A means of meeting this requirement is to optimize the gas exchange process by utilizing cam shafts that complement high speed, wide open throttle (WOT) performance. The resulting valve timing tends to cause high levels of exhaust gas retention, or residual, at low-speed, light-load conditions, giving rise to poor combustion stability and idle quality.
The effects of residual gas level and homogeneity 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.
The total in-cylinder diluent was measured directly using a skip-fire cylinder dumping technique. A sweep of diluent fraction was performed for several engine speeds, engine loads, fuel mixture preparation systems, and ignition timings. An optimum level of diluent, where the combined hydrocarbon (HC) and oxides of nitrogen (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. This was observed by direct analysis of individual-cycle hydrocarbon emissions and combustion performance. Complementary studies investigated how fuel mixture preparation, residual mixedness, intake volume, ignition timing, spark energy and volume affect combustion quality and emissions.
Optimizing the spark timing improved the combustion quality in highly diluted conditions by improving phasing, reducing cyclic variability, and decreasing the burn duration. Similar behavior to stock ignition conditions with regards to hydrocarbon emissions was found; however, improvements in diluent tolerance and combustion quality did not result in reduced emissions.
Residual gas mixing, or the source of diluent, appeared to have little effect on the trends seen in combustion or emissions. The trends were found to be only a function of overall diluent fraction. Optimizing the ignition at high levels of diluent appeared to improve combustion quality more easily in the EGR supplemented cases compared to the maximum overlap cases.
Very slight improvements in cyclic variability, combustion phasing, and heat release rate were noted with increased spark gap and, to a lesser degree, spark energy. Combustion quality reduced significantly at very low energy and hydrocarbon emissions drastically increased as a result.