At the present time, natural gas engines and gasoline engines which are two kinds of typical SI engines have different inherent emission problems. The noxious components which must be reduced first of all in natural gas engines and gasoline engines are NOx and UHC, respectively. It has been well known that the major source of NOx is the combustion temperature and that of UHC is the piston ring crevice volume. The purpose of this study is to reduce NOx emission in a natural gas engine and UHC emission in a gasoline engine by reducing the influence of these major root cause.
The method for reducing NOx emission in this study was the lean burn technology. Lean burn usually accompanies unstable combustion quality though it reduces NOx emission automatically by dropping the combustion temperature. Therefore, investigation was focused on the stability of lean burn. Experiments were performed with a 4-stroke, natural gas engine under the various air-fuel ratio conditions. In order to stabilize the lean operation, the effect of several parameters such as charge swirl flow, direction of the ground electrode and high energy ignition systems were investigated. Shadowgraph visualization tests with a single-shot optical engine provided additional information which can be utilized for discussion of combustion diagnostics interpretation. It was possible to secure a strengthened turbulence and stable flame kernel growth as a result of a swirl vane installation at the inlet port together with a favorable direction of the ground electrode. Some improvement was observed under lean operating conditions with a long spark duration ignition system. The flame jet ignition system displayed a prominent capability for extending the lean limit.
In a gasoline engine, influence of the inter-ring crevice volume on UHC emission was experimentally and numerically investigated. In the experiments, the inter-ring mixture was extracted to the crankcase during the late period of expansion and the early period of exhaust stroke through the engraved grooves on the lower part of cylinder wall. Extraction of the mixture resulted in significant reductions of UHC emission in proportion to the increments of the blowby flow rate, without any losses of efficiency and power. A relationship between the inter-ring mixture and UHC emission was established from the measured results. In addition, a physical model was constructed to predict the gas flows through the piston ring pack, and used to interpret the phenomena related to the results of experiments. Amount of the inter-ring mixture returning to the combustion chamber after exhaust valve open was calculated and converted to the corresponding UHC emission using the relationship between the inter-ring mixture and UHC emission in the experiments. Calculated level of UHC emission caused by the inter-ring crevice was 10-30% of the entire UHC emission over a range of speeds(1250-3500rpm) and loads (bmep 185-556kPa),which showed maximum at 2500rpm, bmep 432kPa, which corresponds to the condition most frequently operated by users.
