Relation between flame propagation characteristics and hydrocarbon emissions under lean operating conditions in spark-ignition engines

Abstract

The objective of this work was the investigation of the hydrocarbon emissions of a spark-ignition (SI) engine under very lean operating conditions. A series of experiments has been conducted in which cycles exhibiting high hydrocarbon (HC) emission were analyzed using the fast response flame ionization detector (FID) in conjunction with a recently developed diagnostic, the head gasket ionization probe. It was found that for relative air/fuel ratios (λ) close to 1.4, single-cycle emissions correlate with single-cycle maximum pressure and indicated mean effective pressure (IMEP), which suggests that around that point postflame oxidation starts becoming less important. For relative air/fuel ratios greater than 1.4, cycles with very high emissions appear. We have divided these partial burns into two types depending on their HC-emission signature profiles and flame propagation characteristics. The first type is a global, slow-burning cycle that does not finish the oxidation of the charge by the time the exhaust valve opens. The second type is a normal burning cycle that exhibits a localized quench phenomenon: although the flame has propagated to the extremities of the cylinder, it has left a region of the charge unburned. The two types of cycles have different IMEP and flame speeds for the same mass fraction of the charge burned, as well as different HC emissions because of the different spatial and temporal location of the partial burn and the varying degrees of postflame oxidation the HC are subjected to.