Sooting Tendencies in an Air-Forced Direct Injection Spark-Ignition (DISI) Engine

Particulate emissions are reported for a 0.31 L single cylinder engine fitted with an air forced direct injection system. Trends in number, size, and mass of engine out particle emissions are examined as a function of injection timing, spark timing, and EGR. Injection timing determines to a large degree the nature of the combustion, with early injection leading to homogeneous like combustion and late injection producing stratified charge combustion. As fuel injection is retarded, at a fixed lean air to fuel ratio, PM emissions decline to a minimum at an injection time well within the compression stroke, after which they rapidly increase. In the heavily stratified regime, the PM increase can be attributed to a growing number of rich zones that occur in the progressively more inhomogeneous fuel mixture. At fixed injection timing, advancing the spark causes a general increase in particle emissions. This trend is opposite that of hydrocarbons, which decrease significantly as spark timing is advanced. A limited examination of the effect of EGR shows it to reduce NOx. However, PM, CO, and hydrocarbon emissions all increase with added EGR, which is linked to a loss in combustion stability. The PM emissions trends for the air assisted direct injection presented here are compared to those previously reported for a production fuel only DISI gasoline engine.