Triple Stratification in a Spark Ignition Engine: The Effect on the Emission at Unthrottled Light Load 1999-01-0575
This paper describes a method to obtain a triple stratification - uncooled EGR / air / air-fuel mixture - in a large prechamber spark ignition engine and the influence on the emission level at unthrottled light load. At this load, the critical problem is the HC emissions. The goal of the obtaining uncooled stratified EGR is the reduction of HC emissions at unthrottled light load.
In the first part of the intake stroke, the exhaust gases are drawn into the cylinder and, in the second part of this stroke, the air. The prechamber comprises 50% of the compression volume and is the place where the stratification air / air-fuel mixture is obtained. The fuel is injected on a hot pin situated in the transfer flow passage (between prechamber and main combustion chamber) during the compression stroke and the air-fuel mixture is carried toward the spark plug. At high EGR levels, a certain quantity of recycled exhaust gas can get into the pre-chamber. In the main combustion chamber, there is hot EGR mixed with air, or only EGR, depending on the EGR level. This charge organization in the combustion chamber allows a high EGR tolerance.
The engine has stable operation up to 55% EGR for BMEP=0.235 MPa. The HC emissions achieve the minimum value at 45% EGR, the reduction (relatively to the 0% EGR) being of 60%. The CO emissions decrease with 30…35% for an EGR value of 25…30%. No powerful influence of the EGR rate on the NOx emissions was observed.