The purpose of this paper is to assess the influence of fuel properties on cycle-resolved exhaust hydrocarbons and investigate the sources of hydrocarbon (HC) emissions in a direct injection stratified charge (DISC) SI engine. The tested engine is a single cylinder version of a commercial DISC engine that uses a wall guided combustion system. The HC emissions were analyzed using both a fast flame ionization detector (Fast FID) and conventional emission measurement equipment.
Three fuels were compared in the study: iso-Pentane, iso-Octane and a gasoline of Japanese specification. The measurements were conducted at part-load, where the combustion is in stratified mode. The start of injection (SOI) was altered in relation to the series calibration to vary the mixture preparation time, the time from SOI to ignition. The ignition timing was set at maximum brake torque (MBT) for each test.
The measurements indicate that overmixing (quenching) at the boundaries of the air/fuel mixture cloud, and undermixing both in the spray center and on the surface of the piston bowl are the dominating mechanisms for HC emission formation in this engine. There is a fuel dependent optimum injection timing that minimizes the sum of HC from overmixing and undermixing. As the mixture preparation time is reduced undermixing increases and consequently carbon monoxide and soot emissions increase.