The Effects of In-Cylinder Flow Fields and Injection Timing on Time-Resolved Hydrocarbon Emissions in a 4-Valve, DISI Engine

Direct injection spark-ignition (DISI) engines have been shown to have much higher engine-out hydrocarbon emissions (HC) than port fuel injected (PFI) engines. A major contribution to the increase in HC emissions is from the in-cylinder surface wetting that occurs as the fuel is injected. A previous study using an optical access engine and a fuel concentration probe demonstrated that the in-cylinder flow field and injection timing have a significant effect on the equivalence ratio at the spark plug. This study continues that work, by using a fast spectroscopic HC emission measurement device (Fast-Spec) to study time-resolved HC emissions from a 4-valve, centrally injected, single cylinder DISI engine. Three flow fields are studied: tumble, reverse tumble and stock. The tumble and reverse tumble flow fields are achieved using shrouded valves. Both early and late start of injection (SOI) timings are investigated. The early SOI timings range from 60° to 180° after TDC of intake at ϕ = 1.0 while the late injection timings range from 90° to 40° before TDC of compression at ϕ = 0.7. The HC emissions are resolved to 3.0 crank angles using the Fast Spec. The engine was run at 750 RPM and unthrottled. The study illustrates that the intake flow and the SOI timing affect the HC emissions. Cases that resulted in greater surface wetting due to intake flow or injection timing or a combination of both had higher HC emissions, as did cases that showed evidence of less mixing.