Fuel Spray Dynamics and Fuel Vapor Concentration Near the Spark Plug in a Direct-Injected 4-Valve SI Engine

The mixture preparation process was investigated in a direct-injected, 4-valve, SI engine under motored conditions. The engine had a transparent cylinder liner that allowed the fuel spray to be imaged using laser sheet Mie scattering. A fiber optic probe was used to measure the vapor phase fuel concentration history at the spark plug location between the two intake valves. The fuel injector was located on the cylinder axis. Two flow fields were examined; the stock configuration (tumble index 1.4) and a high tumble (tumble index 3.4) case created using shrouded intake valves. The fuel spray was visualized with the engine motored at 750 and 1500 RPM. Start of injection timings of 90°, 180° and 270° after TDC of intake were examined. The imaging showed that the fuel jet is greatly distorted for the high tumble condition, particularly at higher engine speeds. The tumble was large enough to cause significant cylinder wall wetting under the exhaust valves for some conditions. The fuel vapor concentration history near the spark plug was very different for the two flow fields; for the high tumble case a richer mixture quickly reached the spark plug and was then diluted before reaching a steady concentration near the end of the compression stroke. For the stock valve configuration the fuel vapor concentration increased gradually throughout compression. The fuel spray imaging provided evidence linking the fuel-spray/charge-motion behavior with the fuel vapor measurements.