Crank-angle-resolved Measurements of Air-fuel Ratio, Temperature, and Liquid Fuel Droplet Scattering in a Direct-injection Gasoline Engine 2010-01-2246

Simultaneous crank-angle-resolved measurements of gasoline vapor concentration, gas temperature, and liquid fuel droplet scattering were made with three-color infrared absorption in a direct-injection spark-ignition engine with premium gasoline. The infrared light was coupled into and out of the cylinder using fiber optics incorporated into a modified spark plug, allowing measurement at a location adjacent to the spark plug electrode. Two mid-infrared (mid-IR) laser wavelengths were simultaneously produced by difference-frequency-generation in periodically poled lithium niobate (PPLN) using one signal and two pump lasers operating in the near-infrared (near-IR). A portion of the near-IR signal laser residual provided a simultaneous third, non-resonant, wavelength for liquid droplet detection. This non-resonant signal was used to subtract the influence of droplet scattering from the resonant mid-IR signals to obtain vapor absorption signals in the presence of droplet extinction.

The sensor was applied to a direct-injection engine in both early-injection homogeneous charge and late-injection stratified charge operation. For the early-injection cases, liquid droplets were detected only during a brief period of the intake stroke just after start of injection. Late in the compression stroke, no droplet extinction was detected, and the vapor mole fraction reached a steady value before passage of the flame kernel through the probe volume, indicating that the fuel was fully evaporated and well-mixed with the intake air. Gas temperature, fuel-vapor concentration, and droplets were then measured for stratified-charge, retarded-combustion timing operation. After subtraction of the droplet signal, repeatable vapor temperature and concentration measurements were obtained for the period from first arrival of the fuel spray until passage of the flame front. Cycle-resolved results show the sensor's value in investigating causes of combustion cycle-by-cycle variability.