LIF Characterization of Intake Valve Fuel Films During Cold Start in a PFI Engine 2002-01-2751
A Laser Induced Fluorescence (LIF) based technique has been employed to examine the transient fuel film behavior on an intake valve during cold start of a PFI engine. Fluorescence from a tracer in the fuel was collected through a Borescope and imaged onto a CCD camera, providing a 2-D image of the fuel film on the valve. The average intensity of the fluorescence, over a Region of Interest (ROI), was taken to be proportional to the total amount of fuel present in the film.
Images were collected (at a fixed crank angle) on every second engine cycle, resolving changes in the fuel film during the cold start transient. Changes in the fuel film were resolved within a cycle by collecting images at varying crank angles during successive experiments.
Results from four fuel mixtures are reported. Two simulated “single component fuels” one with a higher volatility (HV) and one with a lower volatility (LV), were examined. The remaining two mixtures were both designed to simulate a single two component fuel (2CF); in one case the HV component was traced, while in the other the LV component was traced.
The measurements showed marked differences between the behaviors of the fuels. The films of the LV fuel and the LV component of the 2CF were both found to initially grow for the first five to fifteen cycles and then slowly decline until the end of the experiment. The LV films were found to persist even after the intake valve had opened and the intake charge had been swept into the combustion chamber. The HV components demonstrate a similar initial increase in fuel film followed by a more rapid decline. Very little of the HV fuel or of the HV component in the 2CF remained on the valve after the inlet valve opened.