Measurement and Analysis of the Residual Gas Fraction in an SI Engine with Variable Cam Timing

A spontaneous Raman scattering diagnostic was used to measure the residual fraction in a single-cylinder, 4-valve optically accessible engine. The engine was operated at 1500 rpm on pre-vaporized iso-octane at several intake manifold pressures (50-90 kPa). Cam phasing was varied to determine the effect of intake valve timing and valve overlap on the residual mass fraction of the engine. A simple model based on the ideal Otto cycle and 1D gas flow through the exhaust valves was proposed to analyze the results of the Raman experiment. The model showed good agreement (R²=0.91) with the experimental results and demonstrated its potential for use as a method to estimate the residual fraction in an engine from available dynamometer data. The experimental results showed that the residual fraction was reduced at higher manifold pressures due to less backflow through the exhaust valves and varied with intake cam phasing. The variation with intake cam phasing was determined by the gas properties in the cylinder when the intake valve opened. When the engine was stable (Coefficient of Variance (CoV) imep < 3%), changing the manifold pressure did not affect the portion of the residual fraction due to mass trapped at intake valve opening (IVO) and influenced the residual levels by reducing the residual level due to backflow.