Measurement of Residual Gas Fraction in a Single Cylinder HSDI Diesel Engine through Skip-firing

This paper proposes a method of determining residual gas fraction (RGF) by sampling the CO₂ concentration in the exhaust manifold of a single cylinder HSDI diesel engine. During a skip-fire event, the CO₂ concentration in the exhaust gas for the last firing cycle and the subsequent motoring cycle were measured using a fast-response emissions analyzer. The ratios of these two values are shown to be indicative of the RGF.

To simulate the increase in exhaust pressure found with EGR or aftertreatment systems, the exhaust back pressure was elevated using an exhaust throttle. The intake pressure was held constant over a range of engine speed and load conditions. The results demonstrate that the RGF increases linearly with increasing exhaust back pressures for all engine operating conditions. The backflow of exhaust gas into the cylinder and intake manifold during the valve overlap period is found to be the most significant cause of the higher RGFs, especially at higher exhaust back pressures. The measured RGFs are used to validate the results obtained from a commercial 1-D engine flow simulation package.

Analysis of the experimental in-cylinder pressure shows the effects of the residual gas on the combustion. With increased RGF, the ignition delay is reduced due to the higher end-of-compression temperature caused by the hot residual gases. The offsetting effects of charge dilution on ignition delay are insignificant since the RGF never exceeds 9% of the charge. Pumping losses increase linearly with increasing exhaust back-pressure, reducing net indicated thermal efficiency. However, the gross indicated thermal efficiency is not significantly influenced by exhaust throttling.