In-Cylinder CO₂ Sampling Using Skip-Firing Method

Skip-firing (or cylinder de-activation) was assessed as a method of sampling CO₂ from directly in the cylinder at higher speeds than previously possible. CO₂ was directly sampled from one cylinder of a 1-litre 3-cylinder gasoline engine to determine the residual gas fraction using a fast response CO/CO₂ analyser. Acquisition of data for similar measurements is typically limited to engine speeds of below 1300 rpm to allow full resolution of the sample, through the analyser that has an 8 millisecond finite response time. In order to sample in-cylinder CO₂ at higher engine speeds a skip-firing method is developed. By shutting off ignition intermittently during engine operation, the residual CO₂ from the last firing cycle can be measured at significantly higher engine speeds. Comparison of residual gas fraction CO₂ at low speeds for normal and skip-fire operation shows good correlation. This suggests that skip-firing is a suitable method for directly measuring internal exhaust gas recirculation up to at least 3000 rpm. The measurements obtained may provide a useful tool for validating internal exhaust gas recirculation models and could be used to calculate combustion air-fuel ratio from the CO and CO₂ content of the burned gas. These are typically complicated parameters to predict due to the slow response time and sensitivity to hydrocarbons of wide-band oxygen sensors. A differing pattern of residual gas fraction change with increasing speed was seen between normal and skipfire operation.