In-Cylinder CO₂ Sampling Using Skip-Firing Method

Skip-firing (or cylinder de-activation) was assessed as a method of sampling CO₂ directly in the cylinder at higher speeds than previously possible. CO₂ was directly sampled from one cylinder of a 1 L three-cylinder gasoline engine to determine the residual gas fraction (RGF) using a fast response CO/CO₂ analyzer. Acquisition of data for similar measurements is typically limited to engine speeds of below 1300  revolutions per minute (rpm) to allow full resolution of the sample through the analyzer that has an 8 ms 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 RGF 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 RGF change with increasing speed was seen between normal and skip-fire operation.