The Effect of Spark Retard on Engine-out Hydrocarbon Emissions

Using spark retard during a cold-start is a very effective means of achieving fast catalyst light-off. In addition to obtaining faster catalyst light-off, retarding the spark also results in lower engine-out HC emissions. The objective of this research was to understand the reasons for the decrease in HC emissions with spark retard. In order to make the results as unambiguous as possible, the experiments were performed on a dynamometer at constant speed and load conditions using pre-vaporized, premixed gasoline. A zero-dimensional ring-pack crevice flow model was used to determine the mass flows into and out of the piston crevice during the engine cycle. The analysis showed that with spark retard a large fraction of the unburned fuel from the ring-pack re-entered the cylinder before the end of flame propagation, and was consumed by the flame when it extinguished on the cylinder wall. The level of post-flame HC consumption was estimated as the difference between the measured engine-out HC emissions and the unburned fuel re-entering the cylinder after the end of flame propagation, which was taken to be the crankangle of 90% mass fraction burned. Even with the most severe levels of spark retard the majority of the HC consumption, up to 70%, takes place by the flame within the cylinder before exhaust valve opening.