Spark Ignition Engine Hydrocarbon Emissions Behaviors in Stopping and Restarting

Engine Hydrocarbon (HC) emissions behaviors in the shut down and re-start processes were examined in a production 4-cylinder 2.4 L engine. Depending on when the power to the ECU was cut off relative to the engine events, there could be two or three mis-fired cylinders (i.e. cylinders with fuel injected but no ignition). The total HC pumped out by the engine into the catalyst in the stopping process was ∼ 4 mg (approximately equaled to the amount of one injection at idle condition). Because the size of the catalyst was larger than the total exhaust volume in the stopping process, this HC was not observed at the catalyst exit. The catalyst temperature was also not affected. When the engine was purged after shut down (by cranking the engine with the injectors and ignition disconnected), the total exit HC was 33 mg. In a restart 90 minutes after shut down, the integrated amount of HC emissions due to residual fuel from the stopping process was 16 mg. To assess the contribution of the mis-fired cylinders to the residual fuel, the engine was shut down by disabling the injectors but with ignition on at all time so that there was no mis-fired cylinders. The HC pumped out to the catalyst in the stopping process was a factor of two smaller (∼ 2 mg). In the restarting process, the difference (compared to the normal shut down) in HC emissions due to the residual fuel was ∼ 5mg. Thus the mis-fired cylinders contributed to a small but not insignificant amount of HC emissions in the restart process.