| Summary: | To extend the load range over which a natural gas lean-burn spark-ignited engine can operate, a method that uses a partially-stratified charge (PSC) has been proven to be useful. The PSC is produced by injecting a small amount of fuel (less than 5% of the total fuel mass) directly into the engine combustion chamber near the spark plug. This creates a comparatively rich air-fuel ratio near the spark plug which is easily ignited creating a strong flame front that moves through the rest of the homogenous lean air-fuel mixture. To extend the load range even further, a method of PSC that uses two direct injections, one before spark and one after, has been tested and is described in this thesis.
The most effective single-pulse injection (SPI) conditions (pressure, injection size and timing) were used for the injection occurring before the spark then a secondary injection was added after the spark using the same injection pressure. The results showed no improvement in engine performance using the double-pulse injection (DPI) method over the SPI method. Engine performance was similar for the SPI case and the DPI cases where the secondary injection was small and there was a decrease in engine performance for the DPI cases with larger secondary injections.
Although no improvement in overall engine performance was seen using the DPI method employed, there were improvements in IMEP for engine cycles where ignition was successful. If the best primary and secondary-injection conditions could be determined, there would likely be improvements in overall engine performance compared to the single-injection case. More testing is required to determine whether the DPI method can be successful at improving overall engine performance.
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