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Natural gas high pressure direct injection (HPDI) engines represent a technology with the potential for lower engine-out emissions and reduced fuel costs over a diesel engine. This combustion process uses a direct injection of natural gas, into the combustion chamber of a high compression ratio engine, to maintain diesel engine performance. As natural gas will not auto-ignite at typical engine conditions, a small quantity of diesel pilot fuel is used to initiate the combustion event. One potential technique to improve engine performance is the optimization of the intake and exhaust valve timings. To experimentally investigate these effects, tests were performed on a single cylinder engine based on Westport Innovation's 15L HD engine. The intake valve closing time was varied both before and after the standard closing (EIVC and LIVC). Early closing of the exhaust valve was also tested (EEVC).

Retaining the diesel combustion process but burning primarily natural gas offers diesel-like efficiencies from a natural-gas fuelled heavy-duty engine. This combustion event is limited by the injection pressure of the fuel, as this dictates the rate of mixing and hence of combustion. Typical late-cycle direct injection applications are limited to approximately 300 bar fuel pressure. The current work reports on tests for the first time at natural gas injection pressures up to 600 bar. The results show that significant efficiency and particulate matter reductions can be achieved at high loads, especially at higher speeds where the combustion is injection rate limited at conventional pressures. Increases in combustion noise and harshness are a drawback of higher pressures, but these can be mitigated by reducing the diameter of the nozzle gas holes to control the fuel injection rate.