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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.

To maximize payback for operators, it is important that natural gas engines for heavy-duty on-road applications minimize fuel consumption. To directly replace a diesel engine for a given vehicle mass and duty cycle, the natural gas engine also needs to match the diesel's power and torque characteristics. This paper reports the results of a development project to increase the torque and power of Westport's 15L 356 kW pilot-ignited, late cycle direct injection of natural gas engine by 10%, while matching or improving efficiency and maintaining emissions compliance. The strategies evaluated to achieve these objectives were to recover some of the exhaust energy with a power turbine, to increase the injector flow area to avoid excessively long combustion durations and to reduce the compression ratio to keep peak cylinder pressure below its maximum limit.

A semi-empirical turbulent flame growth model has been developed based on thermodynamic equilibrium calculations and experiments in a 125-mm cubical combustion chamber. It covers the main flame growth period from spark kernel formation until flame wall contact, including the effects of laminar flame speed, root mean square turbulence intensity, turbulent eddy size, and flame size. As expected, the combustion rate increases with increasing laminar flame speed and/or turbulence intensity. The effect of turbulent eddy scale is less obvious. For a given turbulence intensity, smaller scales produce higher instantaneous flame speed. However, turbulence of a smaller scale also decays more rapidly. Thus, for a given laminar flame speed and turbulence intensity at the time of ignition, there is an optimum turbulent eddy size which leads to the fastest combustion rate over the period considered.