High Pressure Direct Injection (HPDI) is a promising combustion concept for the medium- to heavy-duty industry to combat climate change. It uses a pilot diesel injection to ignite the main fuel consisting of Natural Gas (NG). Both fuels are injected directly in the combustion chamber using a dedicated HPDI injector. A significant reduction in carbon dioxide and Particulate Matter is achieved due to the use of the low carbon fuel NG. It is seen in literature that a small change in pilot injection can have profound consequences for the HPDI combustion. This research investigates the interaction between the pilot diesel and main NG injection. A relevant Computational Fluid Dynamics (CFD) simulation environment is setup for this purpose. It is observed that the main NG injection needs a certain pilot trigger to ignite. Furthermore, local conditions are derived to investigate driving factors of the ignition of NG on a fundamental level. A homogeneous reactor model is used to study Ignition Delay (ID) behavior by varying the initial temperature as well as concentrations of radicals H and OH. It is observed that both factors influence the ID. The initial temperature has to be higher than 1110 K in order to ignite the NG under enginelike conditions. It is also observed that species mole fractions H or OH encountered in the CFD simulation can reduce the ID up to 5.5 crank angle degrees at a speed of 1400 RPM.

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