Pilot-ignited high-pressure direct injection (HPDI) of natural gas in diesel engines results in lower emissions while retaining high thermal efficiency. As a study of HPDI technique, three-dimensional numerical simulations of injection, ignition and combustion were conducted. In particular, the effects on engine combustion of the injection interlace angle between the pilot diesel sprays and natural gas jets were investigated.Numerical simulations revealed ignition and combustion mechanisms in the engine with different injection interlace angles. The results show that altering the interlace angle changes the contact areas between the pilot diesel sprays and the natural gas jets; this affects the heat release rate. Statistical analysis was done to evaluate the expected value and variance of “closeness” between diesel sprays and natural gas jets for different injector tip configurations.Experiments were done on a Detroit Diesel Corporation two-stroke DDC 6V-92 HPDI engine to validate the simulation model and on a Detroit Diesel Corporation two-stroke single cylinder DDC 1-71 HPDI engine for the interlace angle study. The DDC 1-71 HPDI engine has a structure similar to that of the DDC 6V-92 engine and permits a variety of research testing at low cost. The design of the HPDI injector is such that the gas injection valve, in which the diesel holes are located, rotates during engine operation. The rotation means that the interlace angle changes during operation. Under some conditions, the engine was found to be unstable for an injector with a number of gas holes equal to the number of pilot holes. Consistent with theoretical explanations and statistical evaluations, stability was improved using a number of gas holes different from the number of pilot holes.