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The effects of ambient gas thermodynamic state and fuel composition on the autoignition of natural gas under direct-injection diesel conditions were studied experimentally in a constant-volume combustion vessel and computationally using a detailed chemical kinetic model. Natural gas compositions representative of variations observed across the U.S. were considered. These results extend previous observations to more realistic natural gas compositions and a wider range of thermodynamic states that include the top-dead-center conditions in the natural gas version of the 6V-92 engine being developed by Detroit Diesel Corporation. At temperatures less than 1200 K, the experiments demonstrated that the ignition delay of natural gas under diesel conditions has a dependence on temperature that is Arrhenius in character and a dependence on pressure that is close to first order.

Ambient gas density and fuel vaporization effects on the penetration and dispersion of diesel sprays were examined over a gas density range spanning nearly two orders of magnitude. This range included gas densities more than a factor of two higher than top-dead-center conditions in current technology heavy-duty diesel engines. The results show that ambient gas density has a significantly larger effect on spray penetration and a smaller effect on spray dispersion than has been previously reported. The increased dependence of penetration on gas density is shown to be the result of gas density effects on dispersion. In addition, the results show that vaporization decreases penetration and dispersion by as much as 20% relative to non-vaporizing sprays; however, the effects of vaporization decrease with increasing gas density.