An attempt to further understand the processes of auto-ignition, combustion and ultimately emission formation for a common rail diesel injection system, has lead to an investigation into the effects in-cylinder density and fuel injection pressure have on liquid penetration, vapour formation and auto-ignition delay. Although several correlations for diesel spray penetration at different operating conditions have been presented in the literature, to date the findings are in-conclusive with only limited investigation into the operating envelope expected in present and future high speed direct injection diesel engines.\ud The present paper describes how two high-speed video cameras were utilised to achieved pseudo 3-dimensional imaging of the spray (backlit) and of auto-ignition sites (flame lumines-cence). It also describes how Schlieren imaging enabled vapour phase analysis. Data have been gathered for in-cylinder densities in the range 10 to 50 kg.m-3 and injection pressures be-tween 60 and 160 MPa. New correlations have been established for the penetration of both the liquid and vapour phases of a high-pressure diesel spray injected from a modern VCO nozzle common rail system. By increasing injection pressures and using small injector orifice sizes, it is thought that the injected diesel droplets reduce in size and penetrate further, hence increas-ing air utilisation, thus leading to faster evaporation rates and reduced ignition delay. An ex-perimental correlation for auto-ignition delay against in-cylinder density has been presented, suggesting that as the charge density increases, penetration is reduced but auto-ignition delay is reduced, indicating more favourable conditions for ignition.\u
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