Auto-ignition quality of Diesel-like fuels in HCCI engines

In Homogeneous Charge Compression Ignition (HCCI) engines heat release occurs by auto-ignition and hence the fuel auto-ignition quality is very important. The auto-ignition quality of Diesel fuels is conventionally described by the cetane number. Conventional Diesel fuels are involatile compared to gasoline fuels and mixture preparation becomes far more critical in assessing their behaviour in HCCI engines. This paper considers the relationship between auto-ignition behaviour of Diesel-like fuels in a single-cylinder HCCI engine and the fuel cetane number under different mixture preparation strategies.

Three different fuel injection strategies were used in this study, port fuel injection (PFI), early direct-injection with colliding sprays and late direct-injected in the presence of EGR, akin to the Nissan MK concept. The fuels used were conventional Diesel fuels and model fuels with similar auto-ignition qualities. In the tests with PFI light Diesel-like fuels that consisted of n-heptane and toluene or iso-octane, with a maximum boiling point of 110 °C were used. In the tests with the direct-injection systems heavier Diesel-like fuels, such as conventional Diesel fuels and model fuels of kerosene and aromatic solvent, were used. The tests show that the cetane number can describe the auto-ignition quality of Diesel-like fuels in HCCI combustion well.