Effects of premixed n -heptane from the intake port on the combustion characteristics and emissions of biodiesel-fuelled engines

Abstract

The combustion characteristics and emissions of biodiesel-fuelled direct-injection (DI) compression ignition engines with the addition of n-heptane from the intake port were investigated on a single-cylinder engine. The experimental results revealed that the homogeneous charge compression ignition (HCCI) combustion of the premixed n-heptane had important effects on the overall combustion characteristics and emission levels. The heat release curves exhibited a three-stage combustion mode, which includes the low-temperature reaction (LTR), high-temperature reaction (HTR) of n-heptane, and diffusion burn of biodiesel. On increase in the premixed ratio of the n-heptane, the peak values of both the LTR and the HTR increased linearly. When the overall equivalence ratio is kept constant, the HCCI combustion of premixed n-heptane had a moderate effect on the ignition timing of the biodiesel fuel, and the maximum heat release rate during the third-stage combustion decreased with increase in the premixed ratio. Regarding the regulated emissions, partial substitute of DI biodiesel by port injection of n-heptane caused the carbon monoxide and hydrocarbon emissions to deteriorate in comparison with those of the neat biodiesel engines. For low to medium equivalence ratios, the premixed combustion of heptane resulted in a decrease in the nitrogen oxide (NO _x ) emission, but had little effect on the smoke opacity. For a higher equivalence ratio, it was very interesting to find that both the NO _x emissions and the smoke opacity were suppressed substantially by an n-heptane premixed ratio of 20—25 per cent.