Effects of different cetane number enhancement strategies on HCCI combustion and emissions

Abstract

Cetane number is the accepted indicator for quantifying the autoignition characteristics of diesel fuels in compression ignition engines. Diesel fuel specifications typically require a minimum cetane number to achieve satisfactory combustion behaviour in conventional diesel engines. In contrast, a high cetane number fuel may not be beneficial for implementing high efficiency, clean combustion strategies such as homogeneous charge compression ignition (HCCI). The purpose of this study was to investigate cetane number effects on HCCI combustion and emissions. The experiments were conducted in a single-cylinder, variable compression ratio, cooperative fuel research engine operated in HCCI combustion mode. The fuels were finely atomized and partially vaporized in the intake manifold. The base fuel was a low cetane refining stream derived from oil sands sources. Three different methods were employed to increase the base fuel cetane number, namely hydroprocessing, cetane improver addition, and blending with a renewable fuel component. Results show that the three methods of cetane number enhancement produce significantly different HCCI combustion behaviour. The hydroprocessed fuels exhibited more stable and complete combustion than the base fuel, which resulted in a wider operating region, reduced carbon monoxide, unburned hydrocarbon, and nitrogen oxide (NO x) emissions, and lower indicated specific fuel consumption (ISFC). The main disadvantages of the hydroprocessed fuels were the higher exhaust gas recirculation rates required to retard the combustion phasing, which limits the maximum indicated mean effective pressure for a given intake pressure, and increased knock intensity due to a faster combustion process. In comparison, the other two methods of fuel cetane enhancement increased ISFC compared to the base fuel. The addition of nitrate cetane improver resulted in higher NO x emissions, while blending with a renewable fuel component increased hydrocarbon emissions. The experimental data provide evidence that the magnitude and phasing of low temperature heat release, as well as fuel volatility, play important roles in HCCI combustion.