Assessment of combustion in natural gas dual-fuelled compression ignition engines with dimethyl ether and rapeseed methyl ester pilot ignition

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Compression-ignition engines are known to be more efficient than similar-sized spark-ignition engines because of the higher compression ratios and leaner combustion. The emissions of soot and nitrogen oxides remain the main hurdle in the complete exploitation of these engines. Dual-fuelling is one means favoured for solving the emission problem, in which high-octane fuels are used as the main fuel which is ignited by a smaller pilot injection of diesel or another high-cetane fuel. These dual-fuel engines produce less particulate matter and nitrogen oxides than spark-ignition engines, while retaining the desired compression-ignition engine efficiency. In the present investigation, tests were conducted using a variety of renewable and non-renewable fuels for pilot injection. The pilot fuels employed were conventional diesel, rapeseed methyl ester (known as biodiesel), and dimethyl ether, while natural gas was used as the main fuel. Biogas, or landfill gas, would be the renewable alternative. Pressure versus crank angle traces were obtained, together with their first and second derivatives. These were analysed to determine the crank angle at which ignition began. Diagrams that show the rate of reaction were also plotted for the same purpose. Here, the start of ignition is determined by the trace suddenly changing slope and rising above zero. Using these methods, the ignition delay was determined for different operating conditions and comparisons drawn for different pilot fuels. The characteristic shapes of rate-of-reaction curves were analysed highlighting differences in the combustion processes occurring in single- and dual-fuel diesel engine operation. Emissions data for different operating conditions were also obtained and analysed, showing a tendency for lower emissions of smoke and oxides of nitrogen but increased carbon monoxide and unburnt hydrocarbons to be produced in dual-fuelling. Carbon dioxide was also reduced.