Dimethyl ether and bio-diesel fuels: the potential renewable fuels for CI and HCCI combustion in internal combustion engines

Abstract

Methyl esters derived from vegetable oils, commonly known as ‘Biodiesel’, can be used as an alternative fuel in compression ignition engines as they have very similar physical properties (for example cetane number, viscosity and density) to mineral diesel. In this study, three different vegetables oils (rape seed oil, soya oil and waste oil) were esterified and the resulting biodiesels were then tested in an inline four cylinder direct injection engine, typically used in small diesel genset applications, as blends of 5% and 50% in mineral diesel, or as the pure methyl ester. The engine performance and emissions of the engine were then recorded at five load conditions and at two different speeds. The data were then compared to those obtained under the same operating conditions for mineral diesel. Homogeneous Charge Compression Ignition (HCCI) combustion employs premixed homogeneous air and fuel mixture, but the combustion is ignited by compression. It has the potential to deliver high efficiency, low NOx and particulate emissions. The auto-ignition nature ensures that the HCCI has the potential to burn different fuels. It is therefore a potential alternative technology for diesel applications. However, unlike conventional CI and SI combustion methods, the combustion process of HCCI is predominantly governed by chemical kinetics of the air/fuel mixture. It has been proved difficult to effectively controlling the ignition timing and combustion heat release rate over wide engine speed / load range over recent years of investigation . In the research work reported in this paper, four various diesels type fuels: N-heptane, Dimethyl Ether (DME), Methyl Butanoate (MB) surrogating heavy bio-diesel and Methyl Formate (MF) substituting light bio-diesel were studied using a single-zone combustion model with convective heat transfer loss and detailed chemical kinetics. The effects of different engine parameters such as recirculated exhaust gas, air/fuel ratio, intake temperature, and compression ratio on auto ignition performance were investigated