injection and combustion analysis of rapeseed oil methyl ester (RME) as fuel in a diesel engine

As fossil fuel reserves are depleted daily the interest in alternative fuels increases. Biodiesel is one of the best candidates in this class and its use is expanding rapidly throughout the world. Numerous researchers have been investigating how biodiesel affects combustion and pollutant formation of diesel engines and there is general agreement that its combustion characteristics are similar to those of standard diesel fuel, except for a shorter ignition delay, a higher ignition temperature, and greater ignition pressure and peak heat release. Engine power output is similar with both fuels. As regards emissions, reductions in particulate matter (PM) and carbon monoxide (CO) and increases in nitrogen oxides (NOx) are described with most biodiesel blends. The latter is referred to as the ‘biodiesel NOx effect’. A large part of researchers who explored the effect of biodiesel did so in mechanical injection engines. The first organised studies by the National Renewable Energy Laboratory (NERL) to measure biodiesel speed of sound and establish its impact on injection timing in diesel engines dates back to 2003 and states that the primary mechanism by which biodiesel increases NOx emissions is by an inadvertent advance in the start of injection timing, caused by a higher bulk modulus and viscosity. However, subsequent studies show that NOx emissions also increase in biodiesel-fuelled common rail engines and that in some cases they actually decrease in engines with mechanically controlled fuel injection systems. This implies that the biodiesel NOx effect cannot be explained solely by differences in compressibility and remains an open question. In particular, further studies are needed to better analyse the effect of injection timing, injection pressure and combustion characteristics on NOx emissions in biodiesel fed engines. The present study provides a contribution to the discussion in this field by analysing the injection and combustion characteristics of rapeseed methyl ester in a diesel engine equipped with a pump-line-nozzle (PLN) injection system. In particular experimental results are shown demonstrating that in PNL injection plant the injection advance due to biodiesel higher bulk modulus is less than that theoretically expected. This is explained through a detailed analysis of injection pressure trend showing how pressure in the injection pipe just before injection (i.e. residual pressure) is lower with biodiesel than that with diesel.