Influence of biodiesel-methanol blends on the emissions in the low-temperature combustion of a direct-injection diesel engine using high levels of exhaust gas recirculation

In this study, biodiesel and biodiesel-methanol (BM) blends were tested in a four-cylinder direct-injection diesel engine to investigate the combustion and emissions in low-temperature combustion. With increasing exhaust gas recirculation (EGR) rate, the heat release rate decreases and moves towards top dead centre (TDC); the 50 per cent fuel mass fraction burned point and the start of combustion occur later. For BM blends, the nitrogen oxide (NOx) emissions decrease at each tested EGR level, compared with the NOx emissions for biodiesel. However, the reductions are limited. As the EGR rate increases, the NOx emissions decrease monotonically for all fuels at different engine loads. Generally, the smoke opacity of BM blends increases with increasing EGR rate and then decreases, which is the typical characteristic soot 'hump' of low-temperature combustion. Meanwhile, it is interesting that, with increasing methanol fraction in the blends, the soot hump moves towards the lower-EGR-rate region. Hydrocarbon (HC) and carbon monoxide (CO) emissions increase with increasing EGR rate inevitably. However, it is noted that the use of the biodiesel-10 vol% methanol blend could reduce the HC emissions, compared with the use of biodiesel, or even diesel fuel.