Spray and combustion of waste cooking oil biodiesel in a compression-ignition engine

Abstract

An experimental study was carried out to compare the spray and combustion characteristics of waste cooking oil biodiesel with those of conventional diesel in a common-rail direct-injection compression-ignition engine. The indicated mean effective pressure of approximately 0.7 MPa was tested under an engine speed of 800 r/min. The fuels were injected at an injection timing of −5 crank angle degree after top dead center with injection pressures of 80 and 160 MPa. The experiments were performed to investigate the combustion and emission characteristics in a metal engine, while spray and combustion flame characteristics in an optical engine. The morphological and compositional features of particulate matter were also analyzed by using transmission electron microscopy and thermogravimetric analysis. The direct imaging of the spray indicated a longer injection delay for waste cooking oil biodiesel. However, it exhibited longer liquid penetration length and narrower spray angle than diesel as the spray developed. Waste cooking oil biodiesel combustion showed a longer ignition delay with a slightly lower peak of in-cylinder pressure and heat release rate than diesel due to the deteriorated atomization and evaporation of fuel spray. In terms of emission, waste cooking oil biodiesel showed benefits in reduction of carbon monoxide, unburned hydrocarbon and particulate matter emissions. The particulate matter from waste cooking oil biodiesel was composed of smaller primary particles and more volatile organic fraction than diesel soot. The combustion flame imaging verified the retarded combustion phase of waste cooking oil biodiesel. Waste cooking oil biodiesel combustion showed lower flame luminosity accompanied by shorter visible flame duration, at both injection pressures. The flame spatial fluctuation and flame nonhomogeneity were also lower for waste cooking oil biodiesel.