Engine performance and emissions of furan-series biofuels under stratified lean-burn combustion mode

Abstract

In the recent decade, concerns over CO2 emitted by internal combustion engines is the driven force behind stringent emission legislations. Stratified lean-burn strategy in spark-ignition engines is effective in improving fuel economy by reducing pumping loss and heat loss. In this study, two biofuel candidates, 2-methylfuran (MF) and 2,5-dimethylfuran (DMF), were tested in this combustion mode in comparison with ethanol and gasoline. All experiments were conducted at an engine speed of 1200 rpm and a load of 5.8 bar indicated mean effective pressure (IMEP) with a fixed spark timing of 25° before top dead centre (BTDC). Engine performance and engine emissions of the stratified lean-burn mode (λ (air/fuel ratio) = 1.2 and 1.5) were studied and compared to that of the homogenous mode (λ = 1). The impact of the second injection timing was examined. Results show that the employment of the stratified lean-burn mode could improve the fuel economy by up to 16% if the second injection timing is optimized. The second injection timing also has significant impacts on the IMEP, combustion duration, coefficient of variation (COV), gaseous emission and particulate number (PN) emissions. Generally, combustion duration increases under stratified lean-burn mode, but it decreases if the second injection timing is very late. The COVs for lean-burn mode under a λ of 1.2 are always below 1.5%. However, under the λ of 1.5, COVs significantly increase to 10% when the second injection timing is too early. With the increase of the excess air, the PN concentration significantly reduces by up to 88% for MF, DMF and gasoline under the stratified lean-burn mode. The reduction of the PN is mainly due to the significant reduction of the small particles (<50 nm). However, for ethanol, the reduction of the total PN is not as significant as the other two fuels due to its physiochemical properties.