Diesel engine emissions with oxygenated fuels: a comparative study into cold-start and hot-start operation

As biofuels are increasingly represented in the fuel market, the use of these oxygenated fuels should be evaluated under various engine operating conditions, such as cold-start. However, to-date quantification has been mostly done under hot-start engine operation. By using a custom test designed for this study, a comparative investigation was performed on exhaust emissions during cold- and hot-start with diesel and three oxygenated fuels based on waste cooking biodiesel and triacetin. This study used a six-cylinder, turbocharged, after-cooled diesel engine with a common rail injection system. The results during cold-start with diesel showed lower NOx (up to 15.4%), PN (up to 48%), PM 1 (up to 44%) and PM 2.5 (up to 63%). However, the oxygenated fuels during cold-start showed a significant increase in NOx (up to 94%), PN (up to 27 times), PM 1 (up to 7.3 times) and PM 2.5 (up to 5 times) relative to hot-start. The use of oxygenated fuels instead of diesel during hot-start decreased the PN, PM 2.5 and PM 1 (up to 91%) while, during cold-start, it only decreased PM 1 and PM 2.5 at some engine operating modes and increased PN significantly up to 17 times. In both cold- and hot-start, the use of oxygenated fuels resulted in an increase in NOx emission. For cold-start this was up to 125%, for hot-start it was up to 13.9%. In comparison with hot-start, the use of oxygenated fuels during cold-start increased nucleation mode particles significantly, which are harmful. This should be taken into consideration, since cold-start operation is an inevitable part of the daily driving schedule for a significantly high portion of vehicles, especially in cities.