Abstract
Keywords - Cold start, engine operating conditions, exhaust system operating conditions, simulation, vehicles.
Abstract - Short journeys, mostly consisting of home to work, home to shopping or home to school trips, have large average fuel consumption and emission values (g/km) due, essentially, to cold start. In cold start and warm up the engine has not reached yet its normal operating temperature and enrichment of intake mixture usually occurs. This leads to an increase in engine-out fuel consumption and emissions. Besides, in this period, the catalytic converter does not work properly due to enrichment and because of its warming up time (much less than the engines warm up time because of the high exhaust gas temperatures and lower thermal inertia, but nonetheless non-negligible). Typically it is necessary to reach 350ºC for light off to occur. This leads to high levels of tailpipe emissions. Part warm starts when vehicle is stopped and left for cool down not reaching ambient temperature, typically for less than 6 hours, are also very common in real life driving. A model to predict fuel consumption and tailpipe emissions from cold start, part warm start and warm up regimes, given the availability of data for fully warm engine conditions, is presented. Basically the model accounts for engine warm up/ cool down and for exhaust system warm up/ cool down (exhaust pipe plus catalytic converter). The cold regime duration is defined as the period of time before engine coolant temperature reaches a typical set point and engine oil temperature reaches a typical set point. During this period, correction factors concerning friction and mixture preparation (coolant temperature) are applied to fuel consumption and engine out emissions for the fully warmed-up engine. The cold regime model was introduced in the microscale simulation software EcoGest, as a subroutine. EcoGest was applied to a 2.49 litres
V6 vehicle equipped with an underbody three-way catalytic converter, in order to assess the influence of start up temperature on fuel consumption and tailpipe emissions in FTP75 (bag1, bag2) and NEDC driving cycles. The simulated start temperatures were 5ºC, 8ºC and 20ºC. The main findings allow concluding that fuel consumption penalty is much smaller than regulated emissions penalty. The maximum penalty occurs at 5ºC and is around 6 to 9% for both driving cycles. At 20ºC is around 3 to 5%. The same percentages go to CO2 overemissions. HC emissions are the most sensitive to the cold start. Overemissions can reach up to 156% and 380% over hot emissions with -5ºC start temperature for, respectively, FTP (bag1, bag2) and NEDC. At 20ºC start temperature the same percentages decrease to 106% and 270%, respectively. NOx emissions, always higher at cold start, present a different trend with respect to the cold start temperature, being higher at higher start temperatures. Overemissions vary between 13%, at 5ºC, and 30%, at 20ºC, for NEDC. For FTP75 (bag1, bag2) the same percentages (relative to hot emissions) are 6% and 23%. The smaller overemission percentages in FTP are justified with the fastest catalyst light-off (about 4-5 times faster) due to its higher dynamics.