Impact of Ambient Temperatures on Exhaust Thermal Characteristics during Cold Start for Real World SI Car Urban Driving Tests

Abstract

Thermal characteristics of SI engine exhaust during cold start and warm up period were investigated for different ambient temperatures (-2 to 32 °C). A Euro 1 emission compliance SI car was tested using a real world urban driving cycle to represent typical city driving patterns and simulate ECE15 urban driving cycle. The test car was equipped with 27 thermocouples along the engine and exhaust pipes so as to measure metal and exhaust gas temperatures along the engine, exhaust and catalyst. The characteristics of thermal properties of engine, exhaust system and catalyst were studied as a function of warm up time and ambient temperature. The temperature and time of the light-off of catalyst were investigated so as to evaluate the effect of thermal properties of the catalyst on emissions.The results show that the coolant water reached the full warm up about 5 minutes in summer and 9 minutes in winter after a cold start. Lubricating oil reached the full warm up in 10 minutes in summer and 14 minutes in winter after a cold start. The light-off time of TWC was about 3 minutes in summer and 6 minutes in winter in terms of catalyst substrate temperatures. The determination of catalyst light off has been studied and discussed in terms of catalyst substrate temperatures and gas temperatures. The ambient temperature had little influence on engine out exhaust gas temperatures. The heat loss from the engine out to the catalyst was at highest level in the first 5~6 minutes and after this point the heat available at the catalyst was relatively stable.The thermal properties of the engine and exhaust system had significant influence on emissions. The results indicate that in some urban driving conditions such as short journeys in cities especially under cold weather conditions, the function of catalysts for emission reductions is very limited. INTRODUCTIONIt is well known that a SI engine in cold conditions has much higher exhaust emissions than one that is fully warmed up (1-8). The new European passenger car emissions regulations has removed the first 40 seconds of idle period for the ECE driving cycle, in recognition of the importance of the cold start emissions. The new regulations also include -7°C tests for HC and CO emissions at cold start. It has to be addressed that Euro 1 cars were not developed to pass this cold test since developed prior to new -7°C regulation, and yet Euro 1 cars do cold starts at low temperatures in the real world. Extensive research had been undertaken in the past to investigate the influence of ambient temperature on exhaust emissions (2-8), normally using legislated test cycles and CVS test procedures. It was found that exhaust emissions could be drastically increased, relative to 25°C, at cold ambient conditions. For instance, the hydrocarbon emissions were found increase by 650% at -20°C and carbon monoxide emissions by 800% (8).A satisfactory cold start is very difficult to achieve at subzero ambient temperature conditions in terms of the SI Car Urban Driving Tests

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