Automotive exhaust emissions at low ambient temperature

Cold ambient conditions have a distinct impact on the total emission of the Finnish car park because climatic conditions in Finland are rather severe regarding the use of automobiles. In the present study, the exhaust emission characteristics of three automotive petrol engines with different fuel metering systems were evaluated. Experiments were carried out on an engine and exhaust system combination in a cold chamber at a constant ambient temperature ranging from +20 °C to -30 °C. Test procedures included either constant or variable load mode during a 30 minute run. The test results show that the use of the enrichment devices necessary during cold-start and warm-up period has a major impact on the total emissions during the test In the carburetted non-catalyst engines the share of the initial emissions during cold-start and warm-up phase was smaller than with the fuel-injected catalyst version. The catalytic converter equipped version was also found to be far more sensitive to the low ambient temperature than the normal carburetted versions without exhaust after treatment. The relative increase in CO and HC emissions when the ambient temperature was lowered was in case of the fuel-injected catalyst engine some 5 to 15 times greater than with the normal carburetted version of the same engine. Although the temperature of the converter rose quite quickly after the cold start it did not promote the conversion, because the enrichment circuit of the fuel injection system over riched the air-fuel mixture, thus optimising the driveability, but making the operation of the three-way catalytic converter impossible. The carburetted test engines were found to be quite sensitive to intake air temperature. The test results indicate that the emission behaviour of different fuel metering and emission control systems can be surprising at low ambient temperature. Standardised test procedures for exhaust emissions do not consider the effect of low ambient temperature. Hence, the results of this study indicate a clear need for developing emission test procedures also for low temperature operation of the automobiles.