Light Duty Vehicle CO Emissions During Cold Weather

Recent tests conducted by the Alaska Department of Environmental Conservation and low ambient temperature tests previously conducted by a variety of other organizations indicate that less progress is being achieved in the control of emissions during cold weather than under temperatures similar to those used during EPA certification testing. Although CO emission standards dropped from 15 grams per mile to 7 grams per mile between 1975 and 1981, far less of a change occurred in CO emissions from new vehicles at 20°F. Cold start CO emissions at 20°F are about 60 grams per mile for late model cars at low mileages. The available test data on these cars seem to indicate that results achieved using the standard emission test procedure are poorly correlated with emissions at lower temperatures.

However, the low temperature CO emissions of cars certified at 3.4 grams per mile CO are nearly 502 lower than vehicles certified to a standard of 15 grams per mile. The benefits of the 3.4 gram CO standard on low temperature emission control appears to be due to the fact that manufacturers have been induced to use systems which are much more effective in reducing the degree of mixture enrichment needed during cold start operation.

Test data indicate that cold temperature emission performance from cars equipped with fuel injection systems are clearly superior to most carburetor equipped cars. However, comparable levels of low temperature CO control may be possible with carburetors through the use of electric intake manifold heating grids. Unless new regulations provide an alternative mechanism for encouraging the use of such systems, the achievement of the ambient air quality standard for CO in areas which experience violations during cold weather may depend on the continued requirement for a 3.4 gram per mile standard.