Previous work indicates that depending on the system, the increase in CO and HC emissions after a cold start at low ambient temperature can be 3 to 5 -fold compared to the operation at normal ambient conditions. The performance of a three-way catalyst system operating after a cold start at a low ambient temperature was found mainly to be governed by the rich mixture setting neccesitated by the poor evaporization of the fuel. This results in incorrect exhaust composition for the conversion point of view, as no oxygen is available.
In proprietary engine tests carried out in a special cold test facility the emissions of CO and HC could be decreased by extending the idle or using block heater prior to the cold-start of the engine. Conversion of CO and HC was improved also by adding free oxygen into the exhaust. This was done by injecting air into the exhausts before the converter but after the oxygen sensor. Hence, the closed-loop mixture control done by the ECU was not affected. The air injection was switched off when the engine had warmed up so much that stoichiometric operation and hence also full three-way catalysis was possible. At the ambient temperature of minus 20°C this took about 4 minutes. During a full 30 minute test, which simulates normal urban driving, the CO and HC emissions were lowered by 30% without seriously hurting NOx conversion.
Conversion of CO and HC did not began until the light-off temperature of the converter (about 250°C) was reached. With the standard converter using ceramic substrate this took about 1 minute after the cold start at -20°C. Therefore, further improvement could be gained by swithing over to a comparable unit based on a metal-foil substrate. Because the light-off temperature was then reached more quickly, the conversion efficiency went also further up.
The work is still in progress. In this phase pre-heated metal monoliths are evaluated. Preliminary results show that in order to reach full light-off already at the time of start-up, very high power output is needed.