Here’s why: the dirtiest moments in the internal combustion engine’s operation are when it is starting.
The more times that the internal combustion engine in a hybrid turns off and the restarts, the more times you’re running that engine through its dirtiest phase of operation. On ordinary internal combustion vehicles, a catalytic converter ensures that the emissions components of the initial start exhaust are cleaned up. Hybrids, however, don’t use catalytic converters.
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