Abstract
Hydrocarbon mass emitted, while cold start, becomes significant as vehicle emission restraints tighten. This paper investigates the mechanism of emitted hydrocarbon reduction as forwarding the exhaust valve close timing at the very beginning of cold start, because there are relation between in-cylinder wet and emitted hydrocarbon. The hydrocarbon reduction was due to the backflow of the operating gas into the intake port, boosting the fuel vaporization. Emitted hydrocarbon at engine start is 30% decreased in consequence.