Exhaust gases diluted by air can undergo oxidation reactions in the exhaust manifold of an internal combustion engine. A potential benefit of these reactions is additional heat release in the exhaust manifold which can result in early catalyst light-off during cold start. Since early catalyst light-off significantly reduces hydrocarbon emissions over the federal test procedure (FTP) cycle, the actual oxidation of hydrocarbons and CO in the exhaust manifold becomes a secondary benefit. Air injected into the exhaust manifold is used to oxidize fuel-rich exhaust gases. Oxidation can be initiated by the heat from the exhaust gases as they exit the cylinder. However, since thermal ignition is difficult to accomplish and is very sensitive to exhaust manifold conditions, this work identifies the conditions under which the temperature of the exhaust gases (i.e., thermal ignition) can initiate oxidation. The highest temperature exhaust gases enter the exhaust manifold just after exhaust valve opening and are the most likely to ignite the mixture. In addition to the exhaust gas temperature, there are other competing parameters including exhaust gas composition, gas velocity, and heat loss to the wall, which determine the probability of initiation of oxidation in the exhaust manifold. Here, we introduce a model of exhaust manifold oxidation which allows the isolation and identification of the separate parameters to establish the direction and magnitude of each effect.
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