Comparison of Diesel Oxidation Catalyst Performance on an Engine and a Gas Flow Reactor 2007-01-0231

This paper analyzes and compares reactor and engine behavior of a diesel oxidation catalyst (DOC) in the presence of conventional diesel exhaust and low temperature premixed compression ignition (PCI) diesel exhaust. Surrogate exhaust mixtures of n-undecane (C₁₁H₂₄), ethene (C₂H₄), CO, O₂, H₂O, NO and N₂ are defined for conventional and PCI combustion and used in the gas flow reactor tests. Both engine and reactor tests use a DOC containing platinum, palladium and a hydrocarbon storage component (zeolite).

On both the engine and reactor, the composition of PCI exhaust increases light-off temperature relative to conventional combustion. However, while nominal conditions are similar, the catalyst behaves differently on the two experimental setups. The engine DOC shows higher initial apparent HC conversion efficiencies because the engine exhaust contains a higher fraction of trappable (i.e., high boiling point) HC. Engine DOC light-off is delayed because in the tests performed, engine-out CO was 11-30% higher than on the reactor, causing significant self-inhibition. The engine DOC also experiences ambient cooling, which further delays light-off. Fully-lit HC conversion is lower on the engine because the reactor surrogate exhaust mixture does not include methane (CH₄), which is unreactive at diesel light-off temperatures. Engine DOC heat loss and sample line HC desorption during post-DOC sampling also reduce fully-lit conversion on the engine.