EFFECTS OF SECONDARY AIR ON GLOBAL HYDROCARBON CONSUMPTION RATES IN ENGINE EXHAUST GAS

ABSTRACT

The temperature required to obtain total hydrocarbon consumption within the exhaust port and manifold of an engine, which is operated with secondary air dilution during a cold–start, were computationally investigated using detailed chemistry and thermodynamic properties. Constant pressure calculations were carried out over ranges of equivalence ratio and dilution levels representative of an engine cold-start. The calculations were performed using both iso-octane and n-heptane as surrogate fuels. It was found that the temperature required to obtain total hydrocarbon consumption on a 100 ms timescale is in the 950 to 1000 K range for all pressures, secondary air dilution levels, and fuel types investigated. The dilution levels should be such that the overall O₂/HC ratio in the exhaust gas is greater than 2 in order to achieve HC consumption at the lowest possible temperature. Thus, the biggest challenge for secondary air systems is being able to mix the air with the exhaust gas without decreasing the gas temperature below these critical conditions.

Keywords:

• exhaust port oxidation
• hydrocarbon emissions
• post flame hydrocarbon consumption
• cold-start