Analysis of UHC Emission from a Natural Gas SI Engine Using Fast Response FID and a Heat Release Model 2001-01-3533

Lean burn operation is often used for improving the efficiency of SI engines. However, as a draw back, this method leads to a higher emissions of Unburned Hydro-Carbons, UHC, compared to stoichiometric combustion. In order to gain a better understanding of what is causing the higher UHC emission at lean burn condition, engine experiments have been carried out on a four-cylinder natural gas fueled SI engine. The concentration of UHC in the exhaust manifold and the HC concentration in the vicinity of the spark plug have been measured during the experiments using a Fast Response FID (FFID) analyzer. Using a model describing the outflow from the cylinder during the exhaust stroke and the measured UHC concentration in the manifold near the exhaust valve, the UHC emissions from the individual cycles have been determined. The investigation showed that under lean burn conditions, cycle by cycle variation had a significant importance on the total UHC emission from the engine. Both random, and non-random, variations were observed. The random variations were caused by random variations of, for example, the in-cylinder fluid motion. From the in-cylinder FFID measurement, the excess of air was determined from the individual cycles and it was found that the excess of air was oscillating. The oscillation was probably caused by the λ-control system of the engine and it was leading to significant variations in the UHC emission, especially at very lean conditions.