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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.

Engine experiments have been conducted on a gas fueled SI engine. The engine was fueled with natural gas and mixtures of natural gas and hydrogen containing producer gas in order to examine the effect of addition of producer gas on the combustion process and the engine-out emissions. The experiments showed that addition of producer gas decreased the UHC emission at conditions leaner than λ=1.40. The CO emission was increased by addition of producer gas. This was mainly caused by unburned fuel CO from the producer gas. No effect of producer gas on the NOx emission was detected. Formaldehyde, which is suspected to cause odor problems from natural gas fired engine based power plants, was measured using FTIR. The investigation showed that the formaldehyde emission was decreased significantly by addition of producer gas to natural gas.