FVV Reports

Auszug

The general exhaust gas emission behaviour of gas engines is excellent. However, the partially high emissions of unburnt hydrocarbon (UHC or total hydrocarbon emissions, THC) constitute a significant greenhouse gas contribution. State-of-the-art exhaust gas aftertreatment systems do not seem to provide sufficient long-term stability at sufficiently high conversion rates. THC emissions therefore have to be reduced by means of combustion process optimisation, with the double effect to prevent efficiency losses. For this purpose a combined approach had been pursued in an FVV research project partly funded by the Fachagentur Nachwachsende Rohstoffe (FNR – the central coordinating agency in the area of renewable resources in Germany) to allow a deeper understanding of the THC emission sources: Experimental investigations had been conducted on a single-cylinder research engine to identify individual sources of the THC emission together with a numerical approach to develop a calculation method for unburnt hydrocarbon emissions. An evaluation matrix was developed to show the significance of different THC reduction measures. The major influence is coming from the air-fuel ratio, which also dominates the NO_x formation and the efficiency of the engine. Other important factors are the wall temperature, the crevice volume, the valve timing, the ignition system and the mixture formation, which are all described in detail in the final report. A hybrid model was proposed to integrate the detailed reaction kinetics via separate sub-models coupled with computational fluid dynamics (CFD) methods which describe the three-dimensional flow and turbulence in the engine. Turbulent flame speeds, flame wall quenching and post oxidation of UHC were treated as separate models, and emissions coming either from the walls, the crevices or directly from the inflowing fuel (valve overlap) were studied in detail. For the adjustment of the combustion model, an optical engine had been constructed to investigate into the flame propagation by using different ignition systems and valve timing. The remarkable result of the study is that the application of the hybrid simulation model leads to a reasonable prediction of the unburnt hydrocarbon emission so that the approach can be applied in future for the optimisation of applied engines.

…