Experimental-Numerical Analysis of Nitric Oxide Formation in Partially Stratified Charge (PSC) Natural Gas Engines 2009-01-2783

Lean burn natural gas engines have high potential in terms of efficiency and NOx emissions in comparison with stoichiometric natural gas engines, and much lower particulate emissions than diesel engines. They are a promising solution to meet the increasingly stringent exhaust emission targets for both light and heavy-duty engines.

Partially Stratified-Charge (PSC) is a novel concept which was conceived by prof. Evans (University of British Columbia, Vancouver). This technique allows to further limit pollutant emissions and improve efficiency of an otherwise standard spark-ignition engine fuelled by natural gas, operating with lean air-fuel ratio. The potential of the PSC technique lies in the control of load without throttling by further extending the lean flammability limit. It provides a relatively rich air-fuel mixture (fuel amount is less than 5% of the main charge) in the vicinity of the spark plug, while maintaining an ultra-lean homogeneous charge in the main combustion chamber area. A stable ignition kernel is then allowed to form in this region, to support the subsequent combustion process. The PSC technique does not require any modifications to the basic engine unless the adoption of a special spark-plug: this is one of its main advantages.

Since PSC is a relatively new technique, a thorough understanding of the physical processes involved is mandatory to guide the design process. To that aim, a joint experimental-numerical analysis has been performed in this paper, applied to a Ricardo Single Cylinder Research Engine equipped with the PSC spark-plug. Traditional homogeneous/stoichiometric and lean PSC operating conditions have been compared in terms of performance and NOx emissions, to finally put into evidence the coupling between PSC injection patterns and NOx formation mechanism.