Numerical and Experimental Analysis of Mixture Formation and Performance in a Direct Injection CNG Engine 2012-01-0401

This paper presents the results of part of the research activity carried out by the Politecnico di Torino and AVL List GmbH as part of the European Community InGAS Collaborative Project. The work was aimed at developing a combustion system for a mono-fuel turbocharged CNG engine, with specific focus on performance, fuel economy and emissions.

A numerical and experimental analysis of the jet development and mixture formation in an optically accessible, single cylinder engine is presented in the paper. The experimental investigations were performed at the AVL laboratories by means of the planar laser-induced fluorescence technique, and revealed a cycle-to-cycle jet shape variability that depended, amongst others, on the injector characteristics and in-cylinder backpressure. Moreover, the mixing mechanism had to be optimized over a wide range of operating conditions, under both stratified lean and homogeneous stoichiometric modes. For this reason the experimental activity was supported by the numerical simulations of CNG injection, jet formation and air-fuel mixing, in order to have a deep insight into the injected gas behavior.

Simulations were carried out at the Politecnico di Torino, within the Star-CD environment, and involved the combustion chamber, the intake port and the exhaust port, in order to account for the actual flow field at the start of injection. Fuel injection was simulated by means of the ‘virtual injector’ approach, which was developed and validated at the Politecnico di Torino. Laser-Induced Fluorescence images were used to validate the numerical results. The model was then applied to a variety of test cases, involving part load and full load operating points.

Combustion system optimization was also carried out by means of an experimental investigation, which was performed by AVL on the performance and emissions of a multi-cylinder engine. The single-cylinder and the multi-cylinder engine are both based on a Mercedes-Benz four-cylinder in-line engine and feature a pent-roof combustion chamber, a bowl in piston and a centrally mounted poppet valve injector.

The numerical and the planar laser-induced fluorescence results obtained on the transparent engine revealed a close dependence of the mixture formation process on the injection timing and injector characteristics, and resulted in rather good agreement with the multi-cylinder engine performance and emission results. In fact, an unsatisfactory mixing degree detected in the former engine corresponded to higher HC emissions and cycle-to-cycle variations of combustion-related quantities in the multi-cylinder engine.