Numerical and Experimental Studies on Mixture Formation with an Outward-Opening Nozzle in a SI Engine with CNG-DI

CNG direct injection is a promising technology to promote the acceptance of natural gas engines. Among the beneficial properties of CNG, like reduced pollutants and CO₂ emissions, the direct injection contributes to a higher volumetric efficiency and thus to a better driveability, one of the most limiting drawbacks of today’s CNG vehicles. But such a combustion concept increases the demands on the injection system and mixture formation. Among other things it requires a much higher flow rate at low injection pressure. This can be only provided by an outward-opening nozzle due to its large cross-section. Nevertheless its hollow cone jet with a specific propagation behavior leads to an adverse fuel-air distribution especially at higher loads under scavenging conditions. This paper covers numerical and experimental analysis of CNG direct injection to understand its mixture formation. For this purpose experimental investigations were carried out by the Robert Bosch GmbH using a two-cylinder SI engine at a high load operating point with high scavenging degree. To understand the mixture phenomena the test-bench activities were supported by numerical simulations with the 3D-CFD-tool QuickSim at the FKFS. The experiments included various injection timings and valve overlaps. Additionally, the tests were performed with two different nozzle concepts (outward- and inward-opening injector) to identify the influence of the jet shape on the fuel-air distribution. The simulations also contained these parameters and particularly considered the jet development and flow field in the combustion chamber and the intake port. The test-bench investigations revealed a close dependence of the mixture formation on the injection timing and jet characteristic during scavenging operation. The associated numerical studies resulted in a good agreement with the engine performance and led to a conclusive interpretation of the observed phenomena.