Experimental study on the impact of the jet shape of an outward-opening nozzle on mixture formation with CNG-DI

Abstract

The energy source natural gas is the most promising alternative to the conventional fuels having beneficial properties like reduced pollutants and CO₂ emissions particularly suited for the SI engines. Nevertheless, this environmentally friendly propulsion is met with rejection by the customer due to the deficit in the number of refueling stations and lower power yield compared to gasoline engines. One possibility to raise customers’ acceptance is the improvement of the driveability by increasing the volumetric efficiency with CNG direct injection. However, such a combustion concept puts high demands on the injection system and mixture formation. Among others a much higher flow rate at low injection pressure is necessary which can only be provided by an outward-opening nozzle due to its large cross-section. But this valve concept with a hollow cone jet has a specific propagation behavior with gaseous fuel which completely differs from a liquid one. Thus, the present paper is focused on experimental investigations of the influence of such a jet shape on the mixture formation with CNG-DI. The experiments are performed using a two-cylinder SI engine at stoichiometric homogenous conditions. Additionally, the test-bench results are supported by measurements in a pressure chamber equipped with Schlieren technique and also by numerical 3D-CFD simulations. The last-mentioned ones are presented in a separate paper, named “3D-CFD Analysis on Scavenging and Mixture Formation for CNG-DI with an outward opening Nozzle” from Marlene Wentsch. Nevertheless, the experimental results show a sensitive response of the hollow cone jet to the surrounding combustion chamber walls and thus a significant role of this interaction for the fuel distribution and emissions.