CNG is one of the most promising alternate fuels for passenger car applications. CNG is affordable, is available worldwide and has good intrinsic properties including high knock resistance and low carbon content.
Usually, CNG engines are developed by integrating CNG injectors in the intake manifold of a baseline gasoline engine, thereby remaining gasoline compliant. However, this does not lead to a bi-fuel engine but instead to a compromised solution for both Gasoline and CNG operation.
The aim of the study was to evaluate the potential of a direct injection spark ignition engine derived from a diesel engine core and dedicated to CNG combustion. The main modification was the new design of the cylinder head and the piston crown to optimize the combustion velocity thanks to a high tumble level and good mixing.
This work was done through computations. First, a 3D model was developed for the CFD simulation of CNG direct injection. Numerical tests were carried out on the injection test bench configuration in order to achieve good correlations between calculations and experiments. Once validated, the model was implemented in the numerical setup of the engine and different designs of the combustion chamber were computed to compare mixing level, turbulent energy level, trapped mass and IMEP for homogenous stoichiometric operation. The best design was then manufactured and tested on a single-cylinder research engine.
Comparing test bench results of CNG port injection and CNG direct injection configurations showed the potential of direct injection: - later injection timings lead to higher volumetric efficiency, - later spark ignition timings lead to higher burning speed for a better indicated fuel consumption.
Finally, the test bench results were extrapolated to construct a numerical model of a multi-cylinder engine as well as a vehicle. NEDC cycle simulations using a DI CNG-engine-powered Light-Duty Vehicle have shown a 27% reduction of CO₂ emissions compared to the same vehicle equipped with a diesel engine, making DI CNG engines a credible alternative for the European market.