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.