An experimental investigation of fuel consumption, exhaust emissions and heat release was performed on a prototype 1.2 liter 4 cylinder turbocharged CNG engine, which has been specifically developed and optimized in order to fully exploit natural gas potential. More specifically, the combination of a high CR of 10.1:1 and a Garrett high-performance turbocharger featuring selectable levels of boost produced a favorable efficiency map, with peak values exceeding 35%.
The experimental tests were carried out in order to assess the engine performance improvement attainable through turbocharging and to define the best control strategies for this latter. The investigation included ample variations of engine speed and load, RAFR as well as trade-offs between boost level and throttle position. At each test point, in-cylinder pressure, fuel consumption and ‘engine-out’ pollutant emissions, including methane unburned hydrocarbons concentration, were measured. The acquired data were then processed through a combustion diagnostic tool resulting from the integration of an original multizone heat-release model with a CAD procedure for the burned gas front geometry simulation.
Power density comparable to those of last-generation DI diesel engines were obtained at stoichiometric operations under retarded spark-timings and high boost levels. However, lean-burn operation gave the best fuel efficiency. Concerning the most efficient torque-based control strategy for the turbocharged engine, the best results were achieved by first operating the throttle valve with no boost and then, once WOT condition is reached, by increasing the boost pressure. This procedure allowed minimizing the pumping losses at each operating condition. Finally, based on the conversion efficiencies of 3-way catalytic converters, lean burn operation highlighted a significant increase in tailpipe emissions of methane unburned hydrocarbons and nitrogen oxides with respect to stoichiometric operations.