Experimental and Numerical Analysis of an Active Pre-Chamber Engine Fuelled with Natural Gas

Increasingly stringent pollutant and CO2 emission standards require the car manufacturers to investigate innovative solutions to further improve the fuel economy and environmental impact of their fleets. Nowadays, NOx emissions standards are stringent for spark-ignition (SI) internal combustion engines (ICEs) and many techniques are investigated to limit these emissions. Among these, an extremely lean combustion has a large potential to simultaneously reduce the NOx raw emissions and the fuel consumption of SI ICEs. Engines with pre-chamber ignition system are promising solutions for realizing a high air-fuel ratio which is both ignitable and with an adequate combustion speed. In this work, the combustion characteristics of an active pre-chamber system are experimentally investigated using a single-cylinder research engine. The engine under exam is a large bore heavy-duty unit with an active pre-chamber fuelled with compressed natural gas. In first stage, an experimental campaign was carried out for four different conditions of load and air/fuel ratio, at the same engine speed, then a 3D CFD analysis was realised to evaluate the in-cylinder turbulence and pre-chamber pressure traces. Global engine operating parameters as well as cylinder pressure traces, inside main combustion chamber and pre-chamber, were recorded and analysed. Based on the available 3D and experimental data, a phenomenological model of this unconventional combustion system is developed and validated. The model is implemented in a commercial 1D code. The proposed numerical approach shows the ability to simulate the experimental data with good accuracy, with no case-dependent tuning. The model demonstrates to correctly describe the behaviour of a pre-chamber combustion system under the four operating conditions and to capture the physics behind such an innovative combustion system concept.