Numerical and experimental evaluation of the passive pre-chamber concept for future CNG SI engines

The third decade of the 21st century is set to be one of the most relevant in terms of the transition of methods and forms of using different energy sources, with short and medium-term developments on the road to decarbonization. One of the most widely accepted measures on this path is the reduction of CO2 emissions from internal combustion engines, which is why different concepts are being studied in order to optimize combustion. Furthermore, this research focuses on developing a suitable Spark-Ignition (SI) engine configuration integrating the passive pre-chamber ignition concept (TJI), together with a high compression ratio and the use of the Miller cycle, in order to exploit the benefits of using Compressed Natural Gas (CNG) as fuel. Thus, the novelty of this investigation is the detailed evaluation of the aforementioned SI engine architecture by state-of-the-art CFD simulations and a broad experimental campaign, to understand the advantages of the combined technologies against a conventional SI engine operating with gasoline. The pre-chamber technology is able to overcome the issues associated with the lower laminar flame speeds of CNG, and the reduction of turbulence caused by prematurely closing the intake valve with the Miller cycle. The experimental results showed that the new engine definition achieved higher levels of indicated efficiency compared to the baseline engine (around a 3% increase at high load/speed conditions). Moreover, the addition of EGR allows to further improve the engine performance, extending the load limit in the low-end torque region of the engine map. Finally, a full dynamic 1D model of a current passenger car vehicle was developed to perform transient driving cycle simulations, showing a reduction of 15% in fuel consumption and 25% in CO2 emissions for the new engine definition compared to the baseline engine.