Impact of prechamber design and air-fuel ratio on combustion and fuel consumption in a SI engine equipped with a passive TJI

Abstract

[EN] The increasing concern about Greenhouse Gas (GHG) emissions led the European Union to introduce increasingly stringent limits to the CO2 from road vehicles, with an impact on the sales of passenger cars. Vehicles equipped with Spark-Ignition (SI) engines became more numerous than those equipped with Compression-Ignition (CI) engines, due to the expensive aftertreatment system needed to comply with the restrictive European emission standards. However, SI engines provide lower efficiency than CI engines, due to the low compression ratio and the operation with a stoichiometric air-fuel ratio. Lean combustion can be a solution to increase SI engines' efficiency. Nevertheless, extremely lean mixtures lead to an increase in Cycle-to-Cycle Variability (CCV).The prechamber ignition concept, also known as Turbulent Jet Ignition (TJI), is an attractive solution for lean combustion, without its drawbacks. There are two ways to implement this concept: active TJI and passive TJI. In active TJI, there is an additional fuel supply system inside the prechamber, while passive TJI operates without additional injection. Therefore, passive TJI offers advantages in terms of simplicity, easy packaging, and low cost.In this work, the effects of passive TJI on combustion and performance are investigated by simulation analyses. Particularly, a 1-D engine model was developed to simulate the TJI combustion and validated against the experimental data. Furthermore, model simulations were carried out to assess how the prechamber geometry, in terms of A/V ratio, affects the jet performance, main chamber combustion, and fuel consumption, for different lambda values.The analysis was conducted in a medium-to-high speed and load operating condition, namely 4500 rpm and 13 bar of Indicated Mean Effective Pressure (IMEP), under both stoichiometric and lean mixture. Simulation results demonstrated that the best jet performance and the highest engine efficiency are obtained for medium-tohigh values of prechamber volume and large diameters, both in stoichiometric and lean-burn conditions, defining a common optimum prechamber design regardless of the lambda level.