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Abstract :

Homogeneous lean combustion is a great opportunity to reduce Interna Combustion Engine (ICE) emissions (both greenhouse gases and pollutants) if combined with responsible use. Unfortunately, burning lean mixtures and meeting the demands of ICEs is complicated by low reactions rates, extinction, instabilities and mild heat release. There is therefore a need for breakthrough technologies thwarting the adverse effects of lean combustion to leverage lean-burn strategies in ICEs. The Pre-Chamber Ignition (PCI) concept has demonstrated its capabilities to induce very high burning rates enabling ultra-lean premixed mixtures to be burnt efficiently. This is achieved through the creation of multiple highly turbulent jets of hot burnt gases issuing into the main chamber of the engine. However, the optimization of the size of the pre-chamber orifices is something very complex that is not yet clearly understood. Small holes must be used in order to generate enough turbulence in the main chamber, but these small holes can also inhibit the ignition of the main chamber because of too high jet cooling and/or speed. Therein lies the challenge of this research work: how to design the holes connecting pre- and main chambers to maximize burning rates without exceeding the ignition limit?

To answer this question, multiple numerical tools were used: kinetically detailed Direct Numerical Simulation (DNS), Large Eddy Simulation (LES) and zero-dimensional modelling. DNS was used to build precise knowledge on jet ignition. Especially, it helped to understand how the jet injection speed and temperature govern ignition and revealed specific ignition and combustion flame structures. It also allowed to build models to predict the outcome of an ignition sequence. LES was used to study the whole PCI concept in a real engine. It allowed to analyse the flow entering and leaving the pre-chamber, to measure the cooling and quenching effects in the connecting ducts, and to analyse the ignition and combustion processes for both normal and abnormal combustion cases. Finally, a zero-dimensional model has been developed based on a multi-zone approach. It integrates key sub models to account for thermal effects in the ducts and to predict the outcome of the jet ignition attempts in the main chamber. Therefore, it provides a crucial tool to answer the research question by evaluating the result of multiple PCI designs in term of main chamber ignition at a low computational cost.

Keywords: combustion, ignition, internal combustion engine, pre-chamber, simulation

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