Advanced Ignition Modelling for Pre-chamber Combustion in Lean Burn Gas Engines

Lean burn combustion systems present a viable route to emissions reductions. However, sustainable and conrolled combustion of lean mixtures can be challenging. Scavenged pre-chamber ignition systems aim to address this challenge by creating favourable ignition conditions close to stoichiometry in the spark region. The main lean charge ignition is then delivered by flame jets propagating through the nozzles connecting the pre-chamber to the cylinder. Accurate and fast CFD modelling of the mixture formation and early flame kernel development in the pre-chamber are essential for the design of such systems. The initial stages of ignition in spark-ignited engines typically occur at time scales, temperatures and length scales falling outside of the remit of conventional CFD techniques prompting development of specialised ignition models. A review of the models currently available highlighted a gap in the technology currently available in commercial CFD codes. Simplistic models based on direct energy transfer or fixed temperature kernel development are readily available but these models to not provide an accurate representation of the spar ... mehrk growth. More complex models are also available which are based on complex chemistry/turbulence interaction. While these models can provide accurate solutions they require standalone chemistry solvers or spatial temperature distribution calculations. These can be time consuming to solve, making the models less than ideal for simulations in a production environment. A novel spark model has been developed by Ricardo and implimented into the CFD software VECTIS, allowing for an accurate specification of the spark-ignition process. The model covers all stages of spark discharge from breakdown and the formation of the initial kernel and includes a predictive model for the initial flame kernel size. The flame kernel evolution is computed via a 1D variable temperature model incorporating plasma physics with two-way coupling with 3D CFD. The detailed chemistry effects are included through flame speed and mixture properties tabulation. This paper illustrates the principles and applications of the developed model. The model is then applied to the analysis of a novel pre-chamber ignition system and the results are compared with measurement data. A study is performed to investigate the sensitivity of the results to the input parameters within the spark ignition model.