Study of Different Active Pre-Chamber Ignition Layouts for Lean Operating Gas Engines using 3D-CFD Simulations

Internal combustion engines operating with gas could represent an immediate and cost-effective solution in cutting CO2 emissions thanks to the higher hydrogen to carbon ratio compared to commercial fuels and exploiting the higher knock resistance of methane compared to gasoline. Already successfully used in stationary gas engines, the active pre-chamber ignition for on-road internal combustion engines allows decoupling the mixture realized in the main combustion chamber from the one produced at the electrode. This makes the engine capable of operating in extremely lean conditions and thus increasing its efficiency, while maintaining adequate combustion characteristics, such as low cycle-to-cycle variation and sufficiently short combustion. In addition to this, the adoption of a very lean mixture allows keeping the engine-out NOx under control. Therefore, the design of the pre-chamber, the position of the electrode with respect to the pre-chamber injector and the charge motion generated in the pre-chamber itself, thanks to an ad hoc design of the pre-chamber holes, assume a fundamental role in the new engine development. In this paper, different active pre-chamber designs are studied by means of 3D-CFD Simulation. A single-cylinder engine is reproduced in the calculation environment of the 3D-CFD-Tool QuickSim, allowing multi-cycle simulations and including the test bench peripheries. A conventional piezo-actuated hollow-cone gasoline injector is used to dose the amount of methane in the pre-chamber. Since the pre-chamber could represent a source of NOx and in order to improve cold start operations, the prototype active pre-chamber can be conditioned through a dedicated derivation of the engine water jacket. The 3D-knock model based on detailed chemistry calculation supports the evaluation of the pre-chamber at full load. The performances of the different active pre-chambers are tested both at low load for lean operation and at high load for stoichiometric condition. Following the performance prediction of the first manufactured geometry, the engine can operate ultra-lean up to λ ~ 1.8, with indicated efficiency higher than 42%.