Prechamber autoignition in natural gas engines
Unscavanged prechamber ignition has been developed at the Laboratory of Industrial Energy Systems to reduce the emissions from cogeneration spark ignition gas engines, operating with both natural gas and biogas. Demonstration has been made that a lean burn engine unit equipped with optimized prechambers allows efficient operation, while remaining below the Swiss limits [1,2]. Further development aims at using prechambers operating in autoignition mode similar to HCCI (Homogeneous charge compression ignition). The advantages are on the one hand that there is no need for maintenance and replacement of the spark plugs which usually have a limited life time while on the other hand we are expecting to benefit from the HCCI characteristics, e.g. low NOx emission levels. The prechamber configuration though should be a system easier to control than an engine operating completely in HCCI mode. The investigation of the applicability of this technique for engines used in the transport sector with rapidly changing operating conditions might be a further step in the development. In order to better understand the phenomena, simulations are done taking into account both fluid dynamics and chemical kinetics. Ignition delay measurements with mixtures of methane, ethane and propane have been conducted at the Rapid compression machine facility at the University of Lille at varying stoichiometric ratios. These measurements are used as a basis to choose an appropriate chemical mechanism for combustion simulation. The kinetic solver CHEMKIN has been used for zero-dimensional simulations at TDC conditions of the rapid compression machine to reproduce the ignition delays. Several detailed combustion mechanisms have been applied for these simulations, e.g. GRI 3.0 + RAMEC, n-Heptane and PRF mechanisms (LLNL) and Konnov’s mechanism. The aim is to obtain a reduced mechanism that will be used in combination with computational fluid dynamics. In order to do this, sensitivity and reaction pathway analysis is performed, as well as multi- objective optimization of the kinetic parameters. In cooperation with CFS engineering a coupling between the CFD code NSMB (Navier Stokes Multi Block) and the chemical solver CHEMKIN has been established enabling a simulation of the experiments conducted with the rapid compression machine. Further steps will be the design of a prechamber to be used with an experimental mono-cylinder engine and the simulation of the in-cylinder flow with prechamber to determine the influencing factors for controlling the autoignition event. Behaviour at part load as well as in transient conditions will also be investigated.