Lean large-bore natural gas engines are usually equipped with gas-scavenged prechambers. After ignition and during combustion in the prechamber hot reacting jets penetrate the main chamber and provide much higher ignition energies than electric spark plugs. Although prechambers stabilize combustion, limitations of the concept are observed at very lean main chamber mixtures and large cylinder diameters, which appear as cycle-to-cycle variations of heat release and pressure.

At the Thermodynamics Institute of the Technical University of Munich cycle-to-cycle variations are investigated in an unique periodically chargeable high pressure combustion cell with full optical access to the entire main chamber. Recently, the influence of the ignition timing, the amount of scavenge-gas of the prechamber and the cross section of the prechamber exit orifices on cycle-to-cycle variations have been studied. From the pressure traces characteristic parameters of the combustion process like the ignition probability, the ignition delay and the rate of the pressure rise have been derived. By analysing the emission of OH*-chemiluminescence in terms of reacting area and light emission and on the basis of numerical simulations information on the source of cycle-to-cycle variations is obtained. Finally it is shown that cycle-to-cycle variations can be reduced remarkably by appropriate selection and combination of prechamber geometry and operating parameters.

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