Scarce resources of fossil fuels and increasingly stringent exhaust emission legislation push towards a stronger focus to alternative fuels. Natural gas is considered a promising solution for small engines and passenger cars due to its high availability and low carbon dioxide emissions. Furthermore, natural gas indicates great potential of increased engine efficiency at lean-burn operation. However, the ignition of these lean air/fuel mixtures leads to new challenges, which can be met by fuel scavenged prechambers.
At the Institute of Internal Combustion Engines of the Technische Universitaet Muenchen an air cooled natural gas engine with a single cylinder displacement volume of 0.5 L is equipped with a methane scavenged prechamber for investigations of the combustion process under real engine conditions. The main combustion chamber is supplied with a lean premixed air/fuel mixture. As the excess air restrains a direct ignition by a spark plug, the prechamber must additionally be scavenged with methane to achieve nearly stoichiometric conditions and hence improve the ignition.
To enhance the understanding of the processes occurring, the empirical studies are accompanied by supplementary three-dimensional computational fluid dynamics investigations. The influences of the start of injection and the injection duration are evaluated with regard to the ignition probability and the combustion conditions at the electrodes of the spark plug. Furthermore, the flow and the mixture formation within the prechamber as well as the turbulent kinetic energy are investigated by simulation. The numerical results are validated with experimental data from the test bench regarding parameters such as combustion duration, lean-burn limit and cycle-to-cycle variations.
