The two-stroke SI engine is the predominant driving unit in applications that require a high power-to-weight ratio, such as handheld power tools. Regarding the latest regulations in emission limits the main development area is clearly a further reduction of the exhaust emissions.
The emissions are directly linked to the combustion processes and the scavenging losses. The optimization of the combustion processes, which represents one of the most challenging fields of research, is still one of the most important keys to enhance the thermal efficiency and reduce exhaust emissions. Regarding future emission regulations for small two-stroke SI engines it is inevitable that the emissions of gases causing the greenhouse effect, like carbon dioxide, need to be reduced.
As most small SI engines are carburetted and operate open loop, the mixture formation and the amount of residual gas differs from cycle to cycle [1]. When using conventional ignition systems, the size of the plasma is restricted to the small dimensions of the spark plug gap. In combination with the high flow velocities of small two-stroke SI engines and the variations of the mixture, the cycle-to-cycle variations are very high. Misfiring and delayed combustion lead to high emissions and poor running smoothness, especially at idle speed. Therefore an alternative ignition system was used to enhance the inflammation behavior with a more stable and faster combustion compared to a conventional ignition. At rated power there were improvements in the engine power and also in the engine efficiency due to a better combustion. At idle the most important improvement was the lean misfire limit which could help the fuel efficiency and the reduction of emissions a lot dependent on the field of application of the engine.