Practicability and Influencing Factors of a Lean Burn Mode for Two-Stroke Engines in Hand-Held Powertools

For many applications, such as scooters, hand-held power tools and many off-road vehicles, two-stroke engines are used as a preferred propulsion unit. These engines convince by a good power to weight ratio, a high durability and low maintenance technology and are therefore the first choice in this field of application. In general, already much development effort has been expended to improve those systems. However, an increasing environmental awareness, the protection of health and the shortage of fossil resources are the driving factors to further enhance the internal combustion process of those adapted two-stroke engines. The current focus here is on the reduction of emissions and fuel consumption with an at least constant power output.
An approach to address an improvement of engine efficiency can be covered by applying a lean combustion burn mode. In almost all combustion technology sectors, including furnaces, gas turbines and many internal combustion engines, a lean combustion is utilized. Possible advantages of a combustion process under lean conditions are low pollutant emissions and high efficiencies, because flame temperatures are typically low. Nevertheless, most air-cooled two-stroke engines still operate at wide open throttle with a rich air to fuel ratio. This is due to reliability, durability reasons and the ease of use. For that reason, it is mandatory to determine the key factors for the practicability of a lean burn mode for two-stroke engines. In this context, a new approach to implement a lean combustion is discussed and different influencing factors, which have an impact on lean combustion process, are investigated and analyzed. A thermodynamic loss analysis is performed to highlight the differences. Moreover, further research demands to realize a lean combustion in a series production engine are pointed out.