Advantages and challenges of lean operation of two-stroke engines for hand-held power tools

Today, hand-held power tools facilitate specific jobs in many application areas, e.g. garden/landscape maintenance, construction industry and forestry. This market is highly diversified and demands special tools which are optimized for each application purpose (brush cutters, chain saws, etc.). Most of them are driven by two-stroke SI (spark ignition) engines equipped with a carburetor. High specific power, simplicity, low weight, low costs and compact design are characteristics of these two-stroke engine powered tools. The use of piston control or reed valves, oil-in-gasoline lubrication and loop scavenging is well established. Significant improvements regarding emission behavior have been achieved in recent years due to stratified scavenging systems which have become a standard. In order to respond to future challenges like continuing emissions and fuel consumption reduction the present engine technology has to be further developed. Several research areas are considered in order to improve the overall product, i.e. injection systems, ECU hardware and functionality, exhaust gas after-treatment and combustion systems [1, 3, 4, 5]. To date, the research on combustion systems for hand-held power tools tended to focus on rich mixture setting. The demands for high power and quick engine response are met by a rich mixture in part load and WOT (wide open throttle) operation. Moreover, the rich mixture reduces NOx formation and reduces thermal stress.