Advantages and Challenges of Lean Operation of Two-Stroke Engines for Hand-Held Power Tools

摘要

One of the most significant current discussions worldwide is the anthropogenic climate change accompanying fossil fuel consumption. Sustainable development in all fields of combustion engines is required with the principal objective to enhance efficiency. This certainly concerns the field of hand-held power tools as well. Today, two-stroke SI engines equipped with a carburetor are the most widely used propulsion technology in hand-held power tools like chain saws and grass trimmers. To date, research tended to focus on two-stroke engines with rich mixture setting. In this paper the advantages and challenges of leaner and/or lean operation are discussed. Experimental investigations regarding the influence of equivalence ratio on emissions, fuel consumption and power have been performed. Accompanying 3D-CFD simulations support the experiments in order to gain insight into these complex processes. The investigations concentrate on two different mixture formation processes, i.e. CCI (crankcase injection) and LPDI (low pressure direct injection). Further focus is put onto combustion phenomena of lean mixtures. The main issue addressed in this paper is the impact of mixture setting on fuel consumption. Therefore, the losses from theoretical to effective efficiency of different settings calculated by thermodynamic analyses are explained. Savings potential of fuel consumption and HC emissions are demonstrated. The results indicate that fulfillment of required durability and power density using a leaner mixture setting is a significant challenge. Furthermore, the increase of NO_x emissions is a fundamental drawback. Here, basic research regarding reduction of NO_x engine-out emissions is required.