Strategies to Reduce Scavenge Losses of Small Capacity 2-Stroke Engines, Pressurized by the Common Market Costs 2005-32-0098

More and more restrictive regulations, as the pending Euro3 emission legislation, demand for a lowering of exhaust emissions of two stroke engines.

The reduction of scavenge losses is one of the key requirements for the development of two-stroke engines. Besides it is important to maintain the excellent torque and power behavior of the two stroke units. This publication gives a summary of previous and present development results of test bench and chassis dynamometer measurements and of 3D CFD simulations. All these results are targeted on a reduction of the scavenge losses and therewith HC-emissions. Therefore the focus is put on different injection strategies, such as semi-direct and air-assisted injection.

Additionally, the potential of the high pressure gasoline direct injection is presented, even if this technology is still too expensive for the small capacity engine market, e.g. the European 50 cc scooter class. As a promising alternative to the aforementioned strategies new ideas to reduce the scavenge losses are introduced. These concepts have to be realized by using cost-efficient standard injection components from state of the art intake manifold injection systems. The goal of this type of injection is to achieve Euro3 emission standards with up to 30% lower system costs compared to state of the art air-assisted injection systems.

These new ideas can be described in terms of stratified scavenging. Due to the accurate positioning of the injectors, a stratified mixture preparation or homogeneous mixture preparation can be obtained, before entering the combustion camber. For different load and engine speed variations, the benefits of the different mixture preparations will be pointed out.

A potential estimation of this scavenge strategy will be shown by results of 3D-CFD simulations, comparing homogeneous (air and fuel) scavenging, air-assisted injection, high pressure direct injection, and stratified scavenging.