Environmental concerns demand more stringent emission legislations concerning new power sources for handheld equipment, like chainsaws and trimmers. Today the most common power source is the well-known two-stroke engine. This type of engine has been the natural choice due to low cost and high power density. The two-stroke engine, as we know it today is not able to comply with future emission demands due to its massive hydrocarbon pollutant.
To challenge the future engine manufactures must come up with new cost efficient engine technologies that still deliver the same or improved performance for customer satisfaction.
Manufactures of handheld equipment are players on a very competitive market, where the cost is the main concern in choosing a feasible technology.
This paper describes the development of a sequentially stratified scavenging 25cc trimmer engine. To minimize the hydrocarbon contents in the scavenging losses, known to be in the order of one third of total fuel consumption for a simple two-stroke engine, scavenging is conducted sequentially. The first part contains pure air, thus not compromising emissions, the second part contains air fuel mixture to be retained inside the combustion chamber. An engine like this must have a dual feed system, one ordinary for air fuel and a second one that delivers pure air to the upper part of the scavenging channels. A concept concerning geometrical dimensions derived during theoretical studies gives the basis. Specifications and targets for performance and emissions are stated, and an engine prototype is developed.
The paper will guide the reader through the stages of engine development. Important parameters like the ratio of scavenging air contra air/fuel mixture as well as performance and emissions for each different part of the development stages.
In conclusion, reductions of hydrocarbon emissions in the order of 60% compared to a simple two-stroke engine are within reach without deteriorating engine performance. Unfortunately this is not enough to meet the more stringent emission legislation for this specific engine.