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Because of todays new emissions legislation, a new 45cc Husqvarna trimmer/clearing saw power head was needed. When reducing emissions in a conventional two-stroke engine or a stratified scavenged engine, it is important that the tuning and basic scavenging characteristics of the standard engine are maintained. A dual charge intake system is necessary for the stratified engine but it also creates air fuel delivery issues compared to a standard two stroke engine. With increasing trapping efficiency more spent gases mixes with the fresh charge, creating less favorable combustion properties and thermal loading on the engine. On top of this the sequential stratified scavenging technology introduces a spatial inhomogeneous mix problem between scavenging fresh air, new mixture and spent gases. This all add sensitivity to long term stability due to deposits of carbon both in combustion chamber and exhaust duct, resulting in a change in engine parameters due to aging.

To effectively use Computational Fluid Dynamics (CFD) for engine emission development it is necessary to be able to simulate the scavenging flow in an engine. The CFD model for a stratified charged two-stroke engine is even more complex. This model have been tuned and finally validated with engine tests. A CFD model has been made of the Husqvarna 560XP two-stroke stratified charged chainsaw engine. The model contains piston, cylinder, inlet system ducting and exhaust silencer. The simulation runs with moving deforming mesh with all ports active. The airflow levels have been fine tuned with inlet restrictions similar to those in the air filter holder, which is not completely included in the present model. The results and behaviour of the CFD model has a very good match to the measured values of the finished product. This gives us confidence in the model and several aspects can now be studied that is virtually impossible to capture by other means.

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. Future engine manufactures must come up with new cost efficient engine technologies that still deliver the same or improved performance for customer satisfaction. One of the solutions is the stratified scavenging two-stroke engine, which minimizes the unburnt hydrocarbon contents in the scavenging losses. An engine like this must have a dual feed system; one ordinary for air+fuel mixture and a second one that delivers pure air to the upper part of the scavenging channels.

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.

A sequentially stratified scavenged engine is characterised by the principle that the cylinder is first scavenged by pure air, followed by the air/fuel mixture. The air is introduced into the upper part of the scavenging ducts through a piston port or a reed valve. To take full advantage of the stratified scavenged principle, the scavenging ducts have to be designed in a way, so that they can accommodate all the air that is delivered into the scavenging ducts. When converting a conventional two-stroke engine into a stratified scavenging engine, it is also important that the tuning and basic scavenging characteristics of the standard engine are not deteriorated. In this paper it is shown how these two aspects can be combined. Together with a theoretical approach for dimensioning the volume and length of the scavenging ducts, it gives a guideline on how to design the basic engine layout, for a stratified scavenged two-stroke engine.