A Novel Method of Tuning a Stratified Scavenged Two-Stroke Engine 2006-32-0055
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.
Stratified scavenging introduces many new issues in the development process of the engine. Air/fuel ratio control over the intended speed range becomes more complicated. The relationship between the amount of air that goes through the air head system versus amount of air through the carburettor is another important design factor that does not exist with the standard two stroke engine.
This paper describes a novel way of tuning the dual intake engine, in this case a typical high-speed chainsaw engine designed with two scavenging ducts, and piston-guided valves to deliver the air directly to the upper part of the scavenging ducts.
Results from engine tests run in our engine laboratory will be compared with one-dimensional computer simulations. This makes it possible to explain engine behaviour before and after geometrical changes in the dual intake and scavenging system.
A computer simulation makes it possible to visualize pressure pulses and mass flows inside an engine that would be very hard or impossible to measure without altering the engine and at the same time the measured results.