Ski-Doo RAVE
For a two-stroke cycle engine to have high power capacity at high crankshaft speeds, a high volumetric or breathing efficiency is required and the fresh charge losses must be minimized. This is achieved by opening the exhaust port early (ATDC) and utilizing the resonant effects of the tuned exhaust system to control fresh charge losses.
When an engine of this design is run at a medium speed, efficiency falls off quickly. The relatively high exhaust port effectively shortens the useful power stroke and because the exhaust system is tuned for maximum power, there is a large increase of fresh charge losses.
As a result, the torque decreases along with a dramatic increase of the specific fuel consumption. Higher torque along with lower fuel consumption can be obtained at lower engine speeds if the time the exhaust port is open is reduced (ATDC).
Bombardier-Rotax has patented a remarkably simple system to automatically change the exhaust port height based on pressure in the exhaust system.
Located above the exhaust port is a guillotine-type slide valve. This rectangular valve is connected by a shaft to a diaphragm which is working against the return spring. Two small passages just outside the cylinder in the exhaust port allow exhaust gas pressure to reach the diaphragm.
As the throttle is opened and the engine begins producing more power, the pressure against the diaphragm by exhaust gas will overcome the pressure of the return spring and the RAVE valve will open.
Over the return spring is a red plastic adjustment knob. Turning the adjustment in or out changes the preload on the return spring, which in turn will change the rpm at which the RAVE valve opens and closes. Depending on engine type, the exhaust port height changes a total of 4 to 6 mm when the RAVE valve is fully closed or fully opened.
The RAVE valve does not allow the engine to make higher peak horsepower than an engine not so equipped; it can make moving the horsepower peak higher more practical because of its effect on the rest of the power curve.