Operation Of The Rave Valve - BOMBARDIER 1996 Ski-doo MX Z440 Handbook

OPERATION OF THE RAVE VALVE

(RAVE = ROTAX ADJUSTABLE VARIABLE EXHAUST)
A) THEORY
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. The
result is achieved by opening the exhaust port early (94.5° BBDC) 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 shortened.
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 (item 1). This rectangular valve is
connected by a shaft to a diaphragm (item 2) which is working against the return spring (item 3).
Two small passages in the cylinder just outside the exhaust port (item 4) 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 will overcome the pressure of the return spring and the
RAVE valve will open.
04-23