03-18
SECTION 03 - ENGINE PREPARATION
When an engine of this design is run at a high
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 in-
crease of fresh charge losses at lower speeds. As
a result, the torque decreases along with a dra-
matic 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 sim-
ple system to automatically change the exhaust
port height based on pressure in the exhaust sys-
tem.
Located above the exhaust port is a guillotine-
type slide valve. This rectangular valve is connect-
ed by a shaft to a diaphragm which is working
against a return spring. One small passage in the
cylinder just outside the exhaust port allow ex-
haust gas pressure to reach the diaphragm.
To the outside of 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.
1. Sliding valve
2. Diaphragm
3. Spring
4. Exhaust port
5. Adjustment knob
As the throttle is opened and the engine begins
producing more power, the pressure against the
diaphragm will overcome the pressure of the re-
turn spring and the RAVE valve will open.
VALVE FULLY OPENED
VALVE FULLY CLOSED
947 Engine Only
On this engine, the RAVE valves are controlled by
the Multi-Purpose Electronic Module (MPEM).
The MPEM measures 2 factors to control the
RAVE valves: engine speed (RPM) and its rate of
acceleration.
To open the RAVE valves, the MPEM activates a
solenoid which directs the exhaust pressure to
the diaphragms of the valves.
NOTE:
The exhaust pressure is taken only on the
magneto exhaust port.
To close the RAVE valves, the MPEM desacti-
vates the solenoid which directs the exhaust
pressure to the exhaust resonator.
A18C01A
1
5
3
2
4
F01D7GA
F01D7HA
Summary of Contents for Bombardier
Page 1: ......