available; therefore, it is not always possible to state
exactly at which speed to shift unless all the variable
facts are known. A good rule is "shift down at the same
So called because closing of a valve in the exhaust
engine speed the tachometer indicated immediately after
system retains compressions pressures within the
shifting up." On a steep grade, start down-shift before
exhaust manifolding and engine cylinders to utilize these
the engine actually pulls down to shifting speed,
pressures to reduce speed. Compression braking is
because the truck will lose speed while shifting gears.
most efficient when engine is permitted to turn at same
speeds as for efficient power and may be used as much
Failure to shift down at the right time, or a delayed
and as often as possible.
down-shift will result in the engine failing to reach full
power, and make another down-shift necessary.
JACOBS ENGINE BRAKE
When approaching a hill, open throttle smoothly to
This braking system converts the engine into a
start the up-grade at full power, then shift down as soon
power absorbing air compressor using a master slave
as the engine has dropped to shifting speed. Do not
piston arrangement to open engine exhaust valves near
wait until the engine is below shifting speed. Less gear
shifts will be required and average road speed will be
exhaust. The effect being a net energy loss since work
higher if this is done smoothly.
done during compression is not returned during the
The operator selects a gear which will provide a
The Cummins Diesel is effective as a brake on
balance between engine speed and road speed. If
downhill grades, but care must be exercised not to
engine exceeds maximum rated rpm for a designed road
overspeed the engine going downhill. The governor has
speed, a lower gear can be selected, or intermittent use
no control over engine speed when it is being pushed by
can be made of vehicle service brakes. Selection of a
the loaded vehicle.
lower gear will generally allow complete control of
vehicle by the brake leaving the service brakes in
Never turn off the switch key while going downhill. With
reserve for emergency stops.
the engine still in gear, fuel pressure will build up against
the shut-down valve and may prevent it from opening
when the switch key is turned on.
USE BRAKE AS NEEDED
altitude because the air is too thin to burn as much fuel
EXCESSIVE ENGINE SPEEDS
as at sea level. This loss is about 3 percent for each
1000 ft. (304.8 m) altitude above sea level for a
Use a combination of brakes and gears to keep vehicle
naturally aspirated engine. Most turbocharged engines
under control at all times, and to KEEP ENGINE SPEED
are rated for higher altitudes than naturally aspirated
BELOW RATED GOVERNED RPM.
engines. (See Engine Specification Tables at front of
this manual.) An engine will have a smoky exhaust at
AUXILIARY BRAKING SYSTEM
high altitude unless a lower gear is used so the engine
will not demand full-fuel from the fuel system unless the
Some trucks are equipped with auxiliary braking
engine is altitude compensated by the use of a
equipment which utilize the engine as a braking device
turbocharger. Shift gears as needed to avoid exhaust
to reduce wear on the normal truck brake system.
CUMMINS ENGINE COMPANY, INC. WARRANTY
DOES NOT COVER ENGINE DAMAGE RESULTING
FROM USE OF AUXILIARY BRAKING SYSTEMS
SINCE SUCH DAMAGE CAN BE CAUSED BY
IMPROPER APPLICATION, LACK OF MAINTENANCE,
INCORRECT USE OR MALFUNCTION OF SUCH