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Retarder
Auxiliary Brake Systems Explained
This truck
system is what provides that alternate braking capacity when needed.
While
coming down a mountain grade, extended application of light braking,
results in overtemp of the brake drums and shoes, which reduces
the braking capacity, and this situation is referred to as brake
fade. Extreme brake fade can result in a run-away rig. Retarder
brake systems really shine in this situation, since they can apply
extended light braking action with no brake fade, thereby controlling
the vehicle's downgrade speed, which saves the vehicle service
brakes until they are needed.
A large percentage
of brake applications use less than 20% of the vehicle's braking
capability. Retarders can effect this light braking action when
needed, which will reduce the wear on the vehicle's service brakes.
The retarders are designed with no servicable parts which wear
out, therefore extensive use of retarder systems has no cost.
Engine
Compression Retarders
The engine
compression retarder uses the engine's compression stroke to absorb
energy from the vehicle's motion. Normally the piston compresses
the cylinder air for the next power stroke, and this compression
of the cylinder air takes energy. That energy is normally provided
by another cylinder which is in its power stroke.
The engine
compression retarder uses up vehicle motion energy during the
compression stroke of each piston because the other cylinders
are not producing power during their power stroke time interval.
No (or insignificant) engine power is generated, because the throttle
must be at the idle position for the retarder to operate.
During the
retarder mode of operation, when the piston approaches the top
of the compression stroke, the retarder system opens the exhaust
valves for that piston, which vents the energy which was absorbed
from the vehicle's motion. The retarder works by absorbing energy
during the compression stroke, and throwing away this energy through
the exhaust valves as the piston completes its compression stroke.
The most
popular engine compression retarder is the Jake Brake. This retarder
system is manufactured by the Jacobs Manufacturing Company, and
their product is available for Caterpillar, Cummins, Detroit Diesel,
and Mack engines, just to name a few.
Refer to
the diagram above. The electric signal activates the solenoid
valve. The active solenoid valve applies engine oil pressure to
the control valve. The engine oil presure force moves the ported
piston of the control valve up until the piston port aligns with
the high pressure oil passage to the slave piston. The check ball
in the control valve allows the engine oil pressure to fill the
slave piston and the master piston voids via the high pressure
oil passage. As the cylinder completes its compression stroke,
the cylinder injector pushrod rises and lifts the master piston.
This forces the oil out from above the master piston into the
slave piston. Remember, the check ball in the control valve prevents
the high oil pressure from bleeding back into the lower pressure
engine oil supply.
As the injector
pushrod moves upwards, the slave piston pushes down on the exhaust
crosshead which opens the exhaust valves. This releases the compressed
air which provided the retarder braking action. When the retarder
is deactivated, the solenoid valve vents the engine oil pressure,
which allows the control valve spring to lower it's ported piston,
which seals off the high pressure oil passage.
To activate
the Jake Brake, four requirements must be met, as shown in the
diagram above. The dash switch must be on, the clutch must be
engaged, the throttle must be at idle, and the driver and/or cruise
control must have activated the retarder. Engine
RPM must be kept high for the most retarder effect.
Engine
Exhaust Retarders
The engine
exhaust retarders absorb vehicle motion energy by not letting
the engine exhaust flow out freely. By restricting the engine
exhaust flow, the exhaust cycle of a piston (which should require
very little energy) now becomes a compression cycle for that piston.
During the exhaust cycle of the piston, the intake valves are
closed and the piston is rising. Restriction of the exhaust outlet
(back pressure) opposes the piston's travel upward, and absorbs
vehicle motion energy in the process.
The Williams
Exhaust Brake is the most popular exhaust brake in use. It comes
in two flavors, either the sliding gate version, or the butterfly
version. Both restriction devices are placed after the turbo charger
exhaust outlet, and both devices restrict exhaust flow when activated.
These restrictors normally use the vehicle's system air pressure
to activate. The air control valve is controlled by an electrical
circuit.
To activate
the exhaust retarder, five requirements must be met. The ignition
switch must be on, the clutch must be engaged, the throttle must
be at idle, system air pressure must be adequate, and the driver
and/or cruise control must have activated the retarder. Engine
RPM must be kept high for the most retarder effect.
Engine
Hydraulic Retarders
The hydraulic
brake retarders are a whole different animal. Engine hydraulic
retarders are mounted on the engine. The Caterpillar Brake Saver
is the most popular retarder in this class. This retarder operates
on the torque converter principle.
 In
a normal torque converter, vanes on the input housing (which is
attached to the engine crankshaft) transmit power through the
fluid coupling to the output shaft vanes of the torque converter,
which is connected to the input shaft of the transmission. The
hydraulic retarder transmits power from the rotor vanes (which
are attached to the engine crankshaft) to the retarder housing
vanes (which are attached to the engine block or bellhousing).
This produces a hydraulic drag which absorbs vehicle motion energy
while activated.
To activate
the hydraulic retarder, oil is pumped into the housing, and the
housing vanes exert drag due to the swirling oil, upon the rotor
vanes. To stop the retarder action, simply drain the chamber,
which uncouples the two vanes.
The retarder
oil pressure supply is delivered from a secondary section of the
engine oil pump. The oil used for the retarder comes from the
oil sump of the engine, which must therefore have a larger capacity.
This section of the oil pump is entirely separate from the engine
lubrication section of the oil pump. This oil pump pressure can
be modulated before it reaches the retarder housing. More oil
pressure in the retarder housing means more retarding action.
Less oil pressure results in less retarding action. While the
oil is pressurized within the retarder housing, it is constantly
being circulated through the oil cooler and back into the oil
sump. This is required to remove the heat generated by the retarder
friction.
This retarder
has a BrakeSaver control valve which regulates the retarder action
once it is activated. This control valve in conjunction with a
driver control handle, will set a specific retarder action. As
the engine slows in RPM due to the retarding action, the BrakeSaver
control valve automatically increases the oil pressure in the
retarder housing, which keeps the braking action constant as the
pumping action of the retarder vanes decreases.
The retarder
control valve also provides a proportioning valve function. The
driver control handle will set the retarder force desired, when
in manual mode, but while in auto mode, the driver's release of
both the throttle and the clutch pedal, will apply maximum retarder
action automatically. The proportioning valve uses the larger
of these two pressures to control the retarding action.
As required
with the above retarders, engine RPM must be kept high for the
most retarder effect.
Stand-alone
Hydraulic Retarders
The stand-alone
hydraulic retarder can be easily built into automatic transmissions,
just behind the torque converter. It is placed here so that the
retarding force can be applied to the transmission input shaft.
This retarder can also be mounted behind transmissions when required.
The stand-alone
hydraulic retarder offers advantages over the engine hydraulic
retarder. The weight of this unit can be distributed away from
the engine for better scaling of the axle weight. This retarder
is normally connected to a drive line component or the transmission
output shaft, which means that maintaining high engine RPMs is
not necessary. This also means that gear shifts may not interrupt
retarder action. This retarder normally uses transmission fluid
which is thicker than engine oil, which effects more retarder
capability.
This retarder
works just about the same as the engine hydraulic retarder described
above. It also employes its own oil pump, oil cooler, regulated
retarder action, and it also uses a driver controlled lever or
foot pedal. It can also be controlled by a cruise control unit.
Electric
Retarders
The electric
retarder has been popular in Europe for several years. It is quiet
in operation, is placed within the drivetrain, and provides adjustable
retarder action control for the driver. It does require DC control
power from the vehicle battery charging circuit, in the order
of 10 amps.
In operation,
the electric retarder is a generator whose load is itself. All
power generated by the generator is dissipated back into the generator
as heat. The retarder has cooling turbines which remove the heat
as it is generated. The vehicle's motion energy is converted into
heat by the generator.
The electric
retarders do not place additional requirements upon the vehicle
cooling system, nor do they impact the vehicle's engine lubrication
system. It doesn't restrict the engine's compression or exhaust
cycles in any way. The electric retarder is an easy bolt-on, and
independant option.
The electric
retarder consists of 16 electromagnet coils which surround a rotor
which is connected to the vehicle drive shaft. Gear shifts do
not impact the operation of this retarder, and engine RPMs also
have no impact upon the operation of this retarder.
The retarder
effect is controlled by enabling one through 16 of the electromagnet
coils. The more coils that are activated, the more retarder force
that is generated. There is typically a driver switch which can
select the number of active coils. Recently, electronics has provided
a progressive switch position which enables more coils as the
length of application time increases. There is also a manual driver
control lever option, and a foot brake pedal option. The foot
brake pedal option activates the electric retarder before the
service brakes are applied.
There is
also a low speed sensor which disables the retarder when the vehicle
speed falls below 5 MPH (8KMH).
Conclusion
& Jake Brake Double-Clutch
There are
many retarder options available today. The retarder will normally
make your rig a safer rig on the road and will extend your service
brake life. There is a lot of arguing amongst drivers about whether
or not to use the jake brake during shifting.
During the
shifting double clutch, the conditions are correct for jake brake
activation. If your gear shifts are close together, then the jake
brake will fight you on the engine RPM match while shifting. However,
if the gear shift result in large RPM drops, or if the shift is
on a steep grade which results in rapid truck speed loss, then
use of the jake brake may very well help you to grab that next
gear quicker. In this situation, the truck speed is falling rapidly,
and the engine RPMs are falling normally, so during the double
clutch, the engine RPMs are yanked lower by the jake brake. This
might be helpful to get that next gear in place during a slow
and steep grade climb.
One concern
that we have is the potential for retarder drive wheel slippage
on a slippery road surface. The drive wheel slippage would normally
hold the tractor straight, but with a heavy trailer behind, I
would be concerned that the drive wheel slippage coupled with
the trailer pushing forces could result in a jackknife under extreme
conditions. You may want to keep this in mind. Remember, most
trucks do not have anti-lock retarder computer controls!
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