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Why Add an Exhaust Brake? (and
what makes a good exhaust brake?)
By C.J. Baker
If
you drive a diesel engine-powered pickup, SUV, or motorhome,
you already know that there is precious little braking
effect when you back off the throttle pedal. Few things
can be more terrifying to someone driving a heavily
loaded motorhome or a pickup pulling a trailer than
a long downhill grade that progressively overpowers
the vehicle's braking system as the driver struggles
to keep downhill speed in check. Moreover, when the
vehicle's brakes overheat and can no longer do their
job, continued application only makes the brakes hotter,
further reducing their effectiveness. To his horror,
the driver is knowingly losing control of the vehicle.
Speed continues to climb and there's little or nothing
the driver can do about it – at least not now
that's he's already caught up in this dilemma.
Even
when a vehicle's brakes are capable of keeping the
vehicle from gaining unwanted speed downhill, such
loads impose significant brake wear and leave little
safety margin for additional braking. Even then, sustained
brake use can cause overheating that will damage some
of the braking system components, such as the brake
rotors.
In
all too many cases, the vehicle's brakes simply aren't
up to the challenge of sustained heavy braking. Conventional
brakes are, after all, simply mechanical devices designed
to convert kinetic energy (the vehicle's movement)
into heat energy, thus retarding or slowing the vehicle.
This is typically accomplished by forcing stationary
friction surfaces (either brake shoes or brake pads)
against a rotating machined metallic surface (either
a brake drum or a brake rotor) coupled to wheel rotation.
The more pressure with which the friction surfaces
are applied against the rotating surfaces, the more
heat is generated and the greater the braking effect.
So far so good, but both the friction surfaces and
the rotating surfaces have temperature limits at which
they begin to fail – both as effective braking
components and structurally. The rotating brake drums
or rotors can become literally incandescent, approaching
the point at which the metal begins to lose structural
integrity. By the same token, the stationary friction
materials can get so hot that the composite material
binding the friction materials together actually melts
and begins to boil, releasing a thin layer of liquid
binding material and hot gases that form a lubricating
boundary barrier between the friction materials and
the brake drums or rotors. When this happens, braking
ceases to occur and a phenomenon known as "brake
fade" occurs. Under these conditions, continued
brake application does little or nothing to slow the
vehicle. Fortunately, there are things that can be
done to upgrade the braking systems that are standard
equipment on many trucks and RVs.
Traditional
brake upgrades have consisted of installing bigger
brakes (an expensive procedure, at best) or special
friction materials (if they are available for your
vehicle) capable of higher sustained operating temperature
before brake fade occurs. While both of these approaches
will increase braking capability, one of the most practical
techniques is to utilize auxiliary or supplemental
braking capability to take some of the load off the
conventional brake system.
To
provide some supplemental braking, typically drivers
downshift to a lower gear, allowing the engine to exert
some braking force as the engine attempts to intake
air against a closed throttle. Of course, this only
works for gasoline engine-powered vehicles that have
air throttles. Drivers with diesel engines, which have
unrestricted air intake, face a different problem.
Diesels provide little engine braking unless they are
fitted with either an internal or an external device
that uses the pumping action of the engine to do work.
In either case, such devices on diesel engines are
intended to create a pumping resistance that results
in engine braking.
The
internal mechanism built into some large diesel engines
is called a Jake Brake®, so named after Jacobs
Vehicle Systems® that originated it. This mechanism
opens the exhaust valve of each cylinder at the top
of each compression stroke. Thus, the engine has to
work to compress the air, which is then released into
the exhaust system. Unfortunately, the operation of
Jake Brakes is fairly noisy, making a loud burbling
sound as the vehicle decelerates. This is a fairly
complex and expensive system that is usually found
only on large trucks or some large motorhomes. It is
also something that cannot be retrofitted to a “non-Jake
Brake diesel” since it is incorporated into the
engine’s original design.
An
external supplemental braking system for diesel engines
can economically be retrofitted to the vehicle’s
exhaust system, thus the name “exhaust brake”.
Such a device partially closes the diesel engine's
exhaust when the throttle is released. It does this
by placing a shut-off valve in the exhaust system downstream
from the engine. The valve uses an actuator to close
it against exhaust flow, forcing the engine to pump
against a restricted exhaust to create braking resistance.
Such devices are easily retrofitted to diesel trucks
and motorhomes of all sizes, and an exhaust brake upgrade
can actually pay for itself in reduced brake wear and
maintenance over the life of the vehicle. Best of all,
there’s nothing that requires periodic service
or replacement in a well-designed exhaust brake.
The
primary advantages to exhaust brakes on diesel-powered
vehicles are that they can provide engine braking to
assist deceleration, help control downhill vehicle
speed, reduce brake heat, reduce brake wear, and provide
a greater braking reserve for the conventional brake
system. They are also relatively quiet in normal operation
and use. Of course, these benefits only occur when
the exhaust brake system is operational, which is at
the discretion of the driver.
The
operation of supplemental exhaust brakes varies with
the design. Most such brakes are very elementary and
use a simple on-off switch to trigger a solenoid or
a vacuum actuator to close the exhaust brake valve
— essentially an all-or-nothing braking device. When
installed, some designs create an exhaust restriction
even when not activated, negatively affecting both
performance and fuel economy. Other designs are more
sophisticated. Gale Banks Engineering manufactures
an exhaust brake — the Banks Brake — based on a unique
design that enhances exhaust flow for a power benefit
and better fuel economy under normal driving conditions.
The Banks Brake is also the industry’s only computerized
exhaust brake, featuring the exclusive computerized
brake controller (CBC) module. The CBC functions automatically
when the system is turned on, calibrating throttle
pedal location, “exercising” the valve and
providing fast engine warm up on cold starts, and improving
throttle response after brake activation. The Banks
Brake also produces more braking force than other designs
provide when activated. And for Dodge and Ford diesel
vehicles with automatic transmissions, Banks also offers
an optional electronic device called the SmartLock™ that
automatically locks the torque converter clutch at
a specified point and raises transmission line pressure
while braking for improved holding capacity. By preventing
excess slippage, the SmartLock also reduces transmission
fluid temperature while braking, which greatly prolongs
transmission life. GM and Chevrolet ’01-’03
Duramax diesel trucks already have a similar acting
Tow/Haul feature built into their automatic transmissions
and do not require the SmartLock module.
A
well-engineered exhaust brake also includes safeguards
to prevent any excessive exhaust system overpressure
that might harm the structural integrity of either
the exhaust system or the engine valvetrain. Unfortunately,
not all exhaust brakes are created equal. There are
designs that bypass so much exhaust flow that they
provide little or no braking effect in some circumstances.
Still other designs restrict or disrupt exhaust flow
downstream from the turbocharger, impeding performance
during normal driving. Premium designs, such as the
Banks Brake, combine both system safety and reliability
with effective supplemental braking capability while
simultaneously increasing normal driving performance.
An
exhaust brake should be considered as a worthwhile
safety enhancement for any diesel-powered vehicle,
and a virtual necessity for such vehicles when they
are heavily loaded or towing trailers. Because adequate
braking is so important, the quality and features of
a supplemental brake should be a prime consideration
when purchasing such a device. Thinking that you don't
need an exhaust brake because you've never had a braking
problem in the past could be a risk. After all, you
don't want to discover the problem when you're only
part way down a long grade, gaining speed, and your
vehicle's brakes aren't up to the task at hand (er,
at foot). And even if you never encounter such a critical
situation, the extended brake life an exhaust brake
can provide will reduce your vehicle maintenance and
give you greater peace of mind.
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