[A/S] Light Truck Diesel Engine Exhaust Brakes

["Harvey Barlow" <harveybarlow@xxxxxxxxxxxxxxxx>]

The engine speed of a gasoline engine is regulated by a throttle blade
located inside the carburetor or throttle body.  If the driver calls for
more engine speed (engine power) by pressing harder/farther on the
accelerator pedal, the throttle blade opens farther and more air is ingested
by the engine and either the carburetor or the fuel injection system adds
proportionally more gas to the air-fuel mix and it produces more power.
When the driver removes his/her foot from the accelerator pedal, the
throttle blade in the carb/throttle body closes against engine vacuum,
therefore causing restriction which retards engine speed and, vehicle speed.

On the other hand, a diesel engine has no throttle blade or carburetor.  The
engine's air intake path is always fully open.  Diesel engine speed is
increased or decreased by adding more fuel or reducing fuel injected by the
injection pump.  When the driver removes her/his (politically correct
right?) foot from the pedal, fuel is cutoff but the engine air intake
remains open so little if any retarding effect is provided by a diesel
engine.  Diesel engines offer very little engine braking in their natural
state.

Diesel engines use an exhaust brake installed in the exhaust pipe,
immediately downstream of the turbocharger or downstream of the exhaust
manifold in a non-turbo diesel.  It is a simple ball valve which closes,
blocking discharge of engine exhaust.  When closed, the exhaust brake
provides significant exhaust back pressure which retards engine speed,
therefore retarding vehicle speed.

Modern light trucks can benefit from the use of engine exhaust brakes when
hauling or pulling heavy loads to provide additional engine braking to
assist and save the foot operated service brakes.  Dodge, in their official
literature, recommends the addition of an exhaust brake in manual
transmission equipped 3500 (one-ton rated)trucks when towing heavy loads.
Dodge dealers offer for sale and will install a Jacobs (owned by Cummins I
believe) e-brake or exhaust brake.  Cummins dealers will also sell and
install it.  Various aftermarket manufacturers such as BD, PacBrake, and
others also sell a similar product.   Installation of a Jake or other
exhaust brake in an automatic transmission equipped Dodge truck will void
the warranty.  The reason is: using the engine to decelerate allows the
torque converter to slip and creates enormous heat in the automatic
transmission torque converter.  This will eventually cause major damage.
This fact is true with all automatic transmissions, regardless of
manufacturer, except possibly the Allison.

I had a Jacobs (the original Jake brake) installed in my Dodge by the dealer
before taking delivery.  It has no negative effect on the warranty
whatsoever.  It provides incredible additional braking.  Pulling an 8k
Airstream and grossing over 16k, I rarely have to use the foot brake on
steep downgrades.

Late model Ford PSD trucks with automatic transmissions freewheel when the
driver's foot is lifted from the accelerator pedal.   This is the reason for
installation of the torque locking devices.  These devices lock the
automatic transmission torque converter when applying the exhaust brake so
that engine braking can be connected to the rear wheels.  When freewheeling,
the transmission has essentially disconnected itself from the engine like
depressing the clutch on a manual transmission equipped truck.

An exhaust brake can also be used for faster engine warm-up on very cold
days.  When the exhaust brake is closed, escape of hot burned gasses is
restricted, and the engine warms up faster.  A fast-idle chip will raise
engine idle speed and will slightly decrease warm up time.  In other words,
diesel engine warm-up time can be reduced by either an exhaust brake or fast
idle chip but only the exhaust brake can provide additional braking.  A fast
idle chip has no bearing on engine braking.

I hope this helps explain light truck exhaust brake application.

Harvey Barlow
Lubbock, TX
'01 Dodge-Cummins 3500, '84 Airstream 31'