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Home ►Tech Tips ►Part II: Exhaust Retarders by Ed Purcell
HOW DOES AN EXHAUST BRAKE WORK?
The seasoned diesel enthusiast already knows the why's and how's of diesel engine retarders. For
many the diesel is new to the vocabulary and raises many questions. This article will try and explain
some of the whys and how's of diesel engine retarders.
As stated earlier, the diesel engine inherently lacks a retarding effect in a throttle off position. In a
conventional gasoline engine when you take your foot off the throttle a butterfly in the intake system
closes off the air supply creating a vacuum. This causes the engine to work harder when it's in the
'intake' mode as it tries to draw in air and the result is the engine absorbs energy causing it to
decelerate.
In a diesel engine the throttle controls the fuel flow in the fuel pump and not the air so the engine does
not create vacuum. Even though the fuel and resulting combustion is reduced air is still being pumped
into the engine. The only deceleration characteristic in the diesel engine is the friction of the moving
parts.
There are two differing methods of creating retarding power in a diesel engine (1) engine (or
compression release) brakes or( 2) exhaust brakes. An engine or compression release brake is
designed to affect the operation of the exhaust valves of the engine. This type of brake is more common
on the larger displacement diesel engine 10 liter and larger which are used in the Class 7 & 8
highway (and off highway) trucks (18 wheelers).
The engine brake is a set of cast iron housings that attach to the cylinder head of the engine. The
housings are an electric over hydraulic devise that when activated create a hydraulic rocker lever. This
is achieved through a series of oil galleries, pistons and control valves that are designed to open the
exhaust valve ever so slightly and only for a split second close to top dead center on the compression
stroke of the engine. This allows the compressed air to escape into the exhaust system, which cancels
or negates the following power (rebound) stroke. (That is what creates the 'popping' noise in many big
trucks when they are slowing down.) Simply put, the engine brake changes the engine from an energy
producing device to a very large air compressor.
The exhaust brake is more popular on some of the smaller displacement engines (10L and less). The
exhaust brake works opposite to the engine brake where it holds the compression in the engine
instead of releasing it. Remember the butterfly valve in the intake system of the gasoline engine? The
exhaust brake is a butterfly valve in the exhaust system. The most favorable location to mount an
exhaust brake is to the outlet side of the turbo charger. Instead of the engine working hard on intake it
now has to work hard to exhaust. When in the off position the butterfly valve is open and the exhaust
gases flow normally, but when activated the valve closes and restricts the exhaust flow holding a
portion of the compressed air in the cylinder. After the normal power stroke (rebound) the piston would
again travel up the cylinder expelling the gases. With those gases trapped behind the exhaust brake
the piston meets resistance as it tries to push the gases past the brake.
Try holding your finger over the nozzle of a bicycle pump and at the same time pumping. The resistance
you feel on the handle is similar to the reaction in the engine. The engine piston meets this resistance,
which causes it to absorb energy, causing rapid deceleration. This deceleration translates through the
driveline to the drive wheels slowing the vehicle down.
Although the engine and exhaust brake work on different principals, they achieve the same result an
effective retarding action. It is important to note that not all diesel engines can accept a retarder. The
engine must be designed to facilitate the retarder. Because the engine compression brake attaches to
the top of the cylinder head, underneath the valve cover, the engine manufacturer must take into
account the additional space requirements and components to make the brake work.
The exhaust brake also requires some consideration by the engine manufacturers. The valve train
components must be able to resist the additional backpressure thereby not allowing the exhaust valve
to be held open. Stronger camshafts, pushrods, rocker levers and exhaust valve springs are some of
the components that need to be considered for exhaust braking.
The common key to both methods of engine retarding is engine RPM. The more times each piston
travels through its cycle (intake, compression, power, exhaust) the more retarding power that is
generated. In other words retarding power is directly related to engine RPM ... the higher the RPM the
higher the retarding power. So downshifting is required to achieve the appropriate amount of retarding.
WHY DO I NEED AN EXHAUST BRAKE?
HOW DOES AN EXHAUST BRAKE WORK?
The seasoned diesel enthusiast already knows the why's and how's of diesel engine retarders. For
many the diesel is new to the vocabulary and raises many questions. This article will try and explain
some of the whys and how's of diesel engine retarders.
As stated earlier, the diesel engine inherently lacks a retarding effect in a throttle off position. In a
conventional gasoline engine when you take your foot off the throttle a butterfly in the intake system
closes off the air supply creating a vacuum. This causes the engine to work harder when it's in the
'intake' mode as it tries to draw in air and the result is the engine absorbs energy causing it to
decelerate.
In a diesel engine the throttle controls the fuel flow in the fuel pump and not the air so the engine does
not create vacuum. Even though the fuel and resulting combustion is reduced air is still being pumped
into the engine. The only deceleration characteristic in the diesel engine is the friction of the moving
parts.
There are two differing methods of creating retarding power in a diesel engine (1) engine (or
compression release) brakes or( 2) exhaust brakes. An engine or compression release brake is
designed to affect the operation of the exhaust valves of the engine. This type of brake is more common
on the larger displacement diesel engine 10 liter and larger which are used in the Class 7 & 8
highway (and off highway) trucks (18 wheelers).
The engine brake is a set of cast iron housings that attach to the cylinder head of the engine. The
housings are an electric over hydraulic devise that when activated create a hydraulic rocker lever. This
is achieved through a series of oil galleries, pistons and control valves that are designed to open the
exhaust valve ever so slightly and only for a split second close to top dead center on the compression
stroke of the engine. This allows the compressed air to escape into the exhaust system, which cancels
or negates the following power (rebound) stroke. (That is what creates the 'popping' noise in many big
trucks when they are slowing down.) Simply put, the engine brake changes the engine from an energy
producing device to a very large air compressor.
The exhaust brake is more popular on some of the smaller displacement engines (10L and less). The
exhaust brake works opposite to the engine brake where it holds the compression in the engine
instead of releasing it. Remember the butterfly valve in the intake system of the gasoline engine? The
exhaust brake is a butterfly valve in the exhaust system. The most favorable location to mount an
exhaust brake is to the outlet side of the turbo charger. Instead of the engine working hard on intake it
now has to work hard to exhaust. When in the off position the butterfly valve is open and the exhaust
gases flow normally, but when activated the valve closes and restricts the exhaust flow holding a
portion of the compressed air in the cylinder. After the normal power stroke (rebound) the piston would
again travel up the cylinder expelling the gases. With those gases trapped behind the exhaust brake
the piston meets resistance as it tries to push the gases past the brake.
Try holding your finger over the nozzle of a bicycle pump and at the same time pumping. The resistance
you feel on the handle is similar to the reaction in the engine. The engine piston meets this resistance,
which causes it to absorb energy, causing rapid deceleration. This deceleration translates through the
driveline to the drive wheels slowing the vehicle down.
Although the engine and exhaust brake work on different principals, they achieve the same result an
effective retarding action. It is important to note that not all diesel engines can accept a retarder. The
engine must be designed to facilitate the retarder. Because the engine compression brake attaches to
the top of the cylinder head, underneath the valve cover, the engine manufacturer must take into
account the additional space requirements and components to make the brake work.
The exhaust brake also requires some consideration by the engine manufacturers. The valve train
components must be able to resist the additional backpressure thereby not allowing the exhaust valve
to be held open. Stronger camshafts, pushrods, rocker levers and exhaust valve springs are some of
the components that need to be considered for exhaust braking.
The common key to both methods of engine retarding is engine RPM. The more times each piston
travels through its cycle (intake, compression, power, exhaust) the more retarding power that is
generated. In other words retarding power is directly related to engine RPM ... the higher the RPM the
higher the retarding power. So downshifting is required to achieve the appropriate amount of retarding.
WHY DO I NEED AN EXHAUST BRAKE?
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