Exhaust brake
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Since diesel engines lack a throttle valve on the intake manifold, there is no intake vacuum when the engine is not fueling. The intake vacuum creates the slowing effect felt in gasoline engines when they are going down a hill with the foot off the gas. Many different strategies are used on diesels, the least expensive (and also one of the least powerful) of which is the exhaust brake.
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[edit] Operation
Exhaust brakes (otherwise known as exhaust retarders) are manufactured by many different companies, including competitors Pacbrake and Jacobs. The brakes vary in design, but essentially operate by closing off the exhaust path from the engine, causing the exhaust gasses to be compressed in the exhaust manifold, and in the cylinder. Since the exhaust is being compressed, and there is no fuel being applied, the engine works backwards, slowing down the vehicle. The amount of negative torque generated is usually directly proportional to the back pressure of the engine.
[edit] Performance
Some new innovations increase the exhaust back-pressure by various means, leading to more torque at the flywheel, and therefore more braking power. Braking effectiveness is measured in units of power and is about 60 to 80% of the engine's maximum power output. More performance is usually easily had by down shifting the vehicle (increasing the leverage , or gear ratio of the engine over the wheels).
[edit] Related items
Exhaust brakes should not be confused with engine brakes, which do not hold the exhaust valves closed, although the basic principle of operation is similar.
[edit] Legal implications
Compression braking (see jake brake), a form of engine brake, produces extreme amounts of noise pollution in comparison to exhaust brakes. For this reason, some vehicle original equipment manufacturers prefer to use exhaust brakes, even when the performance is not as good, just because of the noise issues. This is particularly common for delivery vehicles.
Numerous cities, municipalities, states, and provinces banned the use of unmuffled compression brakes.
