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.
Exhaust brakes are manufactured by many different companies, including competitors Pacbrake, GT, Jacobs and Banks Power. The brakes vary in design, but essentially operate by closing off the exhaust path from the engine, causing the exhaust gases 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. More advanced exhaust brakes have exhaust pressure modulation (EPM) that controls the back pressure which in turn improves the braking performance across a range of engine speeds.
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 ratioof the engine over the wheels).
Compression braking, a form of
engine brake, produces excess amounts of noise pollutionin 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 however combination of an exhaust and compression brakes increases their effectiveness while significantly cuts back on noise pollution.
Numerous cities, municipalities, states, and provinces banned the use of unmuffled compression brakes.
* [http://www.bankspower.com/tech_howexhaustbrakeworks.cfm Banks Power exhaust brake page]
* [http://www.pacbrake.com/index.php?page=mechanics PacBrake exhaust brake mechanics]
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