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Posted by Jerry in Idaho on Monday, February 23, 2009 at 01:48:02 :
In Reply to: Re: Clarify... posted by George (AB) on Monday, February 23, 2009 at 01:19:19 :
When the exhaust brake is activated, the exhaust is forced back into the compression chamber. This causes the force of the engine to reverse itself, and slow the engine down. If the valve springs are too weak, the system won't function to complete efficiency.
Here is some reading that might explain it more clearly than I can.
From Wikipedia:
Since diesel engines lack a throttle valve on the intake manifold, there is no intake vacuum when the engine is not using fuel. The intake vacuum creates the drag effect felt in gasoline engines when going down a hill with the foot off the gas. Many alternate strategies are used on diesels, the least expensive (and also one of the least powerful) of which is the exhaust brake.
[edit] Operation
Exhaust brakes are manufactured by many companies, including competitors Pacbrake, 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.
From Wikipedia:
[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).
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