How Engine Brakes Work

Exhaust Brakes

Exhaust brakes slow light duty, diesel-powered vehicles quickly. They also prevent the brakes from overheating on downhill grades, as this causes brake fade and possibly even failure. Using your exhaust brakes properly can help brakes last up to three times longer [source: Jacobs Vehicle Systems].

­Exhaust brakes retard power in a diesel engine, but in an different way than engine brakes. Engine brakes release compressed air through an exhaust valve, but exhaust brakes hold the compression in the engine and slow the crankshaft's rotation, which reduces vehicle speed [source: Lay].


An exhaust brake is typically mounted on the outlet side of the turbocharger and retards the engine's ability to push out or exhaust compression. A butterfly valve in the exhaust brake stays open until it's activated. Then it closes and restricts exhaust flow by keeping it in the cylinder. This causes the piston to force the compression into the exhaust brake, which absorbs the energy [source: Purcell]. A little confusing? Think of it this way: Take a deep breath and hold it in. Now force the air into your mouth and cheeks, but don't let it out. You've just created your own little exhaust brake.

Exhaust brakes don't produce the loud blatting sound for which engine brakes are known. They actually make no sound at all. Exhaust brakes are designed to be used all the time, not just when you need them [source: Purcell]. When used as recommended, they save money by reducing brake service costs and offer added security if you are traveling or pulling a rig in hill country.

­Now that you know how engine and exhaust brakes work, read the next section to learn about engine brake usage.