Engine, exhaust and transmission brakes explained
by Tom Jones
The subject of engine, exhaust and transmission braking can be a little confusing. The situation is compounded by manufacturers' names being attached to perticular types of braking systems. This article, comprising the contributions of several forum members and staff, attempts to demystify the subject.
There are several kinds of supplemental braking systems, namely engine brakes, exhaust brakes and transmission brakes.
Engine brakes work by opening exhaust valves in the engine electrically, hydraulically &/or mechanically. One of the manufacturers markets the system as their Jake Brake and this has resulted in the name being attached to this type of braking system.
Engine brakes are the type banned by communities because of the loud "popping" noise introduced.
Exhaust brakes simply block off free flow of exhaust gases by closing a butterfly valve in the exhaust system, typically on the output side of the turbo charger. A switch on the dash turns the system on or off, but activation of the brake is initiated when zero throttle is detected. At the same time, the system will request the transmission to shift to a lower gear (from 6th to 2nd in my Monaco), although downshift doesn't actually occur until the transmission determines it's safe to do so.
One of the manufacturers markets their exhaust brake system under the name Pac Brake, which gave rise to the name being attached to this type of brake system.
Jacobs, the manufacturer of the Jake engine brake above, also manufactures an exhaust brake under the name Extarder, which probably adds to the confusion surrounding this subject.
Transmission retarders work by impeding (slowing) the revolution of the driveshaft. A few retarders may use straightforward friction, essentially a drum brake on the drive shaft, but the most popular type (the Telma Retarder) is electro-magnetic and slows the shaft via magnetic field action. Allison also offers a retarder integrated into some of their transmissions.
Contributors to this article were Gary Brinck, Jim Dick, Leo Everett, Tom Jones, Ned Reiter and Fred Thomas.