Jake Brakes are used on trucks as an adjunct to the standard wheel brakes. On long downhill runs with a heavy load, the wheel brakes can heat up and fail - the Jake Brakes work directly with the engine to slow the truck. They can also be very loud and are banned from some communities. Here is a great expplanation of what they are and how they work.
From Straight Dope:
What are "jake brakes" and why are they prohibited in some locations?
Turns out this is a rather hot issue with the company who holds the trade name for Jake brakes, but more on that later. First an introduction to the world of engine brakes.
Unlike the wheel brakes you have on your car, the Jacobs Engine Brake (TM) is a "compression release" engine brake used by large diesel trucks, especially on steep downgrades. To understand how it works, remember that a diesel engine has much higher compression than a gasoline engine, typically 15:1. The jake brake slightly opens the exhaust valves when the piston is near top dead center (where ignition normally occurs). On the upstroke, the piston compresses the air in the cylinder to 1/15th its original volume. This creates a lot of drag on the engine. The Jacobs Engine Brake then releases the compressed air, and the energy stored in it, before it can push back on the piston during the downstroke. In addition, releasing the compression prevents any fuel in the cylinder from igniting. (Remember, diesels don't have spark plugs like gasoline engines - they rely on compression alone to ignite the fuel.) So, you've got drag on the upstroke, no power on the downstroke.
Much more at the site - a bit of fascinating history as the manufacturer of the Jake Brake is the Jacobs Manufacturing Company, established in 1903 by A.I. Jacobs, makers of the world famous three jaw Jacobs Drill Chuck. That would be the same chuck that is on 99.9% of all drills today.
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