Air brake over hydraulic brakes

[Danger]

Something like the clatter of a machine-gun? Is that brought on simply by the driver lifting his foot off the accelerator?

It's a bit more complex, but not brain-busting. I'm sure that anyone who drives a standard or an auto with a manual valve body knows that downshifting is a very good way to decelerate or to hold a reasonable speed on a hill. It's referred to as "engine braking" or "compression braking". The Jake takes that a bit farther. If a cylinder doesn't fire, the compression stroke is wasted (as in if a spark plug malfunctions). It actually acts as a "spring" against the rotation of the engine. In normal circumstances, there would be a rebound effect that would see a bit of energy regained by the piston rebounding off of that compressed charge in the combustion chamber. A Jake disables the fuel injectors for specific cylinders, and then opens the exhaust valves to eliminate that rebound.

 

[Pkruse]

Danger's description of Jake brakes is the best yet in this thread. In a gasoline car, if you lift your foot off the gas, the engine will provide a regarding force to slow the car down. If you down shift to make the engine spin faster the braking force is greater.

A diesel provides almost no such regarding force at all because it does not have a throttle.

The Jake brake gives the diesel engine this same engine braking ability, but to a higher degree. Different systems work differently, but they all convert the engine onto an air compressor and discharge thru an orifice to convert energy to heat and noise.