How Does a Diesel-Engine Brake Work?


If you've ever heard the constant, machine-gun like chatter that echoes through the mountains in some areas of heavy traffic, you have some passing experience with diesel-engine brakes. These devices are surprisingly complex, and they do more than provide an extra safety margin for big trucks; they are vital to maintaining safe speed and control while a driver tries to ease a heavy truck down steep mountain grades.

Reverse Horsepower

  • All internal-combustion engines use the pressure of exploding fuel trapped in their combustion chambers (the area between piston tops and cylinder heads) to push down on the pistons. When each pistons moves down, it rotates a crankshaft, powering the engine mechanics. Engine brakes work on a similar but inverse principle; trapping high-pressure gases on the combustion chamber keeps pistons from going up, and creating vacuums keeps them from going down. In theory, any diesel engine can produce more of this "reverse horsepower" than it can produce "regular horsepower."


  • Diesel-engine brakes come in two basic versions: valve-type engine brakes and the oft-misunderstood "Jake" or "Jacobs" brake. One primary difference between them is that Jake brakes (the name actually refers to a specific brand, but it has become a generic reference to any of the type) are built into the engine itself, and valve-type exhaust brakes are an add-on. Valve-types usually are bolted onto the engine's exhaust, just past the turbocharger.

Jake Brakes

  • Jake brake systems are incorporated into the engine's valve train at the factory. These systems control the engine's valves and work by momentarily opening the exhaust valve while each piston is at the top of its stroke, releasing combustion-chamber pressure. When each piston moves back down, it essentially is drawing against a massive vacuum, which can exert more negative pressure on the piston than the diesel fuel exerts positive pressure.

Valve Brakes

  • Valve brakes work on the opposite principle as Jake brakes. A valve-type brake essentially is a large butterfly valve in the exhaust stream that closes on demand, trapping exhaust gases in the cylinders. When a piston attempts to rise against the pressure, the air compresses even further and inhibits crankshaft movement. One closely related type of brake is built into the turbocharger itself. Many diesels use variable-vane turbos that can alter the pitch of the exhaust turbine's blades to increase boost at low RPM. The computer can completely shut the vanes, reducing exhaust flow to almost nothing and increasing cylinder pressure past what an equivalent valve-type brake ever could.

Laws and Caveats

  • Jake brakes almost always are better at slowing a truck than valve types but can cause engine destruction if they are engaged at excessive RPMs. Valve types trap hot exhaust gases in the engine, so they can cause overheating if used in excess. Because they are so loud, exhaust brakes often are prohibited in residential and landslide-prone areas, but many trucks use a combination of Jake brakes and valve-type brakes to help reduce their signature machine-gun clatters to what sound more like loud purrs.

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  • Photo Credit mountain image by Silvia Devina from
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