Have you ever been at a stop or in traffic, and suddenly a semi-truck pulls over next to you, and the sound coming from the truck is so disturbing and loud? It’s not like this is only common with a certain type of semi-truck. It’s a thing all semi-trucks do. So, why do big diesel trucks make such a noise when slowing down?
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The truth is that they have a second brake, a device usually called a Jake Brake, engine brake, or compression/decompression brake. Because of how this engine braking mechanism works, it is very loud.
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The Problem With Slowing Down A Diesel Truck
Diesel trucks can be hard to slow down. A heavily loaded diesel truck has ample momentum, but that’s not the only reason. One problem with a diesel engine is that it can keep pushing the vehicle forward, even when you release the accelerator pedal.
Why? Because a diesel engine does not have a regular throttle body like a gasoline engine. This means that when you release the accelerator, the cylinders fill with air every time the engine spins.
As the crankshaft spins and the positions climb in the cylinder, they compress the air. Then, as the crankshaft continues to spin, these pistons drop, and the compressed air forces them to drop faster. This translates to the crankshaft producing torque and pushing the truck forward.
One solution is to press your clutch pedal and use your hydraulic brakes to slow your diesel truck. But in 1965, Clessie Cummins patented another solution that could slow a diesel truck down quicker and save wear and tear on its hydraulic brakes: the compression-release engine brake.
How Does The Compression Release Engine Brake Work?
A compression-release engine brake, also called a Jake Brake, opens a diesel engine’s exhaust valves at the top dead centre, so it loses compression and does not drive the vehicle forward when you release the accelerator pedal.
If you leave the engine engaged to the drivetrain and activate a compression-release engine brake, the engine rapidly slows the vehicle down. The vehicle’s forward momentum compresses air in the engine’s cylinders.
One downside of the compression-release engine brake is the amount of noise it makes. Each exhaust valve opens at the top dead centre, releasing a stream of compressed air and making a loud bang. As the engine turns, these loud noises sound like machine guns.
What Is A Jake Brake?
An early manufacturer of compression-release engine brakes for diesel trucks was the Jacobs Vehicle Systems company. For this reason, “Jake Brake” became a generic term for diesel engine brakes made by many manufacturers. But Jacobs Brake is actually trademarked.
Compression-release engine brakes are sometimes called decompression brakes. They are also sometimes called compression brakes. Many compression-release brakes allow the driver to pick the number of cylinders affected. This increases the amount of braking force generated. Then the driver chooses a gear, depending on the truck’s speed and terrain.
Finally, they release the gas and clutch and let the compression release brake slow the vehicle down.
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