Why Do Semi Truck Make So Much Noise When They Slow Down?

Have you ever been in stop/go traffic, pulled to a light, and shook you out of your seat with a deafening RatTatTatTat sound? Then when you looked in the mirror for a jackhammer or fireworks behind you, you realized it was just a semi-slowdown. So why let big diesel trucks like this putter slow down? The fact is that they have a second brake, a device that is commonly called, an engine brake, or a pressure/decompression brake. Because of how this engine braking mechanism works, it’s pretty loud.

The problem of slowing down a diesel truck

semi truck curb | Emile Lippi for The Washington Post via Getty Images

Diesel trucks can be difficult to slow down. A heavily loaded diesel truck has plenty of momentum, but that’s not the only reason. One problem with a diesel engine is that it can continue to propel the car forward, even when the accelerator pedal is released.

He does not have a normal system, like a gasoline engine. What this means is that when you release the throttle, the cylinders continue to fill with air each time the engine spins.

As the crankshaft rotates and the pistons climb up in the cylinder, they compress this air. Then, as the crankshaft continues to spin, these pistons begin to fall and the compressed air forces them to fall faster. This translates to the crankshaft producing torque and propelling the truck forward.

One solution is to hit the clutch pedal and use the hydraulic brake to slow your diesel truck. But in 1965, Clessie Cummins patented another solution that could slow a diesel truck faster and save wear and tear on its hydraulic brakes: the pressure-release engine brake.

How does the engine brake work to release pressure?

A semi-truck brakes as it descends a steep hill on the highway, as cars pass by.
semi truck curb | Jakub Porzycki/NurPhoto via Getty Images

The compression release engine brake, also called the Jake Brake, opens the diesel engine’s exhaust valves at top dead center so that they lose pressure and not propel the vehicle forward when the throttle is released.

If you leave the engine engaged to the drivetrain and activate the engine brake to release the pressure, the engine will quickly slow the car. This is because the forward momentum of the car goes into the air pressure in the engine’s cylinders.

One downside to pressure release drive brakes is the amount of noise they make. Each exhaust valve opens at top dead center, releasing a stream of compressed air, and making a loud bang. As the engine continues to spin, these loud noises sound akin to machine guns.

What is Jake Brake?

A semi truck is driving in the downtown area, stone buildings are visible in the background.
semi truck | Robert Nickelsberg / via Getty Images

Jaccobs Vehicle Systems was one of the first manufacturers of pressure release engine brakes for diesel trucks. For this reason, “Jake Brake” has become a generic term for diesel engine brakes made by many manufacturers. But Jaccobs Brake is actually a brand.

A pressure release engine brake is sometimes called a pressure relief brake. They are also sometimes called press brakes.

Many compression release brakes allow the driver to choose the number of cylinders affected. This increases the amount of braking force generated. The driver then selects a gear, depending on the truck’s speed and terrain. Finally, they release the gas and clutch and let the pressure-release brake slow the car.

Can a pressure release brake be quiet?

Engine brake sign | 6 WOWT News via YouTube

Many of the louder pressure release brakes you hear are on trucks with relatively open muffler systems. Modern muffler technology can do an excellent job of quieting Jake Brakes—according to. This is why many cities specify that truck drivers cannot use “unmuffled engine brakes” within city limits.

A truck driver passing through a congested area can also choose to activate a fewer cylinder number of pressure-release brakes and rely more on its hydraulic brakes. Obviously, in an emergency, the pressure release brake is an emergency brake and when you have to stop a speeding truck on its own, it will be very loud. But this is the exception, not the rule.

Then, find out or watch how the pressure release brake works for yourself in the video below:

Finally, watch a road accident caused by engine braking in this latest video:


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