What Is Engine Braking? | YourMechanic Advice

What Is Engine Braking?

What is Engine Braking?

If you've ever driven in the mountains, you may have heard a loud rumble from an eighteen wheeler as it descends. The noise of engine braking may be disconcerting, but without it, large and extremely heavy vehicles would have a very hard time stopping. In fact, engine braking is used to help big rigs reduce the use of normal brakes, which reduces the potential of the mechanical brakes locking up and jack-knifing the truck. While most popular and recognizable in commercial vehicles, a similar approach is also sometimes seen in passenger cars when a driver downshifts down through the gears to slow their vehicle.

How Engine Braking Works

Engine braking reduces the speed of the engine, and therefore, the vehicle. The goal is to prevent the vehicle from picking up speed or going down a decline too fast. This method is most often seen in heavy-duty vehicles and commercial autos such as buses and semi trucks. Engine braking is often referred to as using the “Jake Brake” because of the main manufacturer of commercial engine brakes, Jacobs Vehicle Systems.

Here is how the system works: The driver presses a button to shut off fuel to a certain number of cylinders through the compression-release engine brake. While the cylinders can still receive air, there's no combustion, so they don’t send power to the wheels. When activated, the brake opens exhaust valves in the cylinders after the compression cycle, releasing the compressed air trapped in the cylinders. The mechanical drag of the spinning engine parts slows the engine down, and thus slows the vehicle.

Using an engine brake is important for heavy vehicles because the added weight they carry increases their momentum, making it harder for them to stop. The regular brakes aren’t powerful enough to slow these vehicles down quickly on their own, so engine braking can significantly reduce stopping distance.

Issues and Benefits with Engine Braking

The main problem with engine braking is the noise it creates. The level of noise is at least 10 decibels over the highest level in normal conversation. A vehicle without the proper muffling can emit even higher amounts of noise. Because of this, some states don’t allow engine braking. It’s also common for some diesel regeneration systems to become clogged with excessive carbon debris, which will require cleaning more frequently. Many truck fleets closely monitor the benefits and side-effects to develop engine braking system that are less harmful to emission control systems.

Besides better stopping ability, another benefit of engine braking is that it tends to break up excessive carbon build-up inside exhaust valves, the exhaust manifold, and other exhaust components. It not only helps the engine be more effective at reducing vehicle speed, it can help extend the life of the normal brakes. Fewer instances of brake failure are seen when the engine brake is used, which can improve safety for the vehicle and other automobiles around it. Wear on tires and brakes is reduced, which saves the owner money.

This principle applies to cars as well. While no passenger vehicle is equipped with a Jake Brake, downshifting to use cylinder decompression and the engine's mechanical drag to reduce speed is still used in normal cars. If you've ever watched a car race, you may have noticed drivers downshifting to help them slow before a corner. Skilled manual transmission drivers can do this too, and many of today's paddle-shift automatic transmissions have downshifts that are smooth and precise.


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