How Do Diesel Exhaust Brake Systems Work?

UTI Profile Image Universal Technical Institute May 1, 2020 ·

Large commercial vehicles like tractor-trailers, busses, heavy equipment, and some personal vehicles such as large RV’s and tow vehicles have supplemental braking systems (diesel exhaust brakes and/or compression brakes). Supplemental braking systems are designed to safely slow the vehicle rather than completely relying on the brakes that are positioned at the wheels. Diesel exhaust brakes are a very common supplemental braking option.

Exhaust brakes are not a necessity on smaller diesel trucks and/or passenger vehicles because of their weight. Even when loaded, they are not nearly as taxing on traditional brakes. While not a requirement, many smaller vehicle owners will opt to install them via aftermarket providers.

Compression brakes (commonly referred to as ‘Jake Brakes’), which are identifiable by their signature loud “blat blat blat” sound, are an entirely different system than will be covered in a separate article.

Diesel exhaust brakes operate by physically restricting exhaust flow, creating additional backpressure, thereby slowing engine speed. The physical restriction comes in the form of a butterfly valve installed in-line in the exhaust system actuated by either a servo motor, air pressure or other means as needed to assist in slowing the vehicle. 

What is an Exhaust Brake? Diesel Exhaust Brake Details

If you’ve ever wondered, “what is a diesel exhaust brake?” you’re not alone. A diesel exhaust brake is an intentionally placed controllable physical restriction within the vehicle exhaust system that creates additional exhaust system backpressure, resulting in a reduction in vehicle speed.

A diesel exhaust brake is a system on large, diesel powered vehicle that allows for slowing and stopping without using the vehicle’s wheel mounted brake system. Diesel exhaust brakes, not to be confused with compression brakes, i.e.: Jake Brakes—can assist the wheel mounted braking system, therefore extending the useful life of components like brake pads, rotors and brake drums.

Additionally, the combined use of exhaust brakes and wheel mounted brakes can reduce stopping distance. Specifically, the stopping distance of heavy, fully loaded vehicles is reduced when both braking systems are used in tandem.

Diesel powered vehicles do not benefit from the same level of engine braking as gasoline powered vehicles enjoy. In a typical car or light truck, letting off the accelerator results in vehicle speed reduction beyond simple friction associated with moving an object through air and across a roadway. This slowing is caused by throttle valve closure, creating a vacuum for the engine to operate against.

Diesel engines do not use a throttle valve, as their operation is based on the regulation of fuel, rather than the regulation of air, as is the case with gasoline engines. This difference in engine design partially explains why diesel powered vehicles will “coast” for greater distances when the driver releases the accelerator.

Without a supplemental braking option, the combination of heavy loads and lack of engine braking easily overwhelms wheel brakes. Diesel exhaust brakes often are the preferred choice as they provide stopping benefits, but without the very loud noise associated with compression brakes. An exception would be heavy duty vehicles under significant load. In cases like this, compression brakes would be preferred as they provide the greatest braking force.

How Does A Diesel Exhaust Brake Work?

So, how does an exhaust brake work? Essentially, it works by trapping engine pressure in the exhaust system, which in turn forces the engine to rotate slower (backpressure).

Normally, pistons travel upward in their cylinder bore to force spent exhaust gasses out of the engine via the exhaust valve. When the exhaust brake is applied and the inline butterfly valve is partially closed, significant additional force is required to push the spent exhaust from the engine. This additional force slows the engine’s rotational speed.  As the engine slows, the vehicle decelerates.

In some exhaust brake systems, the vehicle computer controls their operation. In others, the driver can manually actuate the exhaust brake. With either system, the mechanism which closes the butterfly valve may be powered by hydraulic pressure, air pressure or rely on an electronically operated servo motor.

What Makes Exhaust Brakes Efficient?

  • They take advantage of existing systems in the vehicle
  • They do not need to be factory installed as aftermarket application exists
  • They are simple and do not require hydraulic line or pneumatic air pressure to the wheels to stop the vehicle

What Makes Exhaust Brakes Beneficial?

  • They keep wheel brakes from overheating
  • They are quiet, unlike compression brakes
  • They require minimal upkeep while remaining effective

How Does This Apply to Your Career?

Many diesel powered vehicles have supplemental braking systems, such as exhaust brakes. As a diesel technician, familiarity with their use and operation are important skills to have for your career. The braking systems require occasional maintenance and some users report decreased effectiveness over time. Properly inspecting, diagnosing, repairing and maintaining exhaust brake systems are important skills to have and can take you far in this industry.

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