How Do Diesel Exhaust Brake Systems Work?

May 2, 2020 -

Diesel Exhaust Brakes Explained

Large commercial vehicles like semitrailers, buses, heavy equipment, some personal vehicles like large RVs 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 passenger vehicles because of their lower 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 back pressure, 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. 

If the topic of diesel technology interests you, including exhaust brakes, you may enjoy a career as a diesel technician. Keep reading to find out more about these braking systems and how you can get trained at Universal Technical Institute (UTI) for a career in the industry.1

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 back pressure, 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 — 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 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 (back pressure).

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 a hydraulic brake 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

Supplemental Braking System

A supplemental braking system serves as an additional safety feature in vehicles, particularly in situations where the primary braking system may face challenges or require additional support. Typically installed in towed vehicles behind motorhomes or larger vehicles, these systems offer an extra layer of braking control and security.

In the context of towing, a supplemental braking system is designed to assist the towing vehicle in slowing down or stopping the towed vehicle effectively. It operates independently of the towing vehicle's brakes, applying proportional braking force to the towed vehicle when the towing vehicle brakes are engaged.

Supplemental braking systems enhance overall safety during towing by reducing the load and strain on the towing vehicle's braking system while providing more controlled and synchronized braking for both vehicles, ensuring a safer and smoother travel experience.

How Does This Apply to a 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.

Exhaust Brake FAQs

What is the purpose of an exhaust brake system?

The primary purpose of an exhaust brake system is to aid in controlling the speed and providing additional braking power to vehicles, particularly those with diesel engines, when descending steep slopes or navigating downhill terrain.

What are the benefits of using an exhaust brake system?

One primary benefit is the enhanced control and improved safety when navigating downhill slopes or carrying heavy loads. The system minimizes brake fade, heat buildup and wear on the braking components, extending their lifespan and reducing maintenance costs. Additionally, the exhaust brake system aids in maintaining a consistent speed while descending steep grades, ensuring better stability and handling of the vehicle.

Are there different types of exhaust brake systems available?

Yes, there are various types of exhaust brake systems designed to suit different vehicle configurations and engine types. The most common types include pneumatic, hydraulic and electronic exhaust brakes.

Pneumatic exhaust brakes function by restricting the exhaust flow through a butterfly valve in the exhaust pipe, creating back pressure to slow down the engine. Hydraulic exhaust brakes use a similar principle but employ a hydraulic piston to restrict exhaust flow. Electronic exhaust brakes, on the other hand, utilize electronic controls to modulate the exhaust system, providing precise braking force adjustments and integration with the vehicle's engine control unit (ECU).

Train for a Career in Diesel

UTI’s 45-week Diesel Technology program can provide you with the fundamentals needed to service powerful trucks and engines.1,7 Your training doesn’t have to stop there, though — UTI also offers a variety of diesel specialized training programs, which were created in collaboration with leading brands.

To learn more, request information today. We’ll connect you with an Admissions Representative who can help you get started!

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