How Engine Brakes Work

In simple terms, when you push the gas pedal down in a gasoline powered vehicle, you open the throttle or intake valve and increase your speed. When you release the gas pedal, close the intake valve or throttle back, you slow down [source: Nice]. When you go downhill, you let off the gas and press the brake. If the hill is steep, you might even shift into a lower gear to help you slow down and avoid riding the brakes.

­In semitrailers, letting off the gas doesn't slow the rig the same way it slows a passenger vehicle. Using only the brakes to slow a semi on a long, steep, downhill grade may not provide enough stopping power, even when the driver shifts into a lower gear. You're going need a lot more power since a semi typically weighs 80,000 pounds (36,287.4 kg) [source: The Trucker's Report].

In a semi engine, air enters the intake valve and is forced into the cylinder and compressed. As the piston inside the cylinder moves downward, it pushes the energy from the compressed air to the wheels and produces power.

So you're all revved up. Now how are you going to slow down? Enter the compression-release engine brake. When approaching a steep downhill grade, truck drivers will flip a switch in the cab to shut off whatever number of engine cylinders they need to slow the truck. The shut off cylinders receive air, but they do not pass energy back through the system. Instead, the piston pushes the compressed air out the exhaust valve at the top of the cylinder and effectively slows the rig [source: Jacobs Vehicle Systems].

Exhaust brakes will slow you down, but in a different way than engine brakes. Read on to find out how.

Exhaust brakes slow light duty, diesel-powered vehicles quickly. They also prevent the brakes from overheating on downhill grades, as this causes brake fade and possibly even failure. Using your exhaust brakes properly can help brakes last up to three times longer [source: Jacobs Vehicle Systems].

­Exhaust brakes retard power in a diesel engine, but in an different way than engine brakes. Engine brakes release compressed air through an exhaust valve, but exhaust brakes hold the compression in the engine and slow the crankshaft's rotation, which reduces vehicle speed [source: Lay].

An exhaust brake is typically mounted on the outlet side of the turbocharger and retards the engine's ability to push out or exhaust compression. A butterfly valve in the exhaust brake stays open until it's activated. Then it closes and restricts exhaust flow by keeping it in the cylinder. This causes the piston to force the compression into the exhaust brake, which absorbs the energy [source: Purcell]. A little confusing? Think of it this way: Take a deep breath and hold it in. Now force the air into your mouth and cheeks, but don't let it out. You've just created your own little exhaust brake.

Exhaust brakes don't produce the loud blatting sound for which engine brakes are known. They actually make no sound at all. Exhaust brakes are designed to be used all the time, not just when you need them [source: Purcell]. When used as recommended, they save money by reducing brake service costs and offer added security if you are traveling or pulling a rig in hill country.

­Now that you know how engine and exhaust brakes work, read the next section to learn about engine brake usage.

­Engine brakes are used when a semi driver is in stop-and-go traffic and when driving on downhill grades. They are also used on the following vehicles and equipment:

Like exhaust brakes, engine brakes should be used continuously -- not just when traversing a steep decline. This will help to reduce wear on the normal service brakes. And if you've ever needed your brakes replaced, you know that can cost a pretty penny.

­You can hear engine brakes in action while driving on a typical interstate or highway. Drivers regularly switch on the engine brakes to slow down. This creates the blatting sound that ricochets off any surrounding buildings or vehicles.

Engine brakes often get a bad rap due to the noise that is attributed to their use. When a semi uses engine brakes you will often hear a loud blat-blat-blat, sometimes referred to as a Jake Bark because the compressed air is forced through the exhaust valve in the engine's cylinder.

Jacobs Vehicle Systems, the leading manufacturer of engine brakes, says that the noise emitted by a properly maintained rig with an OEM muffler is within the 80 to 83 decibel dB(A) range, which is 10 to 13 decibels above the high range of normal conversation. Jacobs Vehicle System states the main reason for the loud staccato noise that often accompanies their use is improperly muffled vehicles. Straight stack exhaust systems have a sound level that is 16 to 22 decibels higher than properly muffled vehicles [source: Jacobs Vehicle Systems].

­­Legislation has been passed banning the use of engine brakes in some states, but changes are being made to address the root of the problem. Read on to learn the laws and possibly help to create your own.

The United States Environmental Protection Agency (EPA) requires all trucks manufactured since 1988 to produce 80 db(A) of noise at 50 feet (15.2 meters). This range includes the exhaust system. The EPA also prohibits removing noise-reducing equipment from any vehicles [source: Jacobs Vehicle Systems].

Engine brakes are not always to be blamed for the loud noise that is so often heard on the road and can be bothersome to residents along heavy-use highways. Improper exhaust systems can be the problem, and many states are enacting legislations stating this:

If you are bothered by engine brake noise, contact your local authorities and request that legislation be enacted in your state, too.

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Now let's put the parts together to see how engine brakes work as a whole. This diagram shows the basic parts that make up an engine brake system.