Power is both the blessing and the curse of diesel engines, which is where retarders, more commonly known as engine brakes, come into play. Pacbrake explains that since diesel engines lack a retarding effect when the throttle is in the off position, truck brakes have to work extra hard to slow down and stop a vehicle. Add a 6%- to 8%-grade hill to the equation and the result could be overheated brakes or brake fading.
Engine brakes turn diesels into air compressors so they absorb energy instead of produce it. According to Pacbrake, this creates a retarding effect that slows down the engine and the vehicle quickly.
In addition to the safety benefit inherent in slowing down an 80,000-lb. tractor-trailer more quickly, engine brakes can save money by reducing brake wear. Regular use of an engine brake, Pacbrake says, can extend the life of a truck's service brakes by a factor of five. Considering that it can cost $300 to $1,500 to reline the brakes, depending on what components need replacing, engine brakes can save $1,500 to $7,500 annually on brake repairs for the tractor alone.
There are two forms of engine brakes: compression release brakes and exhaust brakes. Though both retard the engine, they do so in very different ways. According to Jacobs Vehicle Systems, a compression brake works via a set of cast iron housings attached to the cylinder head of a diesel engine that hydraulically open the exhaust valve near the end of the upward piston stroke. It also restricts fuel to the injector at the same time. (Truckers often refer to engine brakes as Jake brakes, a copyrighted name that encompasses all of Jacob's engine brake products.)
Energy stored in the cylinder is released to the atmosphere so that when the piston descends on what would normally be the power stroke, no pressure remains in the cylinder to act on the piston. Thus, the engine's energy is transferred from the driving wheels out the exhaust pipe, slowing both the engine and vehicle, Jacobs explains.
Compression brakes also create more noise, a kind of “staccato” or machine-gun sound, causing many communities to restrict or ban their use. Jacobs points out, however, that the brakes do meet the federal requirement that trucks emit less than 80 dB of noise at a range of 50 ft. Many of the complaints are undoubtedly from people who are unaware of the safety benefits the brakes provide.
Exhaust engine brakes function by holding the compression in the engine instead of releasing it. They are typically located on the outlet side of the turbocharger, so instead of making it hard for the engine to intake air, it must work harder to exhaust air. With exhaust gases trapped behind the exhaust brake, the piston meets resistance as it tries to push the gases past the brake. When the engine piston meets this resistance it absorbs energy, resulting in rapid deceleration, explains Pacbrake.
Compared to their compression-brake counterparts, exhaust brakes do not create noise. But they do require the engine to have a high threshold for exhaust backpressure in order to work. So while exhaust brakes can be used on almost all light- and medium-size diesel engines, heavy-duty diesels with 10-liter displacements and higher require compression brakes. Compression brakes also provide twice the retarding power of exhaust brakes, making them the engine brake of choice for heavy Class 8 vehicles.
However, Smartbrake points out that the backpressure threshold on some heavy-duty engines is high enough to let an exhaust brake function properly. Cummins' N-11 engine, for example, has a maximum backpressure of 65 psi, which is perfect for an exhaust brake.
The real issue is noise, although Jacobs believes that the excess noise people complain about is often a result of unmuffled or poorly muffled exhaust pipes. Some companies believe that increasing the exhaust valve ring size in heavy-duty diesels could eliminate the need for compression brakes, thus eliminating the noise issue.
In addition, Pacbrake points out that a big part of the noise problem is generated by a small number of Class 8 drivers who use their engine brakes in the city for regular braking, rather than limiting their use to long, steep slopes. So a third option, according to Pacbrake, would be to equip vehicles with both compression and exhaust brakes — one for highway use and one for city use. Cost is the major drawback to this solution. But as more communities restrict the use compression-style engine brakes, interest in dual retarder technologies under the hood may increase.
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