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The Jake Brake takes its name from the manufacturer who invented the most common implementation of the technology, the Jacobs Company.
The Jake brake is an add-on engine brake for diesel engines. Big semi trailers, the 18 wheel trucks that move everything we use, can weigh as much as 80,000 pounds. Stopping them or slowing them down results in a great deal of wear on the brakes, which have to be replaced frequently. The Jake brake, as an engine system, causes no wear and tear and can help slow the truck before the wheel brakes need to be applied. Its primary use is on long downhill grades where the wheel brakes would otherwise have to be frequently pumped to keep the truck from gaining dangerous speed.
When the driver presses a button in the cab to activate the device, two things happen. First, the switch excites the engine brakes' solenoids. By itself, this would help only a little but it is necessary for the second step. What happens inside of the engine goes roughly like this.
As a four-stroke internal combustion engine, each piston in a diesel normally moves up and down twice in each cycle. For the nit-pickers out there, there are many two-stroke diesel trucks on the road as well. The process begins when the fuel and air valves are closed and the piston moves upward. This compresses the air in the cylinder to as much as 25 times atmospheric pressure. This is much higher compression than a gasoline engine (typically ten times atmospheric pressure) and results in the air getting very hot, about 900 degrees Fahrenheit. At this time fuel is sprayed into the superheated air which immediately begins burning. The second stroke, the power stroke, is the downward movement of the piston as fuel burns. The third stroke is an upward movement with the exhaust valve open to clear out the combustion products while the fourth stroke refills the cylinder with air.
The Jake brake completely changes all this, redefining what the valves do as each piston moves up and down. With the fuel flow terminated, the upward moving first stroke still compresses the air to very high pressure. As we said above, this transfers mechanical energy into heat as the air becomes highly compressed. If nothing else were done, most of this energy would be recovered, except for frictional losses, as the cylinder moved back down and the compressed air expanded. The Jake brake, however, opens the exhaust valve just as the air reaches maximum compression, dumping all of that energy in an almost instantaneous explosive release. The result is a very effective slowing of the vehicle as mechanical energy is converted to heat and then dumped. The Jake brake effectively transforms the internal combustion engine into an air compressor.
It has only one drawback: it is very noisy. You may have heard a semi use the Jake brake without realizing what it was. Sometimes when a truck is approaching a stop sign or stop light it suddenly emits a load roar, very much like a large lawnmower, for five or ten seconds. It is the noise that is causing many towns to ban the use of the Jake brake. Even though tests have shown the decibel level to be about as loud as a large lawnmower, at night or early morning the low frequencies seem to carry a long distance and are very noticeable.
Because it extends the life of wheel brakes and saves money, trucking companies generally lobby against the bans and some towns are compromising by allowing the Jake brake to be used in daylight hours. Yet more and more signs, with the words Jake Brake and the international symbol for "banned", are certain to appear. Since the primary use of the Jake brake is to slow the truck on long downhill grades, the technology will continue to be widely adopted for use on the open road.
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