What is a "jake brake"?

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, all fuel flow to the engine is stopped. 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. 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. 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 a 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.

For MarkM3 - The Jacobs brake operates in the following manner. The Jake brake is turned on - this energizes a solenoid within the Jake head (housing), preventing engine oil from draining back to the oil pan, and the Jake housing is now charged (with oil).

Now, as an engine pushrod comes up to open the exhaust valve, it contacts a master piston, which has its rod extended due to engine oil pressure.

Ordinarily the master piston is retracted and can't contact the exhaust valve bridge.

As the pushrod continues to rise, oil is pushed out of the master piston to a slave piston.

The master piston and slave piston are joined by oil passages within the Jake head and open the exhaust valve (only one exhaust valve is opened usually .. even though a lot of four stroke diesels have two valves actuated by a single bridge).

If #1 exhaust pushrod was coming up on exhaust, the master piston for #1 would activate slave piston on #5 (typical firing order for a six cylinder 153624), which is coming up on compression.

All the compression pressure created in #5 cylinder, is wasted to atmosphere when the exhaust valve on #5 is opened (before top dead center).

As #1 exhaust valve closes, oil from the master piston is retracted, slave piston on #5 retracts, and #5 exhaust valve closes (no bent valves).

The #5 piston goes down with closed valves, creating a vacuum - and we have engine braking.

The Jake brake housing is mounted on the head under the valve covers, and it contains oil passageways and the pistons for operation of the valves in the correct sequence. The oil flow path in the housing is detailed on a tag on the housing.

The Jacobs brake was actually invented by Clessie L. Cummins, as an attachment for his Cummins diesels.
He underwent a "near death" experience in 1931, whilst demonstrating one of his diesel engines, and decided to invent a brake that utilised the potential power in the engine.

Although the Jacobs Engine Brake has been on the market since 1961, the need for it was terrifyingly demonstrated to its inventor, some thirty years earlier.

In August of 1931, Clessie himself, Ford Moyer, and Dave Evans were driving a Cummins diesel powered Indiana truck from New York to Los Angeles attempted to set a new truck speed record across the continent.

All went reasonably well until the descent of the Cajon Pass, on old U.S. 66 leading into San Bernardino, California. A long, winding and steep gravel road, criss-crossed by a busy mainline railroad, almost led to the demise of the truck, its driver Clessie L. Cummins, and a frantic crew.

In Clessie’s words: “About dusk on the fifth day, we reached the top of Cajon Pass west of Barstow, California. Before retiring to the sleeping compartment, Dave had warned me against this thirty-five-mile stretch of mountainous downgrade.
I failed to register when the sign for the Cajon Pass appeared. Soon, however, I realized my error. The brakes wouldn’t hold. Now running in third gear, I tried desperately to get into a lower speed. Nothing doing, I saw I would just have to ride it out. I suddenly saw something moving across the road ahead.
I realized with new alarm that a freight train was cutting across our path. As we raced inexorably toward the crossing and doom, the train’s caboose loomed out of the darkness. Its red lights cleared the highway just as we reached the tracks. We had escaped certain death by inches.”

Clessie spent over two decades on the design of the engine brake, and didn't produce the final design until 1954. He then shopped it around, but found little interest in it from the major engine manufacturers.

In 1957, Clessies brothers son introduced him to the Jacobs Chuck manufacturing Co, which had been in business since 1903, and who produced the 3 jaw Jacobs drill chuck.

Jacobs expressed great interest in manufacturing Clessies engine brake, and drew up a deal with him .. and the Jacobs Engine Brake appeared on the market in 1961.

In 1986, the Jacobs company split into two separate businesses; Jacobs Chuck making drill chucks in Clemson SC - and Jacobs Engine Systems builds the engine brakes in Bloomfield CT.

Very good answer, but I would just like to add that even though they are called "Jake Brakes" there are other manufactures, Pacbrake and Caterpillar make there own. They are called jake brakes because of the original manufacture, Jacobs. But are referred to as a compression brake. Also alot is to do with the electronics, especially with the new engines. Older engines used an idle switch that would only allow positive voltage to go to the jake brakes if you were appling no throttle. Also some of the older engines would allow you to turn on the jakes at an idle, almost killing the engine. This could be bad if the throttle was applied, as it would end up in bent valves or even worse. Newer engines have to meet demands for the jake brakes to work. Such as the clutch pedal has to be disengaged, and no throttle pedal. Also on newer engines it is possible to program the ECM to activate the jake brakes in cruise control to keep the vehicle from exceeding the set cruise control speed and also different variations as to how to get the jakes to operate, like first applying the brake pedal or holding constant pressure on the brakes. However old or new, jake brake is a proven technology that will continue to be on diesel engines to aid in controlled driving of the vehicle.

a brake named jake why of course!!

This isn't an Answer but a question, is a Jake brake the same thing as an Exhaust Brake, if not can you please refer me to where I can find substantial amounts of information for them?