Advice: What's a Jake Brake?

A Jake Brake, or Jacobs Engine Brake, is a device that greatly improves the retarding power of a diesel truck engine when the vehicle is running downhill. It uses engine oil flow and pressure to vary valve timing, to maximise the braking effect caused by engine compression.

Every vehicle has some degree of engine-compression retardation when descending a grade. Diesels have more, because a diesel has a higher compression ratio than a petrol engine, and big diesels have more than little diesels. Size does matter.

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If we look at the four-stroke principle in one cylinder, we'll see how it works. The first stroke is induction, when the descending piston sucks in air – an action that is helped by turbocharging. The second stroke is the compression of that inducted air and, at the top of the piston travel, fuel is injected and combusts, forcing the piston down in what is the third stroke. That leaves the fourth stroke, or exhaust, when the piston travels up and forces out the burnt-fuel gases.

That's the situation when an engine is delivering power. When it's on the 'overrun' there's no fuel being injected, but the strokes occur as above, except that the wheels are rotating the engine, via the transmission. A vehicle-braking effect is caused by the compression of air in the cylinders that resists the vehicle's propensity to run away.

However, in the second stroke the air is compressed, but then expands on the down stroke, reducing the braking effect. The way to stop that expansion from driving the piston down is to open the exhaust valve at the top of the compression stroke to exhaust the compressed air, so that it can't pressurise the piston's descent. The piston will still descend, but won't be helped by expanding air.

Various engine braking systems appeared during the 20th Century, but one man, Clessie Cummins, of Cummins Engine fame, patented a valve-opening device for truck diesels in 1957, after he'd retired from the company he started in 1919.

His original plan was for the engine brake to be produced by Cummins, but in the end it was taken up by Jacobs Vehicle Systems, Inc., which began series production of the Jacobs Engine Brake in 1961. It didn’t take long for the new driving aid to become popular and earn its now-common 'Jake Brake' tag.

Jake Brakes were popular among fleets and drivers, because the powerful engine braking effect took much of the work off the service brakes. North American trucks in the 1970s and 1980s had relatively poor brakes and some prime movers had no front axle braking at all, so the engine brake was most welcome.

Many European and Japanese engine makers also ordered Jacobs Brakes for their engines from the late 1990s.

There's no such thing as a free lunch and the Jake Brake had a particular problem that endeared it to Good Ol' Boys, but not to residents who lived near highway descents. That problem was noise.

The momentary opening of each exhaust valve allowed highly compressed air to escape into the exhaust manifold and this sudden release created a shock wave. Air-powered jackhammers, once used in road maintenance, had a similar issue.

A high-quality muffler could reduce the dreaded 'drrrrrr' rattle of a working Jake, so European and Japanese engine makers managed to muffle Jake noise, but in Australia and North America engine brake noise was often seen as a badge of honour.

There's an unauthorised sticker that states: 'Jacobs Engine Brakes – Waking up your town since 1957'.

The engine-brake noise problem caused communities in the USA and Australia to put signs on their outskirts, requesting drivers not to use their compression brakes.

With the advent of electronically controlled fuel-injection engines in the 1990s, truck makers were able to reduce engine revs, in pursuit of improved fuel economy. The original Jake-equipped engines that ran continuously around 1900-2100rpm were replaced by engines that cruised at 1400-1600rpm.

The Jacobs Brake was designed to work at high revolutions, maximising the number of retarding strokes per kilometre, so at lower revs that braking power was greatly reduced. Drivers had to downshift several gears to keep engine revs high on downgrades so that Jakes worked.

At Jacobs Vehicle Systems the engineering teams were busy working on modern engines, exploiting the power of electronic controls. Engineers had always known that selective opening and closing of inlet and exhaust valves could greatly enhance engine braking power and electronics allowed that to be implemented.

In 2016, Jacobs released the High Power Density (HPD) engine brake. Gabe Roberts, director of product development, explained how it worked…

"At the top of the compression stroke, a conventional engine brake opens the exhaust valve to get the retarding effect, as the energy from the compression stroke is not returned back to the crank.

"With this new HPD technology, we are using cylinder deactivation to produce a second inlet and exhaust valve activated compression release event at the top of the exhaust stroke.

"Two compression-release events for every one cam rotation gives twice as much braking power as a conventional engine brake.

"It provides braking power that is comparable to that of a driveline retarder, but without parasitic losses that impact fuel economy and the significant cost and weight of a retarder."

A lower-cost, less complex version of this so-called 2-Stroke HPD, known as the 1.5-Stroke, was released in 2018, with two-event braking, but without the need to deactivate the inlet valves.

Jacobs Vehicle Systems introduced Cylinder Deactivation (CDA) technology to the European market at the IAA commercial vehicle expo in 2018.

For CDA, cylinder deactivation mechanisms originally designed for HPD are combined with disabled injection in selected cylinders, making the deactivated cylinders act as gas springs that return the compressed energy of the air back to the crank.

CDA reduces emissions by achieving higher exhaust temperatures in the operating cylinders, maintaining after-treatment temperatures when the engine is in low-load operation.

One, two or three cylinders can be deactivated as needed. At the lowest engine loads and with three of six cylinders deactivated, fuel consumption improves by up to a claimed 20 per cent.

We're sure Clessie Cummins would have been pleased to see his engine brake invention developed to such an amazing extent.