What is a Jake Brake?

If you didn’t already, diesel engines have little to no engine braking because they don’t use an intake throttle body like a gas engine does. Instead, diesel engines use fueling as a way to control power output. Depending on the application, some diesel engines have more engine braking than others due to things in the exhaust system, such as a turbo or a diesel particulate filter, but generally, they have minimal engine braking as compared to a gasoline engine.

In the context of a small car, minimal engine braking isn’t a big issue, but what if you’re in a massive truck with an equally massive trailer hauling upwards of 80k of weight and you’re going downhill? Without engine braking, you’ll quickly fry your brakes. Luckily, there is a solution to this simple problem, and that’s the jake brake. So, sit back and get comfy because I’m going to tell you exactly how a jake brake works.

A Jacob’s brake, jake brake, compressions release brake, whatever you want to call it, is actually fairly simple in how it works. To put it really simply, it just uses the engine’s compression against itself to slow down the engine speed and ultimately slow down the vehicle.

Of course, the explanation is simple, but actually understanding it is a little more difficult, so let me break it down into a more easily understandable format, and to do that we first need to get on the same page with how an engine works in the first place.

A four-stroke internal combustion engine works in four stages:

First is the intake stage, where the piston is pulled down by the rotation of the crankshaft and the intake valve opens. This creates a vacuum in the cylinder, which is then filled with air. In the case of a turbocharged engine, it’s not a vacuum that really pulls the air into the cylinder as much as it is the turbocharger forcing air into the cylinder.

The second is the compression stage, where the piston is pushed up and the intake valve is closed. This compresses the air inside the cylinder.

The third is the combustion stage, where the air/fuel mixture is combusted and then that explosion pushes the piston back down. In the case of a diesel engine, the fuel combusts from air pressure, but in the case of a gasoline engine, a spark plug is used to start the combustion process.

Last is the exhaust stage, where the combustion has done its job and needs to be expelled from the engine in order to restart the process.

With that in mind, you might be wondering why simply shutting off the fueling wouldn’t work as an engine brake. In the case of a gas engine, closing the throttle body creates a pumping loss and when combined with the natural drag of the engine operating, provides engine braking, but for a diesel engine, there is no throttle body so your only option would be to shut off the fueling.

Unfortunately, for a diesel engine, shutting off the fueling would not create any engine braking, because there’s no pumping loss because air is still able to freely flow in and out of the engine. Sure, something like an exhaust brake can create exhaust brake pressure to then create engine braking, but in its normal state, shutting off fueling won’t create engine braking in a diesel.

So, what’s the solution then? And that’s where the jake brake comes in. The jake brake fundamentally changes how the four-stroke engine operates on deceleration in order to create engine braking, and it does this by making your engine, which is simply a big air pump, work against itself.

The big change comes between the compression and combustion stages of the engine’s operation. During compression, the air that’s being compressed is pushing back on the piston, because it doesn’t want to be compressed. Ordinarily, this doesn’t really matter because the combustion stage creates more than enough force to overcome this resistance during the compression stage.

But, what if instead of combusting the air, we fully harness the resistance of the compression cycle, and then let it out of the engine before that air spring has a chance to actually start pushing the piston downwards.

To do this, the jake brake simply alters the operation of the engine and opens the exhaust valves near top-dead-center during the compression stage of the engine cycle. This means all that compression force is pushing back on the piston, ultimately slowing the engine, and then that air is released before the piston begins its downwards cycle.

Of course, the downside of this is that it’s only going to provide deceleration to the drive wheels because as the engine is being forced to slow down and the drivetrain is fully engaged, it will force the drive wheels to also slow down. Depending on the context, this can be an issue, such as driving in the snow where you really probably want even braking force across all axles.

The massive upside to this is safer vehicle operation and decreased operational costs. At the end of the day, anything that can help keep a commercial truck at safe speeds downhill is a massive bonus. Plus, by using your brakes less, you ultimately have to replace them less often, so it saves commercial truck drivers a ton of money in the long run.

On the negative side, using a jake brake is generally not legal in town because of the massive amount of noise it makes. On newer trucks, it’s certainly quieter, but on older trucks, it can be pretty insanely loud. For example, this Peterbilt 379 has a Cat motor and the jake brake is super loud.

Of course, the legalities of using a jake brake in a city are different everywhere you go, but most towns and cities don’t allow it. Before we end this article I would like to quickly note that a compression release brake is not the same as an exhaust brake. With an exhaust brake, you’re adding back-pressure to the end to help slow down its rotation and this is either done with an exhaust flap or internally on a VGT turbo, but it’s functionally very different from a jake brake.

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