Engine Brakes & Downsped Drivelines: What You Need to Know – Equipment

How often do you hear drivers complaining about weak engine brakes? Some drivers will say the engine brake provides so little retarding power they have to use their service brakes to help decelerate the truck. In various YouTube videos that purport to be instructional, you’ll hear references to using the service brakes to supplement “weak” engine brakes.

There are drivers advising other drivers how to use a tool that they, themselves, don’t understand how to use.

Like this guy:

“I took one of the new motors for a drive the other day, and I was surprised at how ineffective the [engine] brake was in that truck,” he says, about two minutes into a video that has 520,000 views. “I don’t know whether it was because of the muffler system or just that the [engine brake] was weak on that motor, but I couldn’t even tell if the [engine brake] was engaged in that truck.

“It certainly didn’t slow me down at all. It kind of made a little sputtering noise in the background, but it really didn’t work at all.”

In another video with 192,000 views, the same fellow offers this bit of advice: “You want to keep the engine in the proper rpm range because if you let the thing over-rev and run high, it’ll damage the [engine brake] and it will damage the engine as well, of course.”

Really? He certainly didn’t get that pearl of wisdom from anyone who knows anything about engine brakes. Yet drivers lap that stuff up.

Engine brakes, it seems, are no longer well understood. Many of today’s drivers are not as technically inclined as were previous generations of drivers. In many cases, drivers with less than five years’ experience have only driven automated transmissions. As a consequence, they may not understand the dynamic relationship between gear selection, engine speed, torque, horsepower, and even retarding power. They hardly need to anymore; they are encouraged to let the whizz-bang electronics under the hood do all the thinking.

Engine brakes are much taken for granted these days — like crossmembers. But if drivers don’t understand the fundamentals, if fleets don’t teach drivers how to get the most out of their engine brakes, your service brakes might be paying the price.

To gain some expert insight into the secret life of engine brakes, we spoke with Gabe Roberts, director of product engineering for Valvetrain Technologies with Cummins Engine Components, formerly known as Jacobs Vehicle Systems.

What is an Engine Brake?

The name itself is a bit of a contradiction. An “engine brake” is not a brake at all in the conventional sense. The engine brake hydraulically alters the cylinder valve timing to turn a power-producing internal combustion engine into an energy-absorbing air compressor.

In fact, the proper engineering name for an engine brake is a “Compression Release Engine Brake.” We call them engine brakes for short, or sometimes Jake Brakes, after the name of the company that makes most of them, Jacobs Vehicle Systems. (Jacobs was acquired by Cummins in April 2022, but the name, Jake Brake, will probably stick.)

In the simplest terms, the “engine brake” system alters each cylinder’s exhaust valve open-and-closed cycles while shutting off the fuel supply to the engine. Jacobs has a video with good visuals explaining how this works.

All four-stroke diesel engines work like this with the engine brake turned off:

1. Intake stroke: The piston descends in the cylinder, drawing in air through the open intake valve.
2. Compression stroke: The piston rises in the cylinder, compressing that charge of air against the closed intake and exhaust valves.
3. Combustion stroke: With the piston roughly at the top of the compression stroke, fuel is injected into the cylinder. The fuel ignites under the heat of compression, pushing the piston down. This provides energy to the rotating crankshaft to power the truck.
4. Exhaust stroke: The piston once again rises in the cylinder, pushing the exhaust gas created by the combustion event out through the now-open exhaust valve.

This is how it changes when the engine brake is activated:

1. The intake stroke occurs normally.
2. The compression stroke occurs normally.
3. At the top of the compression stroke, no fuel is injected into the cylinder so no combustion takes place.
4. Also at the top of the compression stroke, after the piston has worked to compress the intake air charge, the exhaust valve opens briefly, releasing the compressed air from the cylinder into the exhaust manifold.

The effect is two-fold.

One, since there was no combustion, no power is produced to drive the wheels.

Two, the energy needed to compress the air in the cylinder comes from the momentum of the truck, transmitted from the wheels through the driveline and crankshaft to the pistons. The effort required to compress the air in the cylinder provides resistance to the rotation of the driveline, thus “retarding” the motion of the truck. (Which is why you’ll also sometimes hear the term “engine retarders.”)

The loud popping sound engine brakes make is the sound of the compressed air escaping from the cylinder through the open exhaust valve. Engine brakes still make that sound, but you can barely hear it today thanks to the sound-attenuating qualities of the diesel particulate filter.

And that fact plays into drivers’ perceptions that today’s engine brakes don’t work as well as they used to. But it’s not the exhaust system that impacts performance — it’s engine speed.          

Factors Affecting Performance

While drivers are often heard complaining about weak engine brakes, there’s no such thing. But the way engine brakes are used does affect their performance.

Roberts is an engineer, and engineers love to illustrate what they say with charts and graphs. He sent this graph showing the engine brake retarding power curves for almost every model of heavy-duty engine in North America. (We removed the make and model information to protect the innocent.)

Shown vertically on the right side is the retarding power measured in kilowatts per liter of displacement. Along the bottom is engine rpm. It’s easy to visualize how the retarding power increases at higher engine speeds.

We’ll get to the difference between two-stroke and four-stroke engine brakes in a moment.

Back to kW/L. This represents how much retarding power the engine brake produces. For example, following the redline from bottom left to upper right, at 1,200 rpm, this engine produces about 12 kW/L of retarding power. If this was a 15-liter engine, you’d multiply that figure by 15 (or multiply by 13 for a 13-liter engine). So, with all cylinders in retarding mode, the engine produces 180 kW of retarding power. In more common parlance, that’s 244 horsepower.

If you follow the red line up to the point where it crosses 2,200 rpm, the output jumps to about 31 kW/L, or 630 hp. At what engine speed do you think that engine brake would feel more effective?

Because engine displacement matters (larger cylinders can hold greater volumes of air), you might not see quite that amount of retarding power from a 13-liter engine. Take the green line for example. At 2,200 rpm, it produces about 28 kW/L, or almost 500 retarding horsepower. That’s still respectable.

The turbocharger calibrations play a part in this, as well. In retarding mode, the turbo works to push more air into the cylinders for the pistons to compress. More air means a higher compression ratio in the cylinder and more retarding power.

And in case you missed this earlier, no fuel is consumed while the engine brake is operating. Zero. Zilch. Nada. The engine brake’s electronic controls shut off fueling to the engine. There is NO fuel economy penalty to running the engine at 2,200 rpm.

Is the Engine Running too Slow?

So, what’s the fuss about weak engine brakes? If your engine brake isn’t practically standing the truck up on its nose, you’re probably running it too slow.

“Complaints about engine brake performance are usually handled by the OEM, but occasionally we’ll go into the field to investigate,” Roberts told HDT. “We often find it’s programmed incorrectly, or the driver doesn’t know how to operate the truck. Everything changes once we coach them on how to run the engine in retarding mode.”

Fleet managers should not be discouraging drivers from running engine brakes at high rpm. They are designed to do just that. We have heard reports of fleets handing drivers engine-overspeed warnings and penalties, making them reluctant to operate it at the higher rpm where it’s designed to operate.

Roberts says the design criteria demands the brake be able to run at much higher rpm (up to 3,000 rpm for some applications) for hundreds of hours under full load. They are tested that way. Running the engine brake at 1,900 to 2,100 rpm is where they work the best, he stresses.

“We design the engine brakes so that it won’t damage itself even under a maximum overspeed condition,” he says. They’ll last forever at normal operating speed.”

That said, Jacobs doesn’t make all the engine brakes under today’s hoods. If there’s any doubt about the normal operating speeds for your make and model of engine, consult the owner’s manual or call your sales rep. To the best of our knowledge, all engine brakes are designed to run at high rpm.

The problem here stems mostly from the use of low-engine-rpm downsped drivelines. As we illustrated earlier, engine brakes don’t deliver optimum retarding power at 1,200 rpm. Drivers would need to downshift at least one gear, maybe two, to get the most out of their engine brake.

When it’s set up correctly, the truck’s cruise control should manage those downshifts based on demand for deceleration when the cruise control is set and engaged. Without active cruise control, the truck will just keep accelerating on a downhill grade, and drivers will probably be inclined to use the service brakes to slow the truck.

Some fleets lock out certain functionality on their automated transmissions to encourage drivers to use cruise control, but that can backfire if those settings aren’t to the drivers’ liking.

High Power Density Engine Brakes

If fleets remain reluctant to operate their engine brakes at the recommended rpm, there is another solution to anemic engine brake performance at low engine rpm.

Jacobs developed the High Power Density Engine Brake specifically for low-rpm engines. Technologies such as low-rolling-resistance tires and better tractor and trailer aero packages have decreased the retarding forces that used to help slow the truck. Those factors, coupled with downspeeding the engine and generally smaller engine displacement, have placed more demands on supplemental retarding technology.

While HPD engine brakes are complex, in the simplest terms, Jacobs uses variable valve opening and closing capability to provide two compression release events for each full rotation of the engine crankshaft. A standard engine brake provides just one. This almost doubles the retarding capability of the engine at maximum rpm.

For downsped, low-rpm engines, HPD engine brakes provide comparable performance to a standard engine brake, but while operating at much lower rpm. And it eliminates the need to downshift two, perhaps three times to get top performance from the engine brake.

Referring once again to the North American Brake Performance Chart, notice the green dotted line, representing one North American engine equipped with an HPD engine brake. Assuming it’s a 13-liter engine, at 1,400 rpm, it produces about 26 kW/L of retarding force, or about 338 kW of power overall. That’s about 460 hp. With a conventional engine brake, that engine (solid green line) needs to run at about 2,200 rpm to achieve that kind of performance.

The engine brake is a useful tool for controlling vehicle speed and reducing brake wear. But if drivers are unfamiliar with how to use the engine brake properly, or if the fleet has locked out certain functionality, they will resort to their service brakes. It’s a good idea to review your drivers’ understanding of these systems and update any knowledge gaps.

If you don’t, they might turn to fellow drivers on YouTube or TikTok for information, and some of that may be questionable at best.