Engine Brakes & Downsped Drivelines: What You Have to Know – Tools

How typically do you hear drivers complaining about weak engine brakes? Some drivers will say the engine brake supplies so little retarding energy they’ve to make use of their service brakes to assist decelerate the truck. In varied YouTube movies that purport to be educational, you’ll hear references to utilizing the service brakes to complement “weak” engine brakes.

There are drivers advising different drivers easy methods to use a software that they, themselves, don’t perceive easy methods to use.

Like this man:

“I took one of many new motors for a drive the opposite day, and I used to be shocked at how ineffective the [engine] brake was in that truck,” he says, about two minutes right into a video that has 520,000 views. “I do not know whether or not it was due to the muffler system or simply that the [engine brake] was weak on that motor, however I could not even inform if the [engine brake] was engaged in that truck.

“It definitely did not sluggish me down in any respect. It sort of made just a little sputtering noise within the background, however it actually didn’t work in any respect.”

In one other video with 192,000 views, the identical fellow provides this bit of recommendation: “You need to preserve the engine within the correct rpm vary as a result of should you let the factor over-rev and run excessive, it’ll harm the [engine brake] and it’ll harm the engine as nicely, in fact.”

Actually? He definitely didn’t get that pearl of knowledge from anybody who is aware of something about engine brakes. But drivers lap that stuff up.

Engine brakes, it appears, are now not nicely understood. Lots of right this moment’s drivers are usually not as technically inclined as had been earlier generations of drivers. In lots of instances, drivers with lower than 5 years’ expertise have solely pushed automated transmissions. As a consequence, they could not perceive the dynamic relationship between gear choice, engine pace, torque, horsepower, and even retarding energy. They hardly must anymore; they’re inspired to let the whizz-bang electronics underneath the hood do all of the considering.

Engine brakes are a lot taken without any consideration lately — like crossmembers. But when drivers don’t perceive the basics, if fleets don’t train drivers easy methods to get probably the most out of their engine brakes, your service brakes could be paying the value.

To achieve some skilled perception into the key lifetime of engine brakes, we spoke with Gabe Roberts, director of product engineering for Valvetrain Applied sciences with Cummins Engine Parts, previously often known as Jacobs Car Programs.

What’s an Engine Brake?

The title itself is a little bit of a contradiction. An “engine brake” is just not a brake in any respect within the standard sense. The engine brake hydraulically alters the cylinder valve timing to show a power-producing inner combustion engine into an energy-absorbing air compressor.

In truth, the correct engineering title for an engine brake is a “Compression Launch Engine Brake.” We name them engine brakes for brief, or typically Jake Brakes, after the title of the corporate that makes most of them, Jacobs Car Programs. (Jacobs was acquired by Cummins in April 2022, however the title, Jake Brake, will most likely stick.)

Within the easiest phrases, the “engine brake” system alters every cylinder’s exhaust valve open-and-closed cycles whereas shutting off the gasoline provide 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. Consumption stroke: The piston descends within the cylinder, drawing in air via the open consumption valve.
2. Compression stroke: The piston rises within the cylinder, compressing that cost of air towards the closed consumption and exhaust valves.
3. Combustion stroke: With the piston roughly on the high of the compression stroke, gasoline is injected into the cylinder. The gasoline ignites underneath the warmth of compression, pushing the piston down. This supplies power to the rotating crankshaft to energy the truck.
4. Exhaust stroke: The piston as soon as once more rises within the cylinder, pushing the exhaust gasoline created by the combustion occasion out via the now-open exhaust valve.

That is the way it adjustments when the engine brake is activated:

1. The consumption stroke happens usually.
2. The compression stroke happens usually.
3. On the high of the compression stroke, no gasoline is injected into the cylinder so no combustion takes place.
4. Additionally on the high of the compression stroke, after the piston has labored to compress the consumption air cost, the exhaust valve opens briefly, releasing the compressed air from the cylinder into the exhaust manifold.

The impact is two-fold.

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

Two, the power wanted to compress the air within the cylinder comes from the momentum of the truck, transmitted from the wheels via the driveline and crankshaft to the pistons. The trouble required to compress the air within the cylinder supplies resistance to the rotation of the driveline, thus “retarding” the movement of the truck. (Which is why you’ll additionally typically hear the time period “engine retarders.”)

The loud popping sound engine brakes make is the sound of the compressed air escaping from the cylinder via the open exhaust valve. Engine brakes nonetheless make that sound, however you possibly can barely hear it right this moment due to the sound-attenuating qualities of the diesel particulate filter.

And that reality performs into drivers’ perceptions that right this moment’s engine brakes don’t work in addition to they used to. But it surely’s not the exhaust system that impacts efficiency — it’s engine pace.          

Elements Affecting Efficiency

Whereas drivers are sometimes heard complaining about weak engine brakes, there’s no such factor. However the best way engine brakes are used does have an effect on their efficiency.

Roberts is an engineer, and engineers like to illustrate what they are saying with charts and graphs. He despatched this graph exhibiting the engine brake retarding energy curves for nearly each mannequin of professional quality engine in North America. (We eliminated the make and mannequin info to guard the harmless.)

Proven vertically on the suitable facet is the retarding energy measured in kilowatts per liter of displacement. Alongside the underside is engine rpm. It’s straightforward to visualise how the retarding energy will increase at greater engine speeds.

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

Again to kW/L. This represents how a lot retarding energy the engine brake produces. For instance, following the redline from backside left to higher proper, at 1,200 rpm, this engine produces about 12 kW/L of retarding energy. If this was a 15-liter engine, you’d multiply that determine 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 energy. In additional widespread parlance, that’s 244 horsepower.

For those who comply with the purple line as much as the purpose the place it crosses 2,200 rpm, the output jumps to about 31 kW/L, or 630 hp. At what engine pace do you assume that engine brake would really feel more practical?

As a result of engine displacement issues (bigger cylinders can maintain larger volumes of air), you may not see fairly that quantity of retarding energy from a 13-liter engine. Take the inexperienced line for instance. At 2,200 rpm, it produces about 28 kW/L, or nearly 500 retarding horsepower. That’s nonetheless respectable.

The turbocharger calibrations play a component on this, as nicely. In retarding mode, the turbo works to push extra air into the cylinders for the pistons to compress. Extra air means the next compression ratio within the cylinder and extra retarding energy.

And in case you missed this earlier, no gasoline is consumed whereas the engine brake is working. Zero. Zilch. Nada. The engine brake’s digital controls shut off fueling to the engine. There may be NO gasoline economic system penalty to operating the engine at 2,200 rpm.

Is the Engine Operating too Sluggish?

So, what’s the fuss about weak engine brakes? In case your engine brake isn’t virtually standing the truck up on its nostril, you’re most likely operating it too sluggish.

“Complaints about engine brake efficiency are normally dealt with by the OEM, however often we’ll go into the sector to analyze,” Roberts informed HDT. “We regularly discover it’s programmed incorrectly, or the motive force doesn’t know easy methods to function the truck. The whole lot adjustments as soon as we coach them on easy methods to run the engine in retarding mode.”

Fleet managers shouldn’t be discouraging drivers from operating engine brakes at excessive rpm. They’re designed to do exactly that. We’ve got heard experiences of fleets handing drivers engine-overspeed warnings and penalties, making them reluctant to function it on the greater rpm the place it’s designed to function.

Roberts says the design standards calls for the brake be capable of run at a lot greater rpm (as much as 3,000 rpm for some functions) for a whole lot of hours underneath full load. They’re examined that means. Operating the engine brake at 1,900 to 2,100 rpm is the place they work the very best, he stresses.

“We design the engine brakes in order that it will not harm itself even underneath a most overspeed situation,” he says. They’ll final perpetually at regular working pace.”

That stated, Jacobs doesn’t make all of the engine brakes underneath right this moment’s hoods. If there’s any doubt in regards to the regular working speeds in your make and mannequin of engine, seek the advice of the proprietor’s guide or name your gross sales rep. To the very best of our data, all engine brakes are designed to run at excessive rpm.

The issue right here stems largely from the usage of low-engine-rpm downsped drivelines. As we illustrated earlier, engine brakes don’t ship optimum retarding energy at 1,200 rpm. Drivers would want to downshift not less than one gear, perhaps two, to get probably the most out of their engine brake.

When it’s arrange appropriately, the truck’s cruise management ought to handle these downshifts based mostly on demand for deceleration when the cruise management is ready and engaged. With out lively cruise management, the truck will simply preserve accelerating on a downhill grade, and drivers will most likely be inclined to make use of the service brakes to sluggish the truck.

Some fleets lock out sure performance on their automated transmissions to encourage drivers to make use of cruise management, however that may backfire if these settings aren’t to the drivers’ liking.

Excessive Energy Density Engine Brakes

If fleets stay reluctant to function their engine brakes on the really useful rpm, there’s one other answer to anemic engine brake efficiency at low engine rpm.

Jacobs developed the Excessive Energy Density Engine Brake particularly for low-rpm engines. Applied sciences comparable to low-rolling-resistance tires and higher tractor and trailer aero packages have decreased the retarding forces that used to assist sluggish the truck. These components, coupled with downspeeding the engine and usually smaller engine displacement, have positioned extra calls for on supplemental retarding know-how.

Whereas HPD engine brakes are advanced, within the easiest phrases, Jacobs makes use of variable valve opening and shutting functionality to supply two compression launch occasions for every full rotation of the engine crankshaft. A typical engine brake supplies only one. This nearly doubles the retarding functionality of the engine at most rpm.

For downsped, low-rpm engines, HPD engine brakes present comparable efficiency to a typical engine brake, however whereas working at a lot decrease rpm. And it eliminates the necessity to downshift two, maybe 3 times to get high efficiency from the engine brake.

Referring as soon as once more to the North American Brake Efficiency Chart, discover the inexperienced dotted line, representing one North American engine geared up with an HPD engine brake. Assuming it’s a 13-liter engine, at 1,400 rpm, it produces about 26 kW/L of retarding pressure, or about 338 kW of energy total. That’s about 460 hp. With a traditional engine brake, that engine (stable inexperienced line) must run at about 2,200 rpm to attain that sort of efficiency.

The engine brake is a useful gizmo for controlling automobile pace and decreasing brake put on. But when drivers are unfamiliar with easy methods to use the engine brake correctly, or if the fleet has locked out sure performance, they may resort to their service brakes. It’s a good suggestion to evaluation your drivers’ understanding of those methods and replace any data gaps.

For those who don’t, they could flip to fellow drivers on YouTube or TikTok for info, and a few of that could be questionable at greatest.