Retarders
'Brakes' on engines, exhaust systems or drivelines can boost safety and cut wear on the rig's service brakes.
ED THOMAS
CONTRIBUTING EDITOR
There are times to go and times to stop. To make it happen, you have the powertrain and the brakes. There are also occasions to simply slow down or maintain a steady speed on a downgrade. This is the function of the retarder, a device increasingly important to trucking.
In the old days, keeping things under control while drifting down hill wasn't easy. Loads were sometimes as heavy as today, but brakes not nearly as strong, and grades on truck routes were steeper than now. The existence of runaway ramps alongside mountain downgrades is testimony of how dangerous this part of driving was, and still is, if drivers don't know what they're doing.
The idea for retarders originated almost 50 years ago with Clessie Cummins, the diesel pioneer. He had retired from Cummins Engine in Indiana and moved to the West Coast. There he became more aware of the need to allow big rigs to go downhill as efficiently as uphill. His answer was the engine compression brake, and he took it to his old company — but was rebuffed. Executives saw no need for it.
But he found a taker for his patented device at, of all companies, Jacobs Manufacturing, which was known for the Jacobs chucks used to secure bits in power drills. Through a nephew who was marrying into the Jacobs family, Clessie met Jacobs' president; he saw the promise of Clessie's invention and agreed to build and market it. Thus was born the Jacobs Engine Brake.
The device has become so popular that its nickname, "Jake Brake," is what most truckers call any engine retarding device, even if it was made by another company. The Jacobs division which still makes the product repeatedly reminds people in the business that "Jake Brake" is a trademarked name and should be used only when referring to the Jacobs Engine Brake. They've even had to warn municipalities against posting "No Jake Brake" signs; "No Engine Brake" is how the signs have to read.
Mack's Dynatard and Cummins' C Brake (which Clessie's namesake put on the market after the Jacobs patents ran out) were once competitors to Jacobs, and caused price cuts in this market. But both now use only Jake Brakes for their diesels. Volvo offers a combination engine and exhaust brake on its own diesels. And Pacbrake makes models for several diesels. There are other types of retarders, too, and we'll describe them below.
How They Help
Retarders are usually equated with safety because they allow a rig to descend a grade with few or no applications of the vehicle's service brakes. The service brakes stay cool and ready for emergencies. Retarders give drivers greater confidence, so are especially popular in mountainous and hilly regions.
Retarders also save wear on brake linings and drums, making them last longer. This saves money in parts and labor. A federally sponsored study in the mid '80s concluded that retarders more than pay for themselves because faster but still-safe downhill speeds cut trip time, and fewer accidents save high out-of-pocket costs and can merit the carrier lower insurance premiums.
Engine retarders may be harder to justify in flatland operations. And they can cause a jackknife if used on slippery pavement. Drivers need to know how to operate them, and to avoid becoming bold. The rig still weighs a lot, so vigilance is always recommended.
How They Work
Downshifting and letting the engine's compression do some of the braking work on downgrades is an old driving technique. But the great weight of a heavily loaded truck overcomes any compression forces. Also, some engines get a bit of fuel even under throttle-off conditions, sometimes resulting in no compression braking at all. Clessie Cummins knew this, and devised a means of enhancing compression by altering the diesel's valve action.
Picture the engine's compression stroke (one of four in the four-cycle system): As the piston travels upward, both exhaust and intake valves are closed, allowing the piston to compress the air that's been forced into the cylinder. At the top of the stroke, fuel is injected into the super-heated air and burns quickly, pushing against the piston as it goes back down the cylinder. After the combustion is complete and the piston begins climbing again, the exhaust valve opens to allow the spent gasses out.
If the throttle is at idle and little or no fuel is injected, the piston will continue its reciprocating motion, dropping down again to create a suction and assist with the air intake process. Then it begins its upward compression stroke again. But if the valve is popped open just as the compression stroke is completed — as the piston has just passed top center — the compressed air escapes. More air enters through the intake valve, and the piston must again work to compress it.
Thus the engine becomes an air compressor, and energy is needed to continue its new "work." That energy is supplied by gravity; it pulls the vehicle down the hill, and its momentum pushes against the wheels and drivetrain and into the engine's crankshaft, and to the connecting rods and pistons.
The escaping of the compressed air creates the popping noise that's characteristic of a compression brake. Because today's engines are turbocharged and more effectively muffled, noise from engine brakes isn't what it once was. But they'll still pop on a loud exhaust system. The sometimes loud backfire-like noise causes many municipalities to ban their use, and more "no engine brake" signs seem to be sprouting outside many towns, especially in the West.
Bolting On Hardware
In the predominant inline six-cylinder diesel, the compression brake housing is bolted atop the cylinder head and its mechanism connected to the engine's valvetrain. Engine oil is the actuating force that engages or disengages the valve action-altering function. Cummins' Signature and ISX engines have two overhead camshafts, one with cam lobes dedicated to the engine brake. This integrated brake claims more retarding power than previous designs, and can itself slow a heavy rig on a steep descent.
Some engines have two-piece heads, with each covering three cylinders, and others three-piece heads, two cylinders per. The compression brake therefore can be made to operate the valves on one, two or three of the heads, and therefore two, four or six cylinders with a three-piece head, or either three or six cylinders with a two-piece head.
The power level can be picked by the driver. He flips one toggle switch to turn the retarder on, then uses another switch to choose between "2-cyl," "4-cyl" or "6-cyl," or between "high" and "low," and gets more or less retarding force. Once on, the retarder will operate every time he takes his foot off the accelerator. It will cut out when rpm's drop toward idle. An exception is a Cat engine; with it the driver has to apply the service brakes to activate a Jake Brake.
Any retarder's output is measured in retarding horsepower. It's possible for a diesel's retarding horsepower to equal its propulsion output, but generally only at higher rpms. As you'd imagine, more power is produced as the engine speeds up and works harder to compress air.
Conversely, retarding force decreases as the engine slows. So drivers will try to choose a transmission gear that keeps revs close to or at the engine's redline for the desired road speed. With the advent of lower-rpm diesels, manufacturers have had to refine their compression brakes to maintain sufficient output.
The Other Types
While the engine compression brake is the principal type used in heavy duty diesels, there are several others:
• Engine-hydraulic — The only one is Caterpillar's BrakeSaver, which bolts onto the rear of the engine ahead of the bell housing. Inside, motor oil is churned between a rotor and stator, creating braking effect. It is less dependent on engine speed than the compression brake, but is not as strong. The BrakeSaver is operated manually with a handle or automatically by releasing the accelerator.
• Exhaust brake — Imposes back pressure in the exhaust system by closing a valve at some point on the exhaust pipe. The valve is actuated by a solenoid or by air on vehicles with an air system. These will vary in power, but those on big-bore engines can be surprisingly effective, especially at high rpm's.
Exhaust brakes can be hung on or near the turbocharger, or downstream a ways. If on the engine, they can work with glow plugs to help speed the engine's warm-up process on cold mornings.
Models for big-bore diesels are offered by Blue Ox and PacBrake, and one Pacbrake model is a combination exhaust and engine brake. Volvo includes a combination exhaust-and-engine brake on its 12-liter Swedish-built diesels sold in Volvo trucks. Several types are available for midrange diesels and are popular on motor homes. Exhaust brakes are standard on some imported midrange diesel trucks.
• Driveline retarder — Installed on the driveshaft between the transmission and rear axle. A hydraulic type absorbs the truck's momentum energy by pumping oil, while the electromagnetic type sets up a magnetic field that grabs at the driveline. Absorbed energy is transferred to the surrounding atmosphere as heat. Switches usually control actuation on modern exhaust brakes and driveline retarders.
No compression brakes are made for midrange diesels, so exhaust brakes and driveline retarders are it for medium-duty trucks. These devices are likewise the only viable possibility for heavy vehicles running at low speeds in stop-and-start service, such as city buses and trash trucks.
Voith, for example, markets its hydraulic retarder to refuse truck operators. Telma has offered an electric retarder, and one of its advantages is that it can also be applied to axles on trailers. But its comparatively high weight has kept it from wide success.
In Europe, driveline retarders have been more popular; some countries require heavy trucks to use some kind of retarder for safety's sake.
Extra Retarding Power
Trucks running in severe service applications — heavy haulers operating in mountainous terrain, for instance — sometimes are equipped with two retarders. Both compression and exhaust brakes have been fitted to some engines — Pacbrake has one model that is actually a compression and turbo-hung exhaust brake working together — or both a BrakeSaver and a Jake Brake on Cat diesels.
There's a limit, of course, and it's partly in what the engine will take in cylinder pressure. A combined compression and exhaust brake will build substantial pressure in cylinders, so the engine maker must approve any installation.
Drive wheel wear is another consideration. The greater the power of a retarding system, the faster the tread on drive tires will wear. Shouldn't the U-joints and axle gears also get some "reverse" wear? Yes, but studies indicate it's almost unmeasurable.
