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AUXILIARY BRAKING

Posted: August 1, 2014

There just aren’t that many competitions where the winner is the guy who stops the fastest. But in the biggest contest of all – survival – the winner is always the guy who stops somewhere just out of harm’s way.

That business of stopping has been made tougher in recent years because so much natural retardation power has been lost by way of aerodynamic trucks and low-rolling-resistance tires. The load on your foundation brakes has increased, but brakes that work too hard get hot, and hot brakes don’t work well for long. So nowadays they need all the help they can get.

Retarders and auxiliary brakes give the foundation brakes a break so that they’ll perform at peak efficiency when you really need them. And some of them are mighty effective. Today’s big diesels provide plenty of go power, but they pack plenty of whoa power in the bargain as well.

The most common auxiliary braking system on heavy trucks today is the compression brake, but by no means does the choice end there. Buyers have the option of spec’ing an electromagnetic driveline retarder, an exhaust brake, or a hydraulic transmission brake. We’ll look at each one here.

Compression Brakes
Clessie Cummins, the founder of Cummins Engine Company, conceived the idea of a diesel-engine compression brake after a near-fatal ride down a mountain pass in 1934. The idea became reality in 1960 with the introduction of the first Jake Brake-brand engine retarder.

The compression brake, in its simplest form, converts an energy-producing diesel engine into an energy-absorbing air compressor. Several factors influence its effectiveness, including engine displacement, compression ratio and the timing of the exhaust valve opening. The power required to compress the air in the cylinder is created by the momentum of the vehicle and transmitted by the driveline to the engine. Large diesels have a compression ratio of around 15:1, meaning that the volume of air within the cylinder is compressed to 1/15 of its normal volume, or to about 500 pounds of pressure per square inch. The act of compressing the air is what provides resistance to the wheels, but the trick to the compression brake is that the energy created during the compression stroke of the piston is dissipated before it can be transmitted back to the wheels.

Using hydraulic pressure, the exhaust valve is opened at what would normally be the top of the piston’s compression stroke. The timing of the valve-opening event is critical to the performance of the compression brake. Ideally, the exhaust valve should open at precisely the same time the piston reaches the top of its stroke, or top dead centre. This provides maximum compression of the air within the cylinder while robbing the piston of energy on its downward stroke. The sudden release of the compressed air is what gives the compression brake its distinctive barking sound.

Compression brakes provide their optimum retarding power at, or near, the engine’s rated speed. With today’s low-rpm engines, it’s often necessary to drop a gear or even two to obtain the expected performance from the brake.

Exhaust Brakes
The exhaust brake is also an auxiliary braking device that increases the braking of the vehicle. The principal is to capture the exhaust pressure and hold it in the cylinders, which creates resistance against the pistons. In a manner similar to the compression brake, the energy required to force the pistons to compress the additional volume of air/exhaust gas within the cylinder provides resistance to the drive wheels through the driveline.

The exhaust brake utilizes a butterfly valve that is usually mounted directly to the outlet of the turbocharger. This valve can be closed partially or completely in order to increase the exhaust back pressure.

Driveline Retarders
The electromagnetic retarders function exactly like an electric motor, but in reverse. A frame-mounted assembly containing a huge winding or coil of copper wire, called a stator, is installed. It actually replaces the center driveshaft support bearing, around the two ends of the driveshaft at that point. Both of the driveshaft ends are fitted with large, cast-iron rotors which act as the target for the magnetic current, or flux, created by the copper winding. When the coils are energized, they produce magnetic fields with alternate polarities. Magnetic flux flows through the coils and discs, creating eddy currents. In turn, these currents create a drag on the rotors, which in turn provide resistance to the drive wheels.

These driveline brakes contain no moving parts and don’t rely on friction. That means there is never any brake fade. The rotors get hot because the momentum of the vehicle works against the resistance created by the alternating electrical current. The braking energy is converted into heat and dissipated through the air-cooled rotors.

Hydraulic Retarders
Still available in a limited number of applications, hydraulic retarders function in a manner similar to the torque converters on automatic transmissions. Caterpillar’s Brake Saver retarder uses engine oil, forced through a diminishing clearance between adjustable vanes in the of the ‘torque converter’ unit to supply resistance to the driveline. In Europe, Volvo uses a transmission-mounted hydraulic retarder and is looking at bringing it to North America. No firm plan yet.

Ups and Downs
There few, if any, downsides to spec’ing an auxiliary retarding device. In severe applications, many operators will use two types of auxiliary brake to manage the task of keeping the vehicle under control.

Loggers and heavy haulers operating in the Rockies often use the electromagnetic brakes in conjunction with compression brakes. With that kind of retarding power they seldom have to touch the service brakes., which is a fantastic safety margin. The electromagnetic brakes remain fairly unpopular here, mainly because of their weight. The larger units, which can develop up to 750 braking horsepower, weigh 950 lb or more. Here, that means a loss of payload. In Europe however, regulators grant an allowance to the gross vehicle weight equal to the weight of the retarder. They recognize the safety value of the units and see no reason to penalize the operator for taking the extra precaution.

Volvo Trucks makes use of two retarding technologies on its VE D12 engines. They use a compression brake of their own design in conjunction with an exhaust brake. It’s a two-stage system, the first being just the exhaust brake. Flicking the switch to stage-two engages the compression brake as well. And it’s a pretty effective combination.

All Bark, No Bite
Over the years, the compression brake has suffered its share of criticism because of a perceived lack of performance on some engine models. Scott Fowler, vice president of market development for Jacobs Vehicle Systems, says that the timing of the exhaust valve-opening event is critical to the retarding power of the system.

“In the past, we had to rely on the exhaust-valve cam lobe to time the event, but the shape of that cam lobe was all wrong for what we were doing,” he says. “Today, with the move to direct injection, there’s a third lobe on the cam shaft, the injector cam, which is the ideal shape to time our event.”

The shape of the injector cam causes the valve to open completely, very quickly and for a brief moment, as opposed to the exhaust cam which is designed to open keep the valve open during the duration of the exhaust stroke.

Then there’s Cummins’ Intebrake, an integral design (by Jacobs and Cummins jointly) complete with a second camshaft with two cam lobes, one dedicated to the injection event, the other to retarder timing. This provides near perfect timing for the operation of the compression brake. John Galbraith, Cummins’ program leader for 15-litre development, says the new ISX series engines can produce whopping great amounts of retarding power. That’s partly a factor of displacement – the larger the bore, the more air can be compressed.

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