Brake Retarders

Brake retarders, otherwise known as auxiliary braking systems, are essential devices for the modern heavy emergency response vehicle (ERV). They have been proven to be so important that NFPA 1901, Standard for Automotive Fire Apparatus, requires them for any rig with a 36,000-pound gross vehicle weight rating (GVWR) and greater.
Christian P. Koop   Christian P. Koop

They all serve the same basic purpose: to assist the foundation brakes in stopping heavy ERVs and commercial over-the-road rigs. However, there are several different makes and types on the market, and how they are mounted and operate all differ. Impact to the rest of the vehicles’ systems and reliability also differs as well as how much retardation output they can produce. In this article, I will briefly cover the history behind some of the different types of retarders we have used in my department and some of my experiences dealing with repairs and maintenance.

Because retarders provide added stopping power, they will also reduce the possibility of brake fade, which I am sure all will agree is huge when it comes to safety. Reducing or eliminating brake fade is the main reason they were invented. Keep in mind that they do a lot more than this because they not only improve or reduce stopping distances but also extend brake life. Extending brake life means cost savings and less time at the shop. Fewer changeouts (crews transferring their gear from a frontline ERV to a spare) is a good thing for the overall operation, the crews, and the communities they serve. These are two extremely important subjects, particularly in today’s highly litigious environment not to mention tighter budgets. To reiterate and make sure I get my point across, retarders not only improve the safe operation of any ERV equipped with these devices, they will also reduce brake maintenance and replacement costs over the life of the ERV while reducing equipment downtime simultaneously.

There are several different types and makes of retarders on the market today, and they differ in operation; efficiency; cost; installation; and, most importantly, to me as far as the fire service goes, how much they can really slow the vehicle down in brake horsepower (BHP) at low-speed operation or intercity service and how quickly they achieve maximum retardation. They run the gamut-ranging from hydraulic transmission output retarders to electromagnetic driveline retarders and engine retarders.

Electromagnetic Induction Retarder

The first type I will cover is the electromagnetic induction retarder from Telma, which is probably the oldest, having been around for almost 70 years. It was developed by an engineer in Europe who saw the need to add a means of assisting brakes to reduce total brake failure because of brake fade. Heavy trucks would lose their brakes in mountainous regions of Europe, resulting in many lost lives and destruction of equipment and goods. These retarders can be mounted in the driveline hanging from the chassis rails or in a “focal mount,” which bolts directly to the differential input housing. This retarder is totally frictionless, and the energy absorbed by these retarders is released directly to the air surrounding them.

In a nutshell, there are a set of fixed nonrotating electrical coils with two metal stators on either side that rotate with the driveline. As the electrical field generated in the coils is induced into the stator, it serves as a driveline brake and will slow and eventually almost bring the vehicle to a full stop. They generally are programmed to turn off or deactivate at a predetermined speed so they do not remain on when the vehicle is stopped. The units we had in service years back had four stages of retardation that applied progressively in direct relation to air brake application pressure or travel of the brake pedal in hydraulic brake applications.

My first experience with these units goes back almost 30 years. We had a number of apparatus and rescue trucks (aka ambulances) that had severe brake failure problems because of excessive weight. All of the units we retrofitted with the electromagnetic induction retarders were driveline-mounted units with the exception of one pumper that received a focal-mount unit. All of the modified units showed improvement in reducing stopping distances and increasing brake life. However, the most dramatic improvements I saw were in a group of 1987 Ford E-350 Type III rescue trucks equipped with 6.9-liter diesel engines that were 2,000 pounds greater than GVWR. As most probably know, brake life also has a lot to do with how the drivers use and treat their brakes. The worst one of these ambulances, prior to the installation of the Telma, was only getting to be averaging about 3,000 miles out of the front brakes and about 7,000 out of the rear brakes. This is not acceptable when it comes to brake life. After the retrofit, the average brake life on this fleet of ambulances went to 12,000 to 15,000 miles on the front axle and 17,000 to 40,000 miles on the rear axle. Thankfully, we never had a serious accident with those units, and we all breathed a sigh of relief when we finally retired those units.

Prior to installation, we had been warned by detractors that we could have electrical system issues because of the new electrical load the Telma would be imparting during operation. However, we did not have any electrical system issues. When the brakes are applied, the coils of the electromagnetic induction retarders use current when they are being energized. However, it is a momentary surge or spike, and it did not affect the batteries or our charging systems. What I did notice was that driveline U-joint life was cut roughly in half, even though it was being greased according to schedule. This probably had a lot to do with the unit being overweight, and the driveline is not only being loaded during acceleration but also during deceleration when the driveline absorbs the braking loads from the electromagnetic induction retarder.

In the larger trucks equipped with heavy duty U-joints, we did not experience this issue. Because this is an electrical unit, the speed at which it becomes fully engaged is extremely fast-there is no lag time, and braking torque is also very high. This is a very important point for intercity operation or any congested stop-and go-operation for ERVs. All brake retarders, not just the electromagnetic induction retarders, have some type of on/off switch on the dash so the driver can turn them off. This is because there is the possibility of rear-wheel lockup in slippery conditions. The switch also gives the driver the option to turn it off when not needed. The modern electromagnetic induction retarders interface with antilock brake systems (ABS) to prevent wheel lockup in slippery conditions, which is good because even if the driver forgets to disengage the unit, it still won’t come on in an ABS event.

Jacobs Brake

The Jacobs Brake, known as the Jake brake, was also developed by an engineer because he saw the need to reduce or eliminate brake fade. Its inventor, Clessie L. Cummins, inventor of the Cummins diesel engine, got the idea for it in 1931 when he was driving a truck through a mountainous region of California and lost the brakes because of brake fade in a steep downhill grade. Miraculously, he and the two others that were with him were not hurt. They missed hitting a train that crossed the road in front of them by inches. That was also lucky for the industry because if Cummins had been killed, who knows how long it would have taken someone to invent this ingenious device-if ever.

To provide a background on how it works, let’s first look at how a gasoline engine helps to slow a car or truck down when the throttle is closed. The throttle plate in a gas engine closes off incoming air and the cylinders, as the pistons go up and down against the closed throttle plate, create a vacuum pumping action that helps slow the vehicle down. During deceleration in a diesel engine, the opposite actually occurs because there is no throttle plate. The air that is going directly into the cylinders is being compressed, and as it pushes back on the pistons, it actually helps to propel the truck forward and will not slow it down. The Jake brake basically converts the diesel engine into an energy-absorbing air compressor of sorts by opening the exhaust valves at a precise time and letting this air escape out the exhaust before it can direct pressure back down on the piston. Cummins’s invention holds open the engine’s exhaust valves using precise timing through hydraulic (engine oil) actuation that is controlled electrically. I am sure most have heard 18-wheelers making a loud machine-gun like clattering noise from the exhaust when slowing down on the interstates. The noise can be quite loud, and in some municipal areas and cities, their use is not allowed. They can be so loud they have been known to trigger avalanches and are also restricted in certain areas during the winter.

Cummins and Detroit diesels were the first engines that were easily adaptable to this technology because they had a separate lobe on the camshaft to actuate the fuel injector. By modifying this camshaft lobe and designing a hydraulic mechanism that would open the exhaust valve at the right time, Cummins created an engine brake that is relatively light and goes virtually unnoticed in the engine with up to 90 percent or more retardation of the engine’s maximum BHP. We installed several of these Jake brakes in some very heavy rigs equipped with 8V92 Detroit engines with Allison automatic transmissions in the mid to late 1980s with limited success in intercity service. Had they been on-highway rigs, I’m sure the results would have been much better. These units proved to be trouble-free with some minor issues with the governor buffer switch adjustments. There are many types available for a wide range of makes and models, and all have different modes of operation available to the driver.

This brings me to the exhaust type of engine brake, which consists of a butterfly valve or knife-type valve located downstream of the exhaust or at the outlet of the exhaust after the turbo. It closes on deceleration and restricts the gases leaving the engine from combusting. When this happens, the pressure buildup inside the cylinders resists the pistons from going up and basically slows down the vehicle, acting like a brake. There are limits to the pressure allowed to build inside the engine, dictated by the engine manufacturers. If these limits are exceeded, engine damage can result from exhaust valves being lifted off their seats and hitting pistons. However, the units are carefully designed to operate within the engine manufacturers’ specifications. They are available from various manufacturers and for a wide range of engines-from light duty to heavy duty-and modern ones interface with the engine electronic control unit (ECU) for precise operation and with automatic transmissions to downshift into specific gears to add more braking power.

They are very effective and can generally produce up to a maximum of 70 percent of the engine’s maximum BHP in certain applications and are most efficient at highway speeds. These units, along with the Jake brakes, have been associated with engine overheating issues. But, the ones we have tested and used never exhibited these problems. I would venture to say that units which have had overheating issues may have had marginal cooling systems or systems that had not been maintained properly. The exhaust piping and exhaust manifold must not have any leaks for this system to operate properly. These units have been around for almost as long as the Jake brake and are also used in conjunction with the Jake in modern over-the-road trucks to help quiet and muffle the Jake. Maintenance involves servicing the butterfly or knife-type valve on a schedule.

Hydraulic Output Retarders

If memory serves me correctly, the first Allison transmission hydraulic output retarders we used were on pumpers in the early 1990s with the Allison heavy-duty electronic transmission. The retarder housing is mounted at the rear of the transmission and essentially oil (transmission fluid) is directed at a vaned rotating wheel very similar to a transmission torque converter. Only in this case, it works for the opposing reason. As the fluid fills the retarder cavity and hits the vanes, it serves to slow the vehicle down via the driveshaft. The energy absorbed by the oil as it hits the vanes in slowing the vehicle down has to be cooled via a coolant-to-oil cooler, generally mounted near or below the radiator.

These units were very effective at high-speed driveline operation, were not affected by engine speed or transmission gear, and could deliver more than maximum engine BHP at the higher speeds. The early versions we used had clutch packs in the retarder housing that did not enjoy a very long service life with our units. However, Allison redesigned these in their World Transmission series (WT) to correct this. Another issue we ran into was short service life of the oil coolers. Part of the reason for cooler failure, I believe, had to do with thermal shock as transmission fluid temperatures could easily go from 200°F to greater than 300°F and quickly back to 200°F during use. If the transmission oil cooler leaked internally, as they were prone to do, engine coolant would contaminate the transmission fluid, causing eventual clutch pack failure. This is because ethylene glycol can dissolve the clutch pack lining material.

Several companies manufactured the early coolers, and some had defects that have since been improved on. However, service life is still not optimal when compared with most other cooling components. Because of this, it is recommended that the coolers be changed at a specified schedule so as to not chance a cooler failure that may result in transmission failure requiring an expensive overhaul. As with all retarders, they have an on/off switch on the dash for the driver that should be turned off in slippery conditions.

These retarders can be configured to apply in several ways. They can be applied at different percentages of application output-either from closing the throttle, brake pedal application, or a combination of the two. For example, you can have it apply or come on at 50 percent when the throttle closes and the remaining 50 percent when the brake is applied. It can also be configured to progressively apply in three steps when the air brakes are applied: Two-psi switch equals 30 percent, seven-psi switch equals 33 percent, and 10-psi switch equals 100 percent retarder engagement. These were only examples as there are more options available. With this type of retarder, the speed to fully apply maximum retardation is related to how fast the retarder cavity is filled with oil and is proportional to driveline or vehicle speed.

ERVs require brake retarders primarily for the sake of safety. I think it is important to understand the theory of how the different retarders operate to understand which type will work best for your budget, operation, traffic conditions, and fleet. Keep in mind that you can use them in a wide ranges of vehicle types-not just the heavy ones-to reap the same benefits. Over the long haul, they will easily return the investment many times over-not to mention risk reduction exposure connected to brake failure.

CHRISTIAN P. KOOP is the fleet manager for the Miami-Dade (FL) Fire Department. He has been involved in the repair and maintenance of autos, heavy equipment, and emergency response vehicles for the past 35 years. He has an associate degree from Central Texas College and a bachelor’s degree in public administration from Barry University and has taken course work in basic and digital electronics. He is an ASE-certified master auto/heavy truck technician and master EVT apparatus and ambulance technician. He is a member of the board of directors of EVTCC and FAEVT and a technical committee member for NFPA 1071, Standard for Emergency Vehicle Technician Professional Qualifications.

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