Auxiliary Braking: Picking the Best of All Goods

So many times in life we are forced to pick the “least of all evils.” So, it is nice when we at the Fire Apparatus Manufacturers’ Association (FAMA) can discuss a topic where you can choose the best from a list of “all goods.” This is the case when it comes to selecting an auxiliary braking system for your next fire apparatus.

National Fire Protection Association (NFPA) 1901, Standard for Automotive Fire Apparatus, requires that any apparatus with a greater than 33,000-pound gross vehicle weight rating (GVWR) must be equipped with an auxiliary braking system. This means a system that assists in stopping the apparatus above and beyond the action of the service brakes located at the wheel ends. Auxiliary braking can be accomplished through the action of the engine, the transmission, or through other means acting on the apparatus drivetrain

Modern fire apparatus, like every other heavy on-road vehicle, are equipped with a service braking system that uses friction to slow the vehicle. This friction comes from physical contact between a consumable material (brake pads or shoes) and a mechanical device (brake drums or rotors). The contact force is created by either hydraulic force (smaller vehicles) or by compressed air (heavy vehicles). The air pressure in heavy vehicles is produced by a compressor driven by the engine.

The downside of friction braking systems is that they create heat that must be dissipated. The heavier the vehicle and the faster the stop, the more heat is created. In heavy braking situations this heat can cause the brakes to fade, or lose their braking power. This can occur if the brakes are used frequently, or consistently down long grades. In extreme braking situations, the components can get hot enough to damage the brakes.

Auxiliary braking systems supplement the service brakes, increasing the stopping power and reducing the likelihood that the service brakes will overheat. The NFPA Apparatus Committee felt strongly that this capability was important in fire apparatus, which are more likely to brake hard and brake frequently. This is the reason it began requiring auxiliary braking systems on large fire apparatus. In addition to these safety benefits, appropriate use of an auxiliary braking system will extend the life of the service brakes, reducing cost and frequency of maintenance.

Common Systems
Commercially available auxiliary braking systems all use some method other than mechanical friction to help slow a vehicle. They also all work by creating a braking force on the vehicle driveline, which in turn transfers force to the tires and then to the road. They all generate heat, but they all control the dissipation of that heat. There are four categories of auxiliary braking systems currently available on fire apparatus:
• Electromagnetic Retarders.
• Exhaust Brakes.
• Engine Compression Brakes.
• Transmission Retarders.

Electromagnetic Retarders
Electromagnetic retarders create their stopping power using the force that can be generated by a magnetic field. You can experience this force if you bring the north or south poles of two magnets together. In vehicle applications, the magnetic fields are created by electricity flowing through a set of coils. The initial electric power to generate the field comes from the vehicle batteries and is replaced by output from the engine alternator. Additional electric power is generated through the rotating action of the system.

Electromagnetic Retarders provide maximum power at higher driveline speeds. As the vehicle speed drops, so does the generated magnetic field. Retarders are generally sized based on overall gross vehicle weight and universal joint capability. They can be sized to provide more retarding than the vehicle’s engine can produce.

Exhaust Brake
Every internal combustion engine takes in air through the air intake and gets rid of it through the tailpipe. If you were to stop up the tailpipe so that no exhaust can escape the backpressure would build up until the engine stopped rotating. This is the affect that the exhaust brake uses to provide braking power.

In modern engines, this back pressure can be created by the variable geometry turbocharger (VGT). A shroud moves within the turbine housing reducing the exit area for exhaust gas. This restriction results in a higher exhaust manifold pressure. Braking force is highest at high engine rpms, decreasing as the engine speed slows.

Compression Brake
An engine compression brake uses a mechanism to open the exhaust valves near the end of the compression stroke of a diesel engine. Exhausting the compressed air from the cylinder takes energy away from the piston as it returns to bottom dead center. Some systems also open a companion cylinder’s exhaust valve during the intake stroke to allow more air into the companion cylinder’s compression stroke. Because there is more starting pressure, it takes more energy to compress the air in the companion cylinder.

Like the exhaust brake, the amount of braking available depends on engine speed (higher rpm results in greater braking force). The maximum braking force a particular engine can produce depends on factors such as the compression ratio and engine displacement. Some engines can create as much braking horsepower as they produce for forward motion.

Transmission Retarder
Where the service brakes use mechanical friction to stop a vehicle, the transmission retarder uses fluid friction. Inside the retarder housing there is a vaned flywheel that rotates at the speed of the transmission output shaft. In normal operation, this flywheel is spinning in a cavity with air around it. This spinning creates minimal drag on the driveline. When auxiliary braking is called for, the cavity is filled with transmission fluid. This fluid is churned by the vanes in the flywheel creating drag. The more fluid in the cavity, the greater the braking force.

This churning action slows the vehicle, but it also heats up the transmission fluid. Chassis designers must take this extra heat into account when designing the cooling system. The transmission cooler capacity is increased, and the rest of the system is designed to accommodate the heat.

Control Methods
While applying the service brakes is a straightforward push on the brake pedal, auxiliary brake control offers many more possibilities. Not all possible control methods are available with all auxiliary brake types, but here are some of the potential methods employed:
• Full on when the accelerator pedal is released
• Full on when the brake pedal is depressed
• On when the brake pedal is depressed, with progressively increasing braking as the pedal is depressed farther.
• Separate hand lever

System Comparison
There are many considerations when selecting an auxiliary vehicle braking system. Each method has its own benefits, disadvantages, and costs. The FAMA Technical Committee has published a complete guide to the selection of auxiliary braking systems that can be downloaded for free at The good news is that FAMA member companies have been installing all of these systems for decades and the technology is mature. Whatever your choice in auxiliary braking systems, you really are picking from the “best of all goods.”

FAMA is committed to the manufacture and sale of safe, efficient emergency response vehicles and equipment. FAMA urges fire departments to evaluate the full range of safety features offered by its member companies.