SECONDARY BRAKING SYSTEMS: A SECOND CHANCE

SECONDARY BRAKING SYSTEMS: A SECOND CHANCE

BY WILLIAM C. PETERS

The members of Squad 4 were having a particularly busy day, responding to alarms from one end of the city to the other. With the unrelenting summer heat, drivers were cruising around with windows closed and air conditioners on high, seeking relief. The apparatus operator remarked to the captain that it seemed as if no one was paying any attention to their warning lights and siren. He had to continuously accelerate and brake to maneuver around the complacent drivers.

Since the squad`s response to a minor fire was canceled, the alarm center dispatched it to a confirmed “working fire” in another part of the city. The plume of smoke off in the distance indicated squad members would be going to work!

As the operator threaded the heavy rig through traffic, he began to notice that the brake`s reaction to his foot pressure seemed different; he had to press harder to slow the apparatus down. A quick check of the gauges showed a steady 120 psi of air pressure, and he began to think that perhaps it was fatigue he was experiencing.

The final approach to the fire scene involved descending a long steep grade. About halfway down the hill, the operator was “standing” on the brakes, and the apparatus still was not stopping. He looked at the captain and said, “Hang on!”

* * *

SECONDARY BRAKING DEVICES

The upcoming (1996) edition of NFPA 1901, Pumper Fire Apparatus, if approved by the organization, will recommend a secondary braking device on apparatus weighing more than 31,000 pounds and require it on units exceeding 36,000 pounds.

Secondary braking devices in the forms of transmission retarders, driveline retarders, and engine brakes, when activated by the driver, apply a retarding force to the vehicle`s drive wheels without the use of friction. These auxiliary braking devices increase safety as well as economy when used properly.

The foundation or friction brakes on an apparatus were designed to fulfill two main requirements–to maintain the vehicle in a stationary position for indefinite periods of time when the parking brake is applied and to bring the apparatus to a standstill in the shortest possible distance without loss of control such as is experienced in a panic stop condition.

The braking duty cycle of fire apparatus on a response is such that the vehicle is accelerated and braked in rapid succession. The friction involved in braking causes heat to rapidly build up in the brake block and drums. This excessive heat can cause a major loss of efficiency in the braking system by affecting the friction surface of the brakes and expanding the drums. Some estimate that brakes that heat to over 520 degrees lose 60 percent of their efficiency. This, of course, increases stopping distance dramatically. On a long downgrade, this results in brake fade. Disc brakes are somewhat less prone to fading, as they dissipate heat better than drum brakes.

Some secondary braking devices provide up to 85 percent of the vehicle`s deceleration prior to the service brake engagement. By keeping the friction brakes cool, they are ready to respond to panic stop conditions, and stopping distances are greatly reduced.

An additional benefit is the reduction of downtime for brake maintenance. Many high-use emergency vehicles require frequent brake lining changes due to premature wear. Using auxiliary braking devices increases lining life by decreasing brake temperatures. The savings on brake maintenance can provide a substantial return on an investment in an engine brake or retarder.

TYPES OF SECONDARY BRAKING SYSTEMS

Secondary braking devices basically fall into three categories: the engine brake, electronic driveline retarder, and automatic transmission retarder. Although each operates differently, the result is the same–a reduction in stopping distance.

All of the auxiliary braking devices interface and are compatible with the antilock braking system (ABS). When wheel lockup is detected, the ABS automatically disconnects the auxiliary brake device and restores it when the situation is under control. A word of caution: Each system has specific operating instructions that apparatus operators should read and understand. Some of these instructions are particularly important on slippery or wet road surfaces.

Engine brake. The Jacobs engine brake, or “jake brake” as it is commonly called, has been in service on most models of commercial trucks and fire apparatus for many years. The jake brake is a hydraulic engine attachment that converts the diesel engine into a power-absorbing air compressor, which in turn provides a retarding effect to the drive wheels. The jake brake can be installed on two- and four-cycle diesel engines.

To understand how the engine brake provides retarding power, let`s compare each cycle of a four-cycle engine, with and without an engine brake.

Stroke 1, intake. Both with and without the engine brake, the intake valve opens and air is pulled into the cylinder.

Stroke 2, compression. Without the engine brake, air is compressed, heat rises, fuel is injected, and combustion occurs, resulting in a rise in pressure. With the engine brake, as the air is compressed and the piston reaches near the top of its travel, the engine brake opens the exhaust valve and the compressed air is released through the exhaust system. No combustion occurs, since the engine brake operates only during a “no-fuel” situation.

Stroke 3, power. Without the engine brake, the high pressure resulting from the firing of the air-fuel mixture drives the piston downward, imparting power to the drive-train. With the engine brake, no power is produced. The energy required to return the piston to the bottom of the cylinder is now derived from the momentum of the vehicle. This two-step process–the elimination of the compressed air in stroke 2 and the use of the vehicle momentum in stroke 3–develops the engine brake`s retarding capabilities.

Stroke 4, exhaust. With and without the engine brake, the piston is in an upward motion pushing out air or exhaust.

The Jacobs engine brake is activated by an electrical dash-mounted switch. On some applications, a multiposition switch is used to provide variable retarding capabilities. In most automatic transmission applications, when the accelerator is released and a no-fuel condition is present, the engine brake engages.

Electrical driveline retarders. The Telma electrical driveline retarder operates on a completely different principle but obtains the same results. Rather than using engine power, it uses a powerful electromagnetic field to slow down and retard the turning of the driveline. The unit is made up of a circular stator that surrounds two rotors and, depending on the application, is securely mounted to the vehicle chassis or rear axle housing. Several electrical coils of alternating polarity are located in the stator.

The vehicle driveshaft is cut, balanced, and attached to the input side of the two rotors. When the retarder is activated, a current flows through the stator coils and a magnetic field is created. This field passes through the rotors, which are spinning inside the stator. The field produces eddy currents in the rotors, which oppose rotor rotation and slow the driveline. The rotors have specially engineered vanes to dissipate heat generated during braking; however, understand that no friction surfaces are involved.

In most applications, the brake pedal controls the retarder. The driver automatically uses the retarder whenever the brake pedal is depressed. Retardation is applied in four progressive levels, and all four are automatically activated in sequence before the vehicle brakes make contact with the drum. An automatic cutoff switch deactivates the retarder as soon as the stop is completed.

The electrical retarder can use up to 200 amps of current in the stopping process; however, this discharge is for a short duration only. Another benefit of the electrical driveline retarder is that it is basically maintenance-free.

Automatic transmission retarders. Integrated into the vehicle`s automatic transmission, transmission retarders have been shown to increase brake life by as much as three times the normal service period, depending on the vehicle and application.

Two types of transmission retarders are the input and output (the latter is more prevalent). The input version operates at the input section of the transmission, between the torque converter housing and the main housing. It is particularly effective in work cycles where downhill speed control is required. An output retarder is mounted at the tail-shaft (output shaft) of the transmission and may be modulated by the vehicle operator.

Typical automatic transmission retarders are made up of a combination of an oil-filled rotor/stator chamber and a clutch pack. The rotor blades are attached to, and turned by, the transmission input/output shaft. When the retarder is activated, fluid enters the cavity and provides resistance to the turning of the rotor blades. This effectively slows the vehicle to a point where the service brakes are needed only for final stopping.

Transmission retarders are operated by air pressure or electrical control activated by the application of the brake pedal, the release of the accelerator, or a separate control lever, depending on the preferred installation. Some purchasers choose to have a percentage of the retarder activate when the accelerator is released and the remainder when the brake pedal is depressed.

Additional transmission fluid-cooling equipment might be necessary when an automatic transmission retarder is specified.

Safely bringing heavy apparatus to a stop places a greater burden on the brakes. A secondary brake system not only will reduce costly brake maintenance but also can give you a second chance when confronted with a panic stop situation. n

WILLIAM C. PETERS is battalion chief, supervisor of apparatus and equipment, and a 20-year veteran of the Jersey City (NJ) Fire Department. He is a member of the International Association of Fire Chiefs Apparatus Maintenance Section, Local 1064 of the International Association of Fire Fighters, and the Fire Engineering editorial advisory board. Peters is the author of Fire Apparatus Purchasing Handbook (Fire Engineering Books, 1994), the booklet Final Farewell to a Fallen Firefighter: A Basic Fire Department Funeral Protocol, and two apparatus chapters in The Fire Chief`s Handbook, Fifth Edition (Fire Engineering Books, 1995).

No posts to display