"No Engine Brake." Have you ever wondered what this sign means when you pass it on the highway?
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A truck's engine brake is also known as a compression brake system. Sometimes it's called an engine retarding device. Retarders are especially helpful for truckers on mountain downgrades.
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But how do retarders work? The compression stroke is one part of an engine's four-cycle system. The intake and exhaust valves are closed as the piston moves up, compressing the air being forced into the brake cylinder. At the top of the stroke, fuel enters the very hot air, burning rapidly and pushing against the piston, which is on its way back down. When the piston begins its ascent, the exhaust valve opens to emit the spent gasses.
Suppose that the valve is popped open when the piston is just past the top center, as the compression stroke is ending. The compressed air is allowed to escape. Then the intake valve lets in more air, and the piston begins to compress it. At that point, the engine becomes an air compression device, with gravity providing the energy needed to run it [source: Newbie Driver].
As the compressed air escapes, the truck makes a backfiring or popping sound. Although more modern systems make less noise, the perception remains that a truck should rely on the air brake system.
Now, let's roll on to take a closer look at how truck and automobile brakes differ.
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Let's begin by considering the ways that truck and car brakes are alike. The purpose of brakes on all types of vehicles is to stop them. Brakes on both trucks and cars work on the principle of friction. Both kinds of vehicles have brake drums, along with their pads and shoes, connected to the vehicles' wheel axles.
Car brakes rely on the brake fluid that flows through the system to work properly. Thus, car brakes are a hydraulic system, relying on fluid. On the other hand, truck brakes depend on compressed air. (Trains and buses also use this type of brake system.)
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A major plus to using air is that it never runs out (as brake fluid can). This means that the air brake system is very reliable -- even if there's a small leak somewhere in the system, it always works.
Most of the newer heavy trucks use a dual air brake system that is not available on automobiles. A single set of brake controls works both of these separate air brake systems. If one system fails, the other will work.
The flaw in the compressed air system of trucks is brake lag. That's the time it takes for air to get through the lines and force the linings to contact the drum. When they push the brake pedal, drivers must get used to the fact that air brakes don't work at once, as they do on a car. Lag time is less than a second, so this is not a major problem.
The truck's air braking system has several tasks. First, it keeps up a steady supply of compressed air. In addition, it must direct that air's flow. Finally, it uses the energy of air pressure and changes it into mechanical force.
One, two, three! A truck's air brake system is really made up of three different brake systems. Let's investigate each of them.
Benefits of Retarders and It's a Westinghouse!Using retarders benefits drivers in two major ways: safety and expense. Being able to go down a steep grade without applying the air brake system means that the air brakes remain cool and ready if needed. That translates into greater safety when stopping. Less usage on mountains means that brake drums and liners don't wear out as quickly, thus saving money in labor and parts.
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You may know the name Westinghouse as an appliance brand. However, the prolific inventor George Westinghouse, Jr., also designed the first practical air brake for railroads. After working in his father's machine shop and serving in the Civil War, he patented the system in . He also patented several other safety devices to improve the railroad system. The Westinghouse Air Brake Company was the first of 60 companies that the creative entrepreneur founded [source: Westinghouse].
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Service brakes are used during normal driving. A sequence of events occurs when a driver pushes the brake pedal.
The driver activates the parking brakes by pulling out one or both of the valves on the dash. (Tractor brakes have a yellow button; trailer brakes have a red button.) The dash button releases the spring inside the brake chamber, thus beginning steps 3-6 listed above.
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The emergency brake system uses parts of the other two systems to stop the truck if brake failure takes place [source: Newbie Driver].
Inside the brake chamber is a powerful spring with about 2,500 pounds of pressure behind it. That spring is held back by a steady and constant airflow in the chamber. The emergency brakes deploy automatically if there's not enough air in the system to hold the spring back. If air pressure drops below 60 pounds per square inch, the low-pressure light comes on. A buzzer may also sound.
Not everyone is happy with the way that S-Cam brakes work. A recent innovation is the use of disc brakes. Let's head down the highway to see how disc brakes work on trucks.
Beware of Brake Fade!When the brake drum becomes overheated, it expands away from the brake shoes. Then the drum and shoes don't make complete contact. The other brakes must work harder to compensate. If the situation continues too long, these systems will also overheat and fade, leading to brake failure. Hot brakes may also smoke or catch fire. Any driver who senses the need for increasing pressure on the brake pedal needs to have the brakes checked for brake fade.
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Some trucks have disc brakes instead of S-Cam brakes. Air pressure works on the brake chamber and the slack adjuster in the same way that it does in S-Cam brakes. However, a power screw replaces the S-Cam. The power screw is turned by the pressure on the brake chamber and the slack adjuster. Then the power screw grasps the disc or rotor between a caliper's brake lining pads.
The many benefits of air disc brakes include the following:
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Still, despite all of the advantages, change isn't always embraced by the masses. In the United States, no regulation demands the use of air disc brakes, which are viewed as a high-end item because they're about twice the cost of drum brakes. Currently, about 95 percent of the U.S. heavy-duty truck market relies on drum brakes. In Europe, more than 80 percent of commercial trucks already use air disc brakes. The complete shift to air disc brakes may require 10 to 15 years.
They say that a picture is worth a thousand words. Let's cruise to the next page to get a glimpse of a diagram of a truck's braking system.
Testing, TestingThe National Highway Traffic Safety Administration (NHTSA) of the U.S. Department of Transportation believes that using air brakes will result in fewer collisions in emergency situations than using drum brakes. The NHTSA states that reducing the minimum stopping distance required by 30 percent would result in a significant number of lives saved. These conclusions were formed following simulator tests conducted at the University of Iowa [source: National Transportation Safety Board].
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