In simple terms, when you push the gas pedal down in a gasoline powered vehicle, you open the throttle or intake valve and increase your speed. When you release the gas pedal, close the intake valve or throttle back, you slow down [source: Nice]. When you go downhill, you let off the gas and press the brake. If the hill is steep, you might even shift into a lower gear to help you slow down and avoid riding the brakes.
In semitrailers, letting off the gas doesn't slow the rig the same way it slows a passenger vehicle. Using only the brakes to slow a semi on a long, steep, downhill grade may not provide enough stopping power, even when the driver shifts into a lower gear. You're going need a lot more power since a semi typically weighs 80,000 pounds (36,287.4 kg) [source: The Trucker's Report].
In a semi engine, air enters the intake valve and is forced into the cylinder and compressed. As the piston inside the cylinder moves downward, it pushes the energy from the compressed air to the wheels and produces power.
So you're all revved up. Now how are you going to slow down? Enter the compression-release engine brake. When approaching a steep downhill grade, truck drivers will flip a switch in the cab to shut off whatever number of engine cylinders they need to slow the truck. The shut off cylinders receive air, but they do not pass energy back through the system. Instead, the piston pushes the compressed air out the exhaust valve at the top of the cylinder and effectively slows the rig [source: Jacobs Vehicle Systems].
Exhaust brakes will slow you down, but in a different way than engine brakes. Read on to find out how.