If you've ever opened the hood of your car, you've probably
seen the brake booster. It's the round, black cannister
located at the back of the engine
compartment on the driver's side of the car.
Back in the day, when most cars had drum
brakes, power brakes were not really necessary -- drum
brakes naturally provide some of their own power assist. Since
most cars today have disc
brakes, at least on the front wheels, they need power
brakes. Without this device, a lot of drivers would have very
tired legs.
The brake booster uses vacuum from the engine to
multiply the force that your foot applies to the master
cylinder. In this edition of HowStuffWorks,
we'll see what's inside the black cannister that provides
power braking.
The Vacuum Booster
The vacuum booster
is a metal canister that contains a clever valve and a
diaphragm. A rod going through the center of the canister
connects to the master
cylinder's piston on one side and to the pedal linkage on
the other.
Another key part of the power brakes is the check
valve.
The photo above shows the check valve, which is a
one-way valve that only allows air to be sucked
out of the master cylinder. If the engine is
turned off, or if a leak forms in a vacuum hose, the check
valve makes sure that air does not enter the vacuum booster.
This is important because the vacuum booster has to be able to
provide enough boost for a driver to make several stops in the
event that the engine stops running -- you certainly don't
want to lose brake function if you run out of gas on the
highway. In the next section, we'll see how the booster works
(and check out a cool animation!).
The Booster in Action
The vacuum booster is
a very simple, elegant design. The device needs a vacuum
source to operate. In gasoline-powered
cars, the engine provides a vacuum suitable for the boosters.
In fact, if you hook a hose to a certain part of an engine,
you can suck some of the air out of a container, producing a
partial vacuum. Because diesel
engines don't produce a vacuum, diesel-powered vehicles
must use a separate vacuum pump.
On cars with a vacuum booster, the brake pedal pushes a rod
that passes through the booster into the master cylinder,
actuating the master-cylinder piston. The engine creates a
partial vacuum inside the vacuum booster on both sides
of the diaphragm. When you hit the brake pedal, the rod cracks
open a valve, allowing air to enter the booster on one side of
the diaphragm while sealing off the vacuum. This increases
pressure on that side of the diaphragm so that it helps to
push the rod, which in turn pushes the piston in the master
cylinder.
As the brake pedal is released, the valve seals off the
outside air supply while reopening the vacuum valve. This
restores vacuum to both sides of the diaphragm, allowing
everything to return to its original position.
For lots more information, check out the links on the next
page.
