Editing Automobile accessory power (section)

==Vacuum==
A commonly available source of power from an internal combustion engine is the partial [[vacuum]] available at the [[Inlet manifold|intake manifold]].  The [[piston engine]] is fundamentally an air [[pump]], and it produces suction and partial [[manifold vacuum]]. 

Manifold vacuum varies depending on engine load and [[throttle]] position, and automobiles use [[vacuum reservoir]]s or "vacuum canisters" to provide a usable source under varying conditions. [[turbocharger|Turbo charged]] and [[supercharger|super charged]] engines do not always produce vacuum; the intake manifold is actually pressurized when the turbo is spinning above a certain speed.

Reservoirs and devices connected to the engine through [[check valve]]s allow pressure to reduce when the engine is generating a lot of vacuum, but do not allow air back in. Vacuum canisters only allow vacuum accessories to be operated for a very short time, and air will leak in after the engine turns off.

The most ubiquitous vacuum-powered accessory is the booster for the power [[Hydraulic brake|brake system]]. The vacuum is only an assist and the brakes can still function, requiring greater force, if the booster vacuum is used up.

Many older vehicles used vacuum-powered [[windshield wiper]]s. Loss of manifold vacuum when the engine was working hard, or at wide open throttle, necessitated using a vacuum booster pump which was usually part of the fuel pump.

Automotive vacuum systems reached their height of use between the 1960s and 1980s. During this time a huge variety of [[vacuum switch]]es, [[Vacuum delay valve|delay valves]] and [[List of automotive vacuum devices|accessory devices]] were created.

As an example, a 1967 [[Ford Thunderbird]] used vacuum for:
* Power [[brake]]s
* [[Transmission (mechanics)|Transmission]] shift control
* Doors for the [[hidden headlamps]]
* Remote trunk latch release
* Rear cabin vent control
* [[Power door lock]]s
* Ventilation air routing
* Control of the [[heater core]] valve
* Tilt-away steering wheel release

Such systems tend to be unreliable with age as the vacuum tubing becomes brittle and susceptible to leaks.