An engine has one or more devices for converting energy it produces into a usable form, electricity connection through the alternator, hydraulic connections from a pump or engine system, compressed air, and engine vacuum; or the engine may be directly tapped through a mechanical connection. Modern vehicles run most accessories on electrical power.
The first electrical accessory connection was supplied by a DC generator. Voltage varied with engine speed and because of technological limitations, complicated mechanical devices were used to regulate it. Even so, voltage at idle was too low to be useful. A lead-acid battery was used to provide proper voltage when the generator could not, and was recharged at higher engine speed or lower electrical load. The automobile self starter was an early engine system to use this.
Early systems were six-volt, but twelve-volt became the standard because it provided greater power with less current. The DC generator was replaced by an alternator with a voltage regulator. Due to mechanical and electrical properties, it is more efficient to convert the alternating current to direct current, and a stable voltage can be produced over a wider range of engine speeds. Higher voltage wiring up to forty-two volts has not been adopted for civilian cars, but military vehicles, such as the Jeep commonly use 24 volts.
Most modern systems, such as power windows, power seats, and power door locks, are electrically powered. Electrically driven power steering systems have been developed and are used in models such as the Chevrolet Cobalt.
In passenger cars, the most common use of hydraulic power is the steering system. Convertible tops may be raised and lowered using hydraulics. Windshield wipers were sometimes hydraulically driven, although this use mostly ceased after the late 1960s. On vehicles with little or no engine vacuum, Hydraulic systems are generally adapted in place of vacuum systems.
The French company Citroën has devised a high-pressure hydraulics system for cars which was used for all manner of systems, even power-adjustable seats.
Manifold vacuum varies depending on engine load and throttle position and automobiles use vacuum reservoirs or "vacuum canisters" to provide a usable source under these conditions. Turbo charged and super charged engines do not always produce vacuum, the intake manifold is pressurized when the turbo is spinning above certain RPM.
Reservoirs and devices connected to the engine through check valves that 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 leaks in after the engine turns off.
The ubiquitous vacuum-powered accessory is the booster for the brake system. The vacuum is only an assist and the brakes function, requiring greater force, if the booster is used up.
Many older vehicles used vacuum-powered windshield wipers. 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.
As an example, a 1967 Ford Thunderbird used vacuum for:
Such systems tend to be unreliable with age as the vacuum tubing becomes brittle and susceptible to leaks.
Pneumatic (compressed air) systems are rarely found in passenger cars. Larger vehicles often use air brakes and the pressure may be used to drive other systems. Windshield wipers, automatic gear boxes, and other common hydraulic or vacuum powered accessories are often adapted. On buses where the engine is often at the rear of the vehicle, compressed air may be used for the throttle and clutch.
Bus doors are typically air powered, as well as the steps and the suspension, allowing the bus to lower itself or "kneel" at stops to allow passengers on or off.