Several different methods of automobile ancillary power exist. The ultimate source of power for most of them is the automobile's main power source—normally an internal combustion engine of some kind—but some way of transferring power to the vehicle's ancillary systems is required. This may be through direct mechanical connection, electricity, hydraulic systems, vacuum, or compressed air.
The trend in modern vehicles is toward making all ancilliaries run on electrical power, simplifying the vehicle's systems and making them easier to control automatically.
Some automobile accessories are connected directly to the engine through gears or belts. These may include ancillary systems that directly service the engine; the water pump, fuel pump, oil pump, cooling fan, etc. Another class of mechanically connected ancilliaries are those that require large amounts of power; for example, an air conditioning compressor.
Also mechanically connected to the engine are components to convert this mechanical power into other forms; for example, an alternator to charge the battery, or a hydraulic pump to drive the power steering.
Automobiles have had electrically driven systems from the beginning, since the spark-ignition engine used in most vehicles requires electricity to generate the spark. Initially this was generated by a magneto and was only of use for ignition, but it was not long before an engine-driven DC generator was fitted. Because of the varying electrical power available from such a generator as engine speed changed, the generator was not connected directly to a car's electrical systems but rather through a lead-acid battery which enabled a more constant, smoothed flow of power and a limited supply of that power even with the engine off. This availability encouraged the use of other electrical accessories, such as lighting (which had previously been through oil lamps or acetylene lamps) and the automobile self starter. In the 1950s, six-volt systems gave way to the more robust and powerful twelve-volt standard as power accessories began to proliferate.
A further advance occurred when the DC generator was replaced by the combination of an alternator and a voltage regulator, first introduced on the 1960 Plymouth Valiant. This gave a more reliable power source capable of handling greater loads, and was the final impetus to electrify more and more vehicle subsystems which had hitherto been driven by other sources of power.
Today, most systems in the average vehicle such as power windows, power seats, and power door locks are electrically powered. The exceptions tend to be the A/C compressor and the power steering, although electrically driven power steering systems have been developed and are being used in a few models such as the Chevrolet Cobalt. Higher voltage wiring, even up to forty-two volts, has been discussed by the industry but never adopted for ordinary cars, reportedly because such high-energy systems could potentially cause harmful or fatal electric shock.
Hydraulic systems in the automobile generally have a hydraulic pump, driven electrically or directly from the engine via a belt. In passenger cars, the most common use of hydraulics is in the power steering system, which is hydraulically driven in the vast majority of vehicles. Convertible tops can be raised and lowered using hydraulics, although other methods, such as electrically, have also been used. Historically, windshield wipers were sometimes hydraulically driven, although this use mostly ceased after the late 1960s. Aside from these, the use of hydraulics on cars has been confined to the French company Citroën and cars built under their influence. Citroën devised a high-pressure hydraulics system for cars which was used to drive all manner of systems, even power-adjustable seats.
In other vehicles, such as heavy trucks, tractors and the like, hydraulic systems are much more common. Hydraulic rams drive dump truck beds, cranes, loaders, 3-point hitches on tractors, and much more.
An easily-accessible source of power from an internal combustion engine is partial vacuum, available by tapping the inlet manifold. The piston engine is fundamentally an air pump, and it produces suction and partial manifold vacuum on the inlet side. This can be used to power accessories or advance the ignition system spark timing.
Inlet manifold vacuum varies depending on engine load and throttle position. Therefore, rather than being connected directly, "vacuum canisters" are sometimes used to smooth things out, also to have a vacuum 'reserve' on turbo charged engines where, when the turbo is active, there exists no vacuum but an overpressure in the inlet manifold.
These canisters are connected to the inlet manifold through one-way valves that allow air to be sucked out when the engine is generating a lot of vacuum, but do not allow air to flow in. They can be viewed as, effectively, a "vacuum battery". Vacuum canisters allow vacuum accessories to be operated for a limited time even when the engine is turned off. (Electrical systems could not do this until the development of retained accessory power allowed power accessories to function after the ignition is turned off.)
The most common vacuum-powered ancillary is the power-assisted braking system. This is often vacuum powered even on modern cars, and is generally connected directly to manifold vacuum for increased reliability. In this case, the vacuum is only an assist; the brakes function (albeit requiring greater force) if vacuum power is lost.
Many older cars used vacuum-powered windscreen wipers; loss of manifold vacuum when the engine was working hard, with wide open throttle, caused these to slow down or even stop when climbing hills. Even in ordinary conditions, their speed varied inversely with the speed of the car—the wipers ran slower at higher vehicle speeds, an inconveient and even unsafe characteristic.
Automotive vacuum systems reached their height of sophistication in the late 1960s, after which their use declined. Electrical power was not trusted as reliable at the time, especially for door locks. As an example, a 1967 Ford Thunderbird used vacuum for:
- Power brakes
- Transmission shift control
- Headlight doors
- Remote trunk latch release
- Rear cabin vent control
- Power door locks
- Ventilation air routing
- Control of the heater core valve
- Tilt-away steering wheel release
Such systems tended to be increasingly unreliable with age, however, as the long runs of vacuum tubing necessary were very susceptible to leaks.
Compressed air systems are rarely found in cars, but larger vehicles often use air brakes. These normally use an electrically-driven compressor and large air tanks. Sometimes air is used to drive other systems as well.