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Automobile accessory power can be transferred by several different means. However, it is always ultimately derived from the automobile

Automobile

An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

's internal combustion engine

Engine

An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...

, battery, or other "prime mover" source of energy. The advent of high-powered batteries in hybrid and all-electrical vehicles is shifting the balance of technologies even further in the direction of electrically powered accessories.

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

Pneumatics

Pneumatics is a branch of technology, which deals with the study and application of use of pressurized gas to effect mechanical motion.Pneumatic systems are extensively used in industry, where factories are commonly plumbed with compressed air or compressed inert gases...

, and engine vacuum

Vacuum

In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

; or the engine may be directly tapped through a mechanical

Machine

A machine manages power to accomplish a task, examples include, a mechanical system, a computing system, an electronic system, and a molecular machine. In common usage, the meaning is that of a device having parts that perform or assist in performing any type of work...

 connection. Modern vehicles run most accessories on electrical power. Typically, only 2% of a vehicle's total power output has gone towards powering accessories. Electrical and hybrid vehicles may use a larger proportion of energy for accessories, due to reduced inefficiencies in the drive train, especially the elimination of engine idling.

Mechanical

Some automobile accessories are connected directly to the engine through gear

Gear

A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....

s or belts. These usually require large amounts of power. The air conditioning

Air conditioning

An air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...

 compressor

Gas compressor

A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...

 has been a familiar example, though new all-electric refrigerant compressors are starting to be used in production vehicles.

Electrical

Early automobiles used a magneto

Magneto (electrical)

A magneto is an electrical generator that uses permanent magnets to produce alternating current.Magnetos adapted to produce pulses of high voltage are used in the ignition systems of some gasoline-powered internal combustion engines to provide power to the spark plugs...

 for ignition, which provided no accessory power.

The first electrical accessory connection was supplied by a DC

Direct current

Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

 generator

Electrical generator

In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...

. 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

Battery (electricity)

An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...

 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

Automobile self starter

A starter motor is an electric motor for rotating an internal-combustion engine so as to initiate the engine's operation under its own power.- History :...

 was an early engine system to use this.

Lighting, which had previously been provided by kerosene lamp

Kerosene lamp

The kerosene lamp is a type of lighting device that uses kerosene as a fuel. This article refers to kerosene lamps that have a wick and a tall glass chimney. Kerosene lanterns that have a wick and a glass globe are related to kerosene lamps and are included here as well...

s or gas lamps

Carbide lamp

Carbide lamps, properly known as acetylene gas lamps, are simple lamps that produce and burn acetylene which is created by the reaction of calcium carbide with water....

, was one of the first common electrical accessories.

Early systems used 6 volts, but 12 volts became the standard because it provided greater power with less current. The original DC generator was replaced by an alternator controlled by a voltage regulator

Voltage regulator

A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. A voltage regulator may be a simple "feed-forward" design or may include negative feedback control loops. It may use an electromechanical mechanism, or electronic components...

. Due to mechanical and electrical properties, it is more efficient to first produce alternating current

Alternating current

In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

 and then immediately convert it to direct current. By regulating the current sent to the alternator's rotor and thus the strength of the magnetic field, a stable voltage can be produced over a wider range of engine speeds.

Higher voltage electrical systems beyond 12 volts have not been adopted for civilian cars, but military vehicles, such as the Jeep

Jeep

Jeep is an automobile marque of Chrysler . The first Willys Jeeps were produced in 1941 with the first civilian models in 1945, making it the oldest off-road vehicle and sport utility vehicle brand. It inspired a number of other light utility vehicles, such as the Land Rover which is the second...

 commonly use 24 volts. Research is ongoing into adopting a new 42V standard for automotive electricity, but the entire electrical system will have to be redesigned and new components manufactured to work with the higher voltage. The main advantage of higher voltage is that electrical components can be made with less metal, saving weight and cost, and improving energy efficiency.

Most modern systems, such as power window

Power window

Power windows or electric windows as well as electric or power window lifts are automobile windows which can be raised and lowered by depressing a button or switch, as opposed to using a hand-turned crank handle.- History :Packard introduced the first power windows in the 1940 Packard...

s, power seat

Power seat

A power seat in an automobile is a front seat which can be adjusted by using a switch or joystick and a set of small electric motors. Most cars with this feature have controls for the driver's seat only, though almost all luxury cars also have power controls for the front passenger seat.In addition...

s, and power door locks

Power door locks

Power door locks allow the driver or front passenger to simultaneously lock or unlock all the doors of an automobile or truck, by pressing a button or flipping a switch....

, are electrically powered. Electrically driven power steering

Power steering

Power steering helps drivers steer vehicles by augmenting steering effort of the steering wheel.Hydraulic or electric actuators add controlled energy to the steering mechanism, so the driver needs to provide only modest effort regardless of conditions. Power steering helps considerably when a...

 systems have been developed and are used in numerous models. High-efficiency all-electric refrigerant compressors for air conditioning are starting to be used, especially in hybrid or all-electric vehicles.

Hydraulic

The engine generally has a hydraulic pump mechanically driven by the engine, but there may also be electrically driven pumps.

In passenger cars, the most common use of hydraulic power has been the steering system

Power steering

Power steering helps drivers steer vehicles by augmenting steering effort of the steering wheel.Hydraulic or electric actuators add controlled energy to the steering mechanism, so the driver needs to provide only modest effort regardless of conditions. Power steering helps considerably when a...

. Convertible

Convertible

A convertible is a type of automobile in which the roof can retract and fold away having windows which wind-down inside the doors, converting it from an enclosed to an open-air vehicle...

 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 adopted in place of vacuum systems.

The French company Citroën

Citroën

Citroën is a major French automobile manufacturer, part of the PSA Peugeot Citroën group.Founded in 1919 by French industrialist André-Gustave Citroën , Citroën was the first mass-production car company outside the USA and pioneered the modern concept of creating a sales and services network that...

 devised a high-pressure hydraulics system for cars which was used for all manner of systems, even power-adjustable seats.

In vehicles such as heavy truck

Truck

A truck or lorry is a motor vehicle designed to transport cargo. Trucks vary greatly in size, power, and configuration, with the smallest being mechanically similar to an automobile...

s and tractor

Tractor

A tractor is a vehicle specifically designed to deliver a high tractive effort at slow speeds, for the purposes of hauling a trailer or machinery used in agriculture or construction...

s, hydraulic systems are much more common. Hydraulic ram

Hydraulic ram

A hydraulic ram, or hydram, is a cyclic water pump powered by hydropower. It functions as a hydraulic transformer that takes in water at one "hydraulic head" and flow-rate, and outputs water at a higher hydraulic-head and lower flow-rate...

s are used for accessories such as dump truck beds, cranes, loaders, and three-point hitch

Three-point hitch

The three-point hitch most often refers to the way ploughs and other implements are attached to an agricultural tractor. The three points resemble either a triangle, or the letter A...

es.

Vacuum

A commonly available source of power from an internal combustion engine is the partial vacuum

Vacuum

In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

 available at the intake manifold

Inlet manifold

In automotive engineering, an inlet manifold or intake manifold is the part of an engine that supplies the fuel/air mixture to the cylinders...

. The piston engine is fundamentally an air pump

Pump

A pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...

, and it produces suction and partial manifold vacuum

Manifold vacuum

Manifold vacuum, or engine vacuum in an internal combustion engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere....

.

Manifold vacuum varies depending on engine load and throttle

Throttle

A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...

 position, and automobiles use vacuum reservoirs or "vacuum canisters" to provide a usable source under varying conditions. Turbo charged

Turbocharger

A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...

 and super charged

Supercharger

A supercharger is an air compressor used for forced induction of an internal combustion engine.The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally aspirated engine, which allows more fuel to be burned and more work to be done per cycle,...

 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 valves 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 brake system

Hydraulic brake

The hydraulic brake is an arrangement of braking mechanism which uses brake fluid, typically containing ethylene glycol, to transfer pressure from the controlling unit, which is usually near the operator of the vehicle, to the actual brake mechanism, which is usually at or near the wheel of the...

. 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 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.

Automotive vacuum systems reached their height of use between the 1960s and 1980s. During this time a huge variety of vacuum switch

Vacuum switch

Prior to effective engine control unit computers, engine vacuum was used for many functions in an automobile. Vacuum switches were employed to regulate this flow, and were commonly controlled by temperature, solenoids, mechanically, or directly...

es, delay valves

Vacuum delay valve

A vacuum delay valve is a valve with a small orifice, which delays a vacuum signal. These are commonly used in automobiles to alter the behavior of vacuum switches, vacuum motors, and other vacuum devices....

 and accessory devices were created.

As an example, a 1967 Ford Thunderbird

Ford Thunderbird

The Thunderbird , is an automobile manufactured by the Ford Motor Company in the United States over eleven model generations from 1955 through 2005...

 used vacuum for:

• Power brake
  Brake
  A brake is a mechanical device which inhibits motion. Its opposite component is a clutch. The rest of this article is dedicated to various types of vehicular brakes....
  
  s
• Transmission
  Transmission (mechanics)
  A machine consists of a power source and a power transmission system, which provides controlled application of the power. Merriam-Webster defines transmission as: an assembly of parts including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a...
  
   shift control
• Doors for the hidden headlights
• Remote trunk latch release
• Rear cabin vent control
• Power door locks
• Ventilation air routing
• Control of the heater core
  Heater Core
  A heater core is a radiator-like device used in heating the cabin of a vehicle. Hot coolant from the vehicle's engine is passed through a winding tube of the core, a heat exchanger between coolant and cabin air...
  
   valve
• Tilt-away steering wheel release

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

Pneumatic

Pneumatic (compressed air) systems are rarely found in passenger cars.
Larger vehicles often use air brakes

Air brake (road vehicle)

Air brakes are used in trucks, buses, trailers, and semi-trailers. George Westinghouse first developed air brakes for use in railway service. He patented a safer air brake on March 5, 1872. Originally designed and built for use on railroad train application, air brakes remain the exclusive systems...

 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

Suspension (vehicle)

Suspension is the term given to the system of springs, shock absorbers and linkages that connects a vehicle to its wheels. Suspension systems serve a dual purpose — contributing to the car's roadholding/handling and braking for good active safety and driving pleasure, and keeping vehicle occupants...

, allowing the bus to lower itself or "kneel" at stops to allow passengers on or off.