An idle air control actuator or idle air control valve (IAC actuator/valve) is a device commonly used in

fuel-injected Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of an injector. This article focuses on fuel injection in reciprocating piston and Wankel rotary engines. All com ...

vehicles to control the engine's idling

rotational speed Rotational frequency (also known as rotational speed or rate of rotation) of an object rotating around an axis is the frequency of rotation of the object. Its unit is revolution per minute (rpm), cycle per second (cps), etc. The symbol fo ...

(

RPM Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines. Standards ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...

). In carburetted vehicles a similar device known as an ''idle speed control actuator'' is used.

Description

The IAC actuator is an electrically controlled device, which gets its input from the vehicle's engine control unit (ECU). The actuator is fitted such that it either bypasses the

throttle A throttle is the mechanism by which fluid flow is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases (by the use of a throttle), but usually decreased. The term ''throttle'' ...

or operates the throttle

butterfly valve A butterfly valve is a valve that isolates or regulates the flow of a fluid. The closing mechanism is a disk that rotates. Principle of operation Operation is similar to that of a ball valve, which allows for quick shut off. Butterfly valves ...

directly. The actuator consists of a linear servo actuator

servomotor A servomotor (or servo motor) is a rotary actuator or linear actuator that allows for precise control of angular or linear position, velocity and acceleration. It consists of a suitable motor coupled to a sensor for position feedback. It also ...

that controls a plunger which varies air flow through the throttle body. The position of the

and hence the amount of air bypass is controlled digitally by the engine ECU. This allows the engine's idle speed to be maintained constant. The linear servo is most commonly a combination of a DC motor,

lead screw A leadscrew (or lead screw), also known as a power screw or translation screw,Bhandari, p. 202. is a screw used as a linkage in a machine, to translate turning motion into linear motion. Because of the large area of sliding contact between t ...

and a digital

optical encoder A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to analog or digital output signals. There are two main types of rotary encoder: absolute and increme ...

. There is essentially no difference in efficiency between the technique of bypassing the throttle butterfly and operating the butterfly itself. The IAC allows the ECU to maintain minimum RPM irrespective of changes in engine load, sometimes referred to as anti-stall feature. Thus the driver can more easily move the car from stand-still by merely releasing the clutch (manual transmission) or the brake (automatic transmission) without having to simultaneously press the accelerator.

Problems with IAC

Although the IAC is supposed to last the vehicle's lifetime, various reasons may cause it to fail/malfunction prematurely. The most common failure mode is partial/complete jamming of the actuator (due to dirt/dust or even oil) where it cannot be smoothly controlled. The result is an engine that fails to maintain idle RPM and frequently stalls. A jammed actuator may be freed simply by cleaning it. However an actuator that has stopped working due to a fault in its

will need replacement. Air leaks in either the stepper housing or pipes will cause elevated idle RPM.

References

{{Automotive engine , collapsed Engine sensors Engine technology