The solenoid-type air control valve acts on signals from the control unit, to bypass a measured airflow around the throttle plate. The position of the valve depends on how much current the control unit applies to the solenoid.
Maximum current flow opens the valve fully to give maximum airflow. This is generally the assumed position for starting.
Thereafter, the taper valve is positioned by pulsing the solenoid winding circuit at a pre-determined frequency.
The amount of valve opening, and therefore the amount of airflow, depends on the ON time of the pulse. A long ON pulse with a short OFF pulse, will produce a high average voltage, and therefore a large opening of the valve.
A short ON pulse with a long OFF pulse will produce a low average voltage, and a small valve opening.
This ON and OFF time is called the duty cycle, and it’s generally expressed as a percentage.
A variation in pulse width, at the set frequency, is called pulse width modulation.
The stepper motor type of idle control positions its tapered pintle, using a screw and nut assembly. The nut is part of a centrally located, permanent magnet armature. It engages with the screw on the pintle.
Rotating the armature extends, or retracts the pintle, which closes or opens the air passage. Rotation occurs by switching current flow, in 2 coils. The pintle turns in forward, or reverse steps, numbered from zero, with the bypass air passage fully closed, up to 255 for maximum airflow.
Such a large number of steps allow the pintle position to be finely controlled.
When the idle speed is controlled by the position of the throttle plate, a DC motor may be used as a variable throttle stop.
The controlling circuit can let current flow through the motor in either direction, to extend or retract the linear actuator.
Solenoid valves can also be used to bypass a pre-determined amount of air around the throttle plate. These may come into operation for specific load compensation. For instance, when the air conditioning compressor is engaged. They open up a passageway of fixed dimension, and do not vary the opening.
