Air Conditioning (A/C) Compressor Control

The variable displacement air conditioning (A/C) compressor is permanently driven by the engine. The flow of refrigerant through the air conditioning (A/C) compressor, and the resultant system pressure and evaporator operating temperature, is regulated by the refrigerant solenoid valve. Operation of the refrigerant solenoid valve is controlled by the automatic temperature control (ATC) module using a 400 Hz pulse width modulation (PWM) signal. The duty cycle of the pulse width modulation (PWM) signal is calculated using the following parameters:

• Air Conditioning (A/C) compressor torque.
• Air Conditioning (A/C) compressor torque maximum.
• Air Conditioning (A/C) cooling status.
• Air Conditioning (A/C) demand.
• Air Conditioning (A/C) refrigerant pressure.
• Ambient air temperature.
• Blower speed.
• Engine cranking status.
• Evaporator temperature.
• Transmission gear status.

When air conditioning (A/C) is selected, the automatic temperature control (ATC) module maintains the evaporator at an operating temperature that varies with the in-vehicle cooling requirement. The automatic temperature control (ATC) module increases the evaporator operating temperature, by reducing the refrigerant flow, as the requirement for air cooling decreases, and vice versa. During an increase of evaporator operating temperature, to avoid compromising the dehumidification function, the automatic temperature control (ATC) module controls the rate of temperature increase, which keeps the cabin humidity at a comfortable level.

When the economy mode is selected, the pulse width modulation (PWM) signal holds the refrigerant solenoid valve in the minimum flow position, effectively switching off the air conditioning (A/C) function.

The automatic temperature control (ATC) module incorporates limits for the operating pressure of the refrigerant system. When the system approaches the high pressure limit, the duty cycle of the pulse width modulation (PWM) signal is progressively reduced until the system pressure decreases. When the system pressure falls below the low pressure limit, the duty cycle of the pulse width modulation (PWM) signal is held at its lowest setting, so that the air conditioning (A/C) compressor is maintained at the minimum stroke, to avoid depletion of lubricant from the air conditioning (A/C) compressor. The protection algorithm is calculated at a high rate, to enable early detection of the rapid pressure changes possible if a system fault develops.