An air conditioning system for motor vehicles includes a blower configured to blow an air into a vehicle room, a PTC heater configured to heat the air blown into the vehicle room, the PTC heater having a heat generation amount adjusted by PWM duty ratio control, a blower air volume change amount measurement unit configured to, when an air volume of the blower is changed, measure a blower air volume change amount per unit time during a predetermined time period from a time point at which the air volume of the blower is changed, and a control unit configured to, when the air volume of the blower is changed, correct a PWM duty ratio of the PTC heater in view of the blower air volume change amount per unit time measured by the blower air volume change amount measurement unit.

A vehicle air conditioner includes an air-heating switching portion that switches between a first air-heating mode of heating ventilation air by a heating heat exchanger and a second air-heating mode of heating the ventilation air by a condenser. The vehicle air conditioner includes a heat-exchange adjustment portion that adjusts an amount of heat exchange between the high-pressure refrigerant and the ventilation air in the condenser, and a heat-exchange control unit that controls the heat-exchange adjustment portion. The heat-exchange control unit controls the heat-exchange adjustment portion to decrease the amount of heat exchange between the high-pressure refrigerant and the ventilation air in the condenser if a condition satisfies in which temperature of the refrigerant in the condenser is equal to or lower than temperature of the ventilation air heated by a heating heat exchanger before passing through the condenser, when the air-heating switching portion switches from the first air-heating mode to the second air-heating mode.

A method of controlling an air conditioning system for a vehicle is provided. The method includes detecting a cooling mode while the vehicle is being driven and comparing a vent discharge temperature of air with a cooling target temperature set by a user. When the vent discharge temperature is greater than the cooling target temperature the compressor RPM is determined and air is introduced into the vehicle without passing through the interior heat exchanger and the electric heater. When the vent discharge temperature is less than the cooling target temperature, the compressor RPM is determined and a door in the system is opened to guide the air through the interior heat exchanger to heat the air is heated and then the vent discharge temperature is compared with the cooling target temperature again, and the electric heater is operated.

A vehicle air conditioner includes an air-heating switching portion that switches between a first air-heating mode of heating ventilation air by a heating heat exchanger and a second air-heating mode of heating the ventilation air by a condenser. The vehicle air conditioner includes a heat-exchange adjustment portion that adjusts an amount of heat exchange between the high-pressure refrigerant and the ventilation air in the condenser, and a heat-exchange control unit that controls the heat-exchange adjustment portion. The heat-exchange control unit controls the heat-exchange adjustment portion to decrease the amount of heat exchange between the high-pressure refrigerant and the ventilation air in the condenser if a condition satisfies in which temperature of the refrigerant in the condenser is equal to or lower than temperature of the ventilation air heated by a heating heat exchanger before passing through the condenser, when the air-heating switching portion switches from the first air-heating mode to the second air-heating mode.