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Mobile HVAC Systems

Mobile HVAC Systems. Heating, Ventilation, and Air Conditioning. HVAC System. Much of the HVAC system is mounted under the instrument panel and provides: heated air for warming the vehicle, fresh outside air for ventilation, and Air conditioning when cooler air is desired.

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Mobile HVAC Systems

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  1. Mobile HVAC Systems Heating, Ventilation, and Air Conditioning

  2. HVAC System • Much of the HVAC system is mounted under the instrument panel and provides: • heated air for warming the vehicle, • fresh outside air for ventilation, and • Air conditioning when cooler air is desired. • Modern systems include a cabin filter to remove small particles like dust or pollen or unpleasant odors.

  3. HVAC Sub-Systems • The HVAC system can be divided into several sub-systems • A/C or refrigeration • Heater • Air Inlet • Air Distribution • Blower for air movement Heater Core Evaporator Blower Mode Temperature Air Inlet Doors Door Door

  4. HVAC Assembly • The HVAC assembly contains the: • Air Inlet Section, • Plenum Section, and • Air Distribution Sections. Distribution Plenum Air Inlet

  5. Cabin Filter Possible Filter Location Possible Filter Location Many modern HVAC systems include a cabin filter to remove dust, pollen, smoke, and bad odors. The filter can be located in the HVAC case or in the air inlet plenum.

  6. Air Flow Control, Flap Doors Blend Air Door Foam Seal Upper Pivot Heater Core This blend air door is a simple flap door. It pivots on the upper and lower pivot points. Evaporator

  7. Sliding Door Old Civic 2001 Civic This style of door slides or rolls on a track or guide. One version is flexible and rolls up somewhat like a window shade. The purpose is to reduce the size of the HVAC case.

  8. Rotary Door Rotary Door To Defrost Temp. Doors This pan-shaped mode door rotates. There are openings in the side and edge of the door, and when the openings in the door align with openings in the case, air will flow to the correct outlet. To A/C Vent Floor Outlets End and side views

  9. Air Inlet Section Air flow into the HVAC system can be either fresh air from the outside or recirculated air from inside the vehicle. This flow is determined by the Air Inlet Door. The amount of air is controlled by the speed of the blower. Blower Fresh Air Inlet In-car, Recirc. Inlet

  10. Fresh Air Cabin Filter Fresh air enters from the plenum at the base of the vehicle’s windshield. Note that the air inlet door is positioned to block air from recirculating from in-car. Fresh Air position is used during most of HVAC system operation, including vent. Blower Fresh Air Inlet: Open In-car, Recirc. Inlet: Closed

  11. Recirculation During Max A/C and Max Heat, the air inlet door is moved to block fresh air. In-car air will enter the blower to be moved through the filter, plenum section, and air distribution section. Fresh Air Inlet: Closed In-car, Recirc. Inlet: Open

  12. Plenum Section Heater Core Evaporator Air is cooled by the evaporator core and can be reheated by the heater core as it passes through the plenum section. Air temperature is controlled by the position of the temperature blend door. Temperature Blend Door Cabin Filter

  13. Cooling the air Heater Core Evaporator When cool air is requested, the temperature blend door is positioned so all of the cool air from the evaporator core bypasses the heater core. Temperature Blend Door

  14. Heating the air Heater Core Evaporator When hot air is requested, the temperature door is positioned so all of the cool air from the evaporator core is sent through the heater core to be reheated. Temperature Blend Door

  15. Intermediate Temperatures Temperatures between cold and hot are obtained by reheating part of the air flow. The warm and cool air flows are mixed to get air of the desired temperature. Animation: HVAC Systems

  16. Reheat Systems Heater Hose Heater Core Evaporator A few modern systems are of the reheat type. All of the incoming air is cooled by the evaporator. The air can then be reheated as it passes through the heater core. Air temperature is controlled by the temperature of the heater core, and this is controlled by the coolant flow control valve. Heater Control Valve

  17. Air Distribution A/C-Heat Door The tempered air flow from the plenum section flows into the Air Distribution Section. Some doors can be set to a mid-point for bi-level (panel and floor) or heat-defrost operation. To A/C Registers To Defrost Ducts Heat-Defrost Door To Floor Outlets

  18. A/C Air Flow To A/C Registers During A/C operation, the air control doors are set to send the cool air to the registers in the instrument panel. Temp. Door: Full Cold A/C-Heat door: Open to A/C

  19. Heating Air Flow A/C – Heat Door: Closed to A/C The air control doors are set to send the heated air to floor discharge during heat operations. Temp. Door: Full Heat Heat-Defrost Door: Open to Heat To Floor Outlets

  20. Defrost Air Flow A/C – Heat Door: Closed to A/C The air doors are set to send the heated air to the base of the windshield during defrost operation. To Windshield Outlets Temp. Door: Full Heat Heat-Defrost Door: Open to Defrost

  21. Control Systems Blower Speed Temperature Mode • The control head determines: • Blower speed • Air temperature (Blend door position) • Operating mode (Air door positions • A/C on or off (A/C Request) Rear Window A/C Request Defrost Request

  22. Blower Speed Control From Control Head Resisters M1 Low B+ Fuse Blower Relay Blower Motor High M2 Blower Speed Switch Blower speed is controlled by routing the current flow through one or more resistors. Note that in this system, high speed has no resistor and that a relay is used to turn the blower motor on or off. Also note that the controls are on the ground side of the motor.

  23. PWM Blower Speed Control Control Head Blower Motor Amplified Motor Feed PWM Signal Blower Speed Controller The control head sends a PWM, pulse width modulated, signal to the Speed Controller that increases the voltage for the Blower Motor. This motor will run at half speed because there is voltage 50% of the time.

  24. Door Position, Mechanical Early systems used mechanical cables to connect the levers at the control head with the air control doors.

  25. Door Position, Vacuum Many systems use vacuum motors to control the position of the doors. The control head includes a vacuum switch to direct the vacuum signal to the proper vacuum motor.

  26. Door Position, Electric Many modern systems use electric motors to control door position. This system uses five motors.

  27. Door Motors Ground Feedback This five-wire actuator motor has two motor feed wires plus three feedback wires to provide a motor position signal to the controller. Some actuator motors use different wire arrangements and circuits. Reference Motor Power

  28. A/C Clutch Control This A/C clutch switched on or off by two different relays. The relays are controlled by the switch in the control head, a high pressure cutoff (HPCO) switch, and a pressure cycling switch mounted in the low side. Clutch circuits vary depending on the vehicle.

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