HVAC
Vehicle HVAC keeps occupants comfortable and windows clear. Learn cabin heat loads (solar, occupants, ventilation, conduction), the refrigerant cycle (R-1234yf, CO₂), heater cores and PTC heaters, heat pumps in EVs, climate control logic and HVAC's impact on EV range.
A car in summer sun can reach 60–70 °C inside; in winter, windows fog and ice. The heating, ventilation and air-conditioning system must cool, heat, dehumidify and defrost — quickly and quietly.
In EVs, HVAC is the largest auxiliary load: heating with resistive heaters can cut winter range by 20–40 %, which is why heat pumps matter.
An analogy
The car's HVAC is a small fridge and a small radiator working together: the fridge pumps heat out of the cabin, the radiator (heater core) puts it back in when needed.
System components
Blower, air-inlet (fresh/recirculation) flap, evaporator (cools and dehumidifies), heater core (engine coolant) or electric heater, blend doors, distribution ducts to face/feet/windscreen, cabin air filter, controls.
A/C refrigerant cycle
Compressor (belt-driven in ICE, electric in EVs) → condenser at the front → expansion valve → evaporator in the dashboard. Refrigerants: R-134a being replaced by R-1234yf (low GWP) or R-744 (CO₂). Reheat for dehumidification wastes energy but clears windows.
Heat loads and comfort
Solar through glass, occupants (~100 W each), ventilation air, conduction through body, and engine/battery heat. Automatic climate control uses cabin, sun and humidity sensors to set blower, temperature and mode; multi-zone systems serve individual occupants.
EV thermal management and heat pumps
EVs have no free engine heat. PTC heaters convert electricity 1:1; heat pumps move ambient heat with COP 2–3. Integrated thermal systems (octovalves) share heat between cabin, battery and drive unit; battery pre-conditioning improves fast charging. Seat and steering heaters warm people directly for far less energy.
Formulas
- Ventilation load:
Q̇_vent = ṁ c_p (T_out − T_supply) - Solar load:
Q̇_solar = G × A_glass × SHGC - Compressor power:
P = Q̇_cooling / COP - EV range impact:
Δ energy per 100 km = P_HVAC × (100 / v_avg)
Worked example: Summer cooling load
Given: Fresh air 0.05 kg/s from 40 °C to 12 °C; 4 occupants × 100 W; Solar 600 W/m² on 1.5 m² glass, SHGC 0.5
Find: Total load and compressor power at COP 2.
- Ventilation = 0.05 × 1.005 × 28 = 1.41 kW
- Occupants 0.40 kW; solar = 600 × 1.5 × 0.5 = 0.45 kW
- Total ≈ 2.26 kW → compressor ≈ 1.13 kW
Answer: ≈ 2.3 kW cooling, ≈ 1.1 kW electric
Worked example: Heat pump vs PTC in winter
Given: Heating demand 3 kW; PTC COP 1, heat pump COP 3; Average speed 50 km/h
Find: Energy saved per 100 km.
- PTC: 3 kW; heat pump: 1 kW → save 2 kW
- 100 km at 50 km/h = 2 h → 4 kWh saved
Answer: ≈ 4 kWh/100 km (≈ 25 % of a 15 kWh/100 km EV)
Common mistakes
- Wrong: Running full recirculation in winter. — Right: Humidity builds and windows fog — keep some fresh air.
- Wrong: Heating EV cabin air only. — Right: Seat/steering heaters warm people with far less energy.
Applications
- Comfort & safety: Fast defogging and demisting are safety requirements.
- EV range: Heat pumps, recirculation and pre-conditioning extend winter range.
- Battery thermal management: Chillers use the A/C loop to cool batteries during fast charging.
Key takeaways
- Loads: solar, occupants, ventilation, conduction.
- A/C cycle: compressor → condenser → valve → evaporator.
- P = Q̇/COP; heat pumps beat PTC heaters.
- HVAC is the biggest EV auxiliary — pre-condition when plugged in.
Going further
- CO₂ (R-744) heat pumps: Work efficiently in very cold climates; transcritical cycles.
- Integrated thermal modules: Multi-port valves route coolant between battery, motor and cabin.
- Thermal comfort models: Equivalent temperature and local comfort guide zonal/radiant systems.