Automotive Fresh Air Systems

Fresh Air Systems in Automotive HVAC

Automotive fresh air systems provide ventilation by introducing filtered outside air into the vehicle cabin. These systems must overcome unique challenges: capturing air at vehicle speeds ranging from stationary to highway conditions, separating water from intake air, filtering contaminants, and maintaining adequate ventilation rates in a compact, noise-sensitive environment.

Cowl Air Intake Design

The cowl area—the junction between the hood and windshield—serves as the primary fresh air inlet for most vehicles due to favorable aerodynamic and packaging characteristics.

Ram Air Effect

Vehicle motion creates dynamic pressure at the cowl intake. The total pressure available increases with velocity according to:

$$P_{total} = P_{static} + \frac{1}{2}\rho v^2$$

where $\rho$ is air density (typically 1.2 kg/m³) and $v$ is vehicle velocity. At highway speed (30 m/s or 67 mph), dynamic pressure contributes approximately 540 Pa (2.2 in. H₂O), significantly reducing blower power requirements.

The cowl intake typically features a raised lip to create a stagnation zone where air velocity approaches zero and pressure maximizes. SAE J1503 recommends intake locations at least 150 mm from the windshield base to avoid recirculation zones.

Intake Geometry

Cowl intake area must balance several factors:

Face velocity at the grille opening should remain below 3 m/s at typical blower settings to minimize intake noise and aerodynamic losses.

Rain and Water Separation

Water separation relies on inertial impaction—exploiting the 800:1 density ratio between water droplets and air.

Separation Physics

As air changes direction through louvers, droplets continue in their original trajectory due to inertia. The separation efficiency depends on the Stokes number:

$$Stk = \frac{\rho_p d_p^2 v}{18\mu D_c}$$

where:

• $\rho_p$ = particle (droplet) density (1000 kg/m³ for water)
• $d_p$ = droplet diameter
• $v$ = approach velocity
• $\mu$ = air dynamic viscosity (1.8 × 10⁻⁵ Pa·s)
• $D_c$ = characteristic dimension (louver spacing)

Effective separation occurs when Stk > 1. For 100 μm droplets at 5 m/s through 10 mm louver spacing, Stk ≈ 1.5, yielding >85% separation efficiency.

Drainage System

graph TD
    A[Cowl Intake Grille] --> B[Inertial Separator Louvers]
    B --> C[Collected Water]
    B --> D[Separated Air Stream]
    C --> E[Drain Channels]
    D --> F[Filter Plenum]
    E --> G[Drainage to Ground]
    F --> H[Cabin Air Filter]
    H --> I[Blower Assembly]

Drain channels must handle the maximum water ingress rate, typically designed for rainfall intensity of 100 mm/hr (extreme conditions). At vehicle speed with 0.04 m² effective catch area, this represents approximately 67 ml/min flow rate. Drain orifice sizing follows standard hydraulic relationships with minimum 8 mm diameter to prevent debris blockage.

Cabin Air Filtration

Modern cabin air filters provide two-stage filtration: particulate removal and gaseous contaminant adsorption.

Particulate Filtration Mechanisms

Three physical mechanisms dominate particle capture:

Interception: Particles following streamlines contact fibers when their radius brings them within one radius of the fiber surface. Collection efficiency by interception:

$$\eta_R = \frac{1}{K_u}(1 + R)\left(\frac{d_p}{d_f}\right)^2$$

where $R$ is the interception parameter and $K_u$ is the Kuwabara hydrodynamic factor.

Diffusion: Brownian motion causes particles <0.5 μm to deviate from streamlines. Diffusion efficiency increases as particle size decreases:

$$\eta_D = 2.6\left(\frac{1}{K_u}\right)^{1/3}\left(\frac{D}{v \cdot d_f}\right)^{2/3}$$

where $D$ is the particle diffusion coefficient.

Impaction: Particles >1 μm with sufficient inertia impact fibers directly. Impaction efficiency depends on the Stokes number for flow around cylindrical fibers.

Filter Performance Specifications

HEPA filters in automotive applications face challenges from limited space and blower capacity. The high initial pressure drop (80-120 Pa) requires more powerful blowers and reduces system airflow by 15-25% compared to standard filters.

Activated Carbon Layer

Gaseous contaminants (NOₓ, volatile organic compounds, ozone) are not captured by particulate mechanisms. Activated carbon layers use physical adsorption onto high-surface-area carbon granules.

Carbon adsorption capacity follows the Langmuir isotherm for single-component adsorption:

$$q = q_{max}\frac{K_{ads}C}{1 + K_{ads}C}$$

where $q$ is the adsorbed amount per unit mass of carbon, $q_{max}$ is the maximum capacity, $K_{ads}$ is the adsorption equilibrium constant, and $C$ is the gas-phase concentration.

Typical automotive carbon layers contain 50-150 g of activated carbon with specific surface areas of 800-1200 m²/g, providing initial removal efficiencies >90% for odors and >60% for NO₂. Carbon saturates after 12,000-25,000 km depending on exposure levels.

Filter Replacement Criteria

Replace cabin air filters when:

• Pressure drop exceeds 200 Pa (0.8 in. H₂O) at rated flow
• Airflow decreases by >30% from clean filter baseline
• Visible contamination or biological growth observed
• Carbon layer saturated (typically 1 year or 20,000 km)

Pressure drop increases approximately linearly with accumulated dust mass until filter begins to densify, then rises exponentially as deep bed filtration transitions to surface cake filtration.

Outside Air Damper Operation

The fresh air damper controls the ratio of outside air to recirculated cabin air.

Damper Positions and Mixing

graph LR
    OA[Outside Air<br/>100% Fresh] --> Mix[Mixed Air<br/>Variable Blend]
    RA[Recirculated Air<br/>0% Fresh] --> Mix
    Mix --> Filter[Cabin Filter]
    Filter --> Blower[Blower]

    subgraph Damper Control
    Auto[Auto Mode<br/>CO₂ Sensor]
    Manual[Manual Mode<br/>Driver Input]
    Recirc[Recirc Mode<br/>0% OA]
    end

Damper actuators use vacuum servos, electric stepper motors, or DC motors with position feedback. Response time typically ranges from 2-5 seconds for full travel to minimize temperature or pressure transients.

Ventilation Rates for Air Quality

SAE J1503 recommends minimum ventilation rates based on cabin volume and occupancy:

$$Q_{vent} = N \cdot V_{person} + V_{cabin} \cdot ACH_{min}$$

where:

• $N$ = number of occupants
• $V_{person}$ = 7-10 L/s per person (15-21 CFM)
• $V_{cabin}$ = cabin volume (typically 3-5 m³)
• $ACH_{min}$ = minimum air changes per hour (0.5-1.0 ACH)

For a typical sedan (4 m³ cabin, 2 occupants):

$$Q_{vent} = 2 \times 8 + 4 \times \frac{1}{3600} \times 3600 \times 0.75 = 16 + 3 = 19 \text{ L/s (40 CFM)}$$

CO₂-Based Demand Control

Modern systems measure cabin CO₂ concentration to modulate outside air damper position. Acceptable CO₂ levels range from 800-1200 ppm above ambient (outdoor typically 400-450 ppm). Human respiration produces approximately 0.3 L/min of CO₂ per person at rest, rising to 1-2 L/min during activity.

Mass balance for cabin CO₂:

$$V_{cabin}\frac{dC}{dt} = N \cdot G_{CO_2} - Q_{vent}(C - C_{ambient})$$

At steady state, the required ventilation rate to maintain target CO₂ level becomes:

$$Q_{vent} = \frac{N \cdot G_{CO_2}}{C_{target} - C_{ambient}}$$

For 2 occupants producing 0.3 L/min CO₂ each, maintaining 1000 ppm above ambient requires approximately 20 L/s ventilation rate, confirming the SAE J1503 guideline.

System Integration Considerations

Fresh air system design must address:

• Filter housing sealing: Leakage bypassing the filter defeats filtration. Gasket compression of 20-30% and seal width >8 mm ensures <2% bypass.
• Blower capacity: Total system pressure drop (intake + filter + ducts + outlets) typically ranges 150-400 Pa. Centrifugal blowers with backward-curved blades provide efficiency of 40-60%.
• Noise control: Intake velocities >6 m/s generate turbulence noise. Plenum volumes >5 liters attenuate pulsations from blower operation.
• Condensate management: Temperature drop across evaporator coils can produce 2-4 L/hr condensate in humid conditions. Drain systems must prevent water backup into filter housing.

References

• SAE J1503: Ventilation Air Change Rates for Light-Duty Vehicles
• SAE J1669: Platform Lift and Wheelchair Lift Design for Stationary Vehicle Applications (cabin volume standards)
• ISO 11155-1: Road vehicles — Air filters for passenger compartments