Automotive AC Compressor Types

Automotive air conditioning compressors convert mechanical energy from the engine or electric motor into refrigerant pressure differential, enabling the vapor-compression refrigeration cycle. The compressor design directly impacts system efficiency, noise, vibration, and thermal management capability under highly variable operating conditions.

Fundamental Operating Principles

Automotive compressors must operate across extreme speed ranges—from idle (600-900 rpm engine speed) to highway conditions (3000-6000 rpm), creating unique design challenges. The compression process follows:

$$W_{comp} = \dot{m} \cdot h_{discharge} - h_{suction}$$

where $W_{comp}$ represents compressor power consumption, $\dot{m}$ is refrigerant mass flow rate, and $h$ denotes specific enthalpy. For typical R-1234yf systems, suction pressures range from 1.5-3.0 bar with discharge pressures of 12-25 bar depending on ambient conditions.

The volumetric efficiency accounts for clearance volume and valve losses:

$$\eta_v = \frac{\dot{V}{actual}}{\dot{V}{displacement}} = 1 - C\left[\left(\frac{P_d}{P_s}\right)^{1/n} - 1\right]$$

where $C$ is the clearance factor, $P_d$ and $P_s$ are discharge and suction pressures, and $n$ is the polytropic exponent (typically 1.1-1.3 for refrigerants).

Swash Plate Compressor Design

The swash plate (axial piston) compressor dominates automotive applications due to compact packaging and smooth operation. A tilted plate mounted on the drive shaft converts rotational motion into reciprocating piston movement.

graph TD
    A[Drive Shaft Rotation] --> B[Swash Plate Tilts]
    B --> C[Piston Reciprocation]
    C --> D[Suction Stroke]
    C --> E[Compression Stroke]
    D --> F[Reed Valve Opens]
    E --> G[Discharge Valve Opens]
    F --> H[Refrigerant Intake]
    G --> I[High Pressure Output]

    style A fill:#e1f5ff
    style I fill:#ffe1e1

Fixed Displacement Swash Plate

The swash plate angle remains constant, typically 15-25°. Piston stroke $S$ relates to swash angle $\alpha$ and plate radius $r$:

$$S = 2r \cdot \tan(\alpha)$$

Displacement volume for $n$ cylinders with bore diameter $d$:

$$V_d = n \cdot \frac{\pi d^2}{4} \cdot S$$

Common configurations include 5, 6, 7, and 10-cylinder designs. The 7-cylinder arrangement provides excellent balance with minimal pulsation.

Variable Displacement Control

Variable displacement compressors adjust cooling capacity by changing the swash plate angle from near 0° (minimum capacity) to maximum angle. This eliminates clutch cycling, reducing thermal shock and improving comfort.

The control mechanism balances three pressure regions:

$$P_{crank} = f(P_{suction}, P_{discharge}, P_{control})$$

A control valve modulates crankcase pressure by bleeding discharge gas. When cooling demand decreases, increasing crankcase pressure reduces the pressure differential across pistons, decreasing swash plate angle.

Capacity modulation response:

• Minimum displacement: 2-5% of maximum
• Transition time: 0.5-2.0 seconds
• Control pressure range: 3-8 bar

Scroll Compressor Technology

Scroll compressors use two interleaved spiral elements—one fixed, one orbiting—to compress refrigerant continuously without valves. This design offers:

• Reduced noise (5-10 dB lower than reciprocating)
• Higher efficiency at partial loads
• Fewer moving parts
• Smoother torque delivery

The compression ratio develops progressively as gas pockets move from outer to inner radius:

$$r_c = \left(\frac{r_{outer}}{r_{inner}}\right)^2$$

For automotive applications, compression ratios of 3.5-6.0 are typical. The orbiting scroll radius $R_{orbit}$ determines displacement:

$$V_d = 2\pi h (R_{outer} - R_{inner}) \cdot R_{orbit}$$

where $h$ is scroll height.

Scroll compressors excel in hybrid and electric vehicles where continuous operation at fixed speeds enables optimal efficiency. SAE J2765 recognizes scroll advantages for electric drive systems.

Electric Compressor Systems

Electric compressors eliminate belt drive dependence, critical for hybrid/electric vehicles and enabling climate control with the engine off. Three architectures exist:

The brushless DC motor efficiency:

$$\eta_{motor} = \frac{P_{mechanical}}{P_{electrical}} = \frac{\tau \cdot \omega}{V \cdot I}$$

Modern inverter controls achieve 85-92% motor efficiency across wide speed ranges (1000-8000 rpm), enabling precise capacity matching without mechanical displacement variation.

Thermal management challenges:

Electric compressors generate heat from motor losses and compression work. Oil cooling circuits or refrigerant injection cooling maintain motor temperatures below 120°C. The heat generation:

$$Q_{motor} = P_{electrical} \cdot (1 - \eta_{motor}) + W_{comp} \cdot (1 - \eta_{comp})$$

Clutch Engagement Systems

Electromagnetic clutches couple the compressor to the engine serpentine belt system. The clutch consists of:

1. Pulley assembly - freewheels on bearing when disengaged
2. Electromagnetic coil - creates magnetic field (12V, 3-5A draw)
3. Friction plate - locks to pulley when energized
4. Hub - connects to compressor shaft

Engagement torque must overcome:

$$T_{engage} = T_{static} + T_{acceleration} + T_{compression}$$

Where static friction torque prevents initial rotation, acceleration torque brings the compressor to belt speed (typically 0.1-0.3 seconds), and compression torque maintains operation.

Clutch cycling impacts:

• On-off cycles: 3-10 per minute typical
• Temperature swing: ±5-8°C at evaporator
• Engagement shock: 20-40 Nm torque spike
• Wear: 50,000-100,000 cycle life expectancy

Clutchless Variable Displacement

Modern systems eliminate the clutch by combining variable displacement with continuous operation at minimum capacity when cooling is not required. This approach:

• Reduces parasitic losses (minimum displacement consumes 0.2-0.5 kW)
• Eliminates engagement shock and noise
• Improves cabin comfort (no temperature cycling)
• Extends component life

The externally controlled variable displacement (ECVD) compressor uses pulse-width modulated control valves responding to electronic climate control signals, enabling integration with thermal management strategies optimizing overall vehicle efficiency.

Performance Comparison

Efficiency values represent isentropic efficiency under SAE J2765 test conditions (35°C ambient, 21°C cabin target).

Application Selection Criteria

Choose fixed displacement with clutch for:

• Cost-sensitive applications
• Conventional ICE vehicles
• Moderate climates

Choose variable displacement for:

• Premium comfort requirements
• Extended idle operation
• Noise-sensitive vehicles

Choose electric compressors for:

• Hybrid and electric vehicles
• Advanced thermal management integration
• Maximum efficiency priority

The trend toward vehicle electrification drives adoption of high-voltage electric compressors with integrated inverters, while variable displacement remains dominant in conventional powertrains where belt-drive packaging and cost advantages persist.

References

• SAE J2765: Procedure for Measuring System COP of a Mobile Air Conditioning System on a Test Bench
• SAE J639: Engine Cooling System Field Test (Alphanumeric Designation)