How an Automotive Variable Displacement Compressor Works

Learn how an Automotive Variable Displacement Compressor Works

A critical component of these systems is the compressor, which comes in various designs. The variable displacement compressor stands out for its efficiency and adaptability. This article is designed help you understand how an automotive variable displacement compressor works and why carmakers switched of to it from a fixed displacement style compressor

Fixed Displacement vs. Variable Displacement Compressors

Traditional air conditioning systems use fixed displacement compressors, which move a constant refrigerant volume regardless of the cooling demand. While reliable, these compressors can be inefficient, operating at full capacity even when the cooling demand is low. The only way to regulate how much volume they output is to cycle the compressor on and off.

A variable displacement compressor, on the other hand, can adjust its capacity to match the cooling demand. This ability to modulate output allows for more precise temperature control, improved fuel efficiency, and reduced wear on system components.

How the Variable Displacement Compressor Works

Basic Structure

A variable displacement compressor resembles a fixed displacement compressor in its basic components but includes additional mechanisms to adjust the displacement. The primary parts include:

• Cylinder Block: Houses the pistons that compress the refrigerant.

• Swash Plate or Crankshaft Mechanism: The swash plate or wobble plate is an angled disc attached to the compressor’s drive shaft. As the shaft rotates, the swash plate’s tilted surface causes pistons to move up and down within their cylinders. The angle of the swash plate determines the stroke length of the pistons and, consequently, the compressor’s displacement.

In a variable displacement compressor, the swash plate’s angle can be adjusted on the fly. This is typically accomplished through a combination of internal pressure differentials and an electronically controlled valve.

• Control Valve  A control valve, often referred to as a capacity control valve or suction throttle valve, plays a crucial role in modulating the compressor’s output. This valve regulates the pressure in the crankcase, which in turn affects the swash plate angle.

When cooling demand is high, the control valve allows more refrigerant to flow into the crankcase, increasing the pressure. This pressure pushes against the backsides of the pistons, counteracting the suction force and causing the swash plate to tilt to a steeper angle. The result is a longer piston stroke and higher displacement.

Conversely, when cooling demand decreases, the control valve restricts flow to the crankcase, reducing pressure. This allows the suction force to dominate, tilting the swash plate to a shallower angle and decreasing displacement.

This cutaway version shows the interior components of a variable displacement air conditioning compressor

How a variable displacement AC compressor changes its output

The key to the variable displacement compressor’s operation is its ability to change the stroke of the pistons, which is controlled by the swash plate or a similar mechanism.

• The Swash Plate Mechanism— In many variable displacement compressors, the swash plate angle determines the piston stroke. When the swash plate angle is steep, the pistons travel a longer distance, increasing the displacement and thus the refrigerant flow. Conversely, a flatter angle reduces the piston stroke and the refrigerant flow.
• The Control Valve: The control valve regulates the pressure differential between the high and low-pressure sides of the compressor. By adjusting this differential, the control valve changes the swash plate angle. This regulation can be influenced by the car’s HVAC control

A Variable displacement compressor spins all the time

VDC compressors don’t have a clutch like a fixed displacement compressor. When the HVAC system isn’t callling for cooling, the swash plate is in a neutral position, so the pistons don’t move; the shaft and swash plate just free-wheel.

To prevent AC drive belt disengagement in the event of a compressor seize, the compressor pulley contains a built-in torque limiter slip clutch. In the event the compressor seizes, the torque limiter slip clutch will shear off the engagement points, allowing the compressor pulley to continue to spin while no longer applying rotational force to the compressor input shaft.

The advantages of a variable displacement AC compressor

• Improved Efficiency: By matching cooling capacity to demand, variable displacement compressors reduce unnecessary power consumption. This translates to improved fuel economy, especially in stop-and-go traffic or during highway cruising when full cooling power isn’t required.

• Enhanced Comfort: The ability to modulate output allows for more stable cabin temperatures and reduced cycling of the compressor clutch. This results in smoother operation and more consistent cooling performance.

• Reduced Wear and Tear: By operating at lower capacities when possible, variable displacement compressors experience less stress and potentially longer service life compared to fixed displacement models.

• Better Low-Speed Performance: Variable displacement compressors can maintain adequate cooling capacity at low engine speeds, improving comfort in heavy traffic or when idling.

• Smoother Operation: The reduced cycling of the compressor clutch leads to less noticeable engagement and disengagement, contributing to a quieter and more refined driving experience.

What goes wrong with a variable displacement AC compressor?

The external control valve or swashplate solenoids are common failures on these newer compressors. They work in conjunction with the low and high-pressure switches, cabin temperature sensors, and the HVAC head to determine when to move the swashplate angle. To diagnose a “no cooling” or “low cooling” condition, you’ll need a shop manual.

Test ECV resistance

Since the ECV is nothing more than an electric solenoid, test using a multimeter. For acceptable readings, refer to the shop manual.

External control valve

For example, The normal ECV resistance for this Hyundai variable displacement compressor should be 10.1 ~ 11.1Ω when the ambient temperature is 77°F. However, resistance may vary due to higher underhood temperatures. The acceptable range is (8 Ω ≤ ECV Resistance ≤ 14 Ω).

If the resistance is outside of the acceptable range, replace the ECV.

©, 2021 Rick Muscoplat