MACS Monthly Newsletter - 2021 - APR5

For A/C, they are sometimes in the third or fourth trip through the comeback
loop with a vehicle. If it is a refrigeration issue with the HVAC system, the first
thing I need to know is if the compressor is VARIABLE or FIXED displacement.

Rather than turn the compressor off as the evaporator approaches freezing, reduce the stroke of the compressor cylinders. Reducing the stroke reduces the displacement. Reducing the displacement reduces the amount
of refrigerant being pumped. Reducing refrigerant volume in the evaporator
decreases the amount of refrigerant available to absorb heat. As a result,
the exact amount of refrigerant is supplied to the evaporator to absorb the
required heat, without the core temperature dropping below freezing.
Remember learning about the Pressure/Temperature relationship? The
lower the refrigerant temperature, the lower its pressure. The higher the refrigerant temperature, the higher the pressure. That Pressure/Temperature
chart is now useful again. I recommend having one on hand for the refrigerants your shop is servicing. Yes, they are different.
For ease of explanation, take a mechanically controlled, variable displacement compressor and set the control valve to maintain suction line pressure
at 30 psi. Put another way, the only thing the control valve is going to do
is command the compressor to maintain 30 psi on the suction port of the
compressor, representing 30 psi in the evaporator core (with minor variables
not worth mentioning here). See Figure 9.

Variable beginnings
General Motors developed their Variable Displacement Orifice Tube (VDOT)
system in the 1980s. The system was developed for many reasons including
drivability, passenger comfort and fuel economy.
Think back. Way back. To the 1990s. Manufacturers were downsizing their
vehicles, engine displacement and number of cylinders was shrinking. Now
take that four-cylinder (carbureted) Chevrolet Cavalier or Oldsmobile Calais
for a drive. Overall performance was less than stellar through the entire rpm
and vehicle speed range.
Turn on the air conditioning and hit the highway. Cruise along at 55 MPH
for a bit and let the cabin temperature stabilize and turn the blower speed
down. How did manufacturers stop their evaporators from freezing? Turn the
compressor off. Let the evaporator warm up a bit and turn the compressor
back on. Systems typically used a thermostatic switch or a low-pressure cycling switch to cycle the compressor and stop evaporator freeze-up.
Another consideration was that " humid smell " discharged from the HVAC
ducts when the compressor is cycled off and the evaporator is warming up.
Many customers find the odour disagreeable and some may become uncomfortable as the humidity in the cabin rises.
On a V-8, compressor cycling was barely noticed. Now cycle the compressor on that small GM with the four-cylinder engine we mentioned earlier.
Customers had real issues with vehicle speed fluctuation, bucking and surging as the compressor was cycled to control evaporator temperature. Now
add a lock-up torque converter to the mix.

MACS Staff

The Variable Solution...100% Duty Cycle Clutch
Many 'fixes' were attempted to reduce compressor-induced speed fluctuation, bucking and surging. Someone came up with the idea to leave the
compressor ON. This idea was used in the 60s and 70s but required an STV,
POA or EPR valve installed at the evaporator suction line. This system is not
practical for modern vehicles.
The solution was the V5 compressor. Variable Displacement five cylinders.
Early versions ran the compressor 100% of the time whenever the A/C was
running, including Defrost. Later versions have deleted the magnetic clutch
on the compressor and permanently drive the compressor input hub using
shear pins to attach it to the belt pulley.

Figure 10: Pressure/Temperature relationship. Left column
represents degrees Fahrenheit. Right column represents
gauge pressure.

Variable benefits
Switching to variable displacement provided the following benefits...
* Speed drop, bucking and surging are eliminated
* Cost savings by eliminating the clutch (some models)
* Reduced belt and tensioner stress
* Continuous oil and refrigerant circulation
* Stable duct discharge temperature
* Improved dehumidifying and stable cabin humidity
* Improved mileage

Figure 10 is taken from a pressure/temperature chart. To use this chart,
look up the refrigerant temperature in the left columns (°F or °C). Follow the
same line to the right and the chart tells you the pressure the refrigerant will
be at, resting, captured in either a tank or in the plumbing of a system (provided the charge is adequate to maintain at least a single of drop of liquid
refrigerant somewhere in the system). In the chart, 29.5 psi (R-134a, second
pressure column) represents refrigerant at 34°F (1.1°C).
We can also use the chart and look up the pressure reading to determine
the temperature of the liquid refrigerant in the evaporator core. If we read 30
psi on our low-pressure gauge, we see 29.5 psi in the chart. Close enough.
The chart tells us that our 30 psi represents close to 34°F (1.1°C). BOTH are
above the freezing point of the condensate / water on the evaporator core
fins. The evaporator will NOT freeze.
Instead of the compressor being turned off, the control valve modulates
the stroke to maintain evaporator pressure and therefore temperature.
Stroke is infinitely variable between minimum and maximum depending on

Variable Operation
We can begin our understanding of variable displacement operation by
discussing a mechanically controlled, variable displacement compressor
and the LOW/SUCTION side of the refrigeration system. Everything about
variable displacement is designed to control low side pressure, and
therefore evaporator temperature.

April 2021

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