MACS Monthly Newsletter - 2021 - APR6

heat load. A GM V-5 compressor can operate at a minimum stroke volume of
0.6 CID up to a maximum displacement of 8.3 or 9.2 CID depending on model.

vides maximum refrigerant flow to move the maximum amount of heat.
Figure 13 shows the wobble plate and pistons in minimum stroke. The

Minimum stroke = LOW heat load
**infinitely variable**
Maximum stroke = HIGH heat load
More on heat load later!

Mike Bailey

Diﬀerent Strokes
The burning question. How do we control stroke?
MOST variable displacement compressors use pressure to control stroke.
The compressor design is typically axial and very closely resembles an axial
hydraulic pump. The piston rods are connected to a wobble plate. The wobble
plate is split with bearings in between so the input can spin while the pistons
wobble plate is forced into linear motion and piston reciprocation as it is held
from rotation with a guide pin. See Figure 11.

Figure 13: The wobble plate shown is on a slight angle, at
Minimum Stroke.

Mike Bailey

wobble plate is almost perpendicular to the cylinder bores. As the wobble
plate turns, pistons are pushed to TDC on discharge. BDC is limited to a small
linear distance...therefore minimum stroke. The spool in the control valve is
routing discharge pressure into the crankcase behind the pistons. This is the
reason the pistons have rings facing both ways. The rings facing the crankcase seal the crankcase pressure in the crankcase. Here, the high pressure
behind the pistons is forcing the pistons toward TDC/minimum stroke. The
wobble plate is designed to stop at minimum stroke and can NOT be forced
to zero stroke. Minimum stroke provides minimum displacement, therefore
minimum refrigerant flow, to raise evaporator pressure/temperature to just
above freezing and avoid condensate freezing on the core.
Why not zero stroke? Zero stroke represents zero refrigerant flow. Zero
refrigerant flow represents zero oil flow. Remember the compressor runs
continuously if the clutch driver is pinned or when the A/C is engaged on
the control head. At zero stroke the wobble plate would still be spinning, but
without oil flow. The wobble plate and shaft bearings would burn up.
In the winter, systems with a magnetic clutch will not engage the clutch if
the ambient temperature is too cold. This is typical of systems with which we
are familiar, including cycling clutch systems.
As mentioned, systems without a magnetic clutch have the compressor
drive hub pinned to the belt pulley. The wobble plate is constantly turning
with the engine running. During the winter in cold climates, the compressor
operates at minimum stroke almost exclusively.
Charge level is exceedingly important for compressors with pinned drive
hubs. As discussed, low refrigerant level represents reduced oil flow. Pinned
drive hubs can not be disconnected and a refrigerant leak will ultimately lead
to compressor failure. Find. Those. Leaks.

Mike Bailey

Figure 11: GM V-5 pistons connected to the wobble plate
and the cylinder block.

Figure 12: The wobble plate shown is on a large angle, at
Maximum Stroke.

Service Variables
Now that we know why variable displacement has taken such a foothold in
the industry and the basic operation of controlling stroke, we can apply this to
testing. Always remember, refrigerant leaks represent shortened compressor
life via reduced oil supply.
Just the nature of a variable displacement system can throw a curve ball
into what is usually a routine diagnosis.
In compressors using a MECHANICAL control valve as seen in Figure 9, the
function of the valve must be considered during initial pressure diagnosis.
The scenario is as follows.

Figure 12 shows the wobble plate and pistons in maximum stroke. The
wobble plate is on an extreme angle compared to the cylinder bores. As
the wobble plate turns, pistons are pushed to TDC (Top Dead Center) on discharge. On intake stroke BDC (Bottom Dead Center) pistons are maximum
distance from TDC...therefore maximum stroke. This is the natural state for
the compressor. The spool in the control valve is exposing the crankcase directly to the suction port and suction action of the pistons. Low crankcase
pressure behind the pistons allows maximum stroke. Maximum stroke pro-

April 2021

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MACS Monthly Newsletter - 2021

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