Skylark L4-144 2.4L DOHC VIN T SFI (1997)
Expansion Valve: Description and Operation
This A/C system uses both a thermal expansion valve (TXV) and a variable displacement 5 cylinder compressor (V-5). The TXV's only purpose is to
regulate the amount of refrigerant flow through the evaporator to achieve optimum performance. Too much flow through the evaporator "floods" (the
refrigerant stays liquid all of the way through the evaporator and boils in the compressor inlet hose), not enough flow and the evaporator "starves" (the
refrigerant boils too early in the evaporator). Both conditions will cause A/C performance to be less than optimal. The goal is to get the refrigerant to boil
around the middle of the evaporator. The TXV achieves this by sensing the evaporator outlet pressure and outlet temperature to control refrigerant flow
to a predetermined factory setpoint. It should be noted that the TXV is a no bleed design (to prevent liquid slugging upon start up). This means that when
the compressor is disengaged the TXV closes off completely and high & low-side pressures take a long time to equalize (they will eventually equalize
due to a very small amount of bleed back through the compressor reeds). It should also be noted that a good TXV on the bench (removed from the
system) will be wide open due to atmospheric pressure and temperature acting upon it.
The V-5 compressor does not need to cycle on and off. Instead it varies the length of its piston stroke and hence changes its displacement to prevent
evaporator core freeze-up. The goal is to run as much displacement as possible for good cooling and yet not too much as to freeze. The V-5 tries to sense
evaporator temperature via its control valve. which knows only one thing: suction pressure. Think of pressure as being roughly equivalent to temperature,
and the suction pressure at the compressor being about the same as the pressure inside the evaporator, and hence the temperature at the evaporator. When
the low-side pressure increases (due to higher evaporator load and/or reduced compressor rpm), the control valve tells the compressor to upstroke to pull
the low-side pressure back down. When the V-5 is all the way to full stroke and can do no more, any additional increase in evaporator load and/or
reduction in compressor speed will result in a corresponding increase in low-side pressure and temperature. When low-side pressure decreases (due to
lower evaporator load and/or increased compressor rpm), the control valve tells the compressor to destroke to keep the low-side pressure and
temperature from going too low (going too low could result in evaporator freeze-up).
The V-5 used in this application has a high rpm destroking mechanism built in. When the trigger speed is reached, the compressor goes to a partially
destroked condition until the rpm is reduced. The action is subtle and probably would not be detected during normal driving conditions.
