I am looking to expand my knowledge on the matter of impact in engine efficiency by using a traditional electromagnetic clutch A/C compressor, vs. the variable swash or "wobble" plate type compressors.

Read somewhere that there's a drop in horsepower, but how does the simpler system (clutch) stand against the "smarter" one (variable swash plate)?

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    This question has no answer. "Climate control" if a marketing term for a system that has an integrated approach to controlling vehicle air temperature in the cabin. "AC" is just a component of that system. – Eric Urban Aug 28 '16 at 23:28
  • This is one complex subject....depends on how the manufacturer implements automatic control of the compressor. – Moab Aug 28 '16 at 23:58
  • @EricUrban Your comment provides good insight (you could possibly convert it to an answer, so that we could create a wiki-like post for other users to be informed about). With regard to your comment, so put simply, "climate control" simply adds the software component that handles all the fan speed and compressor output so that it keeps the temperature at the user-set point. While "AC", is simply the dummy component that gives the user the control of setting the fans' speed and also the compressor's output? So is it safe to say that, "climate control" has greater battery consumption... – Chris Aug 29 '16 at 10:36
  • @EricUrban: (continuing) ..(with the same impact on engine - as "AC"), while "AC" is solely the aforementioned impact on the engine. – Chris Aug 29 '16 at 10:37
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    @Eric Urban More and more compressors are utilizing variable swash plate technology. The Sanden PXE as well as the Denso varieties are getting more and more market share, even on midline vehicles. I say this having just done a Denso clutchless in a 2008 Chrysler Sebring. I edited the question to take it out of the weeds, and now the hive can jump in and answer this question. – SteveRacer Aug 30 '16 at 4:37

There are two states to consider, the A/C on state and A/C off state.

In the off state the cycling clutch is far more efficient. The engine only has to spin an extra bearing. This is compared to spinning the whole swash plate along with all the friction it brings internal to the compressor.

In the on state it's far less clear cut. The swash plate changes the angle to provide just the right amount of pumping at all times. This is more efficient than full on. On the other hand on off on off nature of the cycling clutch should average out to about the same as the swash plate, theoretically that is.


The fixed displacement compressor is a "bang-bang" system, that is, just like a thermostat which only acts on the system's operating status and not also upon its cooling or heating capacity, it's designed to maintain a certain system's property (in that case, the evaporator's temperature) in a certain state (a certain temperature value, or a certain suction pressure value which, for a known refrigerant, matches to a certain evaporator's temperature). It's constantly working to bring the system's property to a precise value. Why? Because, to avoid damage to the system due to too much switching in a too short span of time, the system must feature a certain hysteresis, that is, the value the system should actually resume working at after being stopped due to reaching the desired value, has to be far enough from the desired value that a sufficient time can pass before the system resumes working.

From this standpoint, considering that a warmer evaporator implies an higher suction pressure and therefore an higher discharge pressure (which equates to more work required to the engine to compress the refrigerant vapour), fixed displacement compressors are less efficient than variable displacement compressors (and, to some extent, even other types of compressors, such as the V piston compressor, can be considered variable displacement if equipped with a capacity control system, also known as an unloader, which basically temporarily makes a bank of cylinders useless, i.e. not pumping any refrigerant at all, in some way; capacity control isn't necessarily limited to automotive swash plate or wobble plate compressors).

The latter take the evaporator's temperature in consideration too, instead of just mindlessly pumping any refrigerant they can until the evaporator is cold enough (or the desired vent temperature, if the system is equipped with a thermostat for regulating the vent temperature, is achieved). So the suction and discharge pressures are always steady and at the needed value, not constantly jumping up and down. Hence, once the evaporator's thermal load is stabilized, the engine load is always the same. All of this also thanks to how a thermal expansion valve works by trying its best to keep the refrigerant vapour's superheat steady.

Therefore, they're constantly working to keep the evaporator's temperature at a certain value, rather than bring it to that value.

Externally controlled variable displacement compressors take a step further from internally controlled ones: with externally controlled compressors, the climate control system can adjust the compressor's capacity, and therefore the evaporator's temperature, as needed and in any moment. The latter ones would instead need to be cycled off just like fixed displacement compressors would once the required vent temperature is reached, or the evaporator's air should be heated up again as needed, and both things mean more energy wasted.

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