How do clutchless auto AC compressors work? --- Mathew, Pune, India
Marshall Brain Answered:
It is easy to visualize a traditional car air conditioner compressor as a piston driven by a belt attached to the engine. The belt turns a pulley with an electric clutch. When you turn on the A/C, the clutch engages and the piston starts pumping to compress the refrigerant. See How Air Conditioning Works for details of the refrigerant cycle.
There are two problems with this traditional approach:
1)A compressor needs a fair amount of power. In cars with small engines, the engaging of the clutch to activate the compressor puts a big load on the engine, especially if it is idling.
2)The compressor will produce low flow when the engine is idling (probably not enough) and high flow when the engine is at high RPM (possibly too much).
This paper offers a lot of information about modern automotive air conditioning compressors. To solve problem number 2 mentioned above, one option is a variable compressor. It has a swash plate that can vary the amount of refrigerant that is compressed. See this page for a description of variable displacement pumps and how they work.
As long as you have gone to the trouble of installing a variable displacement pump, you can eliminate the clutch. The swash plate on the compressor can be set to a flat position so that the compressor doesn't do anything as it rotates. To "turn on" the compressor, the swash plate simply moves to an angled position.