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With summer quickly approaching, I find myself wondering why belt-driven AC compressors are so prevalent.

Some food for thought:

  • the laws of thermodynamics support the idea of extracting work from exhaust gases to power an AC compressor
  • the industry is very familiar with turbocharger design
  • there's a solid business case in the form of reduced fuel consumption and emissions

So what gives, why aren't turbo-driven compressors more common? Is this an active area of automotive research? Is there a missing piece to this puzzle that I'm not seeing?

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  • Just wondering if you also give thought to the transfer of heat which occurs through the main shaft of the turbo between the impeller to the compressor side? Also, with turbos being used so much more in engines, wouldn't a turbo driven compressor be a trade off (could do one or the other but not both)? Just food for thought, but not really an answer. – Pᴀᴜʟsᴛᴇʀ2 Apr 1 '17 at 15:28
  • Another thought ... A turbo would be an "always on" situation. The entire design of the compressor system would have to be redesigned to allow for a bypass of some sort. Current A/C design has a clutch which serves this purpose. There's nothing like that for a turbo (currently). The fact current design is fairly efficeint leads me to believe a redesign would be too costly with very little return on the investment. – Pᴀᴜʟsᴛᴇʀ2 Apr 1 '17 at 15:49
  • And also, the turbo is not always producing power, at least in most car engines it is not functioning at low or idle rpm. – Solar Mike Apr 1 '17 at 16:07
  • Btw on the topic of alternative air conditioning systems, check this out. – Jason C Apr 2 '17 at 2:20
  • Could the turbo mech energy be used to drive a small high speed alternator to replenish the cars electrical system? – Autistic Apr 2 '17 at 4:03
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I have found a significant amount of research being done on the topic and one of these papers may contain the answer. Here is a list of some further resources, all relatively recent, all regarding exhaust-driven air conditioners:

Some of those papers, while published, are a little disappointing, but there are a lot of others, check out this google search.

Most of the designs seem to be based on the principles of a Brayton Cycle engine, operating in reverse.

As for why they are not prevalent, an interesting tidbit from that reverse Brayton cycle link, which could be a hint:

As can be seen, the cycle efficiency is not very high (0.85), but the system is relatively simple, and has moreover the advantage of not releasing any greenhouse gases in the event of accidental breakage of pipes.

Air reverse Brayton cycle was until recently widely used in aircraft for in flight cabin air conditioning.

Note that I do not know why aircraft no longer use that cooling method, research into that could also lead to related material.

But the most interesting quote is from the first paper above:

For a typical small-size car with a maximum engine power of 51 kW, turbocompressor impact on the engine is 1,7 kW, which is similar to the typical compressor power consumption of a conventional air conditioning system (about 1,6-1,7 kW according to Kaynakli & Horuz, 2003).

Another aspect to be considered in terms of engine impact is the system weight. The weight of the proposed system was evaluated to be about 11,5 kg, while the conventional system weighs approximately 10,5 kg.

Taking these aspects into account, this first approach indicates that the proposed system would increase fuel consumption, mainly due to higher weight. Nevertheless, considering that system weights are very similar, an optimizing process on proposed system might eliminate such fuel consumption increase.

Which basically states that, at least in that initial design, power consumption was actually similar but weight was increased, and so there was no net improvement over a traditional A/C unit.

Given the date of that paper (2009) and the fact that research is ongoing, it is not unreasonable to imagine improvements have been made (I admit to not having gone over the other papers in detail), but I am presuming that not enough improvements have been made to make a change compelling.


Note: This is assuming that these papers are actually related to what you're describing -- these are turbo-driven systems in that they run off exhaust gas, but have a dedicated turbine, rather than running off the turbine shaft of a turbocharger, although at the end of the day it's essentially the same as far as energy transfer goes, I suppose, and for some of these designs the A/C still needs it's own compressor-side turbine regardless of what's on the other end of the shaft. A lot of these designs do seem to use different refrigerants than common typical A/C's, though.

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