Variable displacement compressors

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Variable displacement compressors

Postby cornbinder89 » Tue Sep 01, 2015 2:53 pm

As this topic came up, I would like to learn more without cluttering up the Toyota thread.
I understand the theory and reason behind why they are used, but not the nitty gritty of the controls. How are the direct drive (no clutch) compressors protected from loss of Freon? DO they go to zero displacement (swash plate vertical)? are they all controlled electrically or are there some mechanical (pressure/flow) based controls.
At one time (15 or 20 years ago) there was a school of thought that all vehicles would go to 48 volt or higher, and all accessories electrically driven. That has not happened for many reasons except possibly for the hybrid/ all electric cars.
The push for smaller engine and less drag is going to make these variable displacement set-ups more common. The old way of having a fixed displacement pump and running it varying speeds isn't going to fly. Storing excess capacity in the form of liquid Freon at high pressure is wasteful. CCOT systems don't store that way (so much, a little) but matching the compressor output to the systems need at real time is the best way.
Working for years with hydraulic systems, I've been around many swash plate pumps. and while they offer some advantages, thy also come with a host of drawbacks. When brute reliability is the main goal, fixed displacement pumps are used.
As I say, I work with heavy truck systems, and older ones at that, where fixed displacement, Tx valve systems are the norm. I would like to hear from those who work on the variable displacement systems. How are they to work on? how hard are they to diagnois and are special (dealer only) tools (scan tools etc) required?
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Re: Variable displacement compressors

Postby Tom Greenleaf » Thu Oct 08, 2015 8:06 am

Thread is now quite old Cornbinder and I've been having troubles logging in for some time now.

I can address some issues with changes expected with guesses on what will be.

First - any configuration of a thing containing gas/vapor is going to have to seal tight. Compressing a gas to change state will yet again have to use seals within or to atmosphere that don't leak.

CAFE or "Corporate Average Fuel Economy" is on the rise again so vehicle makers will once again go for the higher power robbing features A/C included.

I'd expect lots more electrically powered items and the elimination of belts at all. Toyota and others have already designed much of this. A/C by current and original designs for motor vehicles has compressed gas with the highest BTU power that is practical. An issue never really discussed much is we need lots of BTU for mobile A/C so simpler non gas concepts are NOT adequate yet.

One problem is you must change the air inside a passenger vehicle every so many seconds with or without A/C or people would suffocate not unlike a home refrigerator. That means ambient air is constantly being put into cabin area even on recirculate settings it must comply to the minimum.

If not for that I think you could just insulate the heck out of a cabin and fight off just the BTU a human body makes which is substantial by itself. Average human just sitting there can be worth 2,000 BTUs or heat added! Think. Just 5 people in an enclosed space are producing 10,000 BTUs of heat doing nothing. This noted if you've ever been in a crowded room even in a cold area/climate the room gets too hot.

So far it plain take energy to "take away heat" a lot more of an engineering challenge than making heat which is as easy as fire for example in control.

Some facts to consider: Air conditioning to be cool is as I just said "removing heat" rather than adding cool. There's a hidden challenge in that right there. It means that heat went some place whereas heating air doesn't mean cooler something went someplace - see what I mean?

It's been long achieved to remove heat without gasses at all. Ordinary coolers for food for example sold for years just use two heat sinks one with a fan blowing on one side removes heat such that the other is cool to the touch. Very low BTUs produced so would need to be HUGE for thousands of BTU force.
Off topic but if you can tolerate fine details know that Freon is a trademarked product by DuPont so know that if you capitalize "Freon" it suggests a certain gas, usually R-12. The word was entered into the English language for any gas compressed for air conditioning purposes if you do NOT capitalize the word. Like Aspirin was once a brand name by Bayer now without the cap is generic.

For A/C purposes use of the word "refrigerant" is more accurate as it doesn't suggest one type or another.

This is a technical and legal detail that would really matter if writing about an engineering project for A/C which is what you asked. FYI this site does discuss some details but isn't focused on the training and education part of how A/C work to details that involved.

If around ACProf could expound on this for A/C and components than may or may not have patent rights.

This particular board the regulars would get what you mean and that's what counts.

Forgive me, too many years in college now long ago when details mattered! Yes, you could fail a course for mistakes like that!

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Re: Variable displacement compressors

Postby Z2TT » Fri Dec 18, 2015 8:08 pm

By Direct Drive, If you are referring to electric compressors then power would be shut off to them when the pressure switch detects abnormal pressure.

As for how displacement varies - for example a common VD compressor being the GM V5 uses a mechanism to vary the angle of the swashplate to limit travel of the pistons. Less travel = less displacement. They don't totally goto zero displacement but it certainly is reduced a fair amount.
Adds more complexity to diagnosing when there is a fault. Example control valves can become defective in variable displacement compressors.

In terms of efficiency, variable displacement is a great way for it to work - but is the take off of added complexity worth it compared to a clutch cycling system that achieves similar results?
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