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  #1  
Old 09-02-2003, 08:50 PM
Bud
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A/C Efficiency

I've recently moved to Arizona and am thinking of buying a new car. Now that I have to drive in stop and go traffic with temps well into triple digits, air conditioning has become very important.

Would anyone know if the A/C in the current E-Class is any more efficient than that in the W124? Is it pretty much the same system?

Do American cars have more efficient A/C?

How about Lexus?

TIA,
Bud
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  #2  
Old 09-02-2003, 09:27 PM
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The AC system in the 1996- model year MB's is the newer variable pressure compressor system, and it seems to work very well. Even in 100F temps, it seems (in most models) to get to a 50F temp drop, which is serious cooling.

We found the AC in our 1998 E300 very capable and never wanting, even in 100F temps. I imagine the W211 system is probably VERY capable, and as good as any to be found. MB systems were thought of to be quite weak, and the complaints piled up, so I would think later is better for AC power.

I rode in an older high mileage LS400 one very hot day, and it cooled very well.
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  #3  
Old 09-02-2003, 09:55 PM
Bud
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Thanks John, that's just the kind of Info I was hoping to hear.

My hope is that MBUSA will import the E320 CDI next year.
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  #4  
Old 09-03-2003, 06:20 AM
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John,

Could you elaborate more on the variable pressure system? I'm pretty familiar with commercial a/c systems and somewhat familiar with auto a/c systems. I know that sanden is now selling scroll compressors for automotive use, but I've never heard of variable pressure before.

We are designing a bigger (15 ton), constant run a/c system at work that uses copeland scroll compressors, a Sporlan electronic discharge bypass valve, and a head pressure control valve. We needed all of this hardware so the system would run reliably at low and high ambient outside conditions.

We wanted lieberts variable displacement compressors which have a valve release in each cylinder to cut them in and out as needed. These were too noisy for our application, however.

I also am aware that MB has traditionally used thermo expansion valves instead of orifice tubes like most domestic manufacturers. This expands the freon at the maximum allowable flow rate without sending liquid back to the compressor. On a domestic a/c system, you need a suction side accumulator to store the liquid freon that passes the evaporator so it doesn't damage the compressor.

That being said, I own two MB's converted to R134 and neither cools worth a poop. Granted one is 20 yrs old and the other 30, but my impression is that MB's climate control systems are not up to domestic standards.

Sholin
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  #5  
Old 09-03-2003, 10:05 AM
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I can't give you specifics of how the compressor works internally, but here is my understanding:

The compressor runs all the time, and in order to keep the evaporator at optimum temperature, the compressor's "compressing" is variable. In older MB systems the compressor cycles on and off, but the newer models don't. This means that the compressor is running constantly and this is better for compressor.

Steve Brotherton knows more about these systems than anyone I know, and might be able to provide specific details of how the systems work.
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  #6  
Old 09-03-2003, 07:06 PM
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That really is very interesting. I am familiar with constant run systems, but I've never heard of this applied to automotive use. There are some inherent advantages to having the compressor run continuously. The most important one is that it saves wear and tear on the compressor. It also allows for continuous cooling to be available (no delay).

We are designing a constant run system for our radar. It is very tricky to make the compressor always run under all ambient conditions. To make this work, you have to EXACTLY balance the heat load to the cooling capacity. You also have to ensure that you don't return liquid freon to the compressor--very bad.

How we are doing it is with a discharge bypass valve. This works by venting hot compressor gas back to the suction side, raising its pressure. While this sounds very inefficient, it isn't as bad as you would think. Raising the suction side pressure naturally reduces cooling, keeps the evaporator from freezing, and lightens the load on the compressor.

Other systems work on the variable displacement principle by enabling/disabling the required number of cylinders.


Sholin
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