This is a very complex question because these compressors are made to be deliberately inefficient !

The problem is that the compressor has to work over a fairly wide range of evaporator and condenser pressures, and over a wide Rpm operating range as well. To try to keep flow and drive horsepower within a reasonable range, both the compression ratio and valve sizes are designed to intentionally restrict the performance.

Valve sizes are made deliberately small so that as Rpm increases flow through the compressor will not increase at anything like the rate of Rpm increase. If the compressor displacement is oversized you will get reasonable cooling at idle, but pumping horsepower does not then rise excessively to the point of belt slippage at peak Rpm.

Likewise the compression ratio (valve plate clearance volume) is made deliberately large so that flow falls off as the head pressure increases. This is done to limit the maximum head pressure and compressor drive power if the condenser temperature ever becomes dangerously hot.

I am not sure how you would actually rate the efficiency of something like this, because there is such a wide envelope of possible operating conditions. Actual cooling performance in the car is probably more important than mechanical drive power to the designers of these things.

Thanks Warpspeed, very enlightening and informative! I might then get the rest of my answer by looking into an air conditioned vehicle from a quality manufacturer, and which vehicle has a large interior size and a small engine displacement, and see what they grabbed off the shelf...

That sounds like an excellent idea.  No doubt the original vehicle design engineers have gone all through this same exercise, selecting an appropriate compressor and pulley drive ratio.

As you rightly point out, a small underpowered vehicle is not going to have a lot of surplus power available, and airconditioning is now becoming pretty standard even in the smallest cars.

As someone who is INTIMATELY involved with A/C compressors, I have to say that DENSO compressors have teh best reputation for quality.

Thanks guys! I'm not at all surprised that Denso would be in the top category for reliability. I will likely be using a Denso starter and Denso alternator, so I guees it will end up a package deal! I'll try to post back and let you know the results of the experiment.....

Starspangled

Whats the thought on running an electic compressor. I know your putting more of a load on the charging system and thus more load on the engine. But I would think the electric motor would be more effecient being able to run at a contant rpm. Some of the hybrids I think are going this way. With 24 volt systems on the horizon it would seem more feesable.  

edit 42 volt systems

Sorry I didn't get here folks: my new scurity system decided that Engine Tips Forum was one of the bad guys and hid the reply notifications. Thanks for your feeedback!

I've been looking for a Denso electric a/c compressor but can't seem to find any online. Could go to my local toyota dealer but looking for alternative. Even a site with spec could satisfy my curiosity.thx

mike

Haven't got a clue on that query myself, Mike, but hopefully one of the other folks will chime in and help you out.

Strictly from the thermal output to parasitic drag perspective the scroll compressors have about a 10 % advantage over the recips as a class (80% Vs 70%). What Warpspeed said about the performance "clipping" can be the negative about scrolls. Their efficiency just hangs in there to about 7000 RPM and without adequate condenser air flow can cause overpressure catastrophys. The solution is robust pressure limit controls, not difficult but necessary with scrolls.
The Electric / scroll units are so intreguing, but they are not 24/42 volt. I have been working on "direct drive" compressor designs for twenty years and they are all too heavy or too underpowered, until now. Hybrid cars have the magic, 177 - 250 volts. I have a 2005 Prius, But I also can not find a compressor from a wrecked one. With 150,000 out there you would think there would be one that wasn't being rebuilt.

I have a 1989-90 Dodge Intrepid parts car that may have a Denso on it, as they used Denso starter and alternator I think? If it had been a snake......ha ha!