How to improve AC efficiency

[Piwoslaw]

^^ So maybe some insulated thermal mass? And/or phase-change material?

You could "add cool" to it when engine braking, or lengthen the on and off cycles of the A/C compressor, reducing the number of times it turns on. Maybe a second A/C compressor, to get the most out of engine braking?

If facilities are available, you could also charge this "cool storage" via plug in before you start a trip. For this you may want A/C run off of an electrical compressor, not off the engine. Then you could also use batteries to reduce the load on the engine/alternator.

[cRiPpLe_rOoStEr]

Originally Posted by oil pan 4

Yes, European cars tend to have better more efficient A/C systems.
Probably bad something to do with paying $7 a gallon for gasoline in their home domestic market.

And sometimes a noticeably smaller engine for which the burden of an iffefficient A/C system would become more troublesome performance-wise.

[arcosine]

Have a vacuum motor drive a compressor. What about adiabatic expansion of the intake air though the throttle, that might be used for cooling especially if you vaporize a liquid at the lower pressure.

[hayden55]

Not really efficiency of compressor wise but on the Prius this summer I went ahead and did 70% crystalline on the front windshield, and 40% crystalline on the driver and passenger windows. Metered out altogether with the factory tint it netted at 56% front visible, 25% sides visible, 79% energy rejection on the windshield, and 78% energy rejection on the side windows. Also, to save extra money I paid in cash, still got the lifetime warranty in the USA, and skipped tinting the rear windows.
Last summer on short trips around town it was much more bearable to crack the pass window and a rear window for some air flow to avoid using the AC. I honestly never really used the AC around town unless I had somewhere I needed to be a didn't want to be hot when I showed up. Past that I noticed my AC didn't howl as much on the interstate on auto anymore. I could set it to like 73F and not get cold hands and sweaty arms from getting a sun burn and blasted by cold air simultaneously.
I did it in my civic too a long time ago. I drove with the sun constantly on the driver's side so I had them tint the driver's window and got a scrap piece of tint for the front window in front of my torso and hands. Made it a lot more bearable since the AC cooled maybe 2 degrees. lol

[oldtamiyaphile]

Not tinting the rear window on a liftback is like, I dunno something about a horse bolting?

Much more heat gets in through the rear screen than the front, so while tinting the front (windscreen tinting is of course generally illegal) may stop your skin burning, I won't really change in cabin temps.

I put a silver window shade on my Prius cargo cover to reflect heat back out, which can be left in place while driving.

[Joenavy85]

Putting a second receiver/dryer into the system with a mechanical txv instead of a bleeder orifice will give you more refrigerant capacity and better control of it.

[hayden55]

Originally Posted by oldtamiyaphile

Not tinting the rear window on a liftback is like, I dunno something about a horse bolting?

Much more heat gets in through the rear screen than the front, so while tinting the front (windscreen tinting is of course generally illegal) may stop your skin burning, I won't really change in cabin temps.

I put a silver window shade on my Prius cargo cover to reflect heat back out, which can be left in place while driving.

I can see your point, but its mostly direct sunlight onto skin where you feel the heat (sitting underneath a tree vs unprotected is much cooler). Plus, I'm only cooling myself at the front and not the rest of the cabin.

[oldtamiyaphile]

Originally Posted by hayden55

I can see your point, but its mostly direct sunlight onto skin where you feel the heat (sitting underneath a tree vs unprotected is much cooler). Plus, I'm only cooling myself at the front and not the rest of the cabin.

The Prius actually relies on it's A/C to draw cool air into the battery, so you've really got no choice but to cool the whole car. In other cars you could add an air conditioning curtain, to confine the cool air just to the front seats.

Even in my van where I've got a solid steel bulkhead, I find heat from the cargo compartment slowly seeps forward into the cab, so keeping the cargo area cool is almost as important as the cab. If I ever get a sewing machine I plan on adding a nice quilted blanket to aid in insulating the two.

Another thing is you're allowed much darker tint on rear/ rear side windows, so the tint will be more effective.

Particularly on a Prius, if the battery is kept cooler you might get a direct improvement in economy for a very small spend.

[Occasionally6]

For several years I have been using a relay connected in series with the A/C switch and activated via an R-C based timer triggered by a switch on the brake pedal. This means the A/C can be on only when the brake switch is closed with a delay before it is switched off after the pedal is released.

The delay - c. 12 seconds - sets a minimum cycle time for the A/C to be on, so preventing rapid cycling of the compressor, and only a brief, light, touch on the brake pedal to activate when the brakes are not otherwise needed to produce the desired rate of deceleration.

A second switch to bypass the relay is required for driving where the brake pedal is not pressed much eg. on motorways.

Like Teoman, I have found that with the A/C off there is a limited mount of heat storage capacity available in the evaporator and comfort would be improved if this could be increased.

Simply increasing the refrigerant capacity won't work as the compressor is required to pump the refrigerant around the system as well as compress it.

Piwoslaw's suggestion of a phase change material around a heat exchanger could be practical. This is used in the evaporator in cars that implement idle stop-start.

Swapping the evaporator for one that includes a PCM would be expensive, even if you could find one that fits. It might be possible to contrive something that attaches to the vent ductwork, whether behind the dash or in front of it. I'm thinking of something like a small diameter tube and shell heat exchanger in a custom housing/shell - like a section of PVC pipe.

The OEMs use a wax/paraffin as the PCM. For an auxiliary heat storage device I'm guessing a melting/freezing point of 10-15 C would be about right, given that it will be downstream of the evaporator.

The heat storage capacity will be fairly small. The evaporators used as OEM seem to claim around 1 minute of additional cooling. This would be useful but not conducive to pre-charging outside of the car.

[teoman]

Pvc pipe is thermally fairly non conductive

I will measure the on time and the off time of the compressor to get a rough estimate on what it likes and design around that.