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Old 06-25-2021, 08:40 PM   #11 (permalink)
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If you really want efficient A/C and have good fabrication skills, you can try fitting a modern variable displacement compressor. Another strategy is to modify the existing system to reduce its output so it's more efficient when you're driving faster.

If I understand how they work correctly, slightly decreasing the amount of refrigerant in the system would make the compressor (which has a fixed pressure ratio) consume slightly less power at any given speed. It may need to be running at a higher speed before the condenser side has enough pressure to work well.

Another method is an underdrive pulley.

AFAIK, electric A/C is probably best left for retrofitting cars without A/C, since its main advantage is simplification of packaging, not efficiency or cooling effectiveness. You're going to need a pretty crazy amount of battery to run A/C for more than a few minutes.

What sort of driving are you doing where the AC is killing your mpg? At idle, I find A/C only adds a modest amount of fuel consumption (like 0.1gph ish). If it's travelling at high speed with the engine turning faster, I would first look to underdriving the compressor somehow.
Good ideas, thank you! Reducing the refrigerant charge will reduce the compressor's power draw, but I'm not sure doing so would be beneficial since the compressor has to run longer when the system is undercharged. Longer cycles are more efficient if the system is designed for it, but I would think undercharging the system would reduce its efficiency.

I like the idea of getting a variable displacement compressor, but even if I could get one to fit I still wouldn't have a good way to control it since those require a PWM signal from the climate control module to function.

I'm pretty sure underdriving the compressor would improve the system's efficiency at high speeds since it would cause the compressor to run longer cycles and therefore reduce the losses associated with short cycling but it would also reduce the cooling capacity at idle, which I don't want.

I mostly notice the AC's effect on my gas mileage when driving around 35-50 MPH, which I do quite a bit. At such low speeds, the AC's power usage is a larger percentage of the total power required. After the 5 speed swap I was averaging 36-38 MPG without AC, but now that it's hot out and I need AC I'm down to around 31-33 MPG doing the same kind of driving. ~5MPG is pretty significant to me.

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Old 06-25-2021, 10:51 PM   #12 (permalink)
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Besides some hybrid cars, most of the electric AC compressors I'm aware of are meant either to commercial trucks (as they enable sleeping without the engine idling) or motorhomes.
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Old 06-25-2021, 11:13 PM   #13 (permalink)
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Besides some hybrid cars, most of the electric AC compressors I'm aware of are meant either to commercial trucks (as they enable sleeping without the engine idling) or motorhomes.
I saw 12V AC compressors for truck cabs on eBay. The problem is that they are only rated for about 6500 BTU maximum. I'm sure that's fine for keeping a truck cab cool overnight, but not nearly enough to cool off a heatsoaked interior on a 100 degree day with the sun shining on the car.

I haven't been able to find an official spec, but I would guess that my stock AC system is somewhere around 18K BTUs while cruising down the highway with the blower on max. Someone quoted a service manual for a 6th gen Civic that said the system is 14K BTU, but the 6th gen has a half size condenser and my 7th gen has a full size condenser so I could see that adding some capacity.
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Old 06-26-2021, 12:38 AM   #14 (permalink)
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Or... adjust the nut behind the steering wheel... On my wife's Subaru, which has AC, I toggle the system on and off according to road conditions. Downhill = on. Deceleration in gear = on. But off when accelerating and climbing... quite effective for savings... no costs in parts or labor.
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Old 06-26-2021, 12:57 AM   #15 (permalink)
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Or... adjust the nut behind the steering wheel... On my wife's Subaru, which has AC, I toggle the system on and off according to road conditions. Downhill = on. Deceleration in gear = on. But off when accelerating and climbing... quite effective for savings... no costs in parts or labor.
That's a great idea, thanks. Sometimes I shut the AC off while climbing hills to prevent having to downshift, but I never thought of shutting it off during acceleration except at WOT when maximum acceleration is needed. I will try that method and see if I notice an improvement. The effect of the AC on acceleration isn't dramatic like on some cars I have driven, but it's noticeable enough that I could see how eliminating that drag during acceleration could measurably improve efficiency.
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Old 06-26-2021, 01:07 AM   #16 (permalink)
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That's a great idea, thanks. Sometimes I shut the AC off while climbing hills to prevent having to downshift, but I never thought of shutting it off during acceleration except at WOT when maximum acceleration is needed. I will try that method and see if I notice an improvement. The effect of the AC on acceleration isn't dramatic like on some cars I have driven, but it's noticeable enough that I could see how eliminating that drag during acceleration could measurably improve efficiency.
I can feel the car shudder slightly when the AC comes on in her Subaru, so I know it is significant, though I can't measure it. My favorite tactic is the deceleration one: when the engine is warmed up and the rpms are high enough, say 1500+ the ECU will turn off the injectors if I take my foot off the gas. That's when I hit the AC button. The AC load on the drivetrain is literally helping to slow the car for me. The tactic saves the brakes slightly and gives a little free AC.
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Old 06-26-2021, 01:13 AM   #17 (permalink)
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Maybe install a switch on the accelerator that turns on the A/C clutch whenever you take your foot of the pedal.

Maybe: Aluminum foil reflective roof.

Maybe: Water pump and cooling loops in driver's seat that go to a heat exchanger right after the evaporator so you get more of the cooling effect on your body directly.
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Old 06-26-2021, 02:27 AM   #18 (permalink)
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Good ideas, thank you! Reducing the refrigerant charge will reduce the compressor's power draw, but I'm not sure doing so would be beneficial since the compressor has to run longer when the system is undercharged. Longer cycles are more efficient if the system is designed for it, but I would think undercharging the system would reduce its efficiency.
My idea is that the cycles don't really matter as much as the efficiency when the compressor is actually on. If the compressor is pumping too much refrigerant, its efficiency is reduced a lot. With reduced refrigerant charge, it is almost as if the compressor is spinning slower. You have more frictional losses from having to run the compressor more, but the gas cycle itself becomes more efficient. I know, not the best solution...it could be worse.

One idea I came up with for retrofitting a car with no A/C is cooled seats. Some people have modded their stock seats with air channels and fans, but you can also get an aftermarket fan cooled seat pad, and then add a heat exchanger with something so you can cool down the air. An inefficient way to do it is a Peltier cooler. You don't need that much cooling power to actually lower the air temperature to the point where it feels comfortable since the air flow is really slow.

Another way to do it is to use a duct on the car's stock vents and blow it through your seat, and then underdrive the pulley. Since your body is being directly cooled instead of the cabin, you don't need as much cooling capacity.

Yet another idea along the lines of reducing cooling capacity is a mini 12V compressor unit that blows directly on you or through a seat cooling pad. Alternator power is not efficient, but you would be drawing far less power overall compared to the original compressor.
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Old 06-26-2021, 11:12 AM   #19 (permalink)
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My idea is that the cycles don't really matter as much as the efficiency when the compressor is actually on. If the compressor is pumping too much refrigerant, its efficiency is reduced a lot. With reduced refrigerant charge, it is almost as if the compressor is spinning slower. You have more frictional losses from having to run the compressor more, but the gas cycle itself becomes more efficient. I know, not the best solution...it could be worse.

One idea I came up with for retrofitting a car with no A/C is cooled seats. Some people have modded their stock seats with air channels and fans, but you can also get an aftermarket fan cooled seat pad, and then add a heat exchanger with something so you can cool down the air. An inefficient way to do it is a Peltier cooler. You don't need that much cooling power to actually lower the air temperature to the point where it feels comfortable since the air flow is really slow.

Another way to do it is to use a duct on the car's stock vents and blow it through your seat, and then underdrive the pulley. Since your body is being directly cooled instead of the cabin, you don't need as much cooling capacity.

Yet another idea along the lines of reducing cooling capacity is a mini 12V compressor unit that blows directly on you or through a seat cooling pad. Alternator power is not efficient, but you would be drawing far less power overall compared to the original compressor.
Generally longer cycles at only the capacity needed are more efficient than an oversized system short cycling and is better for maintaining a consistent temperature. This is in part why many higher end home AC systems have inverter driven compressors and/or multiple stages.

I think the compressor's frictional losses are negligible compared to the losses from pumping refrigerant as an unloaded compressor spins pretty easily. I like the idea of underdriving the compressor except I don't want to lose significant cooling performance at idle since this car's AC has never cooled great at idle to begin with, even after I rebuilt the entire system.

I think upgrading to a larger and/or more efficient condenser would improve cooling capacity as well as efficiency since the high side pressure would drop, reducing the work the compressor needs to do. I would need to take some measurements, but there is definitely room for a bigger and thicker condenser. The biggest problem would be that I'm sure having a custom condenser made would be very expensive and I likely wouldn't be able to find an aftermarket condenser that would fit and be better enough to be worth buying and having custom AC lines made to be able to use.

I like the cooled seat idea, but unfortunately I wouldn't be able to use it instead of AC as I have asthma and breathing hot humid air for extended periods of time doesn't work for me.
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Old 06-26-2021, 12:10 PM   #20 (permalink)
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Motor ideas, since this is not far off of something I'm messing around with...

Starter-generator from a (mild) hybrid would be compact and comes with an automotive pulley on it already.

Treadmill motors happen to have 17mm shafts on them, which matches most larger alternator pulleys I've come across. Smaller diameter than the starter-generators, but easily twice as long.

Of course, if you're going to run the A/C on it, you could just take it farther and make the engine a mild-hybrid. Nowhere near as straight-forward, mind you.

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