Quote:
Originally Posted by serialk11r
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.
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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.
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.