A/C vs MPG test: impact of air conditioning on fuel economy (2015 Nissan Micra)

[Carbon]

Originally Posted by Daox

A/C systems normally cycle on and off as the pressures in them differ. You're really not adding any additional wear and are probably lessening it from using it less.

This. ^^^

[PaleMelanesian]

I measured my Fit at 10% at 60 mph. 38 mpg vs 42.5. That makes this Mirage's 3.7% loss look good.

On the other hand, your 62 mph numbers look low. 39.8 mpg vs my 42.5, both with AC off.

[MetroMPG]

Note, this is for the Nissan Micra - 1.6L, 4-speed automatic.

The Mirage's MPG is higher.

I confuse them sometimes too. Added a pic to post #1 to clarify.

[Cobb]

Hows the ac performance or does it get that hot and humid up there? I had a 81 240d mercedes made in Canada and the AC sucked at all speeds.

[MetroMPG]

I dunno. Response to heat & humidity is relative.

I've owned around 25 cars, 2 of which have had A/C (including my new winter beater 2007 Civic). I'll turn it on a few times to make sure it works.

But other people I know would wilt in the 3 or 4 weeks of "hot" weather we get each summer.

For those weeks, I put the beaded seat covers in the Firefly, and that's good enough.

I'm sure the Micra's A/C is awesome enough to reverse global warming with the windows left open.

[niky]

I'd stand by the idea that AC becomes less of an issue at speed.

At speed, you have more air going through the condenser. Which means the compressor cycles off more. As Daox stated, it will do this as a matter of course, without input from you. And the wind cools down your car, too.

Perhaps if the compressor was a constant load, and never went off, then it'd be different. You'd need to live somewhere really hot to test this, though.

[Cobb]

You got a point. I know for acceleration it kills it on my old geo and my insight, but once up to speed I dont feel it.

Originally Posted by niky

I'd stand by the idea that AC becomes less of an issue at speed.

For me, my test of ac is does it get my glasses cold enough so when I exit the vehicle they fog up.

[niky]

I haven't run my AC that cold in years.

Typically, I run it so low (when I'm alone) that the variation I see in economy between off and on is pretty small.

[serialk11r]

Originally Posted by niky

I'd stand by the idea that AC becomes less of an issue at speed.

At speed, you have more air going through the condenser. Which means the compressor cycles off more. As Daox stated, it will do this as a matter of course, without input from you. And the wind cools down your car, too.

Perhaps if the compressor was a constant load, and never went off, then it'd be different. You'd need to live somewhere really hot to test this, though.

This is possible. However I think the temperature control on newish cars (newish being like 2004+) is done by adding heat to the air conditioning not cycling the compressor, and I have a strong suspicion my car doesn't cycle the AC either. It blows about equally cold at any speed but I think it may be that the cooler condenser at higher speed is making up for the extra revs wasting energy at the compressor and heating up the system.

I wish there was a way to tell if the AC were cycling, but my instinct is it doesn't on my car because when I manually cycle it, I can immediately feel the air starting to warm up, and that never happens with the compressor on.

[ijames]

My seat of the pants analysis is that a/c efficiency goes up a little with speed, due to the extra airflow over the condenser, but lets ignore that for now. The heat load on the cabin should be constant with speed, so the hp required to cool to a given target temperature is constant as speed varies, and if the bsfc is constant (an assumption but not bad over a narrow range) then the fuel burned per unit of time for a/c is constant. If the fuel burned per unit of time is constant and you increase the speed you decrease the fuel burned per mile in proportion to the change in speed, so the a/c's effect on miles per gallon drops with speed. The extra efficiency we ignored earlier helps make this effect a little bigger since the hp isn't constant but drops a little with increasing speed. One problem is that I did lie when I said that the heat load would be constant. Solar heat load, yes. However, the cabin is cooler than the ambient so there is conductive heat flow into the cabin as well. The R value of the cabin is constant (ignoring the minor nit of air leaks increasing with speed), but increasing speed will transport more heat to the outer skin of the cabin so the conductive heat load will actually rise with speed. Think of the car sitting still, so there is a nice, thick, almost stationary layer of air on the exterior. The temperature of this layer will be slightly cooler than the ambient due to conduction into the cool interior. Now start moving and you blow away more and more of this layer, replacing it with ambient temperature air. How much does this matter compared to everything else? No real quantitative clue, sorry . My guess is not that much for high R values but lots of the cabin wall is single layer glass with an R value of about 1-1.2, so the external boundary condition will matter more, but it still won't trump the primary effect described above. Anyway, my $0.02 worth of analysis.