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

[chumly]

My smart ac switch is a button on the dash, it's easy to just push it and p&g the ac and the car at the same time. It's possible to get better mileage doing this and being cool than just holding a steady speed with the ac off and being hot and uncomfortable with just the window open.

[serialk11r]

I think the decreased fuel usage at higher speed might be an anomaly in data collection or something. If you look at a BSFC chart that has lines going all the way to the 600g/kWh mark, and plug and chug some numbers, you'll notice that the additional fuel used when adding load at idle can be even less than additional fuel used adding the same amount of load in the higher efficiency islands. This makes sense, because when the car is under very low load, an increase in power demand only takes a little bit more fuel because you're derestricting the engine, allowing the combustion to take place at higher temperature, etc.

In other words, the efficiency increase from adding that load is enough to make the marginal fuel usage small. For the same reason that reducing your drag by 10% doesn't increase your mpg by 10%, increasing load by 10% does not decrease your mpg by 10%.

[niky]

Here's some data that actually backs up MetroMPG's testing:

The mpg mythbusters | Auto Express

[serialk11r]

Originally Posted by niky

Here's some data that actually backs up MetroMPG's testing:

The mpg mythbusters | Auto Express

As far as the speed thing goes, I think that actually could go either way. The EPA says in "city" driving your engine tends to be running at higher loads and thus more efficiently. Thus if you buy my argument, it makes sense that on the freeway you'd be using less gas to run the AC.

In that test in particular, I'd expect the "city" portion to go badly as it did. Idle goes up, engine cycling up to 3000rpm (and thus running the compressor very fast) are all going to sap a lot of fuel.

[MetroMPG]

Originally Posted by serialk11r

I think the decreased fuel usage at higher speed might be an anomaly in data collection or something.

I definitely wouldn't rule that out either.

[LeanBurn]

Anyone had any long term data to show the wear and tear on the A/C compressor from multiple on/off cycles in a 30 min trip?

[kafer65]

I worry a bit about cycling my A/C so much, but I noticed that even if I'm coasting down a long hill and using the A/C to engine brake it will still cut in and out on its own so I'm going to try and not worry as much. I'm getting pretty good and using my index finger to hit the botton off without looking from my shift knob when I take off from a stop. I usually run my fan on 3 to pack the cabin full of icy cold air to store for uphill mpg battling with A/C off. Drives my wife crazy when she's along so I'm not as dilligent in that senario.

[digital rules]

Originally Posted by LeanBurn

Anyone had any long term data to show the wear and tear on the A/C compressor from multiple on/off cycles in a 30 min trip?

Great question, was curious about that as well.

[MetroMPG]

As an aside, these Micra results would also apply to the 1.6L Versa Note and Versa sedan. Same platform/drivetrain/components.

[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.