DZA

So, is the heat affecting my gas mileage or is it that i have my windows rolled up? hotter air means less dense air, right? Less dense air means less fuel, right? It has been about 10-15 degrees warmer today than usual. my gas mileage has been a LOT better today.

My driving has been the same. I drive like an old man. I usually drive with all four windows down and the sunroof back and the A/C off, not even the blower on. When I usually drive down the highway, I can't even get to 27mpg. I could reset the gauge and it will immediately even out to 26.8 mpg. The temp is usually around 75 to 80... 82 at the highest.

Today, it's 94 degrees. I had all four windows up and the sunroof closed and the A/C on FULL blast. I reset the mpg gauge (the running average not the current "what you would be making if you drove how you're currently driving"). I make a turn at a traffic light, hit one more traffic light before I drive 10 minutes straight at 55-60 mph (no cruise control). I started off at 38 mpg but assumed it'd even out to 26.8 mpg again. 5 minutes later I'm at 33 mpg. I bump up to 65 mph for 3-5 more minutes and never go lower than 30 mpg. After 10 minutes of no slowing down, my average was actually exactly 30.0 mpg.

The fact that I AVERAGED 30 mpg when I can usually never get better than 26.8. Before this, honestly, I've TRIED to get better and I was able to get 27.1 once with a bit of coasting.

What I don't get is that the A/C was on all the way... I thought A/C hurt the mileage? Did the heat affect it at all? Was it mostly my windows being up?

Avantadrive_Original

heat DECREASES gas mileage, because more engine power is lost as heat when the engine is hotter, meaning less efficiency, meaning more gas required to drive the car. The A/C definitely should take even more away from your gas mileage.

Having the windows rolled up should help because of aerodynamics, but I don't think the difference would be huge by any means.

TonyPNS

I use the a/c too. I've got 420 miles on my current tank.

DZA

Interesting... would you consider 4 mpg increase significant for hwy miles?

TonyPNS

Two Chicago S4 owners drove together from Ohio back to Chicago. One ran with AC and windows up and the other ran without AC and the windows down. The one without AC and windows down had to stop for gas first.

Tgr_Clw

That's because the vehicle's aerodynamics get screwed up with the windows down at high speeds.

Avantadrive_Original

if you see how the air flows over the car, and feel the amount of wind that enters through the windows when driving at high speeds, it's entirely conceivable that your drag would increase enough to drop your mileage. Actually, I wouldn't be surprised to get more than a 4mpg drop now that I think about it.

Have you ever put a roofrack on your car with large objects on it? Same idea, and that definitely reduces your gas mileage!

diagnosticator1

First, the A/C compressor on our B6 A4s is internally controlled, variable displacement type. That means that the amount of refrigerant the compressor pumps to the evaporator is exactly the amount needed to absorb the current heat load on the system. So, once the system has cooled the interior down, the A/C compressor's load on the engine is very low. The A/C compressor drive is continuous while the A/C is on, it is not cycled.
The windows down, sunroof open mode results in a lot more aerodynamic drag on the car than windows closed. The windows open air resistance is much greater load on the engine than the A/C is at set point. There may be a very small reduction in aerodynamic drag at 95 degrees F compared to 45 degrees F.

diagnosticator1

Heat flows from warmer places to cooler places by convection, conduction and radiation. In the case of an operating internal combustion engine, the most recent cooling strategy is to run the coolant as hot as possible. 220 degrees F is not to hot for our A4s. The reason is that the higher cylinder wall and head temps help minimize heat transfer from the combustion process gasses into the cooling system. The less difference in temperature, the less heat flow into the coolant. (There have been many attempts to operate gas and diesel engines uncooled. The reasoning being that if the engine was near the same temp as the burning gasses, then the heat losses into the cooling system will be about zero, and then that much more fuel energy would be available for work output instead of being dumped to the atmosphere.)
The higher coolant temps now used also help minimize exhaust emissions.
The B6 1.8T was the first application of "mapped cooling" by Audi. That is a heater element in the thermostat housing controlled by the ECU. By varying the current to the tstat heater, the ECU is able to control the open/closed state of the tstat and modulate the tstat so that engine temps follow a load related cooling map. The cooling maps call for high coolant temps at light loads and at idle, and cooler temps at high output, high loads. Mapped cooling helps maximize fuel economy and minimize exhaust emissions.

DZA

yea, i remember hearing (come to find out it was from mythbusters) that you'd get better mileage with windows up and A/C on... but 4 mpg is more than I expected. Isn't the highway mileage of a A4 3.0 24... Did they just under estimate or did they have their windows down while testing? hah