Fuel economy in 100+ degree weather.

There are several threads on here regarding high temperatures and impact on mpg. I live on the coast of NC and the temps have been about the same as you. I have had only about a 1-2 mpg drop due to the high heat and having to run the AC full time. It is pretty flat so I do not have a lot of hills, etc. I am still averaging about 48 mpg which is calculated at the pump.

With only 1500 miles on the car, you should see a slight increase in mpg as the car breaks in.

 

depends on what you were getting in non a/c weather. the differential shouldn't be more than 5-6 mpg.

 

Double check your tire pressures. Most of us are running in the upper 30s to mid 40s (PSI).

 

I'm in Huntsville AL and there is a hit due to the AC. My mileage is a little better but I'm often taking the dogs to a nature trail and AC to the trail doesn't make a lot of sense (I'm not walking the dogs from the car!)

Our NHW11 really takes a beating due to the less efficient air conditioning system. But there is a lot to be said for 'stayin alive.'

Birmingham would be a special challenge. You have a lot of steep hills and streets. Getting into the 50 MPG range will be a real challenge.

Bob Wilson

 

115 here in July and yes, mpg goes down when it's hot. It's worth it and I've noticed better mileage after about 2K miles on the car. Take it on a few road trips as it's good for the ICE.

 

I see a very modest hit from AC to my fuel economy when ambient is in the 90's F, but I think it is because I live in a low-humidity area.

I just returned from a 215 mile trip, almost all highway at 55 - 64 mph. MFD shows 63.8 mpg. AC was on most of the time set to 78F, and ambient high 80's F. My fuel economy tends to be higher than most results because the driving is 5000 - 7000 ft altitude.

 

I seem to get minimal drop in mileage w/ AC on on longer rides--perhaps the traction battery keeps cooler, runs more efficiently (and lives longer) when fed cool air.

Q for the engineering folks: Does humidity cause a hit? My SWAG is no, since the car doesn't cool off by 'sweating' (like we mammals do) and drier air is denser, giving a theoretical, at least, lessening in wind resistance at high humidities. Y'know, all that PV=n/RT stuff from high school chem class.

Others' thoughts?
~T

 

What the humidex read when you over 100
At 6:30 am it is 72, humidity at 94% and it feels like 88

And it thick out there

 

What I have noticed with AC in hot weather is that it takes the AC running on high for a while to cool down the cabin. This is the biggest hit on mpg. Once the cabin is cool and the fan and the AC slow down to just a few bars to maintain cabin temp, then I no longer see a hit to mpg but normal operation. So, on short trips you will see a bigger mpg hit than on longer trips.

As for humid air, it is more dense than dry air, so the energy to overcome the heavier air is more.

 

Humid air is less dense. Hence the reason the barametic pressure gets lower as a storm approaches.

 

I'll admit that in Wisconsin we've only been in the upper 80s, almost all summer. That's extremely hot for us. We haven't hit many if any days over 90-that too is unusual.
What I've noticed is that in the winter say below 20 F, I get mileage in the mid to upper 40s, in the summer (both '09 and'10) I have little trouble approaching 70 mpg. Granted, that's usually city driving. No special tricks except easy on the accelerator, look as far ahead as possible for traffic lights and use the same route as often as possible.
This time of year if I'm below the mid 60s mpg, I'm disappointed.
Tony Renier
Green Bay, WI

 

Would humid air create more or less drag on the vehicle at highway speeds?

 

jdcollins,
I am no expert to be sure, but I thought the energy hit from humidity was the energy required to dehumidify the air, not just to cool it down.

 

I am by no means an expert either. I agree with the added energy to dehumidify the air.

I thought for sure that I saw on here where humid air created more drag than dry air, though. Can someone on here answer this for me?

 

You probably read that somewhere but there are lots of opinions on here which everyone should keep in mind.
Anyway, I looked up wikipedia real quick (not a real definitive source either) and found this...
[ame="http://en.wikipedia.org/wiki/Density_of_air"]Density of air - Wikipedia, the free encyclopedia[/ame]

Look under water vapor.

 

In high humidity conditions you are also reducing the amount of oxygen in a given volume of air entering the engine. This will reduce HP/TQ and reduce volumetric efficiency, particularly if barometric pressure drops significantly. Exactly how this affects part throttle efficiency I cannot explain but it definitely has a large effect, negative, on full throttle situations.

jrflip is correct when he says humid air is less dense than dry air at the same temperature. This is because the molecular weight of H2O(18 g/mol) is less than air (29 g/mol). So according to Avogadro's Law, the number of molecules in a given volume of air remain constant at the same pressure and temperature, so by increasing the number of H2O molecules you will decrease the number of O2/N2 molecules and end up with less dense air.

 

If you keep the same part throttle setting you will have less power, if you want the same power as with air with more O2, you use more throttle...up until you hit the limit of full throttle.


A minor nit on your "volumetric efficiency" comment. Outside air pressure and air/water vapor mix aren't normally considered in determining volumetric efficiency. It's the ratio of gas volume moved into the engine/displacement of the piston on the intake stroke.

Intake stroke being defined as volume displaced by the piston during the intake stroke, ignoring valve timing.

 

I know what you are trying to say but I disagree. Air density is part of the VE equation because CFM is based on air density (total volumetric flow rate). This is why VE can change with added pressure via a turbo or supercharger or with an increase in barometric pressure. This is partially why I always run faster in the 1/4mile on cool dry days vs. hot and humid days. The density altitude is much lower on those cool/dry days.