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Have You Wondered if Fuel Economy Increases Just After a Rain?

I don't have documentation, but it seems to be the case.

It should, or any time it is really humid out for that matter. At a given temperature, moist air is less dense than dry air.

Explanation here (http://www.theweatherprediction.com/habyhints/260/), more technical with equations here. (http://www.engineeringtoolbox.com/density-air-d_680.html)

Could the % of oxygen in the dry vs. moist air also mess with the needed air/fuel mixture and possibly MPG that way?

Also, the roads get cleaned up from debris as sand and stuff get washed to the sides. The road is smoother because of this.

I voted yes. It seems that as long as the water isn't pooling up the FE seems to go up.

Have You Wondered if Fuel Economy Increases Just After a Rain?

I don't have documentation, but it seems to be the case.

I have seen my best fuel economy segments on slightly wet road.

It is perfectly reasonable. Everyone know that wet road have less friction, and friction is not only a force preventing the car from sliding/skidding. There is also a force of attraction making the rubber stick to the road (atomic forces, really)

The hotter the asphalt the worse the effect.

Too much water makes the tires do work on moving the water out of the way, and the economy suffers because of that.

Not quite sure about rain, but there is a clear correlation between morning dew and high mileage. I would guess that after a rain, but with dry roads, would be a similar effect.

Update: morning fog is good too, usually. It's bad when you can't see the lights ahead and time them, as I found this morning.

This seems to be the operant theory behind the air mist intake mod that's been kicking around since at least the '70s. It involves using intake vacuum to pull humid air through a "bubbler" and into the engine. Supposedly reduces the amount of O2 available for combustion. Haven't tried it myself, don't know if I'm willing to spend that much of my time on something that smacks of snake oil.

This happens because the rain raises the relative humidity up close to 100% and humid air gives better mileage than dry air. Two reasons for this. Humid air has a higher density altitude than dry air since water is a lower mass molecule than either nitrogen, oxygen, and carbon dioxide, so your car is displacing less mass of atmosphere as it travels through humid air. Also the water vapor displaces some of the oxygen molecules that your ICE would normally inhale, so your ICE has to inject less fuel to maintain the stoch ratio. In essence, derating the ICE's power and increasing its FE. You also get this FE enhancing effect when driving in fog.

I wouldn't agree with the comment above about the roads getting cleaner as a result of the rain. I've seen as much stuff get washed out onto the road as gets washed off of it.

And are we going to even touch whether the water vapor converting to steam during the combustion process will lower engine temp (by absorbing combustion heat) and increase power nominally (through steam expansion)?

This is the kind of conversation that lets my mind get away with itself. Shame on all of you.

One of the heaviest fogs was this morning - 93.5mpg...summertime FE. This time of the year, I'd be very pleased with 87-88mpg.

Some will laugh, but I've toyed with the idea of putting in a fogger.

And are we going to even touch whether the water vapor converting to steam during the combustion process will lower engine temp (by absorbing combustion heat) and increase power nominally (through steam expansion)?


Since water vapor and steam are both water in gaseous form, this conversion doesn't apply. The main effect in derating the engine's power is just from the water molecules displacing other gas molecules (most importantly oxygen) in the atmospheric mix taken into the engine. It would be different if you were talking about injecting fine water droplets (mist) into the intake air.

Do you suppose that maybe the traffic just goes slower in fog and light rain and it may be that more than anything else that increases your mpg?
It was foggy this morning and I was going 45 mph and nobody was passing me!

Since water vapor and steam are both water in gaseous form, this conversion doesn't apply.

Yes, yes, you're right. But I suppose I mis-spoke - the water in the air would be tiny droplets (not vapor) that the engine would convert to steam, and only if they had a chance to make it past the filter...

While it might seem like the wrong way to go from within a hypermiling site, you might want to jump to a NASCAR site and ask the same question plus another. Do they do faster/slower in moist air (assume track is dry), and do they use more/less fuel. If the speed is same, then the fuel will tell you something. If they go faster/slower then the 2nd answer won't be as telling.

These guys are even more concerned than some of us about fuel use (although they don't mind burning fast) and a few extra laps on a tank of gas can be a huge deal. Just cause they guzzle fuel like fool, doesn't mean we can't learn something from them.

> Have You Wondered if Fuel Economy Increases Just After a Rain?

Hi Delta Flyer,

Would you kindly rephrase the poll question? Quite honestly, I've never wondered about the subject matter, so I would have correctly answered, "No." but that wasn't really the intended question, right? ;-) (My first clue about the real question was option 3, hard to tell :-)

cheers,
Bill

> Have You Wondered if Fuel Economy Increases Just After a Rain?

Hi Delta Flyer,

Would you kindly rephrase the poll question? Quite honestly, I've never wondered about the subject matter, so I would have correctly answered, "No." but that wasn't really the intended question, right? ;-) (My first clue about the real question was option 3, hard to tell :-)

cheers,
BillBillLin,

If you could be more specific on the suggested change, I'd be happy to do it.

Time for me will be limited for the next 2-3 weeks as I'm going thru a manditory certification process at work. Most of my pondering will have to be work-related. ;)

Thanks for all the time you've been putting in! That goes for the other mods, too!

> If you could be more specific on the suggested change, I'd be happy to do it.

How about, "Does fuel economy increase just after a rain?" Very minor...

Thanks and cheers,
Bill

How about, "Does fuel economy increase just after a rain?"Done

Hope that Helps

Done

Hope that Helps

Thanks! You've made an OCD person very happy. :)

Cheers,
Bill