My neighbor, who normally gets 50 mpg on a trip, drove 300 miles through heavy rain, and on that leg of his trip managed only 43 mpg. He wants to know why. My own theory is that the car had to do extra work pushing aside water on the road, and a little more work in collisions with raindrops. A wet car is not as aerodynamically clean as a dry one but my guess is that effect, which is significant on sailplanes, wouldn't be significant for the car. In my experience, internal combustion engines run best on damp days, but of course, Prius is shorthand for "a whole new ball game." Has anyone out there had a similar experience and does anyone have hard data or even a better guess on the subject?
Last edited by budput; 08-30-2006 at 01:30 PM.
Reason: to remove a CRLF
Regarding rain and engines running better, well, that's true, particularly in a normal Otto cycle engine, but less so in an Atkinson cycle like the Prius.
Basically rain = cooler weather. Air contracts when it's cooler, thus contains more oxygen, and other gas, molecules per CC of air drawn into the cylinder. With more oxygen in the cylinder you better combustion and more total gas will expand more forcefully under the heat.
The Prius has a Atkinson cycle engine, and if my understanding of them is correct their low compression ratio (compared to expansion ratio) would cause them to draw less benefit from the increased gas density.
As for MPG loss, there's probably a lot of small factors here, most of which have already been covered:
1) higher tire rolling resistance
2) lower tire traction causing some energy lost in slippage (really small)
3) higher wind resistance due to the rain drops (really small)
4) lost momentum that's transfered to the rain when the car hits them.
5) rainstorms are often windy, if there's a headwind this will increase drag.
5) Driving habit changes, although as I see it most folks drive slower in the rain, and that should help, not hurt, mpg.
6) Although cooler air adds HP, it also cools the engine faster, so the Prius must run it more often to keep it warm.
My guess is 4 and 5 are the largest effects, at least in a heavy rain.
My neighbor, who normally gets 50 mpg on a trip, drove 300 miles through heavy rain, and on that leg of his trip managed only 43 mpg. He wants to know why. My own theory is that the car had to do extra work pushing aside water on the road, and a little more work in collisions with raindrops. A wet car is not as aerodynamically clean as a dry one but my guess is that effect, which is significant on sailplanes, wouldn't be significant for the car. In my experience, internal combustion engines run best on damp days, but of course, Prius is shorthand for "a whole new ball game." Has anyone out there had a similar experience and does anyone have hard data or even a better guess on the subject?
I noted exactly the same effect when driving 30 mi in one of the typical summer downpours. Normally for this drive I get 50/52 mpg. For this rainstorm it was 43/45 mpg.
This is just another one of the additions to 'driving knowledge' that the Prius and hybrids in general have brought out. I too theorize that the air is 'thicker' with the droplets in it ( akin to running through a swimming pool - think of the extra effort ) as well as being less aerodynamic as you note and having to push aside the water in the puddles
By driving the Prius I've been able to quantify 5 specific conditions that deteriorate fuel economy plus one that aids it:
short trips ( under 10 min ) : -20%
cold weather driving : -10%
rain storms : -15%
into the wind : -10% ( varies by severity )
with the wind : +5%
highspeed driving ( > 70 mph ) : -10%
I can see you are a careful observer, Kdhspyder. An additional variable that may be important is the interval between short trips. If the interval is short the Prius trick of pumping coolant into a thermos, to be used for a quick warmup in the next trip, is effective. If the interval is long enough to cool the water in the thermos, the trick has a small negative effect because energy is used to operate the pump. -Budput
I drove in rain (and headwind) for nearly 2 days at the end of August. From the western border of Nebraska (overnite in Omaha) to Madison,WI; where the rain finally subsided. Even though there was an overall descent of 4000', my mpg dropped to 44, as compared to about 46 mpg on the uphill trip west to Denver.
So yes, the rain had an effect, as did, I think more-so, the gusty wind. Three out of 4 wind directions are bad...head, left cross and right crosswinds.
I tried the constant "instantaneous mpg" peadal-position technique for a couple hundred miles when traffic was very light. That is, disengaged CC and held 50 mpg on the uphills and downhills. Wierd, but interesting. I was able to push the mpg up from 44+ to nearly 47 before traffic made that procedure an irritant to other drivers.
The Prius sure provides a lot of ways to entertain yourself. Who needs video games when you have a Prius MFD?
The short: Moisture/High Humidity during atomization in theory slows the fuel burn. That becomes a MPG boost. All other things being equal. I won't address the Hybrid factors. I'll leave that to the Hybrid owners experiences.
LT
Effect of rain on gas mileage
Linear Mode
