Fuel Economy in the Rain

I would bet that lower temperatures in the engine bay from all the additional water cooling and increased rolling resistance from water on the road are the main culprits. This would apply to all cars - but no other cars other than Hybrids loop you into to your moment to moment MPG so you probably wouldn't notice in your average car.

 

When driving at speed, air resistance is a significant fraction of your consumption. Thicker air (rain, cold air, wet air, air compressed by the bow wave of trucks, etc) is harder to drive through, so your mileage goes down. Your tires throw rain back up into the air again, that uses energy.

 

Thanks, I was wondering why my mileage was down slightly. I thought it was resistance but wasn't sure, this makes alot of sense.

 

You can expect up to a 10-12 drop in MPG's due to rain and cold weather.

Nothing like warm dry weather for the prius.

alfon

 

Windshield wipers also disturb the aerodynamic.

 

Not true. I've seen an instantaneous MPG display on a Toyota Sienna owned by a friend's coworker years ago.

Nissan's been putting on them cars, see Nissan's new models to get fuel efficiency gauges — Autoblog Green and Nissan/Infiniti News Room.

 

Water vapour is less dense than dry air, so wet air is less dense then dry air. Likewise hot air is less dense than cold air. Any pilot can attest to the increase in density altitude on a hot, humid day.

This means there is less air resistance on hot humid days. Liquid water is a different story. Most of the increase in drag from rain comes from pumping water out from under the tires. There is also some additional drag from hitting water droplets in the air. Windshield wipers are another small factor.

All other factors being the same, rain will also remove heat faster from the engine and exhaust system. In the case of the Prius, this heat must be replaced by burning fuel.

All cars suffer from the effects of rain to some extent, especially the pumping losses from the tires. You see a bigger hit with the Prius because it is so efficient. Low rolling resistance tires show a larger loss when moving water out of the way, and the heat losses are more significant because the Prius has little waste heat.

Tom

 

Gm has been putting them in cars and trucks for over ten years.

 

Often times, rain storms are accompanied by gusting winds.
Winds with sufficient velocity and angle so as to create a resultant
"apparent wind" that is more than 15 deg off the centerline do serious
damage to the aerodynamics of the Prius, a very aerodynamic car.

Wind coming from straight ahead or nearly so effectively just adds to
the air speed the car sees. But in this case, the relative wind -- car
speed over the ground plus wind speed over the ground -- passes
straight back over the car. Aerodynamic drag is the most significant
force trying to slow the car down at highway speeds. And it is not
linear; as the speed of the air passing over/under/around the car goes
up, aerodynamic speed goes up much, much faster.

Winds coming at the car from off the centerline are even worse. They
cause the car to loose a lot of its aerodynamic advantage for passing
efficiently through the air. Air coming from the side pushes on the
flattish side of the car. Most then goes up and over the top of the car
and then spills willy-nilly down the other side to fill the low pressure
area that develops there. In this situation, the Prius is as
aerodynamically dirty as a brick. It takes a lot more fuel to keep the
car moving at speed.

Unfortunately, even wind coming from more than 15 deg off the
centerline from the rear also screws up the aero, and your FE/MPG.
The only good wind comes from essentially straight behind the vehicle.

This matter, generally known as the wind side-slip angle, is discussed in
this document, just over halfway through. Look for this image:



So yes, in the rain there are a number of conditions creating greater
fuel use due to the rain alone. But don't discount the effects of any
accompanying wind. They can be surprisingly large.

 

For more detail on the negative FE/MPG effects of crosswinds, here are
some very interesting graphics that were generated (by someone else)
using the Prius Simulator at privatenrg.

Teaser:


Lots of other very cool Prius specific stuff at this sight too. :rockon:

 

Yep. Had it on an early 90s Olds Toronado.

 

I was reminded of that over the weekend.

Saturday
Austin to Dallas, sunny, 75F, 60mph average: 59mpg

Sunday:
Dallas to Austin, light rain, 65F, 62mph average: 50mpg

All of this was on the same route and traffic conditions both ways and little change in altitude.

I'm still amazed at how easy it is to get close to 60mpg on a dry sunny day, and how quickly it falls even for minor changes in the weather.

 

You got better in the light rain and cooler weather?

 

59mpg in the sun 50mpg in the rain

To the OP
I'm wondering if you run your air conditioner in the rain but not on sunny days?
I have to run mine in the rain even on cool days or my windshield fogs up, but I don't run it on dry days, it would make a significant change in you miles per gallon stats.

 

don't forget to add the affects of having front and rear defroster on to clear the steamy windows.
I find reducing use of 'heated air' will help during rain and colder weather. But you need to see, so pick and choose when to use.

 

Recently we had heavy rain and I noticed sudden drop of fuel economy from 26 km/l to 22 km/l. This was notice during heavy morning / evening traffic (average speed is less than 30 km/h). Therefore effects on air resistance may not that high. I thought one factor was engine was not reaching optimum temp (my commute is only 20km) or engine is getting cool due to lower temp (say 5C less).

Would also be effect on rolling resistance?

What kind of steps we could take to improve FE during rainy weather? Would Grill block help? I do not have a way to measure the temp.

 

Recently we had heavy rain and I noticed sudden drop of fuel economy from 26 km/l to 22 km/l. This was notice during heavy morning / evening traffic (average speed is less than 30 km/h). Therefore effects on air resistance may not that high. I thought one factor was engine was not reaching optimum temp (my commute is only 20km) or engine is getting cool due to lower temp (say 5C less).

Would also be effect on rolling resistance?

What kind of steps we could take to improve FE during rainy weather? Would Grill block help? I do not have a way to measure the temp.

 

I have owned several cars since legally driving in 1966.

No other vehicle has ever had the disparity in mpg than the
Prius.

We currently also have a 2003 Jetta TDI Wagon, (Diesel), 5 speed manual.

I see at the most a 2-3 mpg swing compared to winter and summer.
Overall average is about 53 MPG. Last fillup was 51 MPG,
all winter driving, wet roads, rain, 40-50 degree
weather, nearly continuous rain here in the Northwest since Octobober 2010.

The Prius easily takes a big hit 5-7 mpg during the winter sometimes
even more. Last fillup 47.25 mpg (calculated). During the summer
it would be 53-55 mpg (calculated).

Infact, even on a clear day, if a cloud momentarily covers
the sun you will see a drop of 5 mpg....


al