Rain and Energy consumption

[Kbsilver]

Don't forget you also use energy to accelerate the weight of the water hitting the forward surfaces of the car from zero to the speed of the car.

 

[stevezzzz]

30% may be at the top end of the change-range, but it's not, in my experience, excessive. I consistently drive multi-thousand mile trips, which give me the ability to factor out load, tire conditions and other variables, and year-in, year-out, the three most significant factors in fuel economy variation are road surface, rain and wind. The latter is a roulette wheel, however, in that if it's on your back it gives you a fine boost. I just can't figure out why it never seems to be other than coming right at me!

Somewhat OT, but oh, well...

Pilots know from bitter experience that most any wind is a headwind. While some of this is due to good ol' Murphy, there's a firm basis in geometry: crosswinds also act as a headwind, since the aircraft has to hold a crab angle into the wind to maintain its course over the ground, thus reducing speed-made-good. While a Model S doesn't have to crab into the wind to stay in its lane (not much, anyway), folk wisdom is that a crosswind messes up the car's aerodynamics, raising the drag coefficient and increasing energy consumption per mile nearly as much as a headwind. Then there's the apparent wind effect, which sailors know well: unless the wind is dead aft, the faster you sail, the further forward is the apparent wind blowing over your sails. Whether you're flying, driving or sailing, the wind has to be blowing from well abaft the beam to act as a tailwind.

 

[ggr]

Somewhat OT, but oh, well...

Pilots know from bitter experience that most any wind is a headwind. While some of this is due to good ol' Murphy, there's a firm basis in geometry: crosswinds also act as a headwind, since the aircraft has to hold a crab angle into the wind to maintain its course over the ground, thus reducing speed-made-good. While a Model S doesn't have to crab into the wind to stay in its lane (not much, anyway), folk wisdom is that a crosswind messes up the car's aerodynamics, raising the drag coefficient and increasing energy consumption per mile nearly as much as a headwind. Then there's the apparent wind effect, which sailors know well: unless the wind is dead aft, the faster you sail, the further forward is the apparent wind blowing over your sails. Whether you're flying, driving or sailing, the wind has to be blowing from well abaft the beam to act as a tailwind.

And there's one more effect worth noting. Suppose you're making a round trip and the first leg is straight into the wind. You think to yourself "I'll make up the time I lose on the first leg with a tailwind on the second leg!" But it isn't correct. You spend more time in the headwind than you do in the tailwind, so the effect on your average speed is greater on the headwind leg.

 

[AudubonB]

Then there's the apparent wind effect, which sailors know well: unless the wind is dead aft, the faster you sail, the further forward is the apparent wind blowing over your sails. Whether you're flying, driving or sailing, the wind has to be blowing from well abaft the beam to act as a tailwind.

Oh, I don't know about that, Steve. When I tighten the jib line on the Model S she responds beautifully; I can tack just fine even up to just one point off.
Now, the F-350, with the camper....she's a scow. And crabs like one.

 

[iffatall]

30% may be at the top end of the change-range, but it's not, in my experience, excessive. I consistently drive multi-thousand mile trips, which give me the ability to factor out load, tire conditions and other variables, and year-in, year-out, the three most significant factors in fuel economy variation are road surface, rain and wind. The latter is a roulette wheel, however, in that if it's on your back it gives you a fine boost. I just can't figure out why it never seems to be other than coming right at me!

Somewhat OT, but oh, well...

Pilots know from bitter experience that most any wind is a headwind. While some of this is due to good ol' Murphy, there's a firm basis in geometry: crosswinds also act as a headwind, since the aircraft has to hold a crab angle into the wind to maintain its course over the ground, thus reducing speed-made-good. While a Model S doesn't have to crab into the wind to stay in its lane (not much, anyway), folk wisdom is that a crosswind messes up the car's aerodynamics, raising the drag coefficient and increasing energy consumption per mile nearly as much as a headwind. Then there's the apparent wind effect, which sailors know well: unless the wind is dead aft, the faster you sail, the further forward is the apparent wind blowing over your sails. Whether you're flying, driving or sailing, the wind has to be blowing from well abaft the beam to act as a tailwind.

Just think in terms on relative velocity. When you are moving forward at 65 mph, even still wind hits you at 65 mph. Sure, the wind flowing in your direction will let you consume less energy, but unless the wind flowing in your direction is flowing faster than 65 mph, it would always appear to be coming right at you. Naturally, the wind will appear to be coming at you most of the time.

(Same concept as why rain always seems to fall at acute angle and never obtuse when you look at it through a train window.)

 

[jhs_7645]

I realize that I slow down in the rain automatically whereas most traffic seems not to. Right there is a good bit of that 30%.
--

Actually, slowing down would help with range, not hurt it — unless I mis-understood your meaning.

- - - Updated - - -

Speculation: as wind resistance is the primary energy use factor at highway speed in fair weather, it is more likely that the rain in the air has more effect than rolling resistance - at highway speeds. Excluding puddles, of course. (Most highways have very good drainage.) If high density due to cold is a factor, air doesn't get much more dense than when it is raining.

I’m with Brass Guy here, I think rain definitely has an effect (folks living in the NW get quite a bit of rain to ’sample' ) but rain, combined with a headwind has proven to be *deadly* to range, and many times, heavier rains come along with winds.

As usual, the best advice for getting that range back is to slow down.

 

[stevezzzz]

Just think in terms on relative velocity. When you are moving forward at 65 mph, even still wind hits you at 65 mph. Sure, the wind flowing in your direction will let you consume less energy, but unless the wind flowing in your direction is flowing faster than 65 mph, it would always appear to be coming right at you. Naturally, the wind will appear to be coming at you most of the time.

Eh? Come again? When I talk about a headwind, tailwind or crosswind, I'm talking about airmass motion with respect to the surface of the Earth as it affects a vehicle in motion, not the relative wind flowing over the vehicle. If the airmass is not in motion there can be no headwind or tailwind or crosswind. Airmass motion affects relative wind (without airmass motion, relative wind would always be equal and opposite to the velocity of the vehicle) and thus affects the performance of the vehicle in question.

 

[iffatall]

Eh? Come again? When I talk about a headwind, tailwind or crosswind, I'm talking about airmass motion with respect to the surface of the Earth as it affects a vehicle in motion, not the relative wind flowing over the vehicle. If the airmass is not in motion there can be no headwind or tailwind or crosswind. Airmass motion affects relative wind (without airmass motion, relative wind would always be equal and opposite to the velocity of the vehicle) and thus affects the performance of the vehicle in question.

Yeah, I was responding to AudubonB, adding to what you said.

 

[RDoc]

We just drove back to the Boston area from DC in pretty heavy rain almost the entire way. The effect on energy usage was dramatic. Normally we use about 315 Wh/mi which is about what we used on the way down when not stuck on the Cross Bronx Expressway, but on the way back, when we were averaging more like 70 than 75, we used an average of 405 Wh/mi. On the stretch from Newark, DE to Paramus, NJ we were in torrential rain for part of the trip and even though we were only going around 40 or so, the energy usage was up around 450 Wh/mi. As was mentioned earlier in the thread, I'm pretty sure the main cause of the higher energy usage is the tires pumping water off the road to avoid hydroplaning, but there's also often wind when it's raining.

On the way down, it was about 86F and we had the AC set to 68. On the way back it was about 47 and we had the seat heaters on; in June no less!

I'd suggest that anyone driving long distances in constant rain plan their charging stops and times to account for much higher energy usage.