Reduced Fuel Mileage in the Rain

"poor" upper 40's mpg in the rain.

anyway, yes, the extra rolling resistance due to wet surface would be the largest of the detriments you suffered. Followed by the defrost.

 

Here in the Northwest, with rain, and wet roads temps
in the 40's your MPG's will be mid 40's.

I see a 10 MPG increase in summer with temps in the
60's and dry roads.

al

 

You will notice that the decrease does not happen when the rain starts falling, but when the road surface gets saturated with water.

 

I cant wait for some consistent warm temperatures to see what my Prius can do for fuel mileage. This will be my first summer with the car.

 

Think of it this way. There is water on the road and you tires will pump the water out of their path of travel. They will also pick up water off the road and throw it into the air. All of this requires energy. The energy comes either from your battery, gasoline, or is stolen from your momentum. All of the above will result in lower mpg.

 

Picked up my new Prius II today and drove in the pouring rain from the dealership in Melbourne to Orlando. And I mean POURING rain! Averaged about 42.8 on the highway but when I started driving on the surface roads I got it up to 45.8, still in the rain. I'm wondering how this car is going to do in the hot Florida summer.

 

The best weather requires no use of heat or air conditioning.

The humid weather may require more use of the A/C.

I'm not sure if the humid weather affects anything else like the mass density of air or how the fuel combusts, etc.

 

Humidity will increase density altitude. This effectively decreases HP due to reduced oxygen per unit of air. It essentially makes the car perform like it is at a higher altitude than it really is. Cold and dry days are the best for HP. For Prius efficiency I would think warm, not hot, dry days would be best.

 

In a normal ICE car, higher altitude means lower power but lower drag, higher engine efficiency at partial throttle, and higher mpg.

Warm, yes. Dry, as in no liquid water on the road, yes.

For humidity, I'd guess that high but non-noncondensing humidity would be best for holding down air drag, due to lower air density. But I can't guess on humidity for Atkinson engine efficiency.

 

I was thinking dry as in low humidity. Moisture displaces oxygen and the engine efficiency is reduced. This is why no one like to race, or fly in hot humid conditions.

I think overall engine efficiency is more of a factor than air drag but that is a guess based on drag racing experience. We always get our best ET and MPH on days that are cold with low humidity and high barometric pressure. Conversely, we experience our worst ETs when it is hot, humid and low pressure. For aircraft, the air drag could play a larger role but either way the net outcome is not good.

 

Humid air is less dense than dry air, it reduces aerodynamic drag & reduces pumping losses.

Humid air also slightly reduces the partial pressure of molecular oxygen.

Humid air has a slightly higher specific heat capacity (enthalpy) than dry air, which means it will absorb more energy before increasing in temperature. This could be negligible as the combustion produces also produces water vapor.

 

It reduces power, but not necessarily efficiency. Lack of power is a problem when racing or flying, but it's not an efficiency issue.

Most gas engines are over-sized for normal operation. Because of this, most gas engines spend much of their time pulling air around a partially closed throttle plate. This increases pumping losses.

With a higher density altitude (less oxygen), a normally aspirated gas engine produce less power for any given throttle setting. To get the same power, the throttle is opened more, which has the added benefit of reducing pumping losses and thereby increasing efficiency.

Tom

 

Hi All,

Here is the list of reasons fuel economy is worse in the rain:

Rain has more mass than air, and the car is pushing out of the way - so it needs more power to do this.

Humidiy does not increase air density, it decreases it. H2O has less mass than O2 and N2, because H2 is lighter than O or N. But Rain does increase the density of the mix that hits your car. Any particle of H2O gas displaces particles of N2 or O2 gas. This is why people from hot humid climates rarely have breathing problems when they visit Denver.

Rain on the road has to be pumped out from under the tires, the tires require more torque at a given speed, and that requires more power to turn them, more power requires more grams of fuel per minute.

Rain flows into the cooling system, and evaporates off the cooling system. The heat of vaporization is huge compared to the heat taken off by mere air flowing through the radiator. The Prius control system likes to keep the car above some temperatures, or it runs the engine - more gas. Grill blocking helps this. Maybe the Prius needs a rain baffle in front of the Radiator?

Its a good idea to run your headlights, and many states require this (Illinois is one) when the wipers are used (ie, its raining). That uses more electricity that would go to the wheels.

Defrosting requires electricity to run the AC compressor - less for the wheels - engine has to run more to replace it.


Did I forget anyting?

 

Interesting point. But volumetric efficiency is decreased, and more gas mass has to be sucked in for a given power output , even if the specific pumping loss (energy to intake a given mass) is reduced.

Max power is reduced of course, which is what F8L was getting at, but that is not an issue with regards to efficiency on a roadway. As that is a little different than a race track or taking off a fully fueled aircraft.

I bet its a break even situation with regards to automotive engine efficiency. If there is efficiency improvement in wet weather, its probably related to secondary expansion effects of water droplets being vaporized - ie water injection....

 

The same phenomenon is experienced at altitude, so it's not just a water injection issue. Most ICEs are indeed vastly over-sized for good efficiency, even on our beloved Prius. The Prius gets around this, to a degree, by running the ICE at nearly optimal loads and storing the excess energy in the battery. It helps, but not as much as an ideally sized engine.

Some of the newer gas engine designs eliminate the throttle. I expect this will become more common as gas prices climb.

Tom