Slightly open window?

[MetroMPG]

Originally Posted by stefanv

I tried this for the next few days, and it doesn't seem to have had any measurable effect on fuel economy (my driving is mostly ~90km/h on country highways)

Your moderate speeds (= 55 mph limit for our American friends) lessens the impact too.


FYI, completely anecdotal, but in the same hot weather you're probably talking about, I did a trip at 80 km/h = 50 mph with both windows wide open, and then the same trip with both nearly closed. Same MPG on both trips.



Doesn't prove anything, of course.



I also tested it directly (repeated coastdown testing), and could detect the extra drag of one window down in more controlled conditions.

[teoman]

I do not think that our ground level ozone levels are high. Not many dc motors inside the hood to create arcs either.

How does one check ground level ozone levels?

[cRiPpLe_rOoStEr]

Originally Posted by MetroMPG

completely anecdotal, but in the same hot weather you're probably talking about, I did a trip at 80 km/h = 50 mph with both windows wide open, and then the same trip with both nearly closed. Same MPG on both trips.

It's often pointed out that drag effects become more noticeable at speeds above 80km/h, but I'm sure even a little below that speed they're not negligible at all.

[stefanv]

Indeed, like any drag effect, it's probably proportional to the square of your speed. So at say 75km/h, I'd expect to see about 88% of the window drag I'd get at 80km/h.

[mpg_numbers_guy]

Tried this out today; at 40-65 mph, the only open window configuration that didn't add drag (reduce mileage) was having only one window open about half an inch.

[stefanv]

Originally Posted by redpoint5

Most vehicles have fairly high electrical draw just running the fans, perhaps around 200 W on high...

I finally got around to measuring what my HVAC blower uses. All tests were done with the car off, so voltage was only around 12V:

Setting   Current   TotalPower   MotorPower

Low          2.5A          30W          13W
Medium1      4.2A          50W          34W
Medium2      5.6A          68W          57W
High         9.5A         114W         114W

The difference between TotalPower and MotorPower is what is wasted in the speed control resistors. The power actually reaching the motor is approximately doubled with each increase in setting.

I was wrong about everything going through a 15A fuse. This is true for the low and medium settings, but when the blower is on high, a relay connects it almost directly to the battery through a 30A fuse.

EDIT: At 114W, running the fan adds about 0.06L/100km to my fuel consumption (based on 114W = 0.15hp = 0.04L/h = 0.06L/100km at 67km/h, which is my overall average speed). In US units, where the calculation is more complicated because we're working with reciprocals of fuel consumption, this costs me about 0.6mpg if I was averaging 47mpg.

[teoman]

Thank you for those measurements.

[aerohead]

Originally Posted by teoman

I do not think that our ground level ozone levels are high. Not many dc motors inside the hood to create arcs either.

How does one check ground level ozone levels?

Looking over at GOOGLE,one site is mentioning using a 405 nanometer photometer to detect ozone.
They also mention chemiluminescence detector technology,however they say that measurements can be highly variable.