Thursday, May 3, 2007

Thinking is good. Numbers are good.

I'm seriously jammed for time in my other life right now; luckily there are two excellent quickies available for me.

One - readers will know that I sometimes toss out calculations for someone to do- someone did! In a very early post I asked someone to just quickly figure out how much gasoline is used, worldwide, just to run the airconditioners in the car- every day.

Brian West did take a quick run just today- outstanding! Do please take a look at the original post and his comment # 2; Unthinkable.

The number he gets is pretty scary- actually, there were 2 factors he left out (understandable in quick/dirty first tries) that would make a pretty large difference- that's the number of cars that actually HAVE airconditioning; and, the percentage of time the airconditioners are actually running. I think that would make a big difference it the final number.

Here's his calculation:

"Using some Very Rough assumptions:

cars worldwide = 600,000,000

average daily car mileage = 40m

average car fuel consumption = 27.5mpg

average increase in fuel consumption when using air con = 7.5% (the figures I found varied from 2%-18%)


...656, 250, 000 gallons


or for us more metric antipodeans, that's 2, 483, 906, 250 litres, or enough fuel for quite a road trip.."

THAT'S PER DAY.

Now- I DO think that's going to turn out high- there are a lot of cars out there without a/c, or with a/c that doesn't work. At this point it's a wild guess- but let's guess it's only 1/5 of the cars out there that have working AC - that would make it a mere 131 Million gallons/day.

Then, too- nobody runs their AC all the time; but then there are some that DO - like everyone in TX, FL, AZ - so let's say half again. That gets us to a mere 65 Million gallons/day.

Or - 23 BILLION gallons/year. And that estimate is brutally short of Brian's original - which would be 237 billion... ??

boy, that seems pretty high.... Something else we're missing, so we can do a quick check on whether these numbers make any sense at all - what's the stated dail/annual global gasoline consumption? Somebody's got that number. I tried a quick google, and hit the reason why I ask others to to this; it's a tangled mess, and the only stats I could find quick were "total petroleum products" - in barrels. uh. what?

Points-

a) Doing these kinds of calculations is pretty EASY - don't leave them to the experts- use your own brain and your own googling ability.

b) Is 23 billion gallons/year- and climbing- worth talking about? or 237 BILLION, if Brian's number is closer? (I'll bet mine is actually low- other corrections might well move it back towards The West Number.)

c) Boy, you better check your numbers before publishing....... :-) I put up like 3 different versions here this morning; because... I'm jammed for time..... haste makes waste..... had to do it over and over... and it's probably STILL not right. No time.

d) Regardless; all those little tiny individual actions- that don't make any difference-
MAKE A HUGE DIFFERENCE.


Ok, item 2 - this blog has been given an Award!
Thinking Blogger

:-) Gollyjeepers.

Thanks, More Deliberately. I am honored, humbled, etc. And vastly amused that you think I "probably won't appreciate the recognition..." I'll be chuckling about that a good while. Hey, my crusty grandpa act is working! No- I'm human. A pat on the back feels good. Thanks. And, of course, I'm TOTALLY tickled that I'm #3, and Colin is #4. HA! :-)

(that's intended as purely silly juvenile humor, Colin- nothing more. ) :-)

gotta zoom off for the day-

11 comments:

Lori V. said...

Greenpa, how would you recommend doing the math to figure out how much extra gas is consumed by rolling down windows, thereby creating more drag and less fuel efficiency?

My hubby's car has one of those neat little (extra cost) computers that will tell you actual real-time fuel consumption, as well as average consumption. He's been utilizing it a lot since reading an article about "hypermilers," people who are employing all methods at their disposal to raise their mpg. He's "learning a new driving skill." Anyway, one day we were checking computer readings on a flat stretch of road, and the consumption rate did not vary much at all between A/C on/windows up--A/C off/windows up and A/C off/windows rolled down.

Greenpa said...

Lori- oh, dang, you WOULD bring that up. :-) Yep, I'm aware of the "windows down drag" argument, and there's no doubt that there is something there to discuss. Last I knew, the REALLY serious engineering types were still seriously arguing about that- and where it all shakes out, car by car. That's the crux- individual car aerodynamics. Some are worse, some better; window by window. Irritatingly, some are great with the front windows only down; some with the back only-

Gosh. What if- cars were DESIGNED - for minimum drag? They pretend to be, but they're mostly not. Design Factor #1= Sales. Hey, stick a spoiler on it, that'll sell. Drag, schmag.

And/or; how about this design possibility (worth a million real bucks easy) - a car with CHANGEABLE aerodynamics - features to keep air flow smooth with the windows up - and little spoilers (or whatnot) that pop up to keep drag low with the windows down?

Could be done. And/or - it would also be possible to increase internal airflow to EQUAL windows down- without having the windows down. You just have to get the car built that way.

It's a complicated world. :-)

Greenpa said...

Lori - and another thing!! :-)

Drag is a factor yes- but how about the HOURS people spend sitting at stop lights- with the AC on? And or driving 20 mph on the freeway during rush hour? The drag would NOT be a factor then. And those are large time slots.

Reality really is really really complex.

Lori V. said...

I love coming here, knowing I can find good, introspective conversation! :-)

You are very correct, reality is, indeed, very complex. It's depressing that sometimes we are simply making a "lesser of two evils" choice.

Beelar said...

This comment is just the math- it seems like there's something slightly amiss in the above calculations.

600 million cars * 40 miles/day = 24 billion miles/day

27.5 miles/gal = .0363636... gal/mile

An increase in consumption of 7.5 percent can be represented as a "delta" consumption of 0.075 * 0.03636... = 0.002727... gal/mile. This is the cost in gallons per mile of running a/c if it increases consumption by 7.5 percent.

So, multiplying our number for miles/day times the number for gal/mile, we get a "delta" consumption of 65.5 million gallons/day, or (with 365.25 days/year) 23.9 billion gallons/year.

If only 1/5 (or 0.2) of the cars are running a/c only 50% (0.5) of the time, this conveniently gives us 0.2 * 0.5 = 0.1 = the proportion of daily miles which use a/c. So to get the corresponding daily and yearly usage, just move the decimal over one- 6.5 million gallons/day or 2.3 billion/year. And for those of you living somewhere in the world other than the U.S, thats 3.7854118 liters/gal, giving 25 million liters/day or 9 billion liters/year, just for the a/c.

Beelar said...

Now for a comment not about the math. I didn't check any of the rough assumptions, so can't say anything about them. I think even a 50% a/c usage is a bit high of an estimate.

Also, regarding Lori's observation of the consumption not varying much- I'd believe it, since the 7.5% difference used in those calculations gives (for a 27.5 mpg car) a difference of just .002727... gal/mile, which brings that car down to 25.6. But this is for the mid range of consumption differences- if your car only gets a 2% hit for turning on the a/c, it'd be less noticeable.

Whether the windows or the A/C costs you more gas will vary from car to car, I'm pretty sure. If you're driving a low drag vehicle (like a Prius, say, or even a Corolla) there's a good chance the windows wide open will be more costly. If you're driving a pickup, it's a good bet the A/C will be more costly. And as already mentioned, higher speeds is where windows might get costly. If you're driving around town at 25 mph, it's likely your windows won't make a noticeable difference at all.

Aha- and you didn't think you'd get it in this conversation, but one practical way to make window flow be pretty efficient, for days when it's not super-hot, is to crack the back windows a bit and just turn on the vent fan in front. Not a whole lot of airflow compared to all the windows down on most cars, but it can be enough to keep you cool, without wasting gas or your eardrums.

The point Pa made about designing cars to be low drag with the windows open is quite true. It could certainly be done, and it hasn't been (much- a little bit on some convertibles, e.g. I think the Honda Solara? but just for not mussing your hair, not for fuel economy). Another thing about a/c or no a/c is, all the costs associated with that extra piece of durable equipment (and associated coolants, belts, etc) in your car. I'd say that even if the mileage with the windows down is a bit worse than with a/c on for a particular car, just not having the a/c in the car at all is likely to still be better environmentally.

Greenpa said...

Beelar, it should not be a surprise, is my engineer son- way better with a calculator than I am.

Here is another BIG advantage to doing a quick calculation- it can stimulate others to check your numbers; and ultimately yields a more refined answer. Which is pretty useful.

Beelar's point about not having the AC costs in the car are true. Plus- the whole rig weighs what, 30-50 lbs? Radiator, pipes, etc? You have to carry that weight 100% of the time- which means more gas used up for nothing-

Lori V. said...

I can definitely see the point, and when buying a new car, that will be helpful. However, those of us that already accrued the negative environmental karma (or car-ma... hehehe) of that a/c unit... hmmmm... my birthday Prius is sounding better all the time! :-)

etbnc said...

Lots of good stuff here already -- please keep it up!

I'd say your Thinking Blogger tag is well deserved. I look forward to reading more from your planned topic list.

Cheers

Crunchy Chicken said...

I also wanted to add in that most vehicles produced in the last five years have a lot of "climate control" settings where the A/C goes on automatically and can't be shut off. Annoying if you like the air coming up via the defrost setting but don't need/want the A/C.

Davo said...

Gas ?? mm, one presumes that means petrol. Also have to factor in motor vehicles that run on LPG (gas).