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Monday, March 28, 2005

Cool Runnings

File under the preceding post

Without doing any really controlled experiments, I have noticed a perceptible decrease in mileage when using my car's heater/defroster in the deep winter months. Kind of odd, but cold decreases fuel efficiency in inverse proportion dependence to my feelings of guilt. So when you see people bicycling in below zero weather and start muttering, remember that guilt is a transferrable commodity. Guilt can be neither created or destroyed. In theory, guilt leaves my pedalling brain and enters the nearest passing motorist.

Then we find the disturbing yet entirely predictable outcome for Prius gas mileage. That the energy efficiency flaw in hybrid vehicles turns out to key on heater or air conditioner use makes some intuitive sense. The fact that poorer gas mileage vehicles show less relative impacts on heater use than higher gas mileage vehicles should really come as no surprise. The finding indeed sucks, but then again you can't argue with the math.

Watch for the guilt-free Prius drivers next winter. They will be the ones that turn another kind of green. Either that or they will decide to drive with a few extra layers of clothing.

6 Comments:

Professor Anonymous Anonymous said...

You know, when you run the defrost on a car that has air conditioning, you are running the air conditioning, too, and the compressor for the air conditioning unit puts a significant load on the engine. The only engine additional engine load you get from running just the heater is only the power needed to run the fan (hopefully, the water pump is always working).

2:49 AM  
Professor Blogger Engineer-Poet said...

What anonymous said.

If it takes X amount of energy to run the A/C in a car, you're going to see a bigger difference in MPG with a 45-MPG car than a 15-MPG car.  About 3 times as much.  You get one guess why, and if you can't figure it out you're innumerate and shouldn't be allowed to opine in public.

9:05 PM  
Professor Blogger WHT said...

Funny that the article claims that the mileage goes down to close to 20 MPG. Some would consider this a case of a lack of truth in advertising.

" The hybrids fell as much as 40 percent below the EPA mileage figures for combined city and highway driving during my recent test, which covered a mix of Detroit-area roads."

Innumeracy is a step above illiteracy and I guess you suffer from both. Go find somebody to read you the original article.

7:10 PM  
Professor Blogger Engineer-Poet said...

IIRC, quirks in the EPA test procedures already overstate hybrid fuel economy by 30% or so, so the 40% figure is the sum of the systematic test error and the actual A/C penalty (which is much smaller).  The A/C penalty would rise for vehicles which drive the compressor as an engine accessory rather than via its own independent motor (probably all the Hondas are in this category), forcing the engine to idle and eliminating the idle-shutdown capability.  The Prius drives the A/C off the traction battery.

"Cold and slippery" sounds like a time of heavy traction-control use.  Would I expect to see a large economy penalty if the car was not driven to minimize this?  I sure would.  I'd expect to see Lincolns and GMCs paying penalties too, though perhaps not quite so extreme; yet starting from such a low baseline, the hybrids would still go a lot further on a gallon of fuel.

And that's the point.

9:08 PM  
Professor Anonymous Anonymous said...

We intend to reinact the infamous Jamaican Bob Sleigh team's achievements at a meeting in Belgium in the summer of this year and wondered if you had any advise on where we could obtain such a devise? 'cold and slippery' sounds great, A/C and fuel efficient in the cold conditions, even better.
Ginger Rasta

3:47 PM  
Professor Blogger rasta said...

We intend to reinact the infamous Jamaican Bob Sleigh team's achievements at a meeting in Belgium in the summer of this year and wondered if you had any advise on where we could obtain such a devise? 'cold and slippery' sounds great, A/C and fuel efficient in the cold conditions, even better.
Ginger Rasta

4:17 PM  

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