On Tue, 2006-05-09 at 22:08 +0100, Howard Winter wrote:
> On Tue, 09 May 2006 13:13:31 -0400, Herbert Graf wrote:
> 
> >...
> > Don't get me wrong, I can feel when the compressor turns on and off, I
> > just find it odd that even with a bigger engine the extra energy used by
> > the compressor doesn't appear to change the fuel mileage by any
> > noticeable amount.
> 
> The first time I visited the 'States, in about 1992, I hired a car that had an onboard computer, cruise 
> control, and Aircon.  I decided to play with all three, and set it up on a flat piece of road at a constant 
> speed, then waited for the MPG reading to stabilise (about 33 if I remember rightly).  Then I turned on the 
> Aircon, and it showed a drop of 5MPG.  I repeated the experiment a few times, and it was pretty consistent.
> 
> If you think about it, you are running an extra load from the engine - it *must* use some fuel to do it!  I 
> don't know why your car doesn't seem to show this in practice.

Hmm, interesting, 5MPG equates to about 15%, a huge number IMHO. In my
case it would mean the 7.8L/100km I was seeing on my car should have
gone up to 9L/100km, a HUGE difference which I would most certainly have
been able to measure.

There is one difference I can think of, and that's duty cycle. How hot
was it when you did your experiment? Is it possible that your numbers
may be off since your AC was working so much harder?

Also, how long did you wait? An AC will run ALOT at first to get the
cabin down to a comfortable temp. If the car had climate control the
AC's duty cycle would drop after a while to maintain the set temp. For
cars without climate control the AC blasting that cold will make the
cabin MUCH colder then the occupants want, and they'll turn it down.

My only explanation as to why it doesn't seem to have made a difference
on my old car was the difference was too small to measure reliably.

TTYL

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist