Re: Switch Your Engine Off When In A Queue

Adrian wrote:

Chris Whelan <cawhelan@xxxxxxxxxxxxxxxxxxxxx> gurgled happily, sounding
much like they were saying:

Aircon compressors are much more efficient than they used to be.

But still take a significant amount of power.

A quick google shows them to be a couple of kW. But, of course, they
don't run continuously or at full power other than to initially cool a
car that's been sat in full sun for a while.

What mechanism on a *standard* A/C setup lets it reduce the amount of
power needed?

Cutting the compressor in and out.

But standard A/C does not do that. It's exactly why I highlighted the word
standard in my post.

Standard A/C has a switch on the dash. On or off. Apart from a fault
condition being met, when it's on, it's on. No control system within the
car will modify the the amount of power it takes to drive the compressor,
other than the driver turning it off. Which is not what you said.

I use my A/C extensively, not to reduce heat, but to "condition" (ie
dry) the air. Later versions of my car, and many others on the market,
automatically turn the A/C on as soon as demist is selected for this
very reason.

Indeed, sensing via humidity, then cutting the compressor in and out
appropriately, with the cooled air being re-heated as required.

What you are describing would only happen on a car with full climate
control, not on any car (the vast majority) that just have a standard A/C
setup.

In a very few cars with full climate, a variable output compressor gives
finer control, and here the power needed would *marginally* reduce as cabin
temperature approached set point. However, only a few cars have climate,
and only a few of those have variable output compressors.

Many cars with climate control, although capable of reducing output, and
hence power needs, still run the compressor virtually all the time.

I could supply a link to a large PDF entitled "Impact of Vehicle
Air-Conditioning on Fuel Economy, Tailpipe Emissions, and Electric
Vehicle Range", that amongst other things says:

"The power necessary to operate a vehicle air-conditioning compressor is
significant - it can be greater than the engine power required to move a
mid-sized vehicle at a constant speed of 56 km/h (35 mph)."

<sceptical>

It's a large file to DL however.

s'OK. I'd be interested in seeing it.

OK.

http://www.nrel.gov/docs/fy00osti/28960.pdf

I'm sorry Adrian, but there is no way that a 12vdc motor would
realistically be able to power a vehicle A/C.

Hmmm. I wonder how those fridge units on small vans could possibly work,
then.

Easy. They are engine driven, with an optional standby electrical drive. Oh,
driven by mains electricity of course. Hubbard are the people who are the
chief supplier in the UK.

The larger chilled/refrigerated vehicles have separate diesel driven
refrigeration units.

Chris

--
Remove prejudice to reply.
.

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