Fuel consumption at idle?

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Does anyone know the actual fuel consumption when an engine is at idle?  Say
the very common 3.8 GM engine for instance.

Every gas saving tip articles says not to let your car warm up for long but
they never say how much you are burning.



Re: Fuel consumption at idle?


Edwin Pawlowski wrote:
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Not easy to measure BUT it would be burning a higher amount than when
you let it idle after it had warmed up. The computer looks at water temp
and air intake temps to calculate fuel delivery when in open loop (plus
a few other items) Due to both being colder the ECM adds fuel (just like
the old choke) to keep the engine running. The thinking is that by
driving while it is using that fuel you do two things. One the engine is
working harder so it warms up faster. Plus you are covering some
distance to your destination.

I can tell you that my 4.3 uses about 1 gallon while idling 2 hours.
That is idling with the heater on, 4 ways working and powering 600 watts
of light-bar. I leave it running when on calls to keep the lights on and
so I don't have to head back to the station in a cold rig.

--
Steve W.
Near Cooperstown, New York

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OK, lets call it an ounce a minute.   At $3.58 a gallon, it works out to
.028 per minute. A 5 minute warm-up with a remote starter costs about 14Ę or
so.

Thanks, I was just curious what the price of comfort was.



Re: Fuel consumption at idle?


The cost is much higher than that.  His estimate was based on a warm engine
idling.  A cold engine uses much more fuel than that, much of which washes
oil off of the cylinder walls and contaminates the motor oil, both
contributing to accelerated wear.   An idling engine takes -much- longer to
warm up and remains in this destructive mode longer than one that is being
driven gently for the first few minutes.

Lee Richardson
Evansville, Indiana


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Perhaps, we were taught that many years ago.  I completely agree with older
engines, but I don't think newer ones have the same wear during warm-up as
the '57 Chevy with carburetor and choke coupled with wider tolerances of
rings.  That engine also idled at a very high rpm compared to the computer
controlled injections systems we now have.

As for longer to warm up and stay in "destructive mode", that too is subject
to some analysis.  If an engine idles at 800 rpm and sits for five minutes,
it has turned 4000 times in that period.  Is there more wear than the car
being driven cold at 3000 rpm for two minutes?  I don't know the answers to
that, but it makes some sense that total number of plug firings is going to
be a determinate of engine warm up to operating temperature.



Re: Fuel consumption at idle?


Whether the air-fuel mixture is enriched on a cold start by using a
mechanical choke or by electronic fuel injection really makes no difference.
While there is no doubt the more modern system is capable of metering the
fuel more accurately, the fact remains a cold engine requires a -much-
richer mixture than a warm one.  And an idling engine remains "cold" much
longer than one that is being driven.  Anything that can be done to shorten
the length of time an engine runs cold will lengthen it's life and reduce
fuel cost.   And even in that cold, fuel wasting state, the engine can still
be propelling the car down the road instead of sitting in the driveway.

Lee Richardson


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Again, in principle you are correct, but the electronic fuel measurment is
far superior than a thermostatic choke and light years ahead of a manual
choke.  What I do not know, and evidently you don't either, is how much of a
difference there is.  5%?  50%?  The old mechanical chokes stayed on until
you hit the gas pedal even if it was 20 minutes.  Not so today.  When cold,
my car runs about 150 - 200 rpm more than normal at starup and soon goes
down to normal, the old car was an additional 500+ rpm until I hit the gas
pedal or was driving and used the gas pedal.

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But do we know how much?  The idea of driving slowly goes back decades and I
think the practicality has changed. How rich is rich?  What is the fuel air
ratio?  What is the curve?


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But you did not address my other idea.  How does an engine get warmed up?
By burning fuel, or course, and it still takes a given number of cycles to
get from the cold start of say 0 degrees up to the water temperature of 210
and an even higher oil temperature.  While driving shortens the clock time,
does it lessen the number of revolutions of the engine needed?  If it needs
10,000 revolutions, does it matter if that takes place at 800 rpm over 13
minutes  or 3000 rpm over 3.5 minutes?  What is the fuel consumption either
way?  I know you get 0 mpg while idling, but you've not presented real data
to the above questions.  I doubt that many people know the real answer
outside of some engine design engineers or test lab guys.



Re: Fuel consumption at idle?


The choke may have opened, but the fast idle cam is what kept the idle speed
up till you hit the gas pedal.
Roy
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Thus increasing the fuel consumption at idle compared to the cars today.
Less than when the choke is on, of course.
Thank you.



Re: Fuel consumption at idle?


When in doubt, read the owners manual.  It probably reflects the conclusions
of the manufacturer's engineers who take into account the factors you seem
to be concerned with.  Does yours recommend letting the car idle extensively
on cold mornings before starting out?

Lee Richardson

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Oh, the Owner's Manual. Now there is a wealth of technical information.  I'm
sure I can find things like the proper range of output voltage of the
alternator, the specific gravity of the anti-freeze solution and clearance
for the rod bearings.

So, you don't know the answer to my questions.  No biggie, I don't either.

The owner's manual is not going to give me the information I want.  They'd
rather I didn't idle because then I'd complain that I'm not getting the
mileage as rated on the sticker.  That is not a reflection of the engineers
findings, just a marketing slant to driving.  I do appreciate the laugh
though.
--
Ed
http://pages.cthome.net/edhome /







Re: Fuel consumption at idle?


Edwin Pawlowski wrote:
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no matter how much or how little you actually burn, you are getting 0 MPG

Re: Fuel consumption at idle?



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Some people probably actually care too.



Re: Fuel consumption at idle?


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If my memory serves, our 3.1 Buick uses around .2GPH according to the
Scangauge.

I'll have to hook it up later today when I go out and get back to you
with  the exact number.

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Everybody is talking about exceptions for cold versus hot idle.  If you just
need a
ballpark estimate, factor in the cubic inches displacement, the idle RPM,
and the
typical fuel to air mix ratio.  That should give you a benchmark value at
any rate.


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That would work.  A 3.8 liter four stroke engine at 800 rpm consumes 1520
liters of the mix per minute. the  stoichiometric fuel-air ratio is 14.7:1
so about 103 liters of gasoline vapor is used. Gasoline vapor weighs 3.25
times that of air and air weighs .0807 pounds per cubic foot and about 53
cubic feet or air per minute are burned.

Thanks.



Re: Fuel consumption at idle?


3.1 V6

.9gph@1100rpm dropping to .8gph almost immediately@42degree water temp
air conditioning compressor running.

.4gph@800rpm air conditioning running@185degree water temp.

.3gph@700rpm air conditioning off@185degree water temp.

HTH.


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Yes, thanks.



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