Those are good questions, and I think I have some reasonable answers for them.
Let me expand on a couple of points.
As to air density, yes, colder air is denser than warmer air at the same
humidity. For every 6 degrees F of temperature reduction, there is about a 1
percent increase in density. However, here's the rub: As air density increases,
MORE FUEL must be introduced in order to maintain the correct fuel/air ratio.
This is because the same volume of air that is being drawn into the combustion
chamber has more air molecules in it so you have to have more fuel to maintain
the same fuel-to-air ratio. There is also a resultant increase in power, but in
ordinary city driving, extra power does not translate into increased efficiency
and the net fuel usage still goes UP, as do total emissions. As you mentioned,
older automobiles with non-computerized fuel systems do tend to run lean in
winter months. And yes, this does reduce CO output in those older vehicles
(which are in the minority), but it also increases the nitrous oxide output,
which is another air quality problem in itself.
As you also mentioned, computer-managed automobile engines do automatically
maintain the correct fuel/air ratio in the winter, but they do so by increasing
the amount of fuel to accommodate the increase in air density. This means that
total emissions, including CO, still go up. This is mitigated by various
schemes to pre-heat intake air to help improve fuel consumption and reduce
total emissions in cold ambient temperatures. These systems are fairly
effective (once the engine has warmed up), but not totally effective, due to
several factors, including flow losses through the preheating system.
What I had said in my previous posting was that engines run richer "for a
larger percentage of the time" during the winter, meaning that when you start
an engine at say, 30 degrees F instead of 80 degrees F, it takes significantly
longer for the engine to warm up to operating temperature, and during that
time, cold dense air is being introduced into the combustion chamber, meaning
that the engine has to burn more fuel to maintain the correct fuel/air ratio.
Whether it accomplishes this with a choke or with a computer-controlled fuel
injection system is immaterial. More fuel is still being burned to maintain the
correct ratio, therefore more total CO is emitted (and other emissions as well,
of course). During this warm-up period, the intake air pre-heating systems also
take several minutes to warm up enough to provide hot air, so even newer
automobiles create more emissions while warming up. When you consider that the
average city drive is only about 20 to 25 minutes in length, if an engine takes
10 minutes to warm up, that's a high percentage of driving being done with
extra fuel being burned.
Racers routinely disable the intake manifold heating and the intake air
preheating system and suck cooler intake air from external sources rather than
from the hot engine compartment. This allows them to pack more air molecules
into the combustion chamber, use bigger jets to pump more fuel in, and develop
more power (and create more total emissions in the process).
Some economy recapture can take place when you can use gearing to take
advantage of the increased power that can result from cold, dense air, but this
only helps if you are in a situation where you can really use it. It doesn't
help much in city driving where you're stopped at lights or decelerating half
Hope that clarified things a bit. If not, let me know and I'll have another go
But here's the thing: When the air is cold/dense, larger jets are required
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