Temperature and fuel mileage?

[Brett Dikeman]

At 12:18 PM -0400 4/26/02, John Shost wrote:
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>Volatility is reduced at colder temps so more fuel is injected to compensate.

Actually, no.  Colder air is denser...and thus can be mixed with more
fuel to make more power.

Particularly in turbocharged&intercooled engines, they make much more
power as ambient air temperature drops.  Any turbo'd Audi owner will
tell you, they're total dogs in 90 degree weather, and absolute
monsters around 30-40 degrees.  It was in the high 30's a few days
ago and raining(rain does a -great- job of cooling the ICs on
type44's with their very direct placement), and I was one VERY happy
camper :-)

There are MANY other reasons why mileage goes down in the winter.
I'm about 3 short of a dozen here...obviously some are more
significant than others, but:

-the block, oil  and coolant take more energy to warm up to operating
temperature

-O2 sensor probably takes slightly longer to get up to temperature,
so mixture is off for a little longer each time you start the car

-tolerances at lower engine temperatures are different; one of the
reasons you're not supposed to run a cooler thermostat is that it
increases cylinder wear.  In cooler weather, the engine spends more
time at these cooler temperatures, or may never get up to proper
operating temperature

-oil is thicker at lower temperatures.  Tranny oil/differential oil
is thicker too, and quattros have three differentials versus a normal
car's two.

-increased use of the heater; takes a lot of heat to keep a car's
passenger compartment warm.  That energy has to get "replaced"
somehow.

-increased load from items like the defroster circuit, seat heaters,
etc. which are more likely to get used

-less daylight hours=more headlight usage

-people have a tendency to "let the car warm up"

-cold weather causes you to "loose" a few PSI in the tires, and that
can affect mileage as well.  Rubber is also stiffer at colder
temperatures, so rolling resistance is higher.  If you run winter
tires, they may not be as efficient.

The above almost-dozen reasons are why I never believe people who
tell me they get worse mileage from "winter gas".

Brett
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