cold air

Daniel Stern said:

So did most others who replied to my thread. Some spawned other discussions,
which I didn't really care about so long as the main question was answered
by guys like you, Geoff (interesting point about engine block temp),
fbloogyudsr (fuel doesn't atomize in cold), Brandon, etc.

I just ignored the other responses about road conditions, mileage, and
someone calling me a troll. I guess it takes more self-control for some
people to harness the power of usenet effectively

 

One rule of thumb, which may or may not be accurate, holds that a 20F

So if this is a linear relationship, very close to zero kelvin the power
could easily double.... (obviously assuming the 'air' is still a gas)

I suppose a similar relationship applies to altitude........along with a
plethora of other factors like drag, etc ;-)

Speaking of humidity, I never knew that water displaced air. Aren't there
the same number of O2 molecules in 1cc of dry air as there are in 1cc of
saturated air?

 

The Society of Automotive Engineers has a test standard
(J1349) that applies to net power testing of automotive
engines. The standard includes a formula for an atmospheric
correction factor for WOT power in spark-ignition engines.

Roughly speaking(*), the calculated net power ratio (between
power outputs at two temperatures) is inversely proportional
to the square root of the absolute temperature ratio, i.e.,
colder air produces more power.

To use one of your examples for numbers: power at -15F vs
90F. First, you have to convert the temperatures to an
absolute scale e.g., using the Rankine scale, -15F is about
445R, and 90F is about 550R. The temperature ratio between
the two is then 550/445, or about 1.24. The square root of
the temperature ratio is about 1.11. So, according to the
SAE correction factor, engine power should increase by about
11% when the air temperature falls from 90F to -15F.

(*) The correction formula assumes that *indicated power*
(i.e., total engine power, including internal friction
power) varies with the reciprocal of the square root of the
temperature ratio. It also assumes that friction power does
not change with temperature, so the change in *net power* is
actually greater than is suggested by the temperature ratio.
The effect is fairly small but in your examples, the net
power ratio would increase by 2%-3% using the SAE formula.
IOW, the SAE formula shows a 13% increase in power just by
changing air temperature from 90F to -15F, assuming a
mechanical efficiency of 85% (i.e., a friction loss of 15%).