Posted on 01/02/2010 10:48:10 AM PST by gusopol3
This means that we can expect, on purely theoretical grounds, that our mileage will drop, due to wind resistance from temperature change alone, at -10°F to 81.8 percent of the mileage we experience at 90°F.
(Excerpt) Read more at xenodochy.org ...
He forgets an important variable.
The density of gasoline also increases inversely with temperature. Therefore, at a lower temperature gasoline has more BTU per gallon.
A liquid like gasoline does not change its density as much with temperature as does a gas like air. Denser air should increase the energy yield from combustion, but probably not as enough to match the increase in air resistance. Air drag increases as the fourth power of speed, IIRC.
In any heat engine the greater the temperature difference between input gas and exhaust gas the more work you’ll get out of the engine — all things being equal (the all-important caveat).
More oxygen higher pressure or colder, to mix with the gas in the chamber, hence more boom. My cycle definately gets more torque generated when colder and higher pressure.
Perhaps another contributor is increased losses at low temperatures due to the increase in lubricant viscocity.
Yep. All of these are variables that aren’t accounted for in the chart.
I haven’t done any calculations (and I’m not likely to) myself, so I don’t know which variable is most important and which should be ignored.
Good exercise for a bored and curious engineer though.
My tires lose inflation at colder temperatures!
It seems to me that he has neglected the factors of increased air density and fuel density on combustion efficiency (effect of mild supercharging increases power per stroke), and decreased road friction/stickiness, both tending to increase mileage. Before on-board computers, we used to have to retune our engines for best operation according to the weather.
Not true. The air in the tires condenses from the cold and has less volume. Lower volume translates into a lower psi per given volume. :)
A modern closed loop fuel injected engine will increase the fuel delivery in response to the increase in air density, so that variable evens out. Having to push through heavier air is a bit harder for a car to compensate for. Bottom line, cold weather kills fuel economy. I keep very good records and mine has gone from 32 in the summer to 28 in the winter, down to 26 if I am fighting heavy snow or temps under 0ºF.
The author clearly states that his analysis addresses changes in aerodynamic loss as a function of temperature only.
Many things affect mileage as a function of temperature, but they were not part of this guy’s work.
I have dabbled with a few goodies that impact efficiency on internal combustion motors.
Efficiency, and thus milage (not to mention torque produced per power stroke) greatly and measurably increases when the air is more dense, either due to increased barometric pressure, or due to temp changes
On clear winter days, when the pressure is up, makes for excellent riding (just wear lots of layers of clothes!)
As a bonus, the efficiency of combustion, a more complete burn of all the fuel molecules present, actually increases the milage.
Just to give the author his due, I suspect that a vehicle with increased sensitivity to wind pressure, ie a large cross section offered to the headwind, or a low power to weight ratio would in fact see greater changes associated with the wind drag than with the measured increase in torque applied to the drive wheel(s) on the ground.
Otherwise known as "My tires lose inflation at colder temperatures".
You sound like my son!! :^)
Well, yeah. But it sounds so much more technical and authoritative the other way.
;^)
Actually, that was what I was looking for, because I read the notice on the gas pump today (for thr first time) that said the volume delivered varied with temperature, but I really didn’t know which direction for a non-aqueous liquid. But I’d have to say his table comports pretty well with my driving experince, i.e. 27.5-29 mpg summer/ 25 winter over pretty much tne same roads. I usually see a real drop-off between 40-50 degrees, even with gas bought on successive days from the same station.
My wife’s hybrid Mercury Mariner drops from about 32-34 mpg on the highway to 24-26 mpg when temperatures drop below zero.
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