I'll try to keep this short.
Quote:
Originally Posted by Maury Markowitz
Steady state gas milage is roughly due to two factors, the air resistance which goes with the square of speed, and rolling resistance which goes with the speed. So, roughly, milage is the fuel use at idle dividend by the cube of speed. Roughly. Then you add in things like hills and such.
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Too roughly. It's not (air resistance)
times(rolling resistance), it's (air resistance)
plus(rolling resistance). So,
roughly, gas milage is Av^2+Rv, where A and R are coefficients. By roughly I mean that there is more to that equation, like varying engine efficiency depending on rpms, load, etc.
Quote:
Originally Posted by Maury Markowitz
The part I don't understand is acceleration. It would *appear* that the amount of energy needed to accelerate to a given speed is a fixed, it's simply 1/2*m*v^2. There's no "t" term in there for the time you take, nor an "a" for the acceleration. It's just initial vs. final velocity.
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The formula you gave is for (kinetic) energy, ie that is the energy of a vehicle with mass
m and speed
v. Much better would be to use
force and acceleration: F=ma. Acceleration
a has time in it (time squared, actually), so the greater the acceleration, the more fuel had to be burned to produce enough more force.
Quote:
Originally Posted by Maury Markowitz
But that certainly isn't the case in practice. "gunning it" up to speed appears, according to my milage readout, take considerably more gas than doing so leisurely. And every reference on gas milage says the same thing.
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"Grannying" isn't what you'll find here, nor will you find us telling you to floor it, either. Each engine has a "sweet spot" of efficiency - a certain range of loads (usually ~80%) and rpms (close to max torque) when the amount of fuel used to produce a certain amount of power is lowest. Trying to keep your engine close to that sweet spot yeilds highest efficiency. Search for BSFC maps for more reading.
Edit: MetroMPG beat me, grrr.