Quote:
Originally Posted by redpoint5
There is a balance of engine efficiency, speed, wind resistance, and rolling resistance.
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This part I agree with.
A few notes:
Speed is the only user controllable variable for wind resistance, or rolling resistance... for a given vehicle.
Sense Rolling resistance increases linearly with speed it does not itself directly change the net vehicle energy needed per mile ... going 2x as fast uses 2x the power for rolling resistance but gets there in 1/2 the time ... resulting in the same energy for rolling resistance for any given distance for any given speed ... for a given vehicle weight and Crr for the tires.
So it is the exponential wind resistance effects from speed that always result in less energy for a given distance at a slower speed than a faster speed.
Example Aerodynamics:
40 MPH requires 4x the power of 20MPH ... but only covers the same distance in 1/2 the time ... resulting in 2x more energy spent ( even if it was potential energy ) to travel the same distance.
60MPH requires 9x the power of 20MPH ... but only covers the same distance in 1/3 the time ... resulting in 3x more energy spent ( even if it was potential energy ) to travel the same distance.
Faster always requires more energy to move the vehicle at that speed and slower always requires less energy to move the vehicle at that speed... no mater where you get the energy from.
Quote:
Originally Posted by redpoint5
Good post, but I disagree with this. If you are engine on coasting down a hill, then slower does not take less energy to travel the same distance. Also, going slower up a hill does not always take less energy. If you go 1mph up a steep hill, enormous amounts of energy will be wasted just maintaining position on the hill.
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I'm sorry I was not more clear ... there are multiple interconnected aspects ... but from a vehicle perspective , yes slower is always less energy needed for that given speed ... weather or not it is more energy efficient application for the ICE is a separate issue ... the separate issues that are related to the net result is what I was trying to describe.
Even in your example ... The slower vehicle speed up or down the hill will use less energy to travel the same distance at a slower speed than it will at a higher speed ... going down a hill is just converting the potential energy you stored when you went up the will ... even if you timed it perfectly to come to a near complete stop at the crest of the hill ... going down or up the hill does not change the exponential effects of wind resistance with speed.
In your example you had the engine on while coasting down the hill ... so it becomes a question of while coasting down that hill what is the energy efficiency of the ICE converting fuel energy to useful mechanical energy to move the vehicle? ... that effects the ICE energy efficiency , but not the energy needed for the vehicle to travel a given speed.
There is a set amount of energy needed to lift a specific mass / weight of object up a given hill / distance ... how fast you do it does not effect the amount of energy needed ... just the amount of power needed ... but if you spend 2x the power you will get done in 1/2 the time and you still end up spending the same amount of energy.
All the energy spent fighting gravity to go up the hill is stored as potential energy at 100% efficiency ... Rolling resistance up or down the hill are separate ... Aerodynamic losses up or down the hill are separate ... ICE operating efficiency up or down the hill are separate.
These separate issues each contribute to the net result ... but they each have their own specific directions / functions.
I hope I was a bit more clear this time.