Quote:
Originally Posted by shockj2000
I see weather here is pretty good in the winter. Im going to try it out some this tank as my tank only netted 43 mpg this time around. I coasted from 65 down to 55 today once and it did take some effort to get back up the hill maybe negative idk but i coasted a good 30 secs or so.
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Just keep in mind the break even point isn't a simple static point for all vehicles ... or even the same vehicle under all conditions... and the concept is at the same average speed.
Aerodynamic drag changes with wind speed , air pressure, temperature, humidity, etc.
Different Engines have different BSFC , different pumping losses, etc.
Different driving contexts consume different amounts of energy ... all other things being equal slower is less energy than faster ... energy conservation is more efficient than energy conversion ... etc.
For example of the 75MPH and 25MPH example of aerodynamic concept of that example vehicle 0.25 Cd and 20 Ft^2 ... while it increased the Aerodynamic drag energy consumed by ~2.5 kwh ... that would only be a negative to net FE if that ~2.5 kwh is larger than the gains from reduced Pumping losses and improved % of time at better or best BSFC.
I am not trying to propose a specific set thing ... like 75 to 25 is always too much of a speed variation ... that is not my point ... my point is the reality of the pros and cons ... and it is better to understand how things actually work than a hard set rule that will be wrong in some contexts.
I am referring to the known scientifically confirmed pros and cons of the method ... the break even point will not always be a 50 MPH speed variation ... it will not always be 25 MPH speed variation ... etc.
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For Example:
If the steady state vehicle in the previous example achieved 50 MPG ( so the math is easy at 1 gallon consumed ) and averaged of 28% ICE efficiency ... from average ~36 kwh / gallon gasoline... using ~10.1 kwh per gallon of the chemical energy of the gasoline.
Even with the additional ~2.5 kwh of aerodynamic losses would not result in a net lower FE if the speed varying method can achieve an average ICE efficiency of at least ~35% ... including the efficiency benefits of both time and fuel at better ICE BSFC points and reduced time spent paying for pumping losses.
At that point there would be no net negative FE effect.
But if the increased average ICE BSFC only went from 28% to 32% efficiency ... that is not a large enough gain to offset the aerodynamic penalt in that example.
In different context with different aerodynamic CdA, different conditions, etc... the break even point moves ... it is not static.
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Quote:
Originally Posted by SentraSE-R
You can't produce numbers like these engine on.
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Can't produce those numbers if engine is just off either... use 0 fuel/energy travel 0 Miles... no free lunch.
It isn't the engine off itself that achieved those numbers ... it is how the engine was operated when it was on to push the vehicle , by converting fuel chemical energy into vehicle kinetic energy , and the context of the conditions to achieve whatever average speed you had over that distance.