Quote:
Originally Posted by Tesla
only one concept can be right
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I don't think it's that simple.
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If you look at the BSFC for a given engine ... there is an example posted above ... the best energy efficiency point will have areas around it ... above , bellow, left , right ... which is a result of a long list of factors.
This has two effects from the Engine's perspective:
#1> The same load at lower RPMs can sometimes be less energy efficient than the same load at higher RPMs ... than past a certain point it reverses and higher RPMs at the same load become less efficient... lower RPMs or higher RPMs either one may be good or bad depending on where you are on the BSFC chart.
#2> More load at the same RPMs when you are load bellow the best BSFC point is better for efficiency ... but more load at the same RPMs when you are load above the best BSFC point is less efficient... lower Load or Higher Load either one may be good or bad depending on where you are on the BSFC chart.
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The user's control of the current desired engine load is a function of throttle ... current vehicle external forces , being external are not under the user's control ... although they can be predicted.
The user's control of current desired engine RPM is a function of Gear Choice and current vehicle speed... current vehicle speed is a function of previous choices of both Gear + Throttle.
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From a Vehicle travel perspective.
Slower always takes less energy to travel the same distance.
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In the real world those different effects combine together.
So for example ...
If reducing your vehicle speed reduces your energy per mile needs by 10% , but it moves you ( due to gear and/or throttle ) out of the BSFC best point so that you are ~11% less engine efficient than you are a net of ~1% less or lower MPG.
If increasing your vehicle speed increases your energy per mile needs by 10% , but moves you ( due to gear or throttle ) closer / more into the best BSFC point so that you are ~11% more engine efficient than you are a net of ~1% more or higher MPG.
etc ... etc ... they combine many different ways.
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It is a % difference in current % ... so there is a ~11% difference between 30% and ~33.7% ... just like there is a ~11% difference between 25% and ~28%.
It is not just a difference in % ... so you don't need as big of a jump as 20% to 31% would be ... in order to get ~11% difference ... a 20% to 31% jump would be ~55% difference...
If my gear & throttle choices allow me to jump from 20% to ~31% Engine Efficiency , than I can either increase my MPG by ~55% at the same vehicle energy per mile needs ... or I could get the same MPG at ~55% more vehicle energy per mile needs... or somewhere between the two.
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All the above still applies equally weather you are going up hill , down hill, or on flat level ground.