Quote:
Originally Posted by moorecomp
 Traveling any distance and burning no fuel is always going to be more efficient than traveling the same distance with the engine running.
It is called "paralysis through analysis". |
Not 'paralysis through analysis' at all ... there is a point of diminishing returns... and past a certain point it is less efficient... the point is that it can be useful to know how it works in order to maximize benefits and minimize disadvantages.
It takes energy to get moving from a stop ... you already burned the fuel to get the kinetic energy you are consuming during the coasting portion ... you consume that invested kinetic energy to over come aerodynamic losses and to over come rolling resistance losses ... there is no traveling a distance without spending energy / fuel to do so ... there is no free lunch... having spent the the fuel/energy at a prior point doesn't change this , you still had to spend it.
It is more aerodynamically efficient to achieve a given average speed by a steady speed than it is to achieve the same average speed by fluctuating speed... like it or not ... that's just how aerodynamics works... when it comes to aerodynamics there is a bigger hit to going faster than there is a benefit to going slower... and yes aerodynamics has been tested over and over and over again ... this relationship is well known ... it is just the way it.
Example of concept:
Quote:
Say you have a Vehicle with a Cd of 0.25 ... and a Frontal Area of 20 Ft^2 ... Say for example you want an average speed of ~50 MPH over 50 Miles.
Looking at the aerodynamics.
If you achieve this average speed of 50 MPH via a steady speed of 50 MPH for 1 hour your wind resistance is about ~3.37 kwh of energy consumed... 50 miles covered.
If you achieve this same average speed by traveling for 30 minutes at 25 MPH ... covering 12.5 Miles , consuming ~0.210 kwh of energy ... and the 2nd 30 minutes at 75 MPH ... covering 37.5 Miles , consuming ~5.68 kwh... over the same 1 hour time period you have the same average speed of 50 MPH ... but due to the exponential effect speed has on aerodynamic losses ... your fluctuating speed method used ~5.89 kwh to cover the the same 50 miles at the same 50 MPH average speed.
The fluctuating speed method to cover the same distance at the same average speed consumed ~2.52 kwh more energy due to the exponential effects of aerodynamic losses.
The larger the speed variation the larger the penalty to the speed varying method ... the smaller the speed variation the smaller the penalty ... but it is always an aerodynamic penalty.
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As I already wrote ... there are pros of the engine off method ... maximizing time / fuel spent in better / best BSFC ... and avoiding unnecessary pumping losses... but there are cons as well... there is a point of diminishing returns ... and a point where it breaks even ... and yes ... even a point where taken too far it is a net lower efficiency , and lower over all FE.