Quote:
Originally Posted by TestDrive
Isn't this conflating BSFC (of the engine) with the BSFE and most Optimally FE Cruise Speed (of the vehicle).
Referring to the first chart in the http://ecomodder.com/forum/showthrea...-got-1466.html
I see FUEL CONSUMPTION mapped against engine RPM and TORQUE - nothing involving GEAR RATIOS or MPH.
Assuming the cruise speed in question was already optimally FE prior to the reduction in drag, I think that sums it up nicely.
Of course assuming that the initial cruise speed was already optimally FE can hardly be considered a given. For both my '94 Escort 5sp MT and my '99 Escort 4-door 5sp MT, the most optimally FE cruise speed is about 37 MPH !!! |
The premise of Sovran's report addresses any vehicle operating on SAE cycles.He addresses urban,hwy,and mixed composite cycles.From his charts,the theoretical relationships between drag-reduction and MPG are laid out,however his caveat,and this is what he wants to stress the most,is that without proper gear-matching,the realized improvements from drag reduction may be lower than what is achievable.------------ You'll notice on your BSFC maps,that the island of highest BSFC occurs between either a range of mean piston speed,or engine rpm at a given range of load.The rpm or piston speed equates to a given cruise speed in top gear.At this speed,if the engine does not see this much load,it will operate at a less efficient level.The only way to correct this is to put taller gears in the car,or as MetroMPG has offered,move to a smaller displacement powerplant.