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Testing the limits of 2012 NG

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  • #16
    Excerpts from link found at end of this post:

    We have all heard that there is an optimal speed to travel at to maximize fuel efficiency. But why is that? There ultimately has to be some kind of trade off, wherein we pay a penalty for going too slow, and pay a penalty for going too fast, so the optimum can be somewhere in between. It is easy to see why going to fast is bad, the faster we go, the more we lose out to air resistance. On the other end, if we go very very slowly, this is also inefficient, since it will take a very long time for us to reach our goal, during which we will lose fuel to having the car on and running. As I explored in my answer to How effective is speeding?, a very simple model that does a decent job of fitting measured fuel efficiency curves is to assume that the power drawn by the car takes the form


    here the Av3 term is the cost of air resistance, and P0 denotes our constant power losses due to having the car on.

    Trying to be a bit more accurate this time, let s include rolling friction and rotational losses in our car, using as our model of the force


    The benefit then is that we can use their parameters for a 2004 Honda Civic DX:

    A=105.47 NB=5.4276 N/mpsC=0.2670 N/mps2m=1239 kg

    It remains to determine P0, which I set to P0=6 kW in order to obtain a sane value for our optimal speed. Our resulting model gives, for the fuel efficiency of our model car (terrific graph shown in link):

    (graph shows optimal speed is 42mph, 2500rpm. From standing start, accelerate to 42mph in the first 200m. Shut off engine until 32mph. Restart engine and accelerate to 42mph. Repeat.)

    . . so that our driver just turns on the engine in pulses, and otherwise coasts for most of the journey. This actually cuts our fuel consumption nearly in half . . use 32mph and 42mph as upper/lower speeds.


    • #17
      The pulse-and-glide strategy provides a 20 percent fuel saving:

      Letting off on accelerator reduces fuel flow to engine
      Putting car in neutral eliminates engine braking
      Shutting off engine does both

      Coasting with a vehicle not in gear is prohibited by law in most U.S. states. An example is Maine Revised Statutes Title 29-A, Chapter 19, ยง2064[19] An operator, when traveling on a downgrade, may not coast with the gears of the vehicle in neutral.