EVs vs EPA

Discussion in 'EV (Electric Vehicle) Discussion' started by bwilson4web, Sep 5, 2020.

  1. bwilson4web

    bwilson4web BMW i3 and Model 3

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    Thanks to a recent article "How Tesla scores big range numbers while playing by the EPA's rules" by Dave VanderWerp and Carl Weins, in Car And Driver, Volume 66, Number 3, Sept. 2020, pp. 52-56, we have an opportunity to review how the system works. This issue is on the newsstands and my review add some of my work dating back to Prius days.

    INTRODUCTION

    "... the Model S Long Range Plus can cover a whopping 402 miles by the EPA's yardstick.

    When it comes to EV range, Tesla appears untouchable Despite pouring billions into electric-vehicle programs, established automakers have failed to make up much ground when it comes to the range game, and winning that game remains a critical part of winning over buyers. So how does Tesla do it?"
    (pp. 52)​

    Dave lists these causes:
    • THE ADJUSTMENT FACTOR - I was aware of a general 30% used in the Prius days. Dave points out "the EPA allows automakers the option to run three additional drive cycles and use those results to earn a more favorable adjustment factor." (pp. 54) He goes on to list: 29.5% Model 3 Standard Range Plus (M3SR+ my Tesla), and; 24.4% Model Y.
    • BIG BATTERIES - "... Tesla has the largest battery packs out there." except my M3SR+ has the smallest Tesla battery, listed as 55 kWh in the EPA Test Car Database. Given the excellent data the EPA shares, I wish it had been referenced in the article.
    • EFFICIENCY GAINS - "The key to extending EPA range is to use less electricity to propel the vehicle and recapture as much energy as possible ... whenever the driver lifts off the accelerator ..." He is describing what we call "one pedal driving" which is also shared by our BMW i3-REx. He points out that other manufacturers use different braking and regeneration approaches that may not be as efficient and I agree. Any form of braking that heats the disks and pads is wasting energy. He also points out Tesla has gone from less efficient induction motors towards efficient permanent magnet motors.
    He does a fair job describing the EPA testing procedure including roll-down testing (for dynamometer coefficients) and the "Road Load Pounds Force" as a function of speed. What he missed is if you have the "Road Load" and velocity, you can convert this into drag HP with a simple formula I got at ECOmodder.com:

    drag HP = v * (A + B*v + C*(v*v)) / 375

    With the roll down coefficients from the EPA Test Car Database and this formula, you can generate a Power vs mph similar to my M3SR+ vs IONIQ graph:
    SRM3+_IONIQ.jpg

    From the earliest Prius days to every subsequent car we've own, it turn out that 62.5 mph matches the EPA highway numbers. In contrast, Dave advocates using 75 mph which gives a way lower range than the EPA numbers. But this led to a simpler, analytical approach, 10 mile benchmarks.

    Do three, bi-directional or loop benchmarks to measure the power at three speeds:
    • 15-25 mph - the low speed, primary rolling resistance, drag power. It needs to be ~10 miles starting and ending at the same point(s) without stops.
    • 35-50 mph - a mix, ~50%, of rolling and aerodynamic drag power.
    • 65-75 mph - majority of aerodynamic drag power.
    With three benchmark points, use a quadratic formula solver or a second degree trend line in Excel to get the coefficients for a HP vs mph chart. Everything else such as range and range with reserve is trivial.
    mph_miles.jpg

    Now one thing about the 62.5 mph is how close it is to the 65 mph many semi-trailer trucks use as company policy on the Interstate. So use adaptive cruise control to safely follow and achieve EPA range on a trip. Unlike humans, dynamic cruise control does not get anxious and covers a lot of range ... like the interstate truckers. Play tunes and wait until you get to the next charger or destination.

    SUMMARY

    Dave's article is a fair introduction to those who've never studied the EPA Test Car Database. The additional adjustment protocol for the 29.5% M3SR+ and Prius 30% adjustment was not significant for my car. Still I've used two approaches:
    • compare two or more cars - use the EPA Test Car Database for the roll-down coefficients. Graph the drag power curves and their performance across the speed range becomes trivial. There have been cases of cars with better rolling drag becoming hogs at high speed.
    • benchmark quadratic formula - for cars I own, a trivial way to plot Power vs mph. This is especially useful when tuning a car for efficiency by changing rims, tires, and aerodynamic mods.

    Bob Wilson
    9011 Randall Rd SW
    Huntsville, AL 35802
     
    #1 bwilson4web, Sep 5, 2020
    Last edited: Sep 5, 2020
    Tideland Prius likes this.
  2. markabele

    markabele owner of PiP, then Leaf, then Model 3

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    The Engineering Explained channel on YouTube has a great video about all this as well.