Armchair Hypermiling of a Leaf

Discussion in 'BEV or Battery Electric Vehicle' started by RobertSmalls, Jul 30, 2010.

  1. RobertSmalls

    RobertSmalls Ecodriver

    The topic of trying to squeeze out the most KWh/mi came up here, and it got me thinking on how you would do it.

    Instead of the familiar BSFC plot, EV drivers will study an efficiency curve that looks like this:


    The curve for the Prius has much the same shape, and I'm sure the same will be true of the Leaf. Specific numbers may change, but the shape of the curve won't.

    Note the enormous sweet spot on the '05 Accord's electric motor: 1500-3000RPM, or 1000-3500RPM with a heavy load. That's a very usable sweet spot, and it allows jackrabbit acceleration (by our standards) and hill climbs with less than 1% drop in efficiency.

    The odd thing about the Leaf is it's got one gear ratio and no neutral position on the tranny. That's okay, because an electric motor is easy to spin by hand when not loaded, and losses will be limited to friction in the bearings and aero drag on the rotor inside its case. But you won't P&G or EOC.

    Because it's got one gear ratio, your RPM will be determined only by speed. If you DWL, your operating point on the above plot will move left-right. If you cruise control, it will move up-down. Cruise control moves you to a more efficient point while you make lots of power, then to a less efficient point while you make less power. DWL keeps you at moderate efficiency throughout. CC may beat DWL, especially when you take aero drag into account. You may find yourself driving for constant airspeed instead of constant load.

    The biggest way to squeeze more kWh/mi out of an EV will probably be to drive for minimum aero drag: drive slower. You can also hyperinflate the tires, and reduce accessory loads. Aside from that and extensive aerodynamic modifications, I don't know what else you could do to improve KWh/mi.

    I look forward to the inevitable CleanMPG review of the Leaf, even if that has to wait until next year. If the stated 100mi range figure is based on driving fast, using the A/C, and not exceeding a certain depth of discharge, I'm sure a hypermiler can get more than 200mi by driving very slowly under optimal conditions.

    I'm also looking forward to the (hopefully) inevitable Dept. of Energy teardown of a Leaf. :) Me, I'll wait and see what a Leaf goes for on the used market after it's been in a major rollover. It might turn out to be a cheap way to build an EV Insight. I will name it "Hondatsun EV2". Or maybe not.
  2. jimepting

    jimepting Well-Known Member

    Interesting analysis Robert. I had been wondering when someone would open this subject. With no Leafs in the hands of hypermilers, it is good to ponder in advance. I like that idea of folding the good stuff into an Insight, with it's great aero and light weight.
  3. Bike123

    Bike123 Well-Known Member

    Don't forget i^2*R losses of the motor (rise rapidly at high current (torque) ). Jackrabbit starts, braking, and high speeds will still be penalized. I would expect pulse and glide to be counterproductive.

    Add eddy current losses to the list of parasitics for permanent magnet motors that you can't disengage.
  4. RobertSmalls

    RobertSmalls Ecodriver

    I was hoping that i^2*R would approach zero, as i will be tiny and R is measured in milliohms.

    But this reminds me of something every EV buyer will want to do: Disable automatic regen when you lift off the throttle. The car will come with behavior that mimics an ICE: a significant slowing down at higher speeds when both feet are off the pedals, and creeping forward at low speeds.

    The correct behavior is: zero current when feet are off the pedals. The car will sit and be ready to roll like a car in neutral at a stop, and will glide seemingly forever when you lift off at speed. I presume the car will come with a neutral button that accomplishes exactly this, though it can't disengage the motor. You'll need a simple circuit that engages neutral mode when TPS < 10°.

    I took a second look at the above graph. Consider that for the Leaf, the x-axis will be labeled in mph, not just rpm. Up to a certain vehicle speed (2200RPM in the above case) it looks like you get max. efficiency at part throttle, with more throttle as the speed rises. Beyond that, full throttle. It's a bit odd to extrapolate from Prius to Leaf, so hopefully we'll see exactly what the Leaf's inverter and high-rpm motor can do.

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