Some efficiency observations

A thread I was reading over the weekend got me to thinking, and I tested something out that others might find interesting. In the thread, it was asserted that if you press slightly on the accelerator, such that you are not decelerating and not accelerating, that the car is essentially in neutral. That shouldn't be the case. And I think it should be considerably more efficient to "coast" (when that is your goal) by putting the car into neutral. That disengages the motor, so that there is no back EMF (induced voltage).

If your goal is to slow down, then yes, you want the motor engaged to recapture some energy as regenerative braking. But if you don't want to slow down, then you don't want regenerative braking engaging, and further you don't want the motor engaged when you want to coast.

On my daily commute to work, I've been averaging about 4.3 m/kWh on the way to work (not great - more on that later), with the driving mostly around 40-45 mph with a good deal of hills, and only a couple of stops over the course of 10 miles. Until yesterday, I've always taken the approach of just putting the car in D and leaving it there.

The last two days, I decided to be more active with the "shifter", to put the car in neutral whenever I wanted to just coast (especially on declines when I don't want to slow down at the bottom), to keep it in regular D when I want to use the motor, and to put it in Eco mode when I want to slow down (more aggressive regen - and my car has no B mode).

The efficiency on the way in to work both yesterday and today was 4.8 m/kWh, roughly a 10% increase. I'm going to continue doing this for a while and monitor the efficiency.

On another note - last summer, when I first got the car, I was averaging 4.7-5.0 m/kWh for my round-trip commute, on the OEM steel wheels. I put snow tires on the steel wheels for the winter, and it dropped to around 4.2-4.3 m/kWh (dropping considerably lower as the winter set in).

This spring, I put 17" Juke wheels on, with brand new Yokohoma Avid Ascend low rolling resistance tires, 215/50/17 (10 mm wider than the OEM 16" tires). Over the round-trip, my round-trip efficiency for the past month or so has been 4.3-4.5 m/kWh, about 10% lower than it was last summer with the OEM wheels and tires. I have a few hundred miles on the tires now - perhaps they haven't fully broken in yet? Or does the extra 10 mm width and the bigger wheels (higher moment of inertia, also they have a good deal of open space so they aren't real aerodynamic) really make that much difference?

Hello,

Yes, coasting is better when you need to carry speed - and shifting into neutral (above 7MPH, just shift into Reverse to get neutral instantly) is the easier way to go. Trying to find and then stay in, the neutral zone on the accelerator pedal is not very practical and it is less effective.

But, you have to realize that an electric car never disconnects the motor armature from the wheels. The effect is the same, though, and there is no back EMF because the controller is not in regenerative mode.

For what it is worth, my dash meter is 6.0 miles / kWh at the moment, after 2 days of my regular commute; which is ~26 miles round trip, mostly in 40-45mph roads, with several hills and several traffic lights.

I am using the stock Ecopia tires at 45PSI on the stock 16" steel wheels. They coast very, very well - almost as well as my Nokian Hakka R2 winter tires; which are one of the best RR tires in existence.

How do you manage to average 6 mi/kWh? Have you done some of the aeromods?

No mods at all except the 45PSI in the tires. I have been ecodriving for about 7 years - in my ICE, I was averaging ~50% above EPA (though that was with a lot of aero mods).

I coast in neutral at lots of places, especially on roads that I drive all the time, and I have memorized where I can coast. I take full advantage of all "road candy" aka hills and down slopes.

I rarely use any HVAC - in the winter, I only use the heated seats and heated steering wheel when needed, and the defroster when required. I crack the windows about 1/2" to 1 1/2" when possible - I have found this actually helps aero drag a bit. The passive vents are more effective, when you crack the windows, and this then helps lower the drag a little bit.

I do use the stereo virtually all the time.

I "downshift" into D and then B to slow down, and I try to brake gently. When you have to brake hard - you have already used too much energy. Regen loses less energy than friction brakes - but it still loses energy.

Your (open?) wheels are raising your drag a fair bit. Wider tires add some drag. And the rolling resistance of the tires is critical.

The Leaf's dash display is quite optimistic - the stock 16" tires are smaller than the stock 17", and when I do energy calculations, I use a corrected distance (minus 1.7% at the moment). And measuring the actual charge includes charging losses.

My best measured consumption so far, is just under 207Wh/mile or 4.83 miles / kWh. I will measure the charge this evening and I will see what the 6.0 actually is. Here is my energy log over at EcoModder.com: http://ecomodder.com/forum/em-fuel-log.php?vehicleid=8730

Here is the best data I have seen on wall-to-wheel charging losses: http://www.mynissanleaf.com/viewtopic.php?f=31&t=18924#p409487

My latest charge, corrected for the known odometer error (-1.7%) was just under 205Wh/mile or 4.88m/kWh, while the dash read 6.1 miles / kWh. I don't know why it looks like there is ~22% of wall-to-wheel losses?

NeilBlanchard said:

My latest charge, corrected for the known odometer error (-1.7%) was just under 205Wh/mile or 4.88m/kWh, while the dash read 6.1 miles / kWh. I don't know why it looks like there is ~22% of wall-to-wheel losses?

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Yes, about 15% loss through the charger, and another 5% to pull it out of the battery, with motor and inverter inefficiencies.

msbriggs said:

...This spring, I put 17" Juke wheels on, with brand new Yokohoma Avid Ascend low rolling resistance tires, 215/50/17 (10 mm wider than the OEM 16" tires). Over the round-trip, my round-trip efficiency for the past month or so has been 4.3-4.5 m/kWh, about 10% lower than it was last summer with the OEM wheels and tires. I have a few hundred miles on the tires now - perhaps they haven't fully broken in yet? Or does the extra 10 mm width and the bigger wheels (higher moment of inertia, also they have a good deal of open space so they aren't real aerodynamic) really make that much difference?

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My guess is that the "higher moment of inertia" is a big part of it.

I never understood the attraction of bigger wheels but a lot seem to like them for aesthetic reasons. You can expect to take a hit to efficiency with bigger wheels and tires. In addition to increased drag from the wider profile and, perhaps, the wheel design itself, mass will also play a factor. If the wheel/tire combination is heavier than the stock wheels/tires — especially if the mass is distributed more toward the rim than the axle — it will take more energy to accelerate them to a given velocity. The reason is angular momentum. A few extra pounds on the rim of the wheel/tire can make for a large increase in angular momentum and that takes energy to overcome every time one changes the velocity of rotation. It is less of a factor when cruising at a constant velocity than in varying speed stop-and-go driving, but everyone has to spend energy to get up to cruising speed. And back down when slowing, whether by coasting/drag, regen braking, or friction braking, the energy of rotating mass needs to be dissipated.

Adding mass to the wheels makes for a much bigger hit to efficiency than adding the same mass to the rest of the car. And adding it farther away from the center of rotation is less efficient than nearer the center (due to the moment arm).

FWIW.