Renault Fluence versus Nissan Leaf efficiency & range. - Renault Z.E. Forum
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post #1 of 16 (permalink) Old 07-30-2013, 07:48 AM Thread Starter
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Renault Fluence versus Nissan Leaf efficiency & range.

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Originally Posted by pauledg
The other surprise is that the Fluence, which is large, heavy and surely not designed with aerodynamics in mind, does so well range-wise. It makes me think all this advertising garbage about airflow, funny headlight covers and shiny paint on wing mirrors is just that, advertising garbage that has negligible effect. And since when did streamlining really work at the sort of speeds we are talking about in one of these cars?
Au contraire. The Fluence (much to Leaf owners' surprise, I expect) is more aerodynamic than the Leaf. This may be more out of happy-coincidence rather than design intent, because if you extend the boot to a narrow cross-section then you can really improve the Cd of a car (and, also, it is more aerodynamically stable at higher speeds). So maybe it was a happy outcome, rather than planned design.

The Fluence has a reported Cd of 0.25 (which is the same as the latest Prius, which is a Cd that Toyota have no hesitation in shouting about, yet Renault remain mute), whereas the Leaf has a Cd of 0.28.

Now, the actal aerodynamic cross-sections are about the same, SCx = 0.66m^2 for Leaf (smaller car), 0.67 for Fluence. However, the way aerodynamics works is that you can't model the drag of a car from the bald figures alone - because the Cd is lower on the Fluence then it will gain on aerodynamic efficiency quicker than the Leaf as the speed picks up. The SCx figure is an approximation at particular [lower] speeds. (It's to do with Reynolds numbers and complicated stuff, I think.)

(The Zoe has a SCx of 0.75m^2 ... yeah, wierd, the Fluence has a smaller aerodynamic cross-section than the Zoe!)

It's not altogether a hugh surprise - remember that the Fluence is quite low because the battery is not wedged under the floor (like every other EV?) but is added to the car's length. For aerodynamics, the 'flatter' the cross-section, the better.

The other issue for the Leaf is that it has a permanent magnet motor. I reckon that it is over its V/Hz regime by about 30 mph, so for most of its life, the Leaf motor will be in field weakening, which saps its efficiency (and at very high light loads, can actually demagnetise the magnets). Whereas the rotor coil of the Renault ZEs don't suffer this effect. They may even ramp down the current for the rotor excitation, and get an increase, rather than decrease, in efficiency.

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post #2 of 16 (permalink) Old 07-30-2013, 09:28 AM
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RE: Renault Fluence versus Nissan Leaf efficiency & range.

But surely better aerodynamics is not the full reason of why the Fluence goes further than a Leaf despite having a smaller powered battery?

Does the Leaf leave in a safety buffer zone? Does the Fluence work the battery harder and thus it will have a shorter life? What cooling does the Fluence battery have? Or is it a number of little factors and improvements that all add up to make a noticeable improvement over the Leaf?

(the Fluence does have merits, just a **** shame about the battery lease only model)
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post #3 of 16 (permalink) Old 07-30-2013, 09:54 AM
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RE: Renault Fluence versus Nissan Leaf efficiency & range.

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Originally Posted by GrumpyCabbie
But surely better aerodynamics is not the full reason of why the Fluence goes further than a Leaf despite having a smaller powered battery?
Under certain conditions Renault's motor is more efficient that a Leaf.

Not sure why else.


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What cooling does the Fluence battery have?
Convection mostly.
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post #4 of 16 (permalink) Old 07-30-2013, 01:05 PM Thread Starter
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RE: Renault Fluence versus Nissan Leaf efficiency & range.

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Originally Posted by GrumpyCabbie
But surely better aerodynamics is not the full reason of why the Fluence goes further than a Leaf despite having a smaller powered battery?
I think it'd account for a half of the difference at higher speeds.

The other half down to these field weakening losses.

Remember that EV's have fewer load losses than an ICE. Simpler gearbox (actually, the Fluence appears to have a single 1:10 gear interface, whereas the Leaf has three gear contacts that does not reduce the ratio in one go). No air intakes or exhaust pumping losses. Don't forget coolant and oil pumping takes power too.

So, all in all, more of the actual stored chemical energy delivers real, rolling work to drive the car in an EV (and that's not even beginning to talk about thermal efficiency of converting liquid fuel in the first place). So, really, all that is left to slow the car down is aerodynamic losses. Consequently, aerodynamics plays a bigger part in EV efficiency than it would equivalently do with ICE cars.

I reckon at low speeds, up to 30, the Leaf will be slightly more efficient. Slightly lighter, slightly lower aero cross section. No field weakening. I've not seen the 'urban only' NEDC range for Leaf and Fluence, but my guess would be Leaf wins urban, but Fluence wins extra-urban.

I've no idea if the power electronics might also play a part in the difference - maybe the Fluence power electronics is better/generates less heat? I'd presume they'd be largely the same, but there could be differences.

[hr]
Quote:
Quote:
What cooling does the Fluence battery have?
Convection mostly.
It's a forced-air system, but it doesn't appear to need to run for most of the time.

There is also a filter somewhere in there. There is a 'battery air filter' that appears on the bi-annual service schedule for replacement.

I've plugged in straight after driving and, sometimes, fans start up while it is charging. The exhaust sounds like it is venting under the car, so I presume it is picking up air from within the cabin space? Or maybe it is pulling air from under the car?? [#worried!!!!]

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post #5 of 16 (permalink) Old 07-30-2013, 01:24 PM
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RE: Renault Fluence versus Nissan Leaf efficiency & range.

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Originally Posted by donald
I think it'd account for a half of the difference at higher speeds.

The other half down to these field weakening losses.
What about more aggressive regen? I've driven both the Leaf and the Fluence and the latter is much more aggressive. Could it be that the Fluence is the better car, but the French reliability and battery lease model is causing concerns?

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post #6 of 16 (permalink) Old 07-30-2013, 02:11 PM Thread Starter
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RE: Renault Fluence versus Nissan Leaf efficiency & range.

Quote:
Originally Posted by GrumpyCabbie
Quote:
Originally Posted by donald
I think it'd account for a half of the difference at higher speeds.

The other half down to these field weakening losses.
What about more aggressive regen? I've driven both the Leaf and the Fluence and the latter is much more aggressive. Could it be that the Fluence is the better car, but the French reliability and battery lease model is causing concerns?

The Fluence is more aggressive regen than the Leaf? I find the Fluence to be 'just nice' and quite gentle for regen. I did drive a prototype Leaf, but instantly forgot the experience!

The regen on the Kangoo I test drove was like hitting a brick wall!!

As far as I can figure it out, the M/kWh rating the car feeds back must be an integral of the power-out and power-in to and from the battery. This would therefore NOT be the same as actual energy used from the battery - because it would not recharge the battery with a full kWh if the M/kWh says so.

When slowing down, I will not back off completely but manually limit the regen to 5 to 10 kW so as to give the battery the best opportunity to suck up that energy. This efficiency won't appear on the M/kWh gauge - as far as IT is concerned, it cares not the rate of regen because it will measure all of the energy as having been recovered, which is not true.

[The above is my own interpretation of how the system works - which I conclude because I do not see how the system can predict how much regen energy is actually recovered by the battery in real time.]

So, you can hit a regen power of 30kW for 5 s, sure, but you'll actually get more energy back into your battery with a 15kW regen for 10 s, or better still at 10kW for 15 s. All will appear on your gauge as having recovered the same amount, but the slower regen will be the more efficient (providing, of course, that you gave yourself enough slowing distance to avoid hitting the brakes!).
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post #7 of 16 (permalink) Old 07-31-2013, 03:17 AM
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RE: Renault Fluence versus Nissan Leaf efficiency & range.

Absolutely fascinating stuff and it's making me feel better!

Blue Progress report: 600 miles. Last charge: 107 miles

Latest charge: FAIL (I plugged the car in, set the timer, inserted the timer in the wall socket, switched on and went to bed.
Got up. No charge. Dammit, I forgot to lock the car!)
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post #8 of 16 (permalink) Old 07-31-2013, 06:34 AM
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Regen

I know what you mean about the Kangoo. It reminded me of the old Disney Mark V autopia cars. Lifting off the accelerator just about locked up the rear wheels.

Regen on the 2012 Leaf is mild compared to any of the Renaults.


Zoe uses the brake pedal to control regen. Hard to notice even if you know what the car is doing.


BMW ActiveE is like the Kangoo but with better, finer control. BMW calls it "one-pedal" driving. The i3 has a coast detent in the accelerator pedal.


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post #9 of 16 (permalink) Old 07-31-2013, 06:54 AM Thread Starter
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RE: Renault Fluence versus Nissan Leaf efficiency & range.

I do quite like that about the Fluence, that it is 'old school' in its decel.

It's just like a regular gas-guzzler car as it decels from taking your foot off, just a wee bit more so (and in any case, controllable).

The brakes appear to behave entirely independently (so long as they actually work - hat tip to Paul!!), so there is never any 'mismatch' ['brake-blending'] between braking and regen. This gives a very smooth interaction.

I can imagine that after a fair few miles, these two things might become a little de-synchronised as the performance of the system changes/degrades. Didn't they have this issue with Honda Insights - a few cases where the brakes suddenly altered their effect, mid-braking, as the regen cut in and out, and there were some tail-ending accidents?

(I guess the only thing I am concerned about is whether I'll lose my brake boost vacuum mid-braking, before they get to change this ECU!)

If the Leaf has gentle regen without braking, I guess the regen is controlled by the brake pedal too. I'm not saying one system is better or worse than another, but the way the Fluence does it clearly doesn't seem to hurt its range, by comparison!

I guess the other thing with the Fluence is that you're never having to second-guess what the system is doing, so as to maximise the regen. All you do is take your foot of the accel. You then know your're getting 100% regen. Touch the brake, and you know it is no longer all regen and you're 'wasting' kinetic energy to heat. I suppose with brake blending, you never really know how much is regen and how much is braking.
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post #10 of 16 (permalink) Old 07-31-2013, 03:36 PM Thread Starter
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RE: Renault Fluence versus Nissan Leaf efficiency & range.

OK, so, yeah, looks like Leaf does its regen on the first travel of the brake pedal.

And ... guess what ... they appear to have exactly the 'brake/regen' blending issues I think the Fluence casually sidesteps due to its simplistic engineering:

http://evseupgrade.com/pic/?leaf-brake-tsb
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