SanDust
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Re: Stoaty's Guide to Energy Efficient Driving of the Leaf

Sat Sep 03, 2011 1:27 pm

Stoaty wrote:I am pretty sure I saw that 30% number on MNL. When I looked up Prius regen efficiency, best guesses were 30-50%. If it is better than that, I would like to see a reference. Don't know the real answer, but doubt the 80%. That would be phenomenal.
There are several different numbers that people use. 30% would be at the high end for the overall efficiency increase but too low just for the efficiency of turning kinetic energy into chemical energy. Of course it can't be greater than 90%. A guess of 70% at the highest end wouldn't be bad (80% drivetrain efficiency X 90% charging efficiency).

This is one way efficiency. Roundtrip efficiency would be more like 55% assuming your drivetrain was 80% efficient (80% out X 70% back).

The Prius battery is considerably smaller which will limit the amount of regen it can handle.

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planet4ever
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Re: Stoaty's Guide to Energy Efficient Driving of the Leaf

Sat Sep 03, 2011 3:37 pm

SanDust, can you explain that "80% drivetrain" number. Are you literally talking about losses in the reduction gears and differential? I have no expertise in this area, but I would have guessed less than 5% loss there, since there is no transmission as such. Any losses would have to show up as heat. Does the transaxle really radiate that much heat? It doesn't have active cooling, does it?

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Re: Stoaty's Guide to Energy Efficient Driving of the Leaf

Sat Sep 03, 2011 3:47 pm

planet4ever wrote: Does the transaxle really radiate that much heat? It doesn't have active cooling, does it?
Ray
The three gear set reduction from the motor to the final drive uses one liter of lubricant. There is no effort to provide any cooling whatsoever (no fins cast in the housing, no external coolers, no radiator coolant pumped in, etc).

The motor itself (liquid cooled) would act as a heat sink, however.

SanDust
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Re: Stoaty's Guide to Energy Efficient Driving of the Leaf

Sat Sep 03, 2011 4:07 pm

planet4ever wrote:SanDust, can you explain that "80% drivetrain" number. Are you literally talking about losses in the reduction gears and differential?
All the losses going from the battery to the wheels. That would include the reduction gear and the other mechanical linkages as well as the chemical-magnetic-electrical-mechanical conversion losses. I don't know about the Leaf specifically but 80% is usually assumed to be the efficiency over a range of speeds for an EV. Obviously at low or high speeds you'd have more losses.

Volusiano
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Re: Stoaty's Guide to Energy Efficient Driving of the Leaf

Sun Sep 04, 2011 2:48 am

SanDust wrote:The Prius battery is considerably smaller which will limit the amount of regen it can handle.
Does this mean that the Prius uses the friction brake more often than the Leaf?

Also, I can understand the Prius battery can't handle the last amount of regen from going down hill for a long time, but I have to believe that the Prius battery is designed to be large enough to recapture most of the energy from a typical stop from 45 miles to 0, no? Otherwise, it seems very inefficient not to be able to recapture all the kinetic energy just because the battery is not large enough to store it.

But then again, if the switching point from battery to ICE is 15 miles, is that safe to assume that because the battery is only big enough to propel the Prius to 15 miles, then it's only big enough to capture the kinetic energy from 15 miles to 0?

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JPWhite
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Re: Stoaty's Guide to Energy Efficient Driving of the Leaf

Sun Sep 04, 2011 4:20 am

Volusiano wrote: but I have to believe that the Prius battery is designed to be large enough to recapture most of the energy from a typical stop from 45 miles to 0, no?
You're probably right, but regen comes in two flavors. Braking and coasting.

The regen brakes slow the vehicle down, the drive motor regens the gradual speed loss when coasting. It's my understanding that regen while coasting gets more total energy back than braking does.

I have been simply amazed at the LEAF being able to capture back as much as 25% of energy through regeneration even the way I drive :-) Of course Carwings tells me that, not sure how much we can rely on those statistics, but hopefully its in the ball park.

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garygid
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Re: Stoaty's Guide to Energy Efficient Driving of the Leaf

Sun Sep 04, 2011 8:09 am

For Regen, assuming above average values, the efficiency for some of these steps might be 97% or 98%, and others only 93% or 92% ... so, without real values, let's just assume each is 95% for now:

1. mechanical road to motor
2. motor to electrical
3. AC to DC in the Inverter
4. energy loss into battery
5. energy loss leaving battery
6. inverter converts DC to AC
7. motor drive losses
8. mechanical to road losses

The net efficiency is the product of all of the efficiencies:
0.95 x 0.95 x 0.95 ... (8 terms) =
0.95 ^ 8 = about 66% (a VERY good figure)

In practice, one usually gets lower overall Regen efficiency.
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abasile
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Re: Stoaty's Guide to Energy Efficient Driving of the Leaf

Sun Sep 04, 2011 2:41 pm

Volusiano wrote:
SanDust wrote:The Prius battery is considerably smaller which will limit the amount of regen it can handle.
Does this mean that the Prius uses the friction brake more often than the Leaf?
Yes, unless one learns to brake gently and gradually, the Prius will generally end up using the friction brakes more than the LEAF. With the Prius, regen maxes out at about 20 kW, compared to 30 kW with the LEAF.
Volusiano wrote:Also, I can understand the Prius battery can't handle the last amount of regen from going down hill for a long time
That is correct. However, the Prius does have one sort of advantage over the LEAF on long descents. The ICE (internal combustion engine) can provide compression braking, thereby eliminating or reducing the need for friction braking when going down mountains. With no ICE, the LEAF is forced to rely on friction braking if the battery is too full. So, in the LEAF, one should avoid higher states of charge prior to big descents; then you come out way ahead of the Prius in terms of being able to recapture energy.
Volusiano wrote:I have to believe that the Prius battery is designed to be large enough to recapture most of the energy from a typical stop from 45 miles to 0, no?
Yes, the Prius battery is more than large enough for that. But you need to brake gently and gradually to recapture that energy.
Volusiano wrote:But then again, if the switching point from battery to ICE is 15 miles...
If one accelerates very gently, it is possible to go as fast as 45 mph in the third-generation Prius (2010/2011) without using the ICE. The only problem is, if you drain the battery too far, the Prius will be forced to run the ICE as a generator to recharge the battery, which will reduce overall efficiency.
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Re: Stoaty's Guide to Energy Efficient Driving of the Leaf

Sun Sep 04, 2011 9:38 pm

Stoaty wrote:For those like me who want to conserve as much energy as possible I offer my recipe [...]
6) Look ahead at the speed of traffic and the next 1-2 stoplights. Note whether the lights are "stale" (have been green for a while and likely to change soon) [...]
I use a couple of hints for this:

1. Pedestrian crossing light: if it's white, it means that the my light will be green for a while and there is no need to speed up to make the light and avoid having to stop. If it's blinking red, I may need to hurry up if I want to avoid stopping. (Around here, the pedestrian crossing light changes to solid red at the same time as my light changing from green to yellow.)

2. On a familiar stretch of road at a familiar time of day (e.g. commute), the number of cars at an upcoming red light tells me whether it's about to turn green any time soon. Relatively few cars means that the light has just turned red; relatively many cars means that the light has been red for a while and can be turning green soon.
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davewill
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Re: Stoaty's Guide to Energy Efficient Driving of the Leaf

Tue Sep 06, 2011 11:06 am

LEAFfan wrote:7. To stay in the neutral bubble and to keep your speed constant once up to speed, you can use the CC by going one mph over what you want, then push down quickly on the button and it will put you in the neutral bubble.
You post this a lot, and I have tried this several times but it never works. The car soon slows down 1 mph and starts using power at much the same rate as before.
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