Electric Porsche!

Correct. The power curve for an electric motor will differ from that of an ICE, but they both have a power curve. More speed will translate to more power, up until the point where losses cancel the gains.

Tom

 

yep. edited my post because i could barely understand the point i was trying to make

 

I wasn't able to find a HP vs RPM graph for an Advanced DC electric motor like the ones used in many electric car conversions. However this is for a PMDC (Permanent Magnet Direct Current) motor like the ETek used by many electric motorcycles, scooters, fighting robots, etc.

 

well that shows that efficiency is near the upper range of the RPM range? well ya, but momentum, gravity, and the resulting current velocity all play a part as well. so running efficiently appears to still mean running at or near max RPM's which would be aided by downshifting?

not sure that combining torque (which should be constant) with amps which most definitely is not makes this chart easier to understand

 

Peak power and peak efficiency are not necessarily the same. My understanding is that peak torque comes at low RPM and high current. That would not translate to good efficiency.

Again, this is from reading posts on the EV list and people saying that they upshift for more power. For example to maintain speed on an increasing grade they upshift. Some quotes for the Electric Vehicle Discussion List:


"You do know to hold the trans in lower gears as long as possible, right? Don't upshift too early. Stay in lower gears for lower motor heat and better range, upshift if you run out of steam and/or need faster acceleration. Yes, UPSHIFT for more acceleration!"

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"If you want more horsepower (for climbing a hill or accellerating), a 96v pack only gives you one choice; upshift (like from 2nd to 3rd gear) Note that this is the *opposite* of what you'd do with an ICE. Upshifting slows the motor down, which makes it draw more current. For 2x the horsepower, you need 2x the current; 96v at 380 amps."

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"I'm thinking of reversing the operation of the downshift cable, have it pulled to passing gear for normal driving to keep the RPM up in the efficient region, and let it drop out of "Passing gear" to upshift when I need more torque."

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"My EV ('92 Celica) has a 9" ADC motor, stock 5 spd/clutch/flywheel, and is driven with 14x8V flooded cells through a Raptor 600 controller. Compared to the ICE motor, the DC does pull better at low RPM, and (as you imagined) does seem to "go away" at higher RPM's. With only 112V nominal to work with, I can only draw about 150 Amps near redline (about 5000RPM and 45 mph if I remember correctly). So if I stomp on it in second gear, it pulls hard (at about 400 battery amps), and then eases off as the speed goes above about 30 mph (estimating about 3500 RPM). It continues to pull, but takes forever to get to
40 mph. If I shift "early" (at about 30 mph) my current will stay high, and my speed will increase faster.

Everything I've read tells me to keep the RPM's high, which helps cooling, and range, but if I'm pulling out into traffic, I have to shift at lower RPM's.

If I drive at a steady 40 mph, I can drive in 2nd or 3rd gear. The difference in current drain (for constant speed) is hardly noticeable between the gears, and often I go ahead and upshift, anticipating the need to go faster. (and at lower RPM, I'll have more acceleration at the ready if I need it).

My range is about 35 miles, which is all I need, but if I had to do it over, I would probably go with a higher voltage pack (to give me more torque at the higher RPM's)."


So, if Daniel's EV is like these people's he might be driving it opposite of how he needs to get best acceleration and range. These people claim that to get the best range you need to cruise at high RPM and upshift to a lower RPM for more passing power. This is exactly opposite of what one does in an ICE.

Daniel, If you haven't already I would recommend you join the Electric Vehicle Discussion List. They have moved from direct email to a yahoo group now. It has some very knowledgeable people that can help you with your range and acceleration issues.

 

i have to agree about the overheating. the Tesla didnt it have cooling fans? maybe that is all you need. there are a lot of Zenn owners who installed small fans on their chargers because they seem to kill chargers at a high rate. (i am on my 2nd) but there is a guy who installed a small fan on his and he has over 3 years using same charger as we have

 

Interesting: you still have belts in the ex-engine compartment.
Is that power-steering and A/C compressor? How do you get any
power steering when you're close to a standstill if the main
motor isn't turning very fast, or is that bottom pulley connected
to something else?
.
As far as the hesitation -- here's a question: do your Li modules
have on-board smarts and cut-off transistors? If you have sets
of two in parallel [always seemed like a bad idea but it's done
in any number of laptops], and one decides it's had enough and
disconnects itself from the string, the other one might be able
to still handle the current but you'd see an overall reduction.
Either that your the Curtis was going into some sort of self-
protective current limiting mode, similar to to what was already
mentioned.
.
Very nice new toy, though.
.
_H*

 

As much as I admire your pioneering spirit, Daniel, a Porsche that won't do 70 is kinda...well...neutered. Which I suppose isn't any great harm if it won't get you laid anyway.

Sorry, man. Somebody had to say it. Nothing personal.



Hey, how much to get your knees fixed so you can ride a bike? (May as well get the nastiness over with all at once. I'll try to be nicer tomorrow. ) :redface:

 

Speaking as an old Porsche hand,,, while I applaud the intent to get a cool/green efficient commuter car,, the idea of ripping apart a fairly complex, fairly expensive classic that has and will hold value and put in the hands of some back yard tinkerer is a bit silly.

I can see all kinds of 'minor flaws" that would have to be worked out to make this project viable,, such as ac, power steering issues etc. (does the car have regen capabiltiy?)

It seems to me that for the purchase price of a 2002 911 of say $15k? why wouldn't you spend $2000 on a mint Hyndai, or Honda of the same vintage, put the $13k into the conversion and at the end of the day at least have a car that comes close to your goals,,, including the cost of the project? Somehow the idea of driving around in a 911 "Chick magnet" with the spare tire bungeed down in the back seat, a couple of 50LBS bouncing around in the trunk seems to be making a sow's ear out of a silk purse.

It seems it you are looking to experiment,, a better platform to start with would make better sense. To assume from the back yard tinkerer that you will get X range, X miles per kwh with no beta to compare is optimistic at best,,, foolish at worst. My fear is that sooner than later you will find that the car doesn't come close to expectations,, the headache factor of the quirks out weigh it's cool factor, the battery life and longevity will be poorer than expected and more expensive to replace because of deep draw downs etc, and you will be left with an interesting relic,,, not a 911,, not a EV.

Sorry,, but just one person's opinion.

Icarus.

In the interest of full disclosure,,, I have been know to do similar things myself. The difference is that I squandered my time and energy for the sake of the exercise,,, not my money to let someone else figure out the pitfalls. Some times worked out,,, (solar water heat) often didn't, (small scale wind)

 

I would like to pick up an old Geo Metro and try an EV conversion. For some reason, the 3 cylinder Metro's didn't last long, and assuming they were not crushed, should be cheap

I like to tinker, but taking a pretty car like a 911 and tearing the guts out of it, doesn't sit well with me

 

A Gen 1 Prius would be the best candidate as you have regenerative braking.

 

Indeed. Sorry, I thought it was your knees.

 

This is just a rant.
Unless the power company runs a transmission line from the hydro plant to you house, you cannot tell where the electrons are coming from as the hydro plant is tied to the grid.

My utility company keep sending me letters to ask me to paid 10% more for green electricity, I will not paid a dime more unless they can prove the electricity comes directly from the solar or hybro plant to my house.

 

While this is technically correct,, in fact, and in the real world the issue is somewhat different. If you pay X% more for the promise that YOUR consumption comes from sustainable sources, (PV, wind, GOOD hydro etc) in the aggregate the percentage that you pay for green power will come from green sources. Can the utility claim that the exact kwh that it takes to write and send this comes from green power,,,no,, but the fact that I am willing to underwrite the extra cost for green power, allows the utility to pay extra for it and resell it. It is and does happen in the real world. Witness the wind farms in Eastern WA and OR, that is where my 10% subsidy is going,,, and I am happy to do it.

The reality is if you want to assure that the exact KWH that you use comes from green sources,,, invest in PV solar and get on a net metering program from your utility. Over a calendar year you may put more into the grid then you use,, but even then, you may sell some green power back to the utility and ergo to someone else, and you may indeed buy KWH at night from the utility that is not "green" but in the aggregate,,, you might generate more green power than you will use non-green power.

If you believe that we should pay the real cost(s) of our energy choices,, then by all means you should be willing to pay for them personally. If you accept that coal, or natural gas or nuclear are not sustainable, then you should be willing to pay as much as you can afford to bring about the change that we need to make. (If you cn afford to drive a Prius,, you can afford 10%. Hell you can save that 10% by conservation without breaking a sweat! I'll bet I could walk into your house and find 10% net energy savings within an hour,, most with no change in life style.

If you are just looking for cheap,,, feel free to buy coal fired electricity,,,, and make your kids and grand kids pay the price,,, assuming they will be around to!


Icarus

In the interest of full disclosure once again. We live ~1/2 the year off grid and generate all our electrical power off grid solar. We use Propane for fridge, water heat and cooking. We heat with wood.

The other half we live on utility power. We heat with wood, propane. Our electric bill is ~$25/month, including pumping water for two houses. We do heat a hot tub off of grid power which is far and away our biggest user net/net. We buy the maximum green power that Puget Power allows.
Icarus.

 

Look up "renewable energy credits". It's a bit of a shell game
and rather intangible for the end subscriber, but it *does* push
some numbers around in favorable ways and the hope is that it
shows long-term trends that people are willing to spend their
money on green sources.
.
_H*