[Sxotty]

Originally Posted by Mintmaster

It's not that screwy. DC-DC conversion is really efficient and cheap nowadays.

Do you know any theory behind this? I don't see why, for example, a DC motor would have so much difference in efficiency between generating and driving. Wire resistance is the same for both, AC losses should apply to both equally, and friction should be pretty low for a large motor.

The 3 or so electrical engineers I have talked to about it said the DC-DC conversion was not that cheap, nor efficient, nor light for the kind of power flows in a vehicle. Maybe they were misleading me, if you have any good info I would love to know. There are other reasons that cheap DC-DC voltage conversion at high amp rates would be super awesome in a vehicle, but I was told that it wasn't feasible really (cost effective).

[Simon F]

You would think that if they can manage it in one of these

it could be managed in a smaller vehicle.

[Sxotty]

Why would you think that? The whole point is a locomotive is big, so is a ship, so is the installation at the local electric substation. Cars sell on interior room. Even finding space for the battery is hard.

[Simon F]

Originally Posted by Sxotty

Why would you think that? The whole point is a locomotive is big, so is a ship, so is the installation at the local electric substation. Cars sell on interior room. Even finding space for the battery is hard.

You were saying handling high power was difficult, so I was giving an example of a hybrid vehicle where electronic conversion (at considerably higher power levels) has been done for a considerable number of years.

[update] Here are some that handle 100kW. Not cheap but not ridiculous either.[/update]

[RudeCurve]

Can somebody tell me what this whole argument is about? Current hybrid technology is pretty good, it's only being held back by power density and charge rate for the batteries. This will naturally improve over time.

I guess the Holy Grail is reaching parity with gasoline cars in terms of single charge range vs single gas tank range. On average a standard sized car has a single tank range of about 500 miles.

I think pure EVs like the new Nissan Leaf have a very good market as daily driven cars. If they weren't so ugly I would probably get one and completely not have to worrry about fluctuating gas prices.

[GZ007]

With hundred milions of users charging their EV-s, it wont be realy that much emision free. They just move the emisions to the power plants (majority are still coal plants). Just my reaction to the stupid green advertisements that come with these cars.

The advantage is that in large cities it could help to eliminate smog and of course building a new powerplant is easyer than to negotiate and beg for oil.

[JohnH]

So what's the environmental cost of making these hybrid cars, inparticularly the environmental cost associated with the batteries? Do they break even over their lifetime?

[RudeCurve]

My motive for wanting to purchase an EV is gasoline prices not emisions.

[Sxotty]

Originally Posted by Simon F

You were saying handling high power was difficult, so I was giving an example of a hybrid vehicle where electronic conversion (at considerably higher power levels) has been done for a considerable number of years.

[update] Here are some that handle 100kW. Not cheap but not ridiculous either.[/update]

Your link is not DC-DC converters which is what I was talking about. You have a DC battery and a DC super cap needing to share the same bus. This is not the same thing as a train. A train or boat is a traditional serial hybrid. You can control the rate the engine turns to generate electricity so you have great control over the output. With a supercap you discharge, but then you either need to blend in the battery, or do something else to keep the output matching what the user desires.

When I talked to a company doing power converters they said they had 96-98% efficiency on DC voltage conversion for high power applications, but the whole setup was the size of a vehicle with the cooling system and all that. Now so useful.


As to the other questions others raised the literature now suggests that such vehicles will reduce emissions compared to regular vehicles. Hybrids are comparable to small battery PHEVs, but larger battery PHEVs are likely to have higher emissions than hybrids, though lower than regular vehicles. Even burning coal a PHEV is cleaner during operation than a regular vehicle burning gas. And depending on what EPA does emissions could be even lower.

[AlphaWolf]

and in countries like Canada where 80% of power generation is not coal

[Grall]

Originally Posted by GZ007

With hundred milions of users charging their EV-s, it wont be realy that much emision free. They just move the emisions to the power plants (majority are still coal plants). Just my reaction to the stupid green advertisements that come with these cars.

This is a very common response/fallacy.

Power plants, even the dirtiest of coal fired ones, are far more efficient and "greener" than tens of thousands of petrol-burning internal combustion engines, especially if waste heat is recovered from the plant's heat exchangers and used to warm up buildings during wintertime and hot water supply.

[Frank]

There are three different things brought up:


1. Efficiency of mobile power plants versus huge, stationary ones.

Well, for starters, those power plants don't pay large fuel taxes. But disregarding that, size is everything in efficiency.

And you would want that electricity to recharge your car at night, when some of those plants are simply throwing away generated electricity, because that's cheaper than turning them off and back on, and prevents brownouts.


2. The difference between an ICE used for driving the wheels or a generator.

You want to prevent any direct, mechanical connection between the wheels and that ICE, if you care about efficiency. Use it to drive a generator, and it becomes so much more efficient, that the losses incurred by charging the batteries and using those to drive (a DC/AC converter and) an electric motor still makes it more efficient overall.


3. The differences between different types of electric motors to power the wheels, or to generate electricity.

Well, for starters, an electric motor only uses electricity when delivering torque, in a very linear fashion. While an ICE burns power all the time, even when idling. And even during acceleration the electric engine is better, simply because it has a higher efficiency (about 300%) to start with.

Which makes sense if you think about it: the combustion is a messy, chemical process, that reacts to all the forces working on it, and has a strict, optimal combustion speed for each of those combinations incurred by driving the wheels directly.

Good luck making that somewhat efficient if you cannot control those forces!

As to why some electric motors make better or worse generators (or motors!):


An AC induction motor (the simplest, cheapest and most common one):

Cheap, simple, light, reliable, good torque; what is not to like? It's your basic, cheap workhorse. Not as efficient as brushless or synchronous motors, but still pretty good and much cheaper.

But, as you can see in the animation, there are no magnets or coils on the rotor. So, if you crank it around by braking, not much happens. Because it only works when the coils are inducting a current into the rotor. And for a generator, you want the reverse: you want the rotor to power those coils, not the other way around.


A DC motor:

As there are electromagnets on the outside, it does generate electricity readily.

But the whole design isn't all that efficient for either generating torque or electricity to begin with. The brushes create friction and have overlap, and there's only a few angles where maximum magnetic repulsion is created. Much of the time, it's either powered wrong or pushing in a less than optimal vector.

Strangely enough, they're actually better in generating electricity than converting it to torque, if you want speed control. Because that requires either voltage control (max. efficiency of around 80%), or PWM, which reduces the maximum torque significantly.

Their main benefit is that they're very simple and very cheap. But they're not used if you want efficiency and high torque from low rpm. Like, in a commercial-grade car.


A brushless motor:

Well, they're like an AC motor that you can operate by PWM: no DC/AC conversion needed. And, as they do use permanent magnets, they do produce electricity.

Interestingly enough, they're generally short and wide, while the housing rotates. And because they can deliver very high torque at any rpm, that makes them excellent hub- and brake replacements. They're potentially the best at both driving and generating, when you power them from batteries.

But all the magic is in the electronics and timing. Losses are unavoidable, and the electronics are outright bulky and expensive, compared to the other types. Especially when you want to use them as generators as well.

And the electronics are far more expensive than the motors.


That leaves AC synchronous motors:

If you want efficient, look no further. This is the industry standard, and has been for about a century by now. But they are designed for a single voltage and frequency, which is a problem. Unless you use DC/AC converters and can generate the voltage and frequency you need. But that does incur conversion losses.

In the animation, the rotor uses permanent magnets, which work fine for generating electricity. But the best ones use coils on both the rotor and strator.

Simply because you cannot regulate permanent magnets. They function best at the design spec, and worse at every other speed and power usage.

So, that's why you would want a combination of the brushless and synchronous motor, for the best results.

[Simon F]

Thanks Frank, excellent post. I didn't know there was torque loss when using PWM though I know there are heat problems.

[eastmen]

Originally Posted by GZ007

With hundred milions of users charging their EV-s, it wont be realy that much emision free. They just move the emisions to the power plants (majority are still coal plants). Just my reaction to the stupid green advertisements that come with these cars.

The advantage is that in large cities it could help to eliminate smog and of course building a new powerplant is easyer than to negotiate and beg for oil.

here in new jersey pse&g is rolling out a massive amount of solar panels on telephone polls . So perhaps in the future it wont be at the power plants , or at least not alone

[Sxotty]

Frank remember hub motors are unsprung weight and have significant performance drawbacks. They get rid of gearing losses, but the tradeoff isn't that great. Instead using the motor inboard of the wheel so it is somewhat isolated from road vibration will probably be the way to do unless they can get the motor very robust and very light while still being inexpensive (since in a hub motor you need 2 motors or more).

[Simon F]

Originally Posted by Sxotty

(since in a hub motor you need 2 motors or more).

FWIW the Lotus hybrid concept car used two (inboard) motors which were then driven independently to improve cornering, something which requires a fancy differential in an traditional car. I just wish they'd make it (and then I had the money to buy it)

[RudeCurve]

All modern EVs with individual motors will use torque vectoring since it would basically be free and removes the heavy and parasitic loss of a differential, drive shaft etc.

As for unsprung weight that won't really be an issue if you use smaller motors.

[Mariner]

How about Michelin's peculiar design?

http://www.treehugger.com/files/2008...ectric-car.php

I've seen video clips elsewhere of these active wheels in test cars - they were obviously exaggerating the movement of the wheels to show the range of movement available but it looked most strange!

[Sxotty]

I personally am enamored with a design like the lotus where you have 2 or 4 inboard motors (for fwd/rwd or awd). I am enamored with it b/c I like performance vehicles. The michelin design or similar things have some promise but it is hard to divorce oneself from personal bias. I drive on a significant number of rough roads and look at that design and cringe. It is pretty cook though the ability to change ride height, and so forth is nifty.

I would like to know the weight of the wheels in a voulage though. It is a performance vehicle, but if the reported 35kg of unsprung weight changes significantly it would be important. Also I wasn't clear on whether the unspung weight was w/ or w/o tires. My vehicle wheels weigh 16.5# or 7.5kg. So this setup would double the unsprung weight before a tire is put on. Of course it seems they might be saving weight in the braking system so the overall comparison isn't that bad.

BTW edmunds says the active wheel weighs 95 pounds.

[Frank]

Originally Posted by Sxotty

Frank remember hub motors are unsprung weight and have significant performance drawbacks. They get rid of gearing losses, but the tradeoff isn't that great. Instead using the motor inboard of the wheel so it is somewhat isolated from road vibration will probably be the way to do unless they can get the motor very robust and very light while still being inexpensive (since in a hub motor you need 2 motors or more).

True, but the housing replaces the hub, and the brakes and axle have weight as well. So, it's not as bad as you might think.

Road vibration isn't an issue, as there are no parts inside that make contact with one another.

Like this mini:


EDIT: the motors used weigh 31 kilogram each(!), so that's definitely too much.

Then again, they add a motor to the inside of the hub instead of replacing it, and they put all the electronics inside as well, which is stupid.

I have seen much better ones, but I'll have to Google for them.

[Mintmaster]

Originally Posted by GZ007

The advantage is that in large cities it could help to eliminate smog and of course building a new powerplant is easyer than to negotiate and beg for oil.

Uhh, that's a pretty fucking enormous advantage.

[Frank]

This:

is the idea. Much better and lighter than the ones used on the mini.

But I haven't found the ones I was looking for, yet.

[Sxotty]

Originally Posted by Frank

Road vibration isn't an issue, as there are no parts inside that make contact with one another.

The heads on a HDD don't touch the platter but that doesn't suddenly mean vibration doesn't matter. When you have no parts making contact with each other and slam things around something things go poorly.

BTW mariner already posted something quite similar if not the same.

[JohnH]

Originally Posted by RudeCurve

My motive for wanting to purchase an EV is gasoline prices not emisions.

Ok, so ignoring the environmental cost of building a new hybrid car, how many years would it take to recooperate the cost of a new car?

Don't get me wrong, I've no doubt we need to replace gasoline, but I'm not convinced the rush toward electric vehicles and hybrids has been entirely thought out.

[Sxotty]

John, how much does it cost you for gas? How much for electricity? How far do you drive a day? What is your discount rate?

If you can't answer those questions no one can answer yours.