Chevy Volt drivers average 800 miles between fill-ups.

[Mintmaster]

I haven't looked at the stats of that engine but surely, in stop/start traffic, saving 100kg of mass in a 1~1.5 tonne vehicle would be significant.

I did some math in an earlier B3D thread, and it's something like 2% for every 100kg on the highway. If a refined engine is getting 30% thermal efficiency and this OPOC engine is 29%, then the former will wipe out savings from lower weight. That shows you how tough it will be to match current technology. In the city it will depend on how often you stop, but the EPA test doesn't do much stop and go (which is rather pathetic). A decent engine should use about 0.0005 gallons to get an additional 100kg from rest to 40MPH, so factor that in how you will.

Is that some sort of cross between a pulse jet engine and a more regular turbine?

That's a pretty good way of putting it. What's surprising to me, however, is that they can extract so much more energy from the detonation wave than they can from regular combustion pushing a piston or turbine blades. 60% thermal efficiency is nuts.

Mass is important as it will affect your acceleration/deceleration and though regenerative braking is infinitely more efficient than a set of pads and discs, I see various figures that state only 20% to 30 (or maybe 50%) of the kinetic energy is recovered. Having less mass to start with does help.

That's really shocking to me. Commodity buck-boost converters are 80-90% efficient over a 10x range in power output. Why is it so hard to convert the motor's output during regenerative braking to a voltage that either the batteries or a supercapacitor can store efficiently?

 

[Simon F]

That's a pretty good way of putting it. What's surprising to me, however, is that they can extract so much more energy from the detonation wave than they can from regular combustion pushing a piston or turbine blades. 60% thermal efficiency is nuts.

I suppose, VS a piston, you are going through far fewer changes of direction (rotation only compared to piston head->piston rod ->crank shaft and then back again)
Not sure about it VS a normal turbine.

I looked at wikipedia for the specs of the micro turbines (such as in that Jaguar concept car) and they seem to be about 1% less efficient than, say, an equivalent power diesel, but vastly smaller and lighter. I wonder if the shockwave/explosive combustion (as you must get in an piston engine) is a more efficient method of burning all the fuel and/or capturing the kinetic energy. <shrug>

That's really shocking to me. Commodity buck-boost converters are 80-90% efficient over a 10x range in power output. Why is it so hard to convert the motor's output during regenerative braking to a voltage that either the batteries or a supercapacitor can store efficiently?

It surprised me too. In the past (when I was a youngster) I remember that motors were known not to be very efficient generators/alternators. Obviously technology will have changed a bit, but I wonder how far it has advanced in that regard. Also, dumping all that energy back into a battery will not be efficient (given the differences between charge and discharging) and, to use super capacitors, I estimated I'd need many £k worth of them to capture the kinetic energy of a small, lotus-sized, sports car.

 

[Mintmaster]

I looked at wikipedia for the specs of the micro turbines (such as in that Jaguar concept car) and they seem to be about 1% less efficient than, say, an equivalent power diesel, but vastly smaller and lighter.

We discussed this before, remeber?
http://forum.beyond3d.com/showthread.php?p=1491490#post1491490
Power diesels are north of 40% efficient.

It surprised me too. In the past (when I was a youngster) I remember that motors were known not to be very efficient generators/alternators. Obviously technology will have changed a bit, but I wonder how far it has advanced in that regard. Also, dumping all that energy back into a battery will not be efficient (given the differences between charge and discharging) and, to use super capacitors, I estimated I'd need many £k worth of them to capture the kinetic energy of a small, lotus-sized, sports car.

Electric motors/generators have always had pretty good efficiency. Unless you have a lot of winding resistance (which only comes into play near peak power), you should have well over 90% efficiency. All braking effect (aside from bearing friction) must be shown as power delivery across the generator's terminals.

For supercaps, I don't think the cost is too much. Look here:
http://www.electronicsweekly.com/Ar.../Supercapacitors-see-growth-as-costs-fall.htm
100kJ will capture the energy from a typical car at 30mph, so it should only be a few hundred dollars.

 

[Simon F]

We discussed this before, remeber?
http://forum.beyond3d.com/showthread.php?p=1491490#post1491490
Power diesels are north of 40% efficient.

Yes, I do recall that but in this case I had looked at a reasonably recent report comparing a car diesel engine with an equivalent power, microturbine. It seemed to suggest the difference in efficiency was not that great but, of course, there may have been some important details left out.

Electric motors/generators have always had pretty good efficiency. Unless you have a lot of winding resistance (which only comes into play near peak power), you should have well over 90% efficiency. All braking effect (aside from bearing friction) must be shown as power delivery across the generator's terminals.

Yes I agree that motors are efficient, as are generator/alternators but I get the feeling each are custom made for the purpose, and a standard motor will not function as an efficient generator/alternator and vice/versa.

For supercaps, I don't think the cost is too much. Look here:
http://www.electronicsweekly.com/Ar.../Supercapacitors-see-growth-as-costs-fall.htm
100kJ will capture the energy from a typical car at 30mph, so it should only be a few hundred dollars.

Hmm. I looked at a site quoting current prices but I must admit was considering a 100kph target speed.

 

[Sxotty]

I am still leery of super caps. In practice usable ones have been fairly pricey and the actual recoverable and storeable energy is lower b/c of the voltage profile on discharge.

 

[Simon F]

I am still leery of super caps. In practice usable ones have been fairly pricey and the actual recoverable and storeable energy is lower b/c of the voltage profile on discharge.

I agree that they need to be considerably cheaper. As for the energy - are you saying that they behave differently to normal capacitors or that capaciors aren't batteries?

 

[Sxotty]

I am saying people forget that the energy stored is not all equally accessible. If your vehicle operates from 240V to 300V or so and you have a cap then at first it may be in that range, but the voltage rapidly falls so only about 25% of the energy is easily accessible without doing all sorts of screwy stuff.

 

[Frank]

The efficiency of an electric motor used as a generator greatly depends on what kind of electric motor is used.

You have (roughly) four different kinds: DC, brushless, AC induction and AC synchronous. The first three are good motors, but not so good generators. The last is used as both.

Strangely enough, the synchronous motor that is seemingly the least efficient, with both the strator (the non-moving part) and the rotor powered instead of using permanent magnets for one of them, is actually the most efficient one for both.

Unfortunately, standard synchronous motors cannot start by themselves and only run at a single speed, so they're not used much outside of large, industrial applications, where reliability and efficiency are key.

Then again, if you want to run them off a battery and need a set of DC/AC converters in the first place, a hybrid, brushless, AC synchronous motor would have all the advantages and none of the setbacks. But they're very uncommon.

Induction motors have a strator that functions as a set of magnets by inducting a huge current through them. They're also called asynchronous motors, in that there's always a certain amount of slip (read: lost power) to get the magnetic fields up and running. They're small, simple, cheap and still pretty efficient, so they're often the first choice.

But they don't produce electricity easily, as there's no, or very little, magnetic field without powering the coils, which is needed to induce power to the strator. So, you have to spin it up to (reasonably) high rpm, so the resistance and stray magnetic fields coming from the strator induce power into the rotor, which induces a stronger electromagnetic field in the stator, etc. Not very efficient.

Brushless DC motors are basically DC motors operating like an AC motor, so they could be pretty decent generators as well, although it would require a whole extra set of electronics to extract it and feed it back to the battery.

Then again, if you're running it off batteries, they're potentially the most efficient one for both driving as well as generating, as you don't need (reasonably) inefficient AC generation (sine waves!), but only switching.

But considering that 3-phase DC/AC conversion mostly needs a set (3 or 6, depending) of fast DC/DC convertors where you keep moving the reference voltage, it mostly means that your electronics are more expensive.

So, unless we're talking hub motors, AC is the way to go, and induction has the best price/performance for powering cars, but is bad at reclaiming energy. And brushless synchronous motors a hot research topic.


EDIT: A synchronous AC motor where both the strator and rotor are powered uses brushes to get the electricity into the coils on the rotor. So, a brushless, synchronous AC motor either uses permanent magnets and a frequency regulated AC converter, or is a hybrid between a brushless DC motor and a synchronous motor, which would use brushes to transfer the electricity to the rotor if both the rotor and strator are powered.

You would want to get rid of the permanent magnets, but the brushes are still a problem. Although they're "fixed", in that they always connect to the same wire, so there are no gaps that increase wear, or overlaps that decrease efficiency.

 

[Frank]

Btw, if you want to see some specs of microturbines, look here.

If you want to use a turbine to run a generator, you would want to increase the turbine inside the back that drives the intake fan to nearly 100% recuperation, and use that to drive the generator. Exchange the trust for work. Like the ones inside a helicopter. But that doesn't change the power rating very much.

 

[Mintmaster]

I am saying people forget that the energy stored is not all equally accessible. If your vehicle operates from 240V to 300V or so and you have a cap then at first it may be in that range, but the voltage rapidly falls so only about 25% of the energy is easily accessible without doing all sorts of screwy stuff.

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

You have (roughly) four different kinds: DC, brushless, AC induction and AC synchronous. The first three are good motors, but not so good generators. The last is used as both.

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.

 

[Sxotty]

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]

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]

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