Something I don't understand with Hybrid cars.

[Hearly]

Ok, I was reading about that new car that should be out around 2010, which will get 150 MPG.

Now While I was thinking about that car, an Idea struck me and I wonder why it's never been Thought of, and or is there some practical reason why it's never been done. the Idea below.

Ok most Electrical plants are run by a turbine, how hard would it be to have an Hybrid car that has a turbine attached to the front wheels (one on each wheel, or heck what about having 1 on each of the 4 wheels) which could generate electricty while you drive to power the drive tran? I mean I understand you would never generate more electric that you use, but it would be dang close (and depending on the Power plant used to push the car forward, it might be extremely close)

[miraborn]

A lot deals with Newtonian physics and conservation of energy and momentum. This is why perpetual motion machines don't work. The amount of force you would need to apply to the vehicle to move the rotors could never be countered by enough energy to compensate for the extra force required to turn them. If you put a turbine in the loop, it will put additional drain on the power being used to turn the wheels, which would reduce vehicle efficiency.

If you have ever gone to a science museum and tried the bicycle-connected-to-a-lightbulb experiment, you would understand - the more wattage the bulb requires, the harder you have to pedal . . . and if you connect more lightbulbs the force required to pedal is greater.

[Hearly]

A lot deals with Newtonian physics and conservation of energy and momentum. This is why perpetual motion machines don't work. The amount of force you would need to apply to the vehicle to move the rotors could never be countered by enough energy to compensate for the extra force required to turn them. If you put a turbine in the loop, it will put additional drain on the power being used to turn the wheels, which would reduce vehicle efficiency.

If you have ever gone to a science museum and tried the bicycle-connected-to-a-lightbulb experiment, you would understand - the more wattage the bulb requires, the harder you have to pedal . . . and if you connect more lightbulbs the force required to pedal is greater.

While I do understand that, wouldn't using the car's own momentum to help generate electricity help reduce the amount of gas needed to keep the batteries charged?

[Mistra]

energy is always lost in conversion (trough heat) the larger the generator, the more power is required to keep it running, in turn actually increasing your gas consumption.

When braking, you are interupting the forward momentum your car has, with a regular car, all of the energy used to build up this momentum is lost, the only system that uses this energy are hybrid cars, they run their generators while braking and capture this energy that would otherwise be lost.

see where i'm going?

a hybrid car isn't really a new way of generating power to drive, it's a way to preserve the power you'd otherwise lose when braking. (can't be any other way, cause if it would, you'd have to plug your car in a wall socket every night)


now hydrogen fuel cells, those are cool (i'm waiting for the nuclear fusion powered one myself)

[expedient]

There is inertia to overcome both from the vehicle itself and the turbines when you start to accelerate. Then whilst the car is moving the turbines would provide a braking effect to the momentum (as the road also does ie. friction). So you are putting in more energy in order to overcome the resistance of the turbines than you can gain [hence conservation of energy applies].

This kind of scheme is often used in hybrid and electric cars under braking however, or going down hill whilst keeping a constant speed. Here you can get back some of the energy expended to put back into the batteries.

[borsic]

It all comes down to TANSTAAFL (there ain't no such thing as a free lunch) and Newtons first law of motion (a body at rest or in constant motion tends to remain in it's state unless acted upon by an exterior force).

When you get in your car, it is a body at rest, and to get it in motion you (i.e. the motor) has to provide a force. Once the car is in motion you are unfortunately not home free, because there are forces acting against you, namely wind resistance and friction from the tyres. So you have to provide a continous force, which you do by burning petrol, thereby converting chemical potential energy stored in the petrol to kinetic energy, driving your car forward.

Now, you want to do this as efficiently as possible of course. Unforunately the internal combustion engine has a thermodynamic limit of about 37%, meaning you will loose nearly two thirds of the energy in the petrol (mostly as heat and unburned petrol). The rest is transferred over gears and drive-train to the wheels, loosing a couple of percent on the way. What gets through (~20%) is the energy you have to accelerate your car.

Now take a generator. A generator is the opposite of an electric motor - it takes kinetic (rotational) energy and converts it into electrical energy - electricity. It doesn't generate energy form nothing - the rotational energy it takes in is gone afterwards. So if you connect one to your wheels, it will eat their rotational energy - energy you would have needed for your car. It will go more slowly. Even worse, the generator is not a hundred percent efficient either - we will loose some energy as heat again!

So why do people build hybrids?

Well the 37% efficiency figure I quoted above is the theoretical maximum. All in all your car gets more like 20%. And if the engine is not turning at its most efficient rpm rate, the figures are even worse. The worst is starting the car from a complete stop (which is why stop-and-go traffic eats so much fuel). An electrical motor, on the other hand, is more efficient, and it doesn't care (much) at which rpm it is turning. In fact, an electric motor is so much more efficient that it would be best if we had one in each wheel, and would use a special combustion engine to turn a generator to get electricity for the wheel-motors. This would allow us to use an engine designe to work at a fixed rpm rate, which makes it more efficient. Unfortunately, this would make your car really comlicated and expensive, with 5 motors and a generator.
So a hybrid uses an electrical motor to start and in slow traffic, and the petrol engine for cruising at higher speeds (because we can't build batteries yet that carry enough electricity to go for long distances).

Furthermore, there is braking. Conventional brakes convert kinetic energy - the forward motion of your car - to heat, thereby braking. This is why you have to be careful to use the motor brake when going down the mountains, else your brakes will overheat and -ahem- break. But as we have seen above, there is a way to convert rotational energy to electrical energy - a generator. So you can use to get some energy back into your battery when braking, about 20% if I am not mistaken.

I hope that was helpful
borsic

[Hearly]

Borsic, I do understand how it normally works, the new car that is supposed to be out in 2010 which gets roughly 150mpg, doesn't use a normal engine per say, it uses an electric generator and the gas engine is just used for generating the electric for the car, My thoughts were on this type of a setup (where the gas engine never touches the drive tran) wouldn't it help with generating electricity for the drive tran by having some type of turbine involved with the rotation of the wheels (maybe not used in stop/go traffic but at a highway cruising speed)

Or even why not have a hood scoop that directs air into a turbine, etc, wouldn't some of these types of things help even get higher MPG vs not having it?

http://www.msnbc.msn.com/id/20039854/

Above is a link to the Chevy Volt story I saw on MSNBC

[borsic]

It comes down to efficiency. Let's say that your conventional engine generates 100 horsepower. It will be more efficient (i.e. use less fuel) than a "normal" car engine, because all it has to do is turn the generator, which it can do at a constant RPM. The generator takes those 100hp and generates electricity. The best generators you can mount in cars are about 60% efficient, so now you have 60 hp worth of electricity. This goes to your motor, which is up to 90 % efficient, so you get 54 hp worth of rotational power. If the motors sit directly at the wheels, they will translate that into kinetic energy, which will drive your car forward - on a flat surface, you might reach about 160 km/h, or about 100mph. You can't go any faster, because at that speed the wind resistance will balance out the force your motor can provide - for more speed you would need more horsepower.

Now, if you use a generator to get electricity from the forward motion of your car, this will eat some of the horsepower at the wheels. Let's say you want to get 10hp worth - since your generator is only 60% efficient, this will reduce the hp available at the wheels by 14, meaning now you only have 40hp left!
The same happens if you use a funnel to catch the head wind to turn a propeller which powers a generator - this will increase the wind resistance of your car, "eating" hp again - only worse than the generator, because the propeller is not 100% efficient either, so you will go even slower.

The thing is, a car is a energy conversion system. And at each conversion you loose some energy, because no conversion is 100% efficient. In a normal car you convert fuel to linear motion (the pistons) to rotary motion (the crankshaft) to different rotary motion (the gears) to different rotary motion again (the drivetrain) and finally you have to translate that rotation 90% from drivetrain to wheels. 5 conversions at least, and there are probably many more, but I am a physicist and not a car engineer.

A hybrid car converts fuel to linear motion to rotary motion to electricity (generator) to rotarty motion (wheel motors). Only four conversions, plus the engine gets to be more efficient.

Using any method to get electricity back again from the motion of your car will first of all slow down your car - that energy is not available any more to push against that pesky wind resistance and friction! - and also loose energy again in the conversion. If you want to slow down, why not just back off the gas pedal?

So to answer your question directly, none of those types of things help your mpg, because the car is already as efficient as the engineers can make it. No energy gets wasted, everything is directed into pushing your car forward (well, and your AC). Any method that gets energy from the motion of your car - generator at the wheels, turbine in the airflow, etc. will take energy away from the forward motion, slowing your car down. Worse, it will slow your car down more than the energy you get back, because some is wasted due to inefficiencies.

Things clearer now, or have I managed to confuse you completely?

Btw, that Volt-concept looks awesome. Have you seen theTesla Roadster? An all-electric sports car - brilliant!

[Quindo Ma]

To try and put what Borsic said into a different phrasing, the problem is not using generators to utilize the motion of your wheels. Doing that is not really a big deal overall; there's even a small generator already in the motor to create electricity to keep your battery full.

The big issue with the idea you proposed is that you are already using up your energy to move those wheels in the first place. Unless you have an outside force (such as gravity when going downhill) that will accelerate your car, you will always have to provide power to keep up the motion of going forward.
You know this by the way that you have to continuously keep your foot on the gas pedal if you don't want to slow down.

Now, if you try to use a generator to create electricity from the forward momentum, then you will end up not only having to power the wheels to move forward, but you will also have to power the generator.
This is because if you do that, you will either slow down your speed despite still using the same amount of energy to move, or you will end up having to expend more energy to move at the same speed - all because of the inherent resistances within the generator (as described by the heat loss above by borsic).
In the end you will loose more energy than you gain, since you are not only driving, but powering the generator.

If you really want to test this yourself, get simple bicycle, one where you clamp the dynamo onto the front wheel whenever you want to have light.
Now ride the bike both with and without the dynamo running.
You should feel that you have to push the pedals a little bit harder as soon as you insert the power drain from the dynamo.

Do you see now, why it wouldn't work to do that in a car?

[Spec8472]

http://www.msnbc.msn.com/id/20039854/

Above is a link to the Chevy Volt story I saw on MSNBC

The Chevy Volt is a 'plug in hybrid' - i.e, instead of the batteries always being charged by the onboard generator, they're charged up whilst you're at home by plugging into a wall socket. So, the '150MPG' figure is the upper end, and only if you drive within a few Miles of home. You drop back to regular hybrid mileage ratings if you're not able to plug it in to charge.

As for the air scoop, adding generators to your wheels, etc - they don't and cannot work.

There's a bunch of experiments you can do at home to prove this on a small scale. eg: Wind turbine powering a fan. A lamp being powered by a solar panel

[pfors]

Reading through this thread (particularly Borsic's very well thought through replies) it struck me that there is one form of usable energy available in the car simply because of the inefficiency of the engine/generator: thermal. As I'm sure anybody who's opened the hood after driving has noticed, there are some pretty steep temperature gradients on a car. That could potentially be used to generate electricity through the thermoelectric effect while cooling the engine. Thermoelectric power generation usually isn't too efficient and it would be tricky to catch most of the heat. Still, with a well designed engine I guess it could make an appreciable difference...
Anybody know of any efforts made in this direction?

[ANTIcarrot]

Reading through this thread (particularly Borsic's very well thought through replies) it struck me that there is one form of usable energy available in the car simply because of the inefficiency of the engine/generator: thermal.

Which is where your car gets its heating system from. Using the heat as a source of energy for other things though is problematic. Efficiency is dominated by the difference in temperature between the hottest and coldest parts of the system. Inside an engine compartment there is nothing cold enough to make the efficiency worth the extra cost and weight of a recovery system. Usually it's simpler and more efficient to just add another pulley to the fan belt.

The Chevy Volt is a 'plug in hybrid'

It's more accurate to say it's a parallel hybrid, where both the electric motor and petrol engine are connected to the wheels, rather than a serial hybrid, where the petrol engine runs a generator and the electric motor(s) run the wheels.

For anyone interested in what this technology can really do, this is a report on military hybrids, which unlike the half arsed wishy washy compromises promoted by the car industry, are actually built properly:
http://www.defense-update.com/features/ ... re-HED.htm
I would especially draw people's attention to the Hummer Serial PHEV that has double the acceleration, hill climb speed, and only 70% the fuel consumption of the regular Hummer. There were plans to roll this design out across the entire US military fleet, but the Bush government decided that they were spending too much money on the War in Iraq to save money on petrol - which currently costs $50/gallon in Iraq. Who taught these people logistics?

I also can't help but wonder why all commercial news stories imply that HEVs may one day in theory achieve good performance - and always conveniently forget to mention that they already have, about three or four years ago.

[furry_wolf2001b]

Reading through this thread (particularly Borsic's very well thought through replies) it struck me that there is one form of usable energy available in the car simply because of the inefficiency of the engine/generator: thermal. As I'm sure anybody who's opened the hood after driving has noticed, there are some pretty steep temperature gradients on a car. That could potentially be used to generate electricity through the thermoelectric effect while cooling the engine. Thermoelectric power generation usually isn't too efficient and it would be tricky to catch most of the heat. Still, with a well designed engine I guess it could make an appreciable difference...
Anybody know of any efforts made in this direction?

I have really no idea, but at a guess, the weight gain for anything added are more costly then the gain.
And are electric motors that hot?

The most i think that can be easy to design in for some added power is probably solar cells in the "hull/body".
And i don't really mean tacked on on top of the regular body, but having it, or on a back of perhaps carbon fiber(or whatever) to make the hull.
And that would probably only be efficient if you can get it as thin and light as a layer of paint.

[Mac The Knife]

The Volt is a serial hybrid,,, there is no mechanical connection of the engine to the drive train, only electrical.

[pfors]

Which is where your car gets its heating system from. Using the heat as a source of energy for other things though is problematic. Efficiency is dominated by the difference in temperature between the hottest and coldest parts of the system. Inside an engine compartment there is nothing cold enough to make the efficiency worth the extra cost and weight of a recovery system. Usually it's simpler and more efficient to just add another pulley to the fan belt.

Well, the engine still requires cooling, so the idea would be to let the heat transfer go through a thermocouple (or a series of them). I'm not sure how thin they can be made and still work, but a millimeter or so is not a problem. I don't think the weight is a big problem, rather cost and the possibility that the engine would have to be redesigned for a comparatively small gain. And it may also reduce the effectiveness of the cooling system.

But yeah, the idea was mostly just to see what energy was available in the system that could in principle be used to help the car move.

And are electric motors that hot?

I'm just going off of borsic's values of 60% efficiency for the generator and 90% for electric motors. That leaves 46% which will go mostly to heat.

The most i think that can be easy to design in for some added power is probably solar cells in the "hull/body".
And i don't really mean tacked on on top of the regular body, but having it, or on a back of perhaps carbon fiber(or whatever) to make the hull.
And that would probably only be efficient if you can get it as thin and light as a layer of paint.

I was considering solar cells as well. The benefit there is of course that you're using an energy source outside the car. They're pretty expensive (so far, but there is a lot of research on solar cells these days) and I don't think you'd have so much use of them while driving, but the idea of the car slowly charging itself while parked is rather appealing.