This is just like your example with the banner. Adding the tube makes it more efficient, but then adding the turbine makes it less efficient. It would be better to have the tube without the turbine than the tube with the turbine.
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AeroDynamic Drag, why fight it? Use it
- Thread starter clark
- Start date
This is just like your example with the banner. Adding the tube makes it more efficient, but then adding the turbine makes it less efficient. It would be better to have the tube without the turbine than the tube with the turbine.
The first and second laws of thermodynamics. Basically you always lose some energy out of a system when you convert one type of energy to another. Things like frictions, drag, etc. are the ways these losses pile up.
Adding a turbo to an engine doesn't really increase the efficiency of that engine. In some cases it might increase the efficiency of a car over another engine in the same car that doesn't have a turbo because the turbo allows you to get more power out of a smaller engine. When you aren't running the turbo, such as driving at constant speed on the highway, the engine is running more efficiently because of the smaller displacement of the engine. Turbos tend to put more strain on an engine and will wear it out quicker and when the turbo is running, your fuel consumption is way up.
Additionally a lot of things can be done to make ICE more efficient because they are so inherently inefficient. 50 years ago a typical ICE car was probably about 15-20% efficient with 80% of the energy being lost to heat and noise. Car makers have been forced to make cars more efficient, so they have done things that get them up to close to 35% in some modes of operation. Electric motors on the other hand are inherently more efficient, running over 90% efficient much of the time and even in low efficiency modes they can still be 70% efficient.
windgrins
Buzzy Bee
Hopefully I can help you.
Here's the way it works with turbines and why they don't produce as much energy as they use in general.
If a car (or any other platform) is traveling in still air but moving through it with any sort of a turbine on it, here's what happens:
Most people think from the car's perspective but it is easier to explain it from the perspective of the air or someone standing on the side of the road.
The air over the ground originally has no motion and no energy with respect to the ground.
The air over the turbine is accelerated (with respect to the ground) and disturbed by the turbine. The turbine is actually attached to the car by the hub and for the turbine to continue through the air at the same speed of the car, there is a force placed by the car onto the hub to keep the turbine moving through the air. For every force there is an equal and opposite force. So the car pushes the turbine blades, and the turbine pushes against the motion of the car. And without energy being added, the car would slow to a stop.
But that isn't the whole story. The turbine having an aerodynamic force on the blades, with a force at the hub generates a torque which can be used to turn a generator. Let's say that generator is perfectly efficient (which it never is but forget that for the moment). That generator then produces power which can be used to power the car. But there is a gotcha. The air that was still on the ground has been accelerated in the direction the car is moving, and has also had vortices generated in it. It takes energy from the car to move that air and that energy doesn't come from the air, it comes from the car. So even though some of the energy is recovered and used to drive the car, after the car passes through the air, it always leaves a wake of accelerated (with respect to the original air) and disturbed air. That accelerated air has energy from two things, the bulk motion of parts of it in the direction of the car and the vortices generated which is rotational motion. All that motion is eventually just turned into heat and the motion to produce the heat came from the car.
So to net it all out, the car passing through the air with a turbine generates even more drag (the force opposing the motion of the car) due to turning the turbine because some of that energy goes into the generator (but since it was hypothetically lossless, the car gets that part back assuming lossless motors also) but the remainder of the energy that pushed the air in the direction of the car takes some energy from the car.
So if a car without turbines passes through the air, the air is disturbed and accelerated less (all other things being equal) than the car with the turbine and energy capture device (generator)
So, there is no free lunch. If the two cars were both accelerated to 60 kph, and then allowed to coast with no energy being added, the car with the turbine would slow quicker than the car without the turbine because it generates more drag. So even though a turbine can capture some energy from rotary motion, it takes even more energy to push it through the air (drag) than it captures. It is overall a lossy device.
Hopefully, I haven't confused you further, but that's pretty much the whole story with one or two exceptions.
1st the easy one--There are no lossless generators and motors so the story for the turbine is even worse than the story implies.
2nd) If you add an actual wind over the ground, with some really fancy tricks, you may be able to actually capture some energy with a turbine which is geared to the car wheels (because in this case the air actually starts out with energy with respect to the ground). This is what occurs with the DDWFTTW and DUWFTTW car. But the result is when the car passes through the air in these cases, they have slowed the air with respect to the ground in both cases, having captured some of the energy of the wind! In reality this is the same trick a sailboat does so it isn't all that amazing, You've just built a fancy windcar.
Feel free to ask any clarifying questions.
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Absolutely correct. And there is an interesting situation this creates that most propeller airplane pilots know about. If an engine fails on an airplane so that the engine is no longer driving the propeller, it is much better to stop the propeller from turning (a little counterintuitive) because it produces much more drag than the still propeller. The energy goes into turning the engine with pistons and oil pumps, valves, belts, etc and all sorts of moving parts. This energy slows the plane down quite a bit.
So if a pilot with a two or more engine propeller airplane has an engine failure, they almost always stop the prop from turning on the failed engine (with a process called "feathering") to reduce the drag and to allow the aircraft to fly much better. The difference can be the difference between life and death in some cases!
Here's the way it works with turbines and why they don't produce as much energy as they use in general.
If a car (or any other platform) is traveling in still air but moving through it with any sort of a turbine on it, here's what happens:
Most people think from the car's perspective but it is easier to explain it from the perspective of the air or someone standing on the side of the road.
The air over the ground originally has no motion and no energy with respect to the ground.
The air over the turbine is accelerated (with respect to the ground) and disturbed by the turbine. The turbine is actually attached to the car by the hub and for the turbine to continue through the air at the same speed of the car, there is a force placed by the car onto the hub to keep the turbine moving through the air. For every force there is an equal and opposite force. So the car pushes the turbine blades, and the turbine pushes against the motion of the car. And without energy being added, the car would slow to a stop.
But that isn't the whole story. The turbine having an aerodynamic force on the blades, with a force at the hub generates a torque which can be used to turn a generator. Let's say that generator is perfectly efficient (which it never is but forget that for the moment). That generator then produces power which can be used to power the car. But there is a gotcha. The air that was still on the ground has been accelerated in the direction the car is moving, and has also had vortices generated in it. It takes energy from the car to move that air and that energy doesn't come from the air, it comes from the car. So even though some of the energy is recovered and used to drive the car, after the car passes through the air, it always leaves a wake of accelerated (with respect to the original air) and disturbed air. That accelerated air has energy from two things, the bulk motion of parts of it in the direction of the car and the vortices generated which is rotational motion. All that motion is eventually just turned into heat and the motion to produce the heat came from the car.
So to net it all out, the car passing through the air with a turbine generates even more drag (the force opposing the motion of the car) due to turning the turbine because some of that energy goes into the generator (but since it was hypothetically lossless, the car gets that part back assuming lossless motors also) but the remainder of the energy that pushed the air in the direction of the car takes some energy from the car.
So if a car without turbines passes through the air, the air is disturbed and accelerated less (all other things being equal) than the car with the turbine and energy capture device (generator)
So, there is no free lunch. If the two cars were both accelerated to 60 kph, and then allowed to coast with no energy being added, the car with the turbine would slow quicker than the car without the turbine because it generates more drag. So even though a turbine can capture some energy from rotary motion, it takes even more energy to push it through the air (drag) than it captures. It is overall a lossy device.
Hopefully, I haven't confused you further, but that's pretty much the whole story with one or two exceptions.
1st the easy one--There are no lossless generators and motors so the story for the turbine is even worse than the story implies.
2nd) If you add an actual wind over the ground, with some really fancy tricks, you may be able to actually capture some energy with a turbine which is geared to the car wheels (because in this case the air actually starts out with energy with respect to the ground). This is what occurs with the DDWFTTW and DUWFTTW car. But the result is when the car passes through the air in these cases, they have slowed the air with respect to the ground in both cases, having captured some of the energy of the wind! In reality this is the same trick a sailboat does so it isn't all that amazing, You've just built a fancy windcar.
Feel free to ask any clarifying questions.
- - - Updated - - -
Absolutely correct. And there is an interesting situation this creates that most propeller airplane pilots know about. If an engine fails on an airplane so that the engine is no longer driving the propeller, it is much better to stop the propeller from turning (a little counterintuitive) because it produces much more drag than the still propeller. The energy goes into turning the engine with pistons and oil pumps, valves, belts, etc and all sorts of moving parts. This energy slows the plane down quite a bit.
So if a pilot with a two or more engine propeller airplane has an engine failure, they almost always stop the prop from turning on the failed engine (with a process called "feathering") to reduce the drag and to allow the aircraft to fly much better. The difference can be the difference between life and death in some cases!
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There were many cases in WW II when the feathering mechanism was damaged when an engine was hit and the prop wouldn't feather. The drag from the windmilling propeller on the knocked out engine resulted in the plane going down instead of limping home. It happened many times even on 4 engine aircraft like B-17s and B-24s.
Red Sage
The Cybernetic Samurai
Ah. That moment in Third Grade when you realize a lot of your classmates really have no clue why it is that Wile E. Coyote's plans fail so miserably. ~*le sigh*~
I thought the answer to that was obvious these days: It's a conspiracy with the authors and animators colluding to deny Wylie his rightful due through no fault of his own.
Red Sage
The Cybernetic Samurai
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Red Sage
The Cybernetic Samurai
tga: Yes. I believe so. Thanks to the wondrous plans of Wile E Coyote, Esquire -- along with the Three Dimensional Blackboard of Professor Whoopee -- I developed an intense interest in blueprints, schematics, and industrial design that led to a career as a CAD Operator and Computer Graphic Artist. Gotta give credit where it's due...
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Just to add my two cent, the overall aerodynamic drag of a car is the Cd (coefficient of drag) x the frontal area of the car. There are massive misunderstanding out there that the drag of a car is just Cd, or just the area. It is actually BOTH, and you have to multiply time together.
windgrins
Buzzy Bee
Surprisingly, there is a way to harvest energy with turbines or propellers but only if there is a real wind blowing over the surface/ground.
I'm not saying this is practical but it can be done as illustrated by the DDWFTTW/DDUFTTW car which has no motor at all but only runs on wind power. Obviously sailboats run on wind power (wind moving with respect to the water) so there is no PM aspect to this, just slowing of the wind over the surface.
What you can't do, as many posters report, is gain any energy from only the relative wind of the car without an actual wind over ground (this is wind blowing with respect to the reference frame of the car only, not the ground-In other words what you feel when you put your hand out the window of a moving car on a no-wind day)
With a large turbine/propeller on the top of the car, one can hook electric generators/motors to the car and depending upon the real wind (wind blowing over the ground), the turbine can be used to either source or sink electricity and act as a turbine or a propeller. This is because the turbine or propeller combined with the wheels electric motors can indeed harvest some of the energy of the wind over the ground. Note that the reason that it is not PM is because the wind is ultimately slowed over the ground and the car gets to use some (but not all-due to losses) of the energy in the car. The actual wind has to be strong enough to make up for all losses before there is any gain to be had.
But, if there is no real wind blowing over the ground to harvest energy from, any turbine/propeller arrangement is just less efficient than the car without it. All of this is probably highly impractical in real passenger cars but if there is a real wind over the ground, it is possible to make the car benefit from it. In still air, its just a loss.
I'm not saying this is practical but it can be done as illustrated by the DDWFTTW/DDUFTTW car which has no motor at all but only runs on wind power. Obviously sailboats run on wind power (wind moving with respect to the water) so there is no PM aspect to this, just slowing of the wind over the surface.
What you can't do, as many posters report, is gain any energy from only the relative wind of the car without an actual wind over ground (this is wind blowing with respect to the reference frame of the car only, not the ground-In other words what you feel when you put your hand out the window of a moving car on a no-wind day)
With a large turbine/propeller on the top of the car, one can hook electric generators/motors to the car and depending upon the real wind (wind blowing over the ground), the turbine can be used to either source or sink electricity and act as a turbine or a propeller. This is because the turbine or propeller combined with the wheels electric motors can indeed harvest some of the energy of the wind over the ground. Note that the reason that it is not PM is because the wind is ultimately slowed over the ground and the car gets to use some (but not all-due to losses) of the energy in the car. The actual wind has to be strong enough to make up for all losses before there is any gain to be had.
But, if there is no real wind blowing over the ground to harvest energy from, any turbine/propeller arrangement is just less efficient than the car without it. All of this is probably highly impractical in real passenger cars but if there is a real wind over the ground, it is possible to make the car benefit from it. In still air, its just a loss.
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