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I don't know if this is on the correct site, but I have a question about cars and their energy. When heat is transformed into mechanical energy, that mechanical energy can produce electrical energy via the alternator. That electrical energy can be used to produce mechanical energy again. Can we not just keep that cycle going when using a car? We would use so much less fuel as it would only be used when starting a car.

P.S. This is a question for my VCE class at school. This class is not a mechanical class but Systems Engineering. For all I know, cars already do that.

Thanks, Rian

  • Are you trying to describe a gas-electric vehicle or a gas-hybrid (one is more like diesel-electric train and one has both gas and electric motors)? – Bob Cross Feb 6 '17 at 23:01
  • Take a look at what a hybrid car does. But, really, there is no free lunch. A hybrid vehicle only uses what's available to them and stores the excess or recaptured energy into batteries through braking. – Pᴀᴜʟsᴛᴇʀ2 Feb 6 '17 at 23:02
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    You will always, always have losses somewhere in the drivetrain (friction is a big driver of this). At high speeds wind resistance is energy expended that you will never recover. There is no perpetual energy machine. – silentsod Feb 6 '17 at 23:06
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    What you are describing is a perpetual motion device. That's impossible. Energy conversions ALWAYS have losses. – tlhIngan Feb 7 '17 at 1:10
  • There is always mechanical loss so you have to put more energy in to overcome this. – Steve Matthews Feb 7 '17 at 9:19
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No, we can't just keep the alternator running in order to produce electricity forever. We need something to be a prime mover: the thing that provides the first energy (i.e. the car's combustion engine.) From there we siphon off some of the mechanical rotation in order to spin a generator (the alternator.) We also lose some amount of energy when making that transformation, because the process isn't terribly efficient. Speaking of inefficient, most small 4-cycle gasoline engines are so inefficient and transforming the energy stored in fuel into mechanical energy useful to turn the transmission that it's a headline when someone makes a 38% efficient one. A lot of what's left can be attributed to waste heat (or not-quite-as-wasted heat if it's winter and you're blasting the heater.)

So you always need a source of energy. In traditional automobiles, the source is the fuel, either gasoline or diesel usually. Sometimes when I'm going down a big hill in my manual transmission car I will put the car into a relatively low gear and coast. This is called engine braking and what's really happening is I'm allowing the weight of the car to fight the low gearing, causing the engine to have to turn over. There's a lot of friction so it takes a considerable amount of energy to do this, so the car goes downhill more slowly than it would if I were in neutral. And the best part is, for that short period of time I'm engine braking I'm not using any fuel at all, because I'm off the accelerator totally. The engine is well above idle speed so it doesn't need to inject any fuel! And the alternator is still turning of course, so I'm getting a free ride thanks to my temporary fuel source: gravity. However, what goes up must come down so it's surely a net zero effect or worse in the end.

I'm feel like I'm rambling now but it's a good question and an interesting one to try to answer!

  • Thank you for helping me with my question. Upvoted and confirmed as answer. – Rian Webster Feb 7 '17 at 1:30
  • You know, it's not really that thorough of an answer but it was just some thoughts off the top of my head. – Tedwin Feb 7 '17 at 2:24
  • Well it told me that my idea is wrong. That's all I needed to know. – Rian Webster Feb 7 '17 at 3:25
  • Hah! Fair enough. – Tedwin Feb 7 '17 at 4:09
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To expand on tlhlngan's comment you're describing a Type 1 perpetual motion machine:

  • Convert mechanical energy into electrical energy by using the engine to spin the alternator
  • Convert electrical energy into mechanical energy by using a motor to turn the engine

There are a few issues with this:

  • energy conversions are not 100% efficient in the real world
  • even if conversions are 100% efficient, it takes energy to keep shafts rotating and working against other frictional forces (pumping losses, eddies)
  • even if frictional forces are absent, the best you can do is oscillate between turning the engine or turning the alternator (sort of a flywheel effect). The moment you want the car to move, it represents additional work that will irrecoverably consume whatever energy is present in this electro-mechanical flywheel.
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I had a similar idea when I was young and learned about electromagnetic induction: install a powerful magnet to the moon and a coil on the earth. The changing magnetic field will give you free energy!

Of course, there's such a thing as the law of conservation of energy. It is probably the most well-tested law of physics. Numerous attempts have been made to obtain free energy, with all of them failed.

How would my scheme fail, then? Obviously, powerful enough magnets and big enough coils are not feasible, but even if they were, it would simply extract energy stored in the earth-moon system. Quite similar to what is done using tidal power, with the exception that my scheme is not feasible but tidal power is. Both of them extract energy from the earth-moon system which actually has plenty of energy.

In your case, conversion efficiencies of the various mechanical->electrical and electrical->mechanical conversions are not 100%. Because they are less than 100%, a given amount of mechanical energy turns into less electrical energy and even less mechanical energy, so the engine in this system will not rotate forever, and definitely will not allow you to divert free energy from this system to other purposes. If you could get 100% conversion efficiency with no friction, then in that case it would rotate forever, but attempts to extract energy would slow down the rotation which would gradually stop then.

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