I argue with my friend and cannot say that he is wrong although I feel it somehow. But I don't have good arguments against him. I want to know who is right and why.

The problem is

Let's imagine we have a car and we put fuel in equivalent of A joules of energy.

We drive this car until the fuel is over and drive until the car stops (of we put on brakes and stop it).

So my friend says that all (100%) this A joles of energy will be converted into thermal power (temperature energy, heat energy) either when engine works or when we gain some kinetic energy and after we're trying to stop all kinetic energy converts to heat energy. He says because of conservation of energy theorem and because of type of energy eventually in this case converted to heat (when you run engine, when you try to use stop force etc).

So his assumption: 100% of our fuel energy equivalent will be eventually one way or another converted into thermal energy.

I guess this not correct, I feel there is some work / activity is done and not all fuel energy converted into heat through this driving process.

Am I correct or all energy of fuel converts to heat energy. And why? Please explain, thanks.

  • $\begingroup$ If the car drove up a hill than there would be an increase in potential energy too. Most of the energy is lost is as air movement ... but eventually this becomes thermal too! Your friend is mostly correct. $\endgroup$ – PhysicsDave Jul 23 '19 at 18:11
  • $\begingroup$ Yes that's correct note about the height, let say we are on plane so there are no up or downs. $\endgroup$ – andymur Jul 23 '19 at 18:24

I guess this not correct

It's not, but it's pretty close to correct. A tiny bit will be turned into sound, there are also more minor mechanical losses like wear on the brake pads and tires. But yeah, the vast majority ultimately turns into heat.

And that's largely due to one very simple fact: of the energy in the fuel you put into the car, something like 99.9% of that is literally turned into heat when you burn it (there's some tiny losses out the tailpipe and others). Of that 99.9, only about 15 to 30% of it turns into motion. The rest is heat lost through the radiator, the top of the engine, or the tailpipe.

Yes, cars waste that much energy. So even if the rest didn't turn into heat, you're probably about 85% of the way there already.

As to the rest, most of it ultimately warms up the air you drive through, a smaller amount warms up the tires, and depending on what sort of driving you're doing, the brakes.

| cite | improve this answer | |
  • $\begingroup$ Just about every thermodynamics text that details power plant analysis and ICE gives roughly those values, plus 1. $\endgroup$ – user207455 Jul 23 '19 at 18:23
  • $\begingroup$ Maury, thanks for the answer! You wrote that 99.9% energy of fuel converts to the heat energy, I agree. But then you wrote 15-30% converts to energy of motion. That's my point. This 15-30% never will be converted into heat energy (or at least not 100% of this 15-30%) am I correct? $\endgroup$ – andymur Jul 23 '19 at 18:44
  • $\begingroup$ That 15-30% goes into overcoming friction ie aerodynamic, tires, gears, shafts, bearings, seals etc which eventually gets to heat... $\endgroup$ – user207455 Jul 23 '19 at 19:05

This is an interesting question, and I wouldn't necessarily call your friend wrong; though there are definitely many caveats.

So for starters, one thing your friend seems to have not considered is that you can use a car to change your potential energy. If you spend that fuel to move the car somewhere of a higher elevation, you've increased your potential energy as well, so not all of the fuel spent in that case goes towards heating. You also may displace some rocks or dirt into a higher potential energy as you drive by; which would be more energy that doesn't go into heat.

You could also possibly say that your effect on the surrounding air is not exactly thermal energy. The car provides some kinetic energy to the air as it is displaced; this will depend highly on shape. The issue with that is that it's hard to really track that additional kinetic energy in the air, and will just result in a small amount of heating when all is said and done.

Which brings me to the major thing I wanted to point out; which is the reason why I wouldn't necessarily call your friend wrong. Assuming the laws of thermodynamics absolutely hold (specifically the second law), then it is theorized that at some point we may reach the heat death of the universe. This in a way could be considered as everything converted into a thermal equilibrium; where no more work could be done, and all the energy is "thermal"; or random. In the absolute timescale of the universe, your friend might be correct, according to some proposed models.

| cite | improve this answer | |
  • $\begingroup$ The part about changing potential energy is, of course, true, but it's probably not relevant in the long run unless you never go back down. On the way down, you don't generate as much new heat as going up, but you still give up the potential energy and all of the fuel, except for some small bits maybe as you describe, went into heat sooner or later. $\endgroup$ – Brick Jul 23 '19 at 18:23
  • $\begingroup$ @Brick That's why I tried to put attention to the time scales near the end, and even with the kinetic energy of wind. Assuming the second law leads to heat death, eventually his friend is right. Absent that assumption; his friend is still coming from the right place; but I just had to point out the obvious cases like changes in potential energy, and that the air is also put into motion by the vehicle. $\endgroup$ – JMac Jul 23 '19 at 18:35
  • $\begingroup$ Yeah, I just think there are interesting time scales that are greater than a weekend-length trip to the mountains and shorter than the time until the heat death of the universe that might be more relevant given the nature and level of the question. I think a non-physicist reader might take away some perpetual-motion type idea from the way the first part is written and not understand the end at all. But strictly speak, what you've said is correct. $\endgroup$ – Brick Jul 23 '19 at 19:18
  • $\begingroup$ @Brick I'm really at a loss how anyone could get "perpetual motion" out of my answer. Can you clarify so I can try to fix it? $\endgroup$ – JMac Jul 23 '19 at 19:22
  • $\begingroup$ Faulty logic starts here: "If you spend that fuel to move the car somewhere of a higher elevation, you've increased your potential energy as well, so not all of the fuel spent in that case goes towards heating." Now I think (wrongly!) I can go down for "free" because of that potential energy I stored. And somehow by going up and then down I generated less heat than if I had driven level, which means I have "extra" energy from this process somewhere. (Really more total energy went into heating when you drove up than if you had driven level, which is "paid forward" to when you come down.) $\endgroup$ – Brick Jul 23 '19 at 19:29

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.