Question about thermodynamics

[u/DMayleeRevengeReveng]

I had a theory that the reason an internal combustion engine has to be so inefficient is this. The piston can only extract work from the fraction of the gas’s internal energy stored in linear momentum perpendicular to the piston’s surface.

So as the piston is extracting work from that particular “reservoir,” the only way to extract more work would be if the molecules spontaneously ordered themselves so that, all of a sudden, more of their energy is channeled into that “reservoir.” I.E. to get perfect efficiency, all the molecules would have to spontaneously assume linear motion directly at the piston surface.

And that’s just too improbable, so we never observe it occurring.

I am told this is wrong, because internal energy is re-equilibrated between modes (such as linear momentum) rapidly enough that spontaneous generation of order is not required to do a next increment of useful work.

But the only alternative explanations I’m getting are just, “well, the engine cycle has to produce an entropy increase, so it has to reject heat.” Okay, I get that, that’s a statement of the second law.

But if my original theory is incorrect, what is actually happening at the statistical, molecular level that requires the power stroke of the piston to reject a certain amount of heat each cycle?

[u/Saibot393]

In the Carnot process (the theortically most efficient thermodynamical process) we bascially use the transfer of gas from hot to cold to power a mechanical system (the same goes for any other such system). The problem ist, that we can only really use the heat difference between our hottest and our coldest state to power our engine. So the inefficiency basically arises from us not being able to go from a given heat source to absolut zero, normally only far above zero, so efficiencies below 1.

Regarding your idea with the ordered gas molecules: When we look at the actualy mechanical work our molescules can achive only the force/preasure they can can apply and the change in volume that causes matter. The reason being that mechanical energy ist just a force applied over a certain distance. So it doesn't matter if the molecules do not hit our actual piston, the moleculs that hit the piston are the only ones that somehow change the energy balance, while all other (in an ideal system) simply keep their energy as heat (i.e. random movements) as they can not move the piston chambers walls.

[u/DMayleeRevengeReveng → u/Saibot393]

That makes more sense to me. I think that makes sense of the other reasons I was told, which just didn’t click.

[u/John02904]

So for an ideal internal engine the power stroke is an adiabatic process. Volume increases pressure decreases. Pressure is the same in the gas. The amount of pressure or energy or velocity of the gas particles on the cylinder walls is decreasing. That kind of goes against your theory. Its not the linear momentum of the particles pushing downward on the piston that is only affected.

[u/DMayleeRevengeReveng → u/John02904]

I understand that. I’m not really defending the theory. I understand it’s incorrect.

The theory was just that, the pressure against the walls can’t be “captured” (unless the molecules somehow spontaneously reorder themselves, which is what I felt was entropically/statistically prohibitive) so the internal energy stored in those modes (parallel to the piston surface) would be lost as heat. And that made sense to me.

But yeah, I’m not saying the theory must be accurate.

[u/mehmin]

Let's say the whole of the gas internal energy is used up so that its temperature is now 0K during the power stroke.

The next stroke is the exhaust stroke, but what happens when you open the valve to the atmosphere? It's the air from outside that's going to come in instead of the gas inside goin out, and it'll stop the cycle.

To stop the air from outside, the gas inside must be at least the same temperature with the outside air, hence the limiting factor of the cold reservoir.

[u/DMayleeRevengeReveng → u/mehmin]

That makes perfect sense. Thank you.

[u/03263]

to get perfect efficiency, all the molecules would have to spontaneously assume linear motion directly at the piston surface.

Isn't that basically what happens, because the walls don't blow out. Only some of that pressure on the walls gets converted to waste heat, but not too much, at least not enough that they fail. It's just not exactly spontaneous, but following the path of less resistance.