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New thermoelectric generator converts vehicle exhaust heat into electricity, boosting fuel efficiency
(www.inceptivemind.com)
this post was submitted on 14 Feb 2025
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Technology
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Basically, slap Peltier modules on the exhaust pipe. This ain't gonna do much. We can invent a thousand applications for Peltier modules, until there is a massive technology breakthrough in terms of semi-condutor materials, it's kinda pointless.
Reusing heat energy from exhaust is what turbos are doing for 120 years now.
This concept isn't new either. Factories have been using very similar methods to use the heat of the exhaust gasses to power the sensors and whatnot on top of their smoke stacks for some time now, for example.
I thought turbos converted air (exhaust) kinetic energy into mechanical energy, not the heat itself. If exhaust was cold, the turbo would still work, no?
Not saying that these Peltier devices are going to be game changers, but it sounds like they do something different from turbos in that they are capturing additional wasted energy.
Yes, and no. Heat and kinetic energy are fundamentally all just energy. What we call heat is, technically, the kinetic energy of molecules vibrating around.
When exhaust gas passes through a turbocharger, it is both slowed and reduced in pressure, resulting in it coming out slightly cooler than when it entered. This device is using a different method of getting energy out of the exhaust gas, but it's fundamentally still the kinetic energy of those very energetic exhaust gas molecules bouncing against one side of the thermoelectric generator and giving up their energy into it. I would still expect the exhaust gas to come out of it slightly cooler and slower.
Your explanation about where the energy comes from with turbochargers sounds wrong to me.
You're skipping a crucial step here. The exhaust gases get pushed through input of the exhaust gas impeller on the turbocharger by the movement of a piston in the engine during the exhaust cycle. This "work" isn't free. Its energy that comes from the other pistons on their combustion cycle. If there is more resistance on the exhaust coming out of the engine (which there is to drive the turbocharger impeller), that energy must be added (robbed) by the energy at the crankshaft that ultimately powers the wheels.
The extra boost of power we experience in an engine from using a turbocharger is that the turbocharger allows more oxygen to be put into the combustion chambers (and the engine puts more fuel in at the same time). The extra energy is from burning - - more fuel in the same period of time than without turbocharging. The fuel is the source of the energy, the turbocharger isn't recovering any energy.
The article is covering technology is actually recovering energy turning heat (thermal energy) back into electricity (electrical energy).
The "pushing" (exhaust stroke) isn't particularly relevant.
When the valves close at the beginning of the compression stroke, the pressure in the cylinder is atmospheric: zero psig. The valves don't open until the piston has risen (compression) and fallen (power) again. Without combustion, the pressure at the time the exhaust valves open is again at atmospheric. The gasses were compressed, and re-expanded.
With combustion, the pressure at the bottom of the stroke is substantially higher than atmospheric: the combustion event has radically increased the pressure of those gasses. At the end of the power stroke, just before the exhaust valves open, the pressure inside the cylinder is still extremely high.
It is the expansion of those gasses - not the "pushing" of those gasses - that drives the turbo.
I think it might be beneficial to think about the next evolution in aircraft propulsion. The turbocharger operates by expanding gasses through a power turbine, and using that energy to drive a compressor turbine. Remove the cylinders and pistons from the path, carefully tune those turbines, and you have a turbojet.
If the pistons are "pushing" the turbocharger, the turbojet would be impossible. It is the expansion of the gasses, not the displacement of the pistons, that drives the turbocharger.
The exhaust gases are at a high pressure after combustion due to combustion heat. The turbo does indeed increase exhaust pressure, and therefore extracts some work from the crank but it's extracting significantly more from the high pressure of the expanded hot gas. It's not "free" because it's energy that is usually just wasted in a naturally aspirated engine. There are many examples of engine configurations where a turbo is used to boost efficiency by reducing displacement.
There were systems on old aircraft engines which used exhaust power recovery turbines geared directly to the crank. Those wouldn't physically function under your concept.
The increase in manifold pressure doesn't just increase oxygen in the cylinder. It also increases the manifold pressure, or the total mass of gases. The increase of oxygen does allow for more fuel and total energy in the ignition event but the extra ~~inert~~ gas also expands when heated. So both play a factor in increasing mean effective pressure, and therefore energy output per cycle (power).
Edit: im tired... Bad wording, adding inert gas to increase intake mass doesn't help.
I'll admit I'm at the edge of my knowledge here, but are you saying that if we were increasing the pressure in the cylinder from, say pure nitrogen (or another inert), instead of atmosphere (which contains oxygen), and we kept the same amount of fuel from natural aspiration, we're still get the majority of the benefit of turbocharging even overcoming the parasitic portion of extra energy needed during the compression cycle and the exhaust cycle against the turbocharger impeller?
No. The heat of combustion increases the gas temperature. But this temperature increase is relative to the mass of the gas. The heat is relative to fuel/oxygen mass combusted. (Combustion energy + Ideal gas law)
Add mass without adding combustion, you get lower pressure and temperature out. So you get less boost from the turbo and make more work for the compression cycle.
The major point of the turbo is to use wasted heat to add more oxygen by packing more air in. So it's a bit of an odd question to answer. The point is there's a lot of energy wasted in a naturally aspirated engine's exhaust. Turbos mostly use that wasted energy, and not power from the crank.
Oh yeah, the turbo is going to have an efficiency ratio for converting exhaust pressure into boost. So that added backpressure on the exhaust is going to be offset in the intake stroke by that ratio. Not important to the point, hat a tidbit. These things are so complicated lol.
I’ve wondered about this. If this is so, and heat is molecules moving back and forth, how do the molecules stop, change direction, and then accelerate in the other direction, stop, change directions again, and go back, over and over and over?
It's pretty much like billiards. They just bounce. Different chemicals (types of molecule) are different phases at different temperatures e.g. nitrogen is a gas at room temp, water is liquid. Stuff that's a gas at room temp just has less bonding forces (and often mass) than liquids or solids. So they don't take as much heat to go fast. There's a lot of heat even at room temp, and even at -40deg. The temperature for nitrogen to sit in one place is -210C or -346F.
Yeah, I do apologize - I'm somewhat simplifying my explanation because when you start going into the full detail, it just brings up more questions.
So yes, like the other comment says, the particles are constantly bouncing into other things.
Nothing to apologize for - thank you for elaborating.
Molecules interact with each other. Energy is transferred as they bump around. If you were to follow a single molecule it would move around randomly. What we can measure is usually the average of many molecules.
So it’s less of vibrating and more about smashing around into things?
This is easier to envision with a gas: like a chamber of balks all ricocheting like mad. It’s harder to envision for a solid. But I guess a molecule will be up smack against its neighbors, getting repelled, not so much bounding freely?
So in a solid, you can imagine each atom connected to each other by springs (bonds). They can vibrate on these springs. If they vibrate too much (by heating) then they can break the bonds and escape as a gas. Gasses basically have too much energy to bond again.
Even solids are mostly nothing. This is why neutron stars are so dense - there is a lot less nothing between the neutrons, largely due to gravity.
Here's another way to think about it. A gas is like a bunch of balls bouncing around a room, hitting the walls and occasionally each other. A solid is like a ball pit, but the balls are vibrating. There is still a lot of bouncing, but most of themstay together.
Gotcha, thanks. It doesn't sound particularly revolutionary as far as energy capture goes, since 40W is probably negligible on today's cars.
NGL though, on an old '75 Honda motorcycle I used to own, an extra 40W would have been amazing. The alternator was too tiny to keep the electronics running and battery charged, and I had to turn the headlight off whenever possible to keep the battery charging.