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Comment on New thermoelectric generator converts vehicle exhaust heat into electricity, boosting fuel efficiency

partial_accumen@lemmy.world ⁨1⁩ ⁨week⁩ ago

Your explanation about where the energy comes from with turbochargers sounds wrong to me.

When exhaust gas passes through a turbocharger,

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).

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Rivalarrival@lemmy.today ⁨1⁩ ⁨week⁩ ago

The exhaust gases get pushed

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.

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untorquer@lemmy.world ⁨1⁩ ⁨week⁩ ago
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).

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- partial_accumen@lemmy.world ⁨1⁩ ⁨week⁩ ago
  
  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.
  
  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?
  
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  - Rivalarrival@lemmy.today ⁨1⁩ ⁨week⁩ ago
    
    even overcoming the parasitic portion of extra energy needed during the compression cycle and the exhaust cycle against the turbocharger impeller?
    
    Let’s assume the contrary. Let’s assume it can’t. Let’s assume the turbocharger is a net drag on the engine, and any gains are only from enabling the engine to burn more fuel. If this is all true, then the turbocharger should not be able to function without the reciprocating engine. Without the “push” from the pistons during the exhaust stroke, the turbo shouldn’t be able to turn.
    
    If we can show that the turbo can not only spin without the piston engine, but that additional energy can be harvested, we will have disproven this assumption.
    
    So, let’s get rid of the pistons. Plumb the intake manifold directly to the exhaust manifold. We have one combined intake/exhaust manifold. We stick a couple spark plugs into that manifold and turn it into a combustion chamber.
    
    Now we have air passing through a compressor turbine, into a combustion chamber and then through an exhaust turbine. Sound familiar?
    
    Engineers discovered that some turbos were capable of producing more power than the engines they were attached to. They discovered that the reciprocating engine was a drag on the turbo.
    
    That discovery gave us the jet engine.
    
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  - untorquer@lemmy.world ⁨1⁩ ⁨week⁩ ago
    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.
    
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