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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

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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