Quote:
Originally Posted by serialk11r
Do turbo engines not have higher BSFC at full load? A naturally aspirated engine with decent compression has only 2 bar absolute pressure left at the end of the exhaust stroke, which amounts to less than 10% of power if you were to extend the expansion to let it reach atmospheric pressure.
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Yes, a turbo engine does have a higher BSFC but what is the point we're arguing for a performance engine? This isn't a prius forum
As for the higher pressure left at the end of the expansion stroke, you can't simply pick one point to look at. You'll also see that the peak pressure of combustion is MUCH higher on the entire combustion stroke. So take your delta P.
Quote:
Originally Posted by serialk11r
Whatever energy left in the cylinder of a turbocharged engine at the end of expansion stroke is far greater than what is needed to compress the intake air.
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well of course it is, it's taking in more than it's own swept volume. That energy is also part of what drives the turbo and this is where turbo sizing also comes into what portion of the range you want to target (just like any engine will have)...so are you turbocharging for fuel economy or power?
Quote:
Originally Posted by serialk11r
A turbo motor eating 2 times as much air (and thus fuel) at 14psi typically can't produce even 70% more power right? The only efficiency "gain" is that it's better than a supercharger working on the same engine producing the same boost, still not as good as a bigger naturally aspirated engine.
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Power gains is on a case by case situation, obviously. Now that you're looking at power from a larger NA engine that has far greater pumping and frictional losses at points other than full throttle, you're going away from all those points that you were hitting on with wasted energy. So now you're taking those trade offs, weight, package size, etc to try and make the same power as the smaller turbo engine.
Quote:
Originally Posted by serialk11r
Anyhow I just read your other post more carefully again, and you said the only source of additional loss with a turbo is increased thermal loss, which I don't think is right? You blow more pressure out the exhaust because there's simply a lot more of it, you have to enrich fuel and pull timing or compression, anyways higher BSFC at full load is the result. Thermal losses through the block aren't even proportionally increasing with power, so that's not a source of "inefficiency".
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Have you ever seen data on the where the total energy goes in an engine or more so various types of engines? I think you'd like to see how minimal a lot of them are and also how similar they are too.
The thermal losses through the block are always very small.
Pressure out of the exhaust isn't a comparable as it isn't a unit of energy and is also a physical characteristic of the exhaust system. I can put a 4" exhaust and lower the pressure, does that change the energy waste out of the tailpipe? OR I can cool it and reduce the pressure...does that mean that I didn't lose as much energy? no.
Quote:
Originally Posted by serialk11r
My bad my bad, slight correction: when boost pressure is higher than exhaust backpressure the turbo is putting a certain amount of energy back into the system, so that's how a turbo does better. Okay but that doesn't change the fact that a ton of pressure left behind in the cylinder that the engine cannot use. Anything wrong with what I'm saying sir? If your answer is yes, then I'll go back and read that physics textbook again.
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what pressure is left behind in the cylinder? are you stating that after the exhaust stroke that their is still lots of pressure? If so, I'm going to guess that you have never put pressure transducers in an engine. Cam overlap does a lot to control this and contrary to what most people think (they think of things as a static picture), scavenging effect still happens on a turbo engines, though they normally have much less overlap, because of the pressures at hand.
If you're now concerned with residual mass fraction, think of how small of a mass fraction that is in comparison to the incoming charge air.
Now how are we off on a geeky engine/thermo design debate?
If you want to see the trends of best BSFC and fuel consumption, looks at the truck diesel industry (as it's usually easier to find data from them than the train or boat world). Their is a reason they run turbos and not 32L NA diesel engines.