Quote:
Originally Posted by Formula413
But isn't the power of the expansion used to move the piston in the firing stroke? How would just heat (and not pressure) turn the turbine?
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You are both right.
If you put a turbo in the exhaust stream, yes, the piston has to push the exhaust gasses out of the cylinder against the back pressure of the turbo.
However, the opposite is also true. Think of the piston at BDC after the intake stroke. Now think of it at BDC after the power stroke. After the power stroke, the gasses have combusted and expanded due to the increase of temperature. Typically, this expansion is large enough that even at the bottom of the power stroke there is still pressure within the cylinder. As soon as the valve opens this pressure is released and the potential energy is lost.
Thus, the turbo has the advantage of being fed by a higher volume of gasses than simply that displaced by the exhaust stroke. See, the gain comes from the pulse of exhaust gasses that jets out as the exhaust valve opens and the piston is still stationary.
So yes, backpressure increases the work that the engine must do. However, a portion of the engine gained by turbines would have been lost anyhow. This is why intercooled turbos win compared to intercooled superchargers, even at the same operating parameters. Turbos are partially driven by otherwise uncaptured energy.