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A turbo is good, an impulse turbine can collect the exhaust pulse pressure (not sure if there's a term for this) without much restriction and will muffle the exhaust without wasting as much energy as a muffler. The only problem is under part load operation there isn't much energy to collect, and because it arrives in pulses you can't collect as much of it.
That's where the steam generation comes in. An ideal gasoline engine that has no cooling losses, no friction, and has full expansion of the burnt charge will still dump 30% of the energy as heat into the exhaust. Even under full load, the leftover pressure that is being blown out only accounts for a small amount of the total energy that can be extracted from the fuel. As the exhaust blows through the restriction of the exhaust valves, it loses energy, and what's left can only be converted at less than 50% (?) efficiency in an impulse turbine.
If you can generate electricity with steam heated by exhaust at just 10% efficiency, you can recover 3% of the overall fuel energy. If you consider that good engines are running in the high 30s (percent efficient), this is a pretty good amount of power.
I think in Formula 1 they are thinking of using turbocompounding (using a turbine to fully expand exhaust gases as you describe), and someone told me they saw a 7% efficiency gain. This means the turbocompound makes the same amount of power as a 10% efficient steam generator at full load, except street cars rarely run full load.
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