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Most modern gasoline engine designs are near 40% efficient, and nowhere close to 12%. Even the big pushrod cam-in-block V8 in GM's Corvette, which lacks cam phasing, is in the mid 30's %, and modern transmissions keep engines in that range much more of the time.
Unfortunately, burn speed is asymptotic, and we're pretty far along that already. Smaller combustion chambers improve burn speed, as does increasing pressure (compression or boost), but you run into knock limits with gasoline and have to use a different fuel.
I think Mazda is thinking along the right lines with their compression ignition gasoline engines. You can largely bypass the inherent losses of the burn speed of gasoline by intentionally causing detonation at the optimal crank angle (which ends up being later than the current target of ~18%). Without doing this, we're effectively pretty close to the limits of gasoline engines. Maybe not "combustion" engines, but specifically those that rely on burning gasoline.
Going above... maybe 60%, is highly improbable, unless we start using exotic materials for engines. By simply compression air (nevermind combusting gasoline) you heat up the charge, and that concentration of heat then conducts through the piston and cylinder walls, and a large chunk of it is lost, just by virtue of having been compressed. Add to that, you will never extract 100% of energy from expansion. As long as what comes out of an engine's exhaust valves is above ambient, you're losing a huge amount of energy in the exhaust. Even combined cycle power plants, which concentrate waste heat and run it through the system once or even twice more, are only up around 60%.
Burn speed is not low hanging fruit in modern engines.
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