DCB, a steady output generator will be more efficient than a variable output generator, period. The 75% is the power efficiency not the energy efficiency. The generator will use a different amount of energy from the gasoline not just 18hp. Power doesn't equal energy. You're not counting the fuel efficiency.
25hp is the standard number repeated for getting 55mph steady. The JourneytoForever hybrid went 45mph on a 7hp generator. 60mpg at 45mph on the generator's power alone. A good example of a homebrew serial hybrid. Diesel engine, but at least someone tried.
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-Allch Chcar
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