Bob, are you speaking of gasoline engines, or gas turbine engines? In my limited experience with gas turbines, they have a very wide speed range, easily responding to a load signal to throttle rapidly from minimum to full speed. But they are expensive, complicated, and not widely available in small sizes.Diesel would be best - long way to go to maintain a gas engine. Generally speaking gas engines are absolutely useless if there is a load step of more than perhaps 8% of total capacity - they die.
A few years ago, I experimented with a small Capstone natural gas turbine/generator package, 65 KW, about the size of a large toolbox. That sucker could fly, from 5000 to 25000 RPM in about 8 seconds, made to set up in your backyard and sell power back to the utility company. I see they have a 200 KW unit now.
http://www.microturbine.com/prodsol/products/
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