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Originally Posted by Arragonis
Isn't that more or less what a Prius does ? Smaller, more efficient but less powerful engine with battery backup when you need it. Select PWR mode and you get more.
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Yes and no. The electrics in all the gas-electric hybrids are much smaller than the gas engines. Less than half the power. In the Prius the combined output is 36 hp more than the gas engine (98 hp) So the Prius electric motor produces 1/3 as much power as the gas engine.
The idea in this thread is good, and could be used in current hybrid cars. But some of the hypotheticals are not realistic. All the gas-electric hybrids that are designed for economy use an Atkinson type engine. The efficiency is higher than Otto cycle and run at wide open throttle for most of their operating range. Atkinson type engines have a very wide sweet-spot. The Prius, for example, has it's best efficiency (about 33%) between 1500 rpm and 3000 rpm, and the efficiency only falls below 31% past 4500 rpm. But below about 1000 rpm efficiency collapses.
The batteries in hybrid cars are expensive, heavy, and bulky so the designers have opted for the smallest battery that provides an improvement in economy. About 2 hp-hr. If the Prius engine runs at 1000 rpm it can charge the battery at 20 hp. From 25% charge to 75% charge is 1 hp-hr or 3 minutes of running time. The Prius is a pretty efficient car and probably needs about 4 hp to cruise at 40 mph. So the battery could push the car for about 15 minutes on 1 hp-hr. This isn't really like the p&g we're used to.
By the way, operating like this is very efficient. If you could find a route that allowed you to cruise at 40 without hills or stops, you could get about 150 mpg.
-mort