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Old 12-22-2011, 12:13 AM   #19 (permalink)
Ken Fry
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Originally Posted by Frank Lee View Post
The engine might be 32% but it's putting the power to the ground via generator then battery then electric motor then transmission(?) then wheel vs engine/trans/wheel.

For that putting the ICE power to the wheels seems like a no-brainer.
Hi Frank,

The Zing production version has 40 electric HP. But it doesn't need anywhere near that to cruise along at 60 mph (where it uses about 8HP). It uses 40 hp to accelerate really hard and to and go up steep hills. Most of the time, the motor puts out very little HP, which it can do efficiently -- that's the key advantage of an electric motor.

An engine, however, does not operate at light load efficiently. This is why a Lamborghini, a very small car, gets rotten mileage: in ordinary driving, where it is using 2-4% of maximum power, the efficiency is awful: 6%, 7%, 8%, etc. Thus the 12 mpg rating in the city. A Prius of the same weight gets 50 mpg, due mainly to hybridizing.

The hybridizing idea is to make a very small engine run at or near full load, where it is most efficient. In the Zing! the engine runs at full efficiency or not at all. If I had a 40 HP gasoline engine (like a detuned Geo Metro or Daihatsu) I'd spend a lot of my time operating at less than 20% power, where efficiency is poor (around 12%).

You are right that there are losses in the generator, motor, and controller. (The batteries are mainly out of the picture at a 60 mph cruise). With the Zing!, I get efficiencies of 32%(eng) x 90%(gen) x 98%(cont) x 90%(mot) = 25.4%. So I'd expect that the Zing! would get 50 mpg with a 40 hp gas engine, and about double that as a hybrid, using an engine that operates at 8HP continuous.

Obviously 8 HP is not enough for good performance... thus the batteries, which permit 40 hp outputs for hill climbing and acceleration.

Where does all this fall flat? Suppose I want to drive across the country at 85 miles per hour, the Zing!'s top speed. The engine is not large enough to support that for hours at a time. Your average cruise speed needs to be more like 65. Plan in advance for really big mountains -- this is why the Volt has a mountain mode to accumulate charge before you get there.
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