Are forced induction engines inherently more efficient?

Question

Please let me know if the following is correct, or where I am making invalid assumptions:

1. Larger (by piston volume) piston engines are always less fuel efficient than smaller engines of the same design.

2. The reason larger piston engines are less efficient is that larger pistons (and related mechanisms) sweep larger surface areas and therefore have higher friction loses throughout their cycle.

3. For a given piston volume forced induction will always provide more power per unit volume, all else held constant.

4. Therefore, forced induction engines can always be more fuel efficient than naturally-aspirated engines that are capable of the same power levels.

Nope, I'm talking about turbochargers and superchargers.

Answer 1

It's not all about friction. Forced induction does increase volumetric efficiency, meaning that a smaller engine can produce power equivalent to that of a larger conventionally aspirated engine. Because larger engines waste more energy at part throttle and idling (pumping losses), overall efficiency improves.
But if by "the same design" you also mean using the same fuel-air mixture density, statement 1 is false. Of course larger pistons have more absolute friction energy loss, but not relative to the energy produced, which is what's important here. Assume two pistons have the same bore but piston A has twice the stroke of piston B. Then piston A has twice the energy produced per stroke and twice the piston-sliding friction energy loss, so the friction loss per unit energy produced is the same. Now assume both have the same stroke but A has twice the bore or 4 times the swept volume. Since piston-sliding friction depends on the ring contact length, piston A has twice that kind of friction loss for 4 times the energy, so it's more efficient. With more fuel-air mixture density due to forced induction, the energy per stroke increases while the friction energy loss stays the same.
However, another major effect of forced induction is to raise efficiency in the sense of energy produced relative to fuel energy consumed, regardless of friction. A higher density of fuel-air mixture means a higher combustion temperature, which has been shown by Carnot to increase theoretical efficiency. Per ref. 1, Carnot theoretical efficiency equals (Thot - Tcold) / Thot, so the higher the combustion temperature (Thot) the greater the theoretical efficiency. All other things remaining the same, increased theoretical efficiency produces increased real efficiency. This effect is fairly easily applied in diesel engines but for gasoline engines requires careful matching of characteristics, and, unfortunately, a reduction in compression ratio, often resulting in a net reduction in efficiency (ref. 2).
For typical, variable-power use, a well-designed hybrid car has all the advantages among systems using only fossil fuel as the energy source. The most efficient engine running conditions are at full throttle and low rpm. If the hybrid design uses its engine only under these conditions to charge the battery as needed, it can achieve excellent efficiency. And if the engine is a turbocharged diesel, ooh-la-la!

Answer 2

I am not sure but I think the word you meant to use was injection. From the context of the question I believe you are discussing the difference between an engine with a carburetor or an engine with injectors. In the real world problems are complex and there are seldom straight forward answers. The cleanliness of the available fuel would potentially affect the choice of carburetor versus fuel injection. Clogged injectors would affect the ability of the engine to run. I am sure that an experienced mechanics could explain many other conditions that might make the fuel injection method less efficient.

I hope this helps and please excuse me if I have misunderstood what you were asking.

Answer 3

Ramp up the compression. Upgraded cams, springs, valves. CP pistons($200 each and each), Eagle rods($200 each and each), Balanced or sturdy Crank shaft, all new bearings(can in straightforward terms have precisely .002" hollow between crank and bearing for suitable working consequences), piston rings, Bore, Deck, and hone block. Even w/ all this one spec of dirt in engine in the process carry collectively can mess each and every factor up