I have this weird misconception that the engine size is related to the amount of fuel being burned in one cycle(4-stroke engine). I know this is stupid but how does engine size relate to the amount of fuel being burned in one cycle. For example I drive a Ford Ka with a: 1.3i Endura-E engine OHV 1.3 L I4. So I thought that in one cycle 1.3L of fuel is burned, but this is not the case so can someone please explain how they are related.
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With regular driving the Air/Fuel ratio is kept constant at 14.7:1. So if a larger engine needs more air (more swept volume) per cycle then yes it will require more fuel.
So the question boils down to which engine has more friction per cycle, and which car has more weight to carry around. The friction is measured in FMEP (friction mean effective pressure) which gives you the pressure required in the pistons to keep the engine spinning at a constant rate. FMEP times volume = Power per cycle needed to fight friction.
Note that most of the mechanical losses come from the friction between the piston and the bore. The more pistons you have (V6 vs. I4) the more friction you will have. Note that the #of camshafts, #of valves and #of journal bearings also affect friction.
This is more complicated than a simple ratio. I present as evidence exhibit A: the comparison between the BMW M3 and the Toyota Prius on the Top Gear test track.
In short, the Prius was driven at its top (not very high) speed and the M3 remained right behind it for ten laps. The resulting fuel economy results were:
That's the comparison between a 1.5 liter 4 cylinder vs. a 4 liter V8 and the bigger engine was significantly more efficient.
The money quote from Jeremy Clarkson was "It isn't what you drive that matters, it's how you drive it."
In short, if you're trying to compare efficiency between two engines, size is not the deciding factor.