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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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The 1.3 L number is the volume of the cylinder, not necessarily how much fuel is being burned. So basically, you could fit 1.3 Liters of fluid in the cylinder. – Annath Jul 11 '11 at 20:55
Why are engines shown in this way? – Dean Jul 11 '11 at 20:58
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@Dean, engines are sized in terms of the air volume to which fuel is added primarily because air is the dominant factor. Fuel is an additive to the air, not the other way around. – Bob Cross♦ Jul 11 '11 at 22:27
Thats where i've been going wrong then. – Dean Jul 11 '11 at 22:47
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Litres are a measure of volume of any type, not just liquid quantity. People used to use cubic centimetres for engine size, but "litres" is easier to say and the number is the same anyway (albeit with a decimal in a different spot). – staticsan Jul 15 '11 at 0:29

2 Answers

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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.

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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:

  • 17.2 mpg Prius
  • 19.4 mpg M3

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.

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The difference there being the Prius was being driven flat out, way past what it was designed to do efficiently whereas the M3 was cruising along because it has the power and gearing to go much faster. If they did the test 20MPH slower it'd been a different story. – ManiacZX Jul 12 '11 at 20:03
@ManiacZX, exactly. The size of the engine was insufficient to say "big = bad, small = good." Even with the hypothetical 20 MPH, it becomes a question of apples vs. oranges. Is the Prius running on battery? If so, its MPG = infinity for a while (i.e., not a good comparison). – Bob Cross♦ Jul 13 '11 at 3:05
Also of note is that there's a lot more to factor in when considering the fuel economy of a car, than just the drivetrain. There's the weight and aerodynamic profile of the whole system, tire pressure and design, additional running accessories, etc. – Iszi Jul 14 '11 at 19:55

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