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.
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:
- 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.
All other things being equal- same car body shape, mass, road, tires, cylinders, strokes, fuel/air system and ignition etc, a larger engine operating at middling RPM and power will require slightly more fuel to overcome friction than a smaller engine also comfortably operating at middling rpm and power. For example a Triumph Herald with inline 4 cylinder, 949 cc motor, versus the same car with an 1149 cc motor. Compare to a Vittese with a straight 6 of larger displacement, they're going to be comfortable at two different speeds.
Clarkson was and is an entertainer. A Prius howling along at 80 MPH with unknown hybrid system setup (no regenerative braking? Topped up battery to start?) following a course of hard acceleration, turns and braking, can be driven to a crap MPG. A BMW throbbing along at a similar speed, driven perhaps not so hard because it would go faster or slower given the same inputs, will produce a happier MPG.
If you need to magnify the effect, let some air out of the Prius tires and pump some into the BMW's. Sand bags and water bottles could also have a role.
My 2.8L 6 cylinder Corrado was sleeker, heavier and vastly more powerful than my 1.8l 4 cylinder VW Golf. At a steady 55, driven gently, the Golf delivered 35-40 mpg in California's Central Valley. The Corrado could touch 30 mpg under the same conditions, but only with constant attention to never having fun. 65mph and < 27mpg was more typical. At 65, 30-35mpg for the Golf.