Displacement not affecting fuel economy?

[stephenl9000]

Hello,
I was playing the game and I noticed something peculiar, so I decided to investigate. I found that increasing the number of cylinders an engine has does not affect the fuel ecoomy of the engine.
I ran an inline 4, inline 6, and flatplane V8 against each other and, with the same settings, they all used the same amount of fuel. To me, it seems that if an inline 4 uses 300 g/kwh than throwing another two cylinders on the end of it would use two cylinders more fuel, so the inline six would get 450 g/kwh. However, in the game, the inline six gets the same fuel economy as the inline 4. *Note* All three engines had the same cylinder size (86mm bore/86mm stroke).

Also, I made a 1.5 liter inline 4. After that I revised the engine and lengthened the stroke (so it became a 2.4 liter), and the engine used less fuel . Again, this seems counter-intuitive because a larger displacement engine needs more air and therefore more fuel. Could you please adress this conern, or explain why a larger displacement engine can use the same amount of fuel as its smaller counterpart?

Thanks

[Killrob]

Good observation, but not true
g/kWh is the inverse efficiency of an engine, not its fuel economy!
And ask yourself this: what is more efficient, engine A or or engine 2A? Well, both cases have the same efficiency. (Your example of I4 -> I6 -> V8)

Why don't we give actual fuel economy but a weird efficiency stat? Because you cannot say much about fuel economy from just having an engine on an engine stand... it depends on what car you put it into!

Cheers!

[darkjedi]

actually engines that have more cylinders use more fuel at idle and light loads. at full throttle for the same displacement an engine with more cylinders will have the same efficiency than one with less.

[stephenl9000]

Oh I get it, so for every certain amount of KWh an engine produces, it will use 1 gallon of gas, and because a larger engine would make more KWh, it would use more gas. Thanks for the reply!

[Daffyflyer]

Exactly! What isn't noted there is that large displacment, high power engines have sucky economy at cruise, because you have to have the throttle SO far closed to make a small enough amount of power, that the engine has to work really hard pulling air passed a closed throttle. Engines are most effecient near wide open throttle at low RPM. So ideal would be a low revving engine that needed full throttle to cruise at 100kph at say 2000rpm. It'd get amazing fuel economy but would accelerate like crap as it'd have no spare power besides what it needed to cruise.

Turbo engines are good for this though, as they can cruise at quite high throttle settings with a tiny bit of boost, thus NOT sucking against a closed throttle. And then when you need power they just grab a lungful of boost and off you go. This is why loads of manufacturers are going for small turbo motors these days.

[Daffyflyer]

So in otherwords, displacment WILL effect fuel economy, but not at wide open throttle (where the engine designer measures it)

The best way to think of the current measure, is if you ran the engine flat out at peak power for 1hr, how much fuel would it use per KW/HP that the engine generated.