2L 4Cyl. vs. 2L 8Cyl What's more fuel efficient?

[Regenerit]

So I've run into the question of engine efficiency (in terms of fuel) this time in terms of equal displacement engines but with a different amount of cylinders.

2L I4 vs 2L V8.
What would be more efficient and why?

My guess would be the 4 cylinder because it has fewer moving parts (maybe less friction?) but then wouldn't it be easier to burn all of the gas in a .25L V8 cylinder due to the smaller size vs. burning all of the gas in a .5L cylinder in a I4 due to the smaller area in which to burn the fuel (less time necessary for each spark to ignite flame and have it travel less distance to burn all fuel multipled by the fact that its 8 sparks by half the area of the I4..... so yeah. I'm also rooting for the V8 being as efficient if not more than the I4... what's your take, everyone and why?

[tjts1]

It depends on a lot of things, not just the engine configuration. Internal friction, bore stroke ratio, cams, manifold, injection system, peak required RPM, engine management, etc
Also why sell yourself short? How about a 1.5L V12 from 1947

[drmiller100]

if you are considering only MPG, the 4 cylinder will be quite a bit more efficient at lower rpms as it has a LOT less surface area for the combustion chamber.

if you are considering only horsepower, the 8 cylinder will probably make more ultimate horsepower as there is more circumference of the valves for more air flow.

if you are after horsepower, some people will pay a lot of money to have more horsepower, so there is a hope the added complexity and prices will be overcome by a higher sales price.

if you are after economy, then you are selling to people who want to save money (be cheap) so saving production costs is paramount.

[drmiller100]

should have explained - heat is lost to surface area, power is made by PSI pushing on the piston times the length it effectively moves (stroke, sort of).

so, there is a square/cube thing going on - for a given CC, the fewer pistons you have, the less surface area you have for heat to be lost to.

air gets in and out of the combustion area through valves, which are effectively limited by the circumference of the valves, times the number of valves, with bonus for more lift and longer lift. too long of lift, and you lose economy.

[big shafe]

Based on what calculations do you figure a 4cyl has less surface area? For the same stroke or piston diameter a 4cyl has more cylinder surface area than an 8cyl.

If you are talking about combustion chamber then it depends on the compression ratio.

The real answer is, it depends on what you hold constant in engine design to compare them with each other.

[big shafe]

Also, how can the circumference of the valves be bigger in the V8 when the cylinders have to be smaller for the same total displacement? (unless you want to give the 8cyl extremely small stroke vs a 4cyl with large stroke)

[ProDarwin]

Originally Posted by big shafe

Based on what calculations do you figure a 4cyl has less surface area? For the same stroke or piston diameter a 4cyl has more cylinder surface area than an 8cyl.

Huh? If you fix the stroke, a 2L 8cyl would have ~50% more cyl. wall area than a 2L 4cyl.

That said, a 2L 2cyl would be even more efficient and rattle like a god damn Harley. But that 2L v8 would purrrr like a kitten.

[big shafe]

Ok so if both have 2L of total displacement, each cylinder of a 4cyl engine has 0.5L displacement. Each cylinder of a 8cyl engine has 0.25L displacement.

Volume for a cylinder: V=pi*r^2*h
Surface area of cylinder: A=2*pi*r^2+2*pi*r*h

For the 4cyl and stroke of 1cm, 0.5L = 500cc
500 = pi*r^2*1
r = 12.62cm
As = 2*pi*12.62^2+2*pi*12.62*1
As = 1079.98cm^2

For the 8cyl and stroke of 1cm, 0.25L = 250cc
250 = pi*r^2*1
r = 8.92cm
As = 2*pi*8.92^2+2*pi*8.92*1
As = 555.98cm^2

[big shafe]

Ok crap I forgot to sum up the cylinders, so the 4cyl does have more surface area. Only about 2% more, not 50%.

[tortoise]

Do the math with a realistic bore/stroke ratio, say 1/1 for both engines.