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

[tjts1]

Originally Posted by tortoise

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

BINGO

[big shafe]

Well thats why I said it depends one what you hold equal. I calculated for around 1:1 b/s the difference is around 21%.

[3-Wheeler]

Originally Posted by 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%.

The math that you did only shows part the heat rejection process inside a cylinder.

For example, the "core" heat of the larger diameter cylinder is higher in temperature than the smaller diameter cylinder, and this in turn allows less heat to escape to the outside cylinder wall during operation.

This is why a single cylinder engine of the same displacement will always show a theoretical improvement in engine operating efficiency over a multi-cylinder engine.

What Mr Miller pointed out earlier, multi engines are preferred for more power output, since they allow higher valve area per cylinder volume, and higher rpm's, BUT at a lower operating efficiency. Back in the late 60's Honda was famous for it's family of multi-cylinder race engines, including a 250cc 6-cylinder.

This is why locomotive engines employ such large piston diameters; higher core cylinder heat and less cylinder wall losses.

I work with pressure decay leak testers, and this effect is very important to accurately setting up these testers for proper leak-down numbers. When you pressurize a part for leak down testing, the internal air heats up, and you need to allow for this heat to normalize before making a proper leak down measurement. The larger the part, the more core heat it has and the wait time for this heat to dissipate is not linear, strictly based on volume. The part shape also dictates surface area, and also affects heat rejection to the outside.

All things being equal the 4 cylinder should out-perform the 8 cylinder in efficiency.

Jim.

[big shafe]

Do you have any or can point me to any data that supports this? I would think temperature gradients would be hard to quantify. I also think it would be highly dependent on fuel/air mixture distribution and fluid flow characteristics.

I could possibly see that for similar compression ratio's but not sure about large compression ratio differences.

[tjts1]

4 cylinder
8.6cm stroke 8.6cm bore
1998cc
surface area: 348.5cm x 4 = 1394cm^2

8 cylinder
6.83cm stroke, 6.83cm bore
1998.4cc
surface area: 219.7cm x 8 = 1757cm^2

With equal total displacement and identical bore/stroke ratio, the engine with the fewest cylinders ALWAYS has the least total surface area. The 4 cylinder has less surface area than the 8 cylinder.

Originally Posted by big shafe

Do you have any or can point me to any data that supports this? I would think temperature gradients would be hard to quantify. I also think it would be highly dependent on fuel/air mixture distribution and fluid flow characteristics.

I could possibly see that for similar compression ratio's but not sure about large compression ratio differences.

The bigger the cylinder volume the lower the BSFC. Its a well established linear relationship.

[drmiller100]

and don't forget to include the surface area of the stroke.

[tjts1]

Originally Posted by drmiller100

and don't forget to include the surface area of the stroke.

I included top, bottom and cyl walls.

[Gealii]

im going with the I4 cuz that straight motors tend to put out alot of torque which would allow the user to hold a speed easier but accelerate slower

[some_other_dave]

I have done a little bit of reading on a similar subject, but it was concerned with power and not with efficiency. If you check F1 engines from about a decade ago, you'll find that everyone settled on 10-cylinder engines. I forget which displacement formula this was, but it was not in any of the forced-induction eras. (Possibly 3-liter?)

It turned out that there was a definite maximum in the power produced by a V10 over the V8 and V12 designs, which is why everyone eventually went to the V10. I believe it was a tradeoff in internal friction with higher numbers of cylinders versus better breathing with higher numbers of cylinders. There could have been something with the lower overall masses of the moving parts when you have a larger number of cylinders.

So the differences: This was cost-no-object engineering, optimizing only for power and not at all for efficiency.

I suspect that there are similar considerations and tradeoffs when talking about efficiency, but I don't really know exactly what they would be.

I'm pretty sure a large I4 would be more efficient than a little V8 though.

-soD

[smokey442]

BMW published a paper in this subject including optimum bore and stroke. The v8 wins out for reasons of lower heat rejection rate and accessory losses (oil pump and waterpump)