Internal engine geometry and physics

[RobbieNelson]

Here a question to which I don't know the answer off the top of my head. If we put our thoughts together, I'm sure we can figure it out. Or perhaps someone already knows the answer.

Why is the bore and stroke of most engines close to square? By square, I mean that the bore diameter is close to the stroke length.

My evo has a bore of 85mm and a stroke of 88mm.

[RallyEX]

Here a question to which I don't know the answer off the top of my head. If we put our thoughts together, I'm sure we can figure it out. Or perhaps someone already knows the answer.

Why is the bore and stroke of most engines close to square? By square, I mean that the bore diameter is close to the stroke length.

My evo has a bore of 85mm and a stroke of 88mm.

Given the minimal amount of Physics that I can recall from high school and college, I came up with the following reasons:

1. Balance - an engine with a much smaller bore but a long stroke would be a very tall, narrow engine with a high center of gravity making it much harder to keep balanced and reduce vibration.

2. Space constraints - An engine with a huge bore but very short stroke would be a very oddly shaped engine, as would an engine with a very small bore and a very long stroke.

3. Efficiency - It seems to me that an engine with a very large bore but a short stroke would not be efficient. It would require much larger intake and exhaust valves to get all of the air and fuel in and the waste gases out. Also, it would require much more fuel. Also, a clean burn over such a large surface area with that much fuel would be tough to achieve with a single standard spark plug, even on an upgraded ignition system.

Those are just my thoughts, I could be wrong.

[narfdanarf]

I agree with Tim that one of the main concerns is the size/shape of a engine in a commuter car.

It seems like somewhere I read that you can do some dividing/multiplying/figuring and get a value called "n". When you get this value it explains how well your car works in certain situations. For example a high "n" value means you probably have to have larger intake ports, your cylinder walls are going to wear prematurely, and will use more gas. Whereas a small "n" value means you will probably have smaller intake ports/less power, less gas, butt a more optimum angle for piston/cylinder life.

Keep in mind that I have not read up on this for some time, and I'm only typing by the seat of my pants. It may be completely backwards or even non-relevant. I'll do some research though if someone would like? Instead of just trying to figure it out?

[Davis Silver Sti]

I could only think of "balance" as being a potential answer.

Having a more equal sized bore and stroke would give an even powerband.

Some theory could be said that having a shorter stroke will give one a higher reving engine at the expense of less torque. v/v.

However, it would be interesting to see what kind of bore and stroke ratios there are in the JPTCC (Japanese touring car series) or some other series that have modified race engines.

I need to look up my STi specs as I've never really paid attention to these numbers. Heck, my pistons are laying flat anyhow...who knows what that does to the above theory of shorter stroke = higher rev's.

[Feedman]

I agree with Tim that one of the main concerns is the size/shape of a engine in a commuter car.

It seems like somewhere I read that you can do some dividing/multiplying/figuring and get a value called "n". When you get this value it explains how well your car works in certain situations. For example a high "n" value means you probably have to have larger intake ports, your cylinder walls are going to wear prematurely, and will use more gas. Whereas a small "n" value means you will probably have smaller intake ports/less power, less gas, butt a more optimum angle for piston/cylinder life.

Keep in mind that I have not read up on this for some time, and I'm only typing by the seat of my pants. It may be completely backwards or even non-relevant. I'll do some research though if someone would like? Instead of just trying to figure it out?

I am sure someone can confirm this, but I have heard Hondas particularly B-series have a great rod/stroke ratio. Meaning that they don't apply alot of pressure agaisn't the cylinder wall via the piston. This aids in allowing B-Series to withstand more pressure such as boost vs the D-series.

8)

[Don juan futon]

The answer to your question is as follows:

A square engine is basically as you describer, a smiliar bore and stroke. It is basically a compromise between low rpm torque and high rpm power.
It also provides good low speed torque with high speed power. And finally, it allows use of higher final drive ratios (lower numbers) for fuel economy, yet still maintains drivability in slow city driving.