Now I've just started looking at my mother-in-law's Rover engine (possibly valves or head gasket problems...) I got thinking about engine building.

I know the difference between bore and stroke, and was wondering the effects of increasing them.

Presubably with an increased bore, there will be more force generated by the cylinder due to an increased cross section area, but probably less pressure (or the same pressure) on the piston itself (force/area). Should the compression ratio stay the same in this case? (all other things being equal)

For the increased stroke, I get a bit more confused. Surely changing the stroke will first change the compression ratio. Therefore if it increases, surely there is more pressure on the pistons and more force generated.

Say two 2.5 engines were made from a 2.0 engine. (Both to be turbocharged) One by changing the bore and one by changing the stroke. What sort of parameters would each engine exhibit?

Sorry for the essay... Probably not such an easy question now, so if there are books that explain it better, I wouldn't mind reading them. Any discussion welcome!

 

How long do you have?

Larger bore or large stroke will increase capacity. You would have to increase the combustion chamber volume and/or piston dish to maintain the same C/R.

Increased stroke will give increased leverage

Increased bore (area) will give increase force for the same pressure.

Both will draw in extra charge, but the bore, stroke and conrod dimensions will alter the scavenging (breathing) and volumetric efficiency due to a number of reasons. Things like changes in the piston speed curve, peak speed etc.

There's all sorts to consider, and you could read many books on the subject, I've only scratched the surface.

Paul

 

Thanks I knew it would be a large subject.
I've got plenty of time, but access to a relatively poorly stocked library...

I'm quite interested in the theory, even though there may never be the funding to have a go for real. (It's good programming practice writing models anyway )