Bore/Stroke question

[cvcc_wagon]

i have a question about bore and stroke, i have done some looking around this forum and founds some stuff be still have a couple questions. how does the ratio between the two relate to power output? what i mean is that you can have 2 engines with the same displacement but one will have a smaller bore and longer stroke and the other will have a short stroke and larger bore, i realize that an engine with a shorter stroke will have a higher redline but my question is will these two engines have similar power and if not what will be the difference.

[454Casull]

For a given displacement, a higher bore/stroke ratio will tend to give more peak power (though torque is sacrificed).

[SaabJohan]

Torque decreases because of that it gets harder to reach high brake mean effective pressures at high speed; it's not because of the stroke and therefore the crankradius gets shorter as sometimes believed, the larger piston area will compensate for the shorter stroke. The difference in torque from a 3 liter F1 engine and any normally aspiranted 3 liter engine isn't therefore that large, but the torque from the F1 engine on low revs will be poor since it's designed for high speed running.

[cvcc_wagon]

so what you are saying is that with a short stroke engine ther will be a loss in torque but since it has the capibility to rev higher it will have similar power output? and also what you were saying about the crankradius, you are saying that the shorter arm will not alone be responsible for the loss in torque? i thought that was the reason why someone would put a crank with a longer stroke in an engine

[454Casull]

Originally Posted by cvcc_wagon

so what you are saying is that with a short stroke engine ther will be a loss in torque but since it has the capibility to rev higher it will have similar power output? and also what you were saying about the crankradius, you are saying that the shorter arm will not alone be responsible for the loss in torque? i thought that was the reason why someone would put a crank with a longer stroke in an engine

Increasing the length of the crank throws (radius) will increase stroke while keeping bore diameter constant. There isn't any loss in piston crown area, allowing for the same area for the expanding gas to act on, and the increased stroke allows more gas to be burned, increasing pressure.

[454Casull]

Originally Posted by SaabJohan

Torque decreases because of that it gets harder to reach high brake mean effective pressures at high speed; it's not because of the stroke and therefore the crankradius gets shorter as sometimes believed, the larger piston area will compensate for the shorter stroke. The difference in torque from a 3 liter F1 engine and any normally aspiranted 3 liter engine isn't therefore that large, but the torque from the F1 engine on low revs will be poor since it's designed for high speed running.

Thanks for clearing that up, SaabJohan. I have a question, then; if a continuously-variable-lift valve actuation assistance mechanism is installed so that the lift is ideal for all RPM, is intake timing/duration a very big factor (in the production of torque at any given RPM)?

[ivymike1031]

Originally Posted by 454Casull

Thanks for clearing that up, SaabJohan. I have a question, then; if a continuously-variable-lift valve actuation assistance mechanism is installed so that the lift is ideal for all RPM, is intake timing/duration a very big factor (in the production of torque at any given RPM)?

Is your question "if we can control lift, then do we need to worry about timing?"

If that's what you're asking, then the smart-alec answer is "no, just use zero lift for the times when you'd like to have the valve closed."

Barring the smart-alec response, then yes, intake and exhaust timing are still important. Overlap period is probably the most critical factor to look at (EVC-IVO), but EVO and IVC are important as well.

To be honest, if I had to choose between two systems that worked equally well mechanically, where one allowed me to control valve timing, and the other allowed me to control valve lift, I'd choose the valve timing mechanism every time.

[SaabJohan]

Originally Posted by 454Casull

Thanks for clearing that up, SaabJohan. I have a question, then; if a continuously-variable-lift valve actuation assistance mechanism is installed so that the lift is ideal for all RPM, is intake timing/duration a very big factor (in the production of torque at any given RPM)?

Valvetiming and lift are both important, an advantage with variable valvelift is that it can replace the throttle. But for maximum gain we will need to adjust the timing also. Adjustable timing is however very simple, at least the systems used today, and the gain in power from those alone are very small.

If we could completely adjust timing and valvelift as we want there will still be other things that affect the filling of the cylinders, the piston speed and intake design (including length of pipes and all that).

[ivymike1031]

Originally Posted by ivymike1031

To be honest, if I had to choose between two systems that worked equally well mechanically, where one allowed me to control valve timing, and the other allowed me to control valve lift, I'd choose the valve timing mechanism every time.

I may have been unclear in my previous statement - when I said "allowed to control timing" I meant a system that would allow EVO, EVC, IVO, IVC to vary independently, so that I could change timing and duration (with fixed peak lift).

Incidentally, the throttle can also be replaced by variable valve timing, to an extent (eg- miller cycle).

[454Casull]

Originally Posted by ivymike1031

I may have been unclear in my previous statement - when I said "allowed to control timing" I meant a system that would allow EVO, EVC, IVO, IVC to vary independently, so that I could change timing and duration (with fixed peak lift).

Incidentally, the throttle can also be replaced by variable valve timing, to an extent (eg- miller cycle).

Yep. Saab's "Saab Variable Compression" seems a bit unnecessary, because something that varies intake duration can do the same thing.

[SaabJohan]

Originally Posted by 454Casull

Yep. Saab's "Saab Variable Compression" seems a bit unnecessary, because something that varies intake duration can do the same thing.

Saabs variable compression engine varies the compression ratio and boost dependent on engine load, this allows the use of a very small powerful engine without the disadvantage of running with a low compression ratio. Variable intake duration can change the engine power but it will have the disadvantage of a constant CR. Saabs SVC engine has shown a reduction in fuel conumtion of 30%, I haven't heard of any variable duration/timing/lift system that can decrease the fuel consumption by that much, at least not alone.

We'll see what happends to the solenoid valve actuation technology since this should freely be able of adjusting timing, duration and lift. We can even make the engine running as a twostroke for increased power. Should be interresting together with a variable compression engine, preferbly with VNT turbocharging and an exhaust recirculation/direct injection system similar to "Saab Combustion Control".

[ivymike1031]

[quote=SaabJohan]Variable intake duration can change the engine power but it will have the disadvantage of a constant CR. QUOTE]

That's not really an accurate statement - if you close the intake valve many degrees bbdc, shortening the intake stroke, you've dropped the effective compression ratio. Geometric compression ratio remains the same, of course, but you have the desired effect on both engine power output and efficiency. Again, as used in Miller Cycle engines.

[SaabJohan]

[quote=ivymike1031]

Originally Posted by SaabJohan

Variable intake duration can change the engine power but it will have the disadvantage of a constant CR. QUOTE]

That's not really an accurate statement - if you close the intake valve many degrees bbdc, shortening the intake stroke, you've dropped the effective compression ratio. Geometric compression ratio remains the same, of course, but you have the desired effect on both engine power output and efficiency. Again, as used in Miller Cycle engines.

I meant geometric compression in my post above.

The SVC engine drops it's compression ratio so it can use more boost and deliver more power under high load conditions, hence to increase airmass/combustion. If you instead do as you have described we have decreased the effective compression ratio because of the airmass/combustion have dropped, so it won't be able to work as the SVC engine; that is, to be able to change airmass/combustion without sacrificing effective compression (since the geometric cr can be changed) and therefore efficiency". This will make the most of the downsizing concept.

To shortening the intake stroke is quite similar to throtteling but it has the advantage of that pumping losses aren't that large.