Bore/stroke ratio & volume efficiency & torque curve shape 2

[GSXRFrance]

Hello,

If we keep the same valve diagrams, does the bore/stroke ratio have any influence on the torque curve shape or the volume efficiency especially at low revs ?

Regards

 

[patprimmer]

A bigger bore will unshroud the valves, even if the valve size remains constant, unless the valves are very small to start with.

Regards

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[richdubbya]

A longer stroke/smaller bore will produce more low speed torque and be slightly more heat effecient and have a slightly better combustion.

The VE is affected by the bore as Pat stated above

 

[strokersix]

richdubbya,

Please explain why longer stroke/smaller bore produces more low speed torque.

I think this is a myth. Within reasonable limits the torque output of an engine should be related to many factors other than bore/stroke ratio.

 

[richdubbya]

of course there are many factors, but just comparing strokes with all other parameters equal a longer stroke produces more low end torque. It is because the longer crank arm has more leverage and the friction increase is small at low rpm.

If this is a myth, all the manufactures of vehicles, tractors, lawn mowers, aircraft etc have been terribly wrong all these years.

Now if by low speed you mean a drag engine, where low end is 6000 rpm plus then that is another story.

 

[Azmio]

It has been reported in an IC text book that at low rpm, a relatively bigger stoke to bore ratio will yield slightly higher volumetric efficiency. This is due to the fact that deeper piston travel and faster piston speed will create low pressure region for air to be sucked in.

However, when it comes to torque curve shape, I dont think that stroke to bore ratio will have significant effect to the overall torque curve. It will slightly increase the achievable volumetric efficiency at low rpm but it's not enough to create extra bump or anything.

To change the torque curve significantly, you need to vary the intake and exhaust configurations or to change the cam timing or lift.

 

[j2bprometheus]

Bore to stroke ratio has more to do with the top engine speed.

Engines designed to spin fast tend to have high bore to stroke ratios so that they can breathe well and to keep piston speed under its limit and keep friction low.

Example: The Ferrari Enzo has a bore/stroke ratio of 1.23 (big bore, short stroke). It's a naturally aspirated V-12 with the red line at 8,000 RPM.

Engines that spin slow (like very large diesel engines) have low bore to stroke ratios. Why? Engine speeds are low, so air velocities are low, so pressure drops are low.

 

[Azmio]

Bigger bore to stroke ratio will make it possible for an engine to have bigger intake and exhaust valves. Bigger intake valves will ensure that the choking point is not located at the port and valve areas.

With bigger B/S ratio, it is also possible to keep the mean piston speed below the limit at say 19,300 rpm.

High performance bike engine that I know have 1.5 b/s ratio

High performance car engine that I know have 2 b/s ratio

 

[al1]

I believe that you will find that longer stroke to bore ratio engines typically have shorter rod to stroke ratio. The shorter rods have more favorable rod angles to multiply the higher cylinder pressure sooner (and therefore also effectively longer) in the power stroke thus produce more low speed torque. Remember, that the starting cylinder pressure of the combustion process lowers precipitously as the displacement increase from TDC. Offset wrist pins also do the same thing. The net results are more low-end power due to the above, however (like in everything else) volumetric efficiency falls off sooner as the RPM increase due to faster displacement changes at or near TDC. But this faster displacement change of the shorter rod to stroke ratio at or near TDC increases low RPM volumetric efficiency by having more pressure differential till it starts to choke.
al1

 

[Azmio]

I would rather have big number for the L/R ratio. It should be more than 3.