Rod ratio and dwell time... doesn't make sense

[Stan Weiss]

Less ratio means you have better flowing heads...
More ratio means you have poorer flowing heads...



Stan

[ptuomov]

Less ratio means you have better flowing heads...
More ratio means you have poorer flowing heads...

Relating to my specific interests and the Koenigsegg writeup, how much does the rod ratio impact the knock resistance? I’m thinking about pump gas engine running say 1.5-2.0 bar of boost and geometric compression ratio of about 8:1. The four valve heads flow way more than required for 5l engine with 100mm bore and 78.9mm stroke. Suppose that one would move the piston pin up by 10mm and thus increase the rod ratio from 1.9 to 2.0 with 160mm rod. What would happen?

What would happen to the squish velocity near the TDC? How would the required ignition advance change? With some reasonably relevant combustion model, how would the maximum unburnt end gas temperature change? In general, would the engine become more knock resistant?

[David Redszus]

Less ratio means you have better flowing heads...
More ratio means you have poorer flowing heads...

Relating to my specific interests and the Koenigsegg writeup, how much does the rod ratio impact the knock resistance? I’m thinking about pump gas engine running say 1.5-2.0 bar of boost and geometric compression ratio of about 8:1. The four valve heads flow way more than required for 5l engine with 100mm bore and 78.9mm stroke. Suppose that one would move the piston pin up by 10mm and thus increase the rod ratio from 1.9 to 2.0 with 160mm rod. What would happen?

What would happen to the squish velocity near the TDC? How would the required ignition advance change? With some reasonably relevant combustion model, how would the maximum unburnt end gas temperature change? In general, would the engine become more knock resistant?

Given an engine with a B & S of 100mm x 78.9mm, a squish area ratio of 50% (70.5mm dia bowl), clearance of .040", and 6000 rpm, then...
a 1.9 rod ratio yields a squish velocity of 44.4 m/s @ 9.7 deg BTC and ATC.
a 2.0 rod ratio yields a squish velocity of 44.168 m/s @ 9.8 deg BTC and ATC.

Rod ratio has virtually no meaningful effect on squish velocity.
These squish velocity numbers are on the high side and would be even worse if the engine speed is increased.

Less ratio means you have better flowing heads...
More ratio means you have poorer flowing heads...

The difference in piston air demand for an engine with a rod ratio of 1.5 compared to 2.0 is about 2.3%, which occurs at max piston speed crank angle. Higher air demand does not automatically produce higher flow numbers. But the potential is there.

Once again the data indicates that rod ratio is meaningless and should be ignored. Remember not to confuse a change in stroke (instead of rod) while computing rod ratio. Stroke must be held constant.

[ptuomov]

Thanks David.

It seems that Koenigsegg aggressively minimizing the piston compression height was probably motivated by other things than rod-to-stroke ratio. Well, it’s a 1000hp 5.0L production engine, so the loading on the piston skirts may be one rod-to-stroke ratio related consideration. More likely, though, the main consideration was to keep the piston weight down. 8250rpm with 95mm stroke in a production engine is a challenge that probably requires some very light pistons.

[Stan Weiss]

David,
Using your 1.5:1 and 2.0:1 (which is a huge difference) looking at the cc's change above the piston from TDC to BDC the max is not that great but the shape of the curves is where the real difference.

Stan

ab-cvc-15-20.gif

[kimosabi]

Longer rods are more of a mechanical and angular benefit than just power. I'd go as far as saying piston sidethrust decrease for engine longevity is also a factor. You also get a lighter piston.

Power output is not all that makes a happy engine.

[digger]

Longer rods are more of a mechanical and angular benefit than just power. I'd go as far as saying piston sidethrust decrease for engine longevity is also a factor. You also get a lighter piston.

Power output is not all that makes a happy engine.

Have you calculated the change in angle ? You'd be surprised how little difference in makes as often you normally have a relatively small window that the rod length cAn fall into due to other constraints. It's fine to look at extreme differences but that's accademic in most cases

It was alluded to earlier but increased stroke more drastically negatively influences the things than a lower rod ratio. It so happens that almost always increased stroke brings lower rod ratio. most have trouble differentiating between the two.

Also the temperature and rpm has more effect on piston volumes at and around tdc due to change in rod length when you're talking under 1/2".

[ptuomov]

Longer rods are more of a mechanical and angular benefit than just power. I'd go as far as saying piston sidethrust decrease for engine longevity is also a factor. You also get a lighter piston. Power output is not all that makes a happy engine.

Ok here’s a question. Suppose that the stroke is given and the deck height is given. Suppose further that the compression ratio requires a small shallow dish in there. What rod length and compression height should I pick if I want to maximize high rpm and high gas load reliability, understanding that a great many considerations are going into it.

For practical purposes, the 1980’s factory stock solution ended up with 56mm compression height, 150mm rod length with 78.9mm stroke and 100mm bore. What would be today’s solution with new piston technology and higher than stock rpms but with the same bore and stroke?

[Brian P]

Whatever fits in the engine.

Longer rods mean less side thrust for a given torque but you can accomplish that or more (at the OEM level) by moving the crankshaft centerline off the cylinder centerline "desaxe", and many newer engine designs do this.

Longer rods on an inline-four reduce the second-order vertical vibration component but probably not by enough to eliminate a demand for balance shafts where the situation warrants.

Within practical limits there is usually little choice in the rod ratio and the few percent that you can change it means the practical differences are background noise, hence "whatever fits".

The current trend towards wanting higher thermal efficiency at the OEM level because of CAFE is pushing things towards narrower bore, longer stroke, desaxe to reduce friction a little, higher compression, direct injection, more tumble in the intake charge to promote faster combustion, etc. The desire to keep the engine compact despite the longer stroke means minimizing the deck height and that means shorter rods. How much shorter, "whatever fits", the side effects are noise compared to the effects of the other changes. The new-generation Toyota "Dynamic Force" engines are worthy of study.

[ptuomov]

A lot of compression heights and a lot of rod lengths fit. Which ones to pick? Does the piston skirt length matter, along with allowable piston weight?

[David Redszus]

Whatever fits in the engine.

Longer rods mean less side thrust for a given torque but you can accomplish that or more (at the OEM level) by moving the crankshaft centerline off the cylinder centerline "desaxe", and many newer engine designs do this.

Rod ratio does have an effect on piston side forces. Peak side forces are shown below.

Rod ratio 1.5
Thrust side = 3992 N
Anti-thrust = -3128 N

Rod ratio 2.0
Thrust side = 2892 N
Anti-thrust = -2179 N

100 N = 22.5 lbf

As Brian indicated, there are many important considerations to modern engine design beyond rod ratio.

Pin height placement is another factor to consider. The piston center of gravity and pin height will determine
the piston cross-over angle and impact force. Worn pistons are often attributed to piston rocking but actually
side loading forces and pin location are responsible.

[ptuomov]

Whatever fits in the engine. Longer rods mean less side thrust for a given torque but you can accomplish that or more (at the OEM level) by moving the crankshaft centerline off the cylinder centerline "desaxe", and many newer engine designs do this.

Rod ratio does have an effect on piston side forces. Peak side forces are shown below.

Rod ratio 1.5
Thrust side = 3992 N
Anti-thrust = -3128 N

Rod ratio 2.0
Thrust side = 2892 N
Anti-thrust = -2179 N

100 N = 22.5 lbf

As Brian indicated, there are many important considerations to modern engine design beyond rod ratio.

Pin height placement is another factor to consider. The piston center of gravity and pin height will determine
the piston cross-over angle and impact force. Worn pistons are often attributed to piston rocking but actually
side loading forces and pin location are responsible.

This is extremely interesting. What are the important data points to estimate the side loading force, especially the peak force? Are BMEP, stroke, bore, rod length? Is there a reasonable approximate geometric formula for this that takes into account the quantitatively important issues?

The tribological computation of pressure on the skirt is a whole another bag of worms, but I am just interested in getting a simple approximate formula for this side loading force. It appears to me that there are real benefits in reducing the piston side loading from moving the piston pin up, if in the above example we see about 27% reduction in the side loading. That would allow for a lot shorter skirt with the same pressure on the piston skirt and bore wall, so I am thinking this is significant.

[Belgian1979]

Keeping stroke constant is only interesting in a theoretical approach. In practice you will get a change in stroke as well on either side of the spectrum. I you change both to a longer rod and a shorter stroke, the difference become more pronounced, especially in terms of piston side loading.

[Stan Weiss]

Whatever fits in the engine. Longer rods mean less side thrust for a given torque but you can accomplish that or more (at the OEM level) by moving the crankshaft centerline off the cylinder centerline "desaxe", and many newer engine designs do this.

Rod ratio does have an effect on piston side forces. Peak side forces are shown below.

Rod ratio 1.5
Thrust side = 3992 N
Anti-thrust = -3128 N

Rod ratio 2.0
Thrust side = 2892 N
Anti-thrust = -2179 N

100 N = 22.5 lbf

As Brian indicated, there are many important considerations to modern engine design beyond rod ratio.

Pin height placement is another factor to consider. The piston center of gravity and pin height will determine
the piston cross-over angle and impact force. Worn pistons are often attributed to piston rocking but actually
side loading forces and pin location are responsible.

This is extremely interesting. What are the important data points to estimate the side loading force, especially the peak force? Are BMEP, stroke, bore, rod length? Is there a reasonable approximate geometric formula for this that takes into account the quantitatively important issues?

The tribological computation of pressure on the skirt is a whole another bag of worms, but I am just interested in getting a simple approximate formula for this side loading force. It appears to me that there are real benefits in reducing the piston side loading from moving the piston pin up, if in the above example we see about 27% reduction in the side loading. That would allow for a lot shorter skirt with the same pressure on the piston skirt and bore wall, so I am thinking this is significant.

Some other factors are

Piston Weight
Rod Weight
RPM

Stan

[ptuomov]

This is extremely interesting. What are the important data points to estimate the side loading force, especially the peak force? Are BMEP, stroke, bore, rod length? Is there a reasonable approximate geometric formula for this that takes into account the quantitatively important issues?

The tribological computation of pressure on the skirt is a whole another bag of worms, but I am just interested in getting a simple approximate formula for this side loading force. It appears to me that there are real benefits in reducing the piston side loading from moving the piston pin up, if in the above example we see about 27% reduction in the side loading. That would allow for a lot shorter skirt with the same pressure on the piston skirt and bore wall, so I am thinking this is significant.

Some other factors are

Piston Weight
Rod Weight
RPM

Stan

Clearly, the reciprocating mass (piston assembly and some fraction of the rod), or something like that, and RPM should be there in the formula before it can pop out the final number.

Let’s throw in the pin offset for good measure to generate some internet traffic!