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

[ptuomov]

If the discussion moved to friction and ring seal thenit might be different to other rod ratio threads and be remotely interesting/useful

I agree.

[naukkis79]

Why should I care about part of the cycles air flow? Should not what matters be the trapped air / fuel mass at IVC?

Stan

Because airflow changes aren't instant, airflow signal have speed of sound and nothing enters intake runner before signal comes end of it, only pressure in runner lowers. So engine needs lots of vacuum at first filling crank degrees to start incoming flow, and it takes same amount of time time for flow to stop. So flow is not reversing when cylinder is full but when flow starts to happen at end of runner. Pressure rising in runner of course can trap air from cylinder so keeping intake runner volume as low as possible increases VE.

For accelerate flow to mach1 speed there have to be about 0.55 bar vacuum, less pressure differential means less flow speed. To achieve that engines first have exhaust that pulls vacuum to cylinder - and then there's length of intake manifold runner which present latency to flow start. So with high flowing head you need longer intake tract to tune flow at lower rpm range than with low flowing setup.

With short runner and high flow intake it's possible that cylinder fills way before BDC and flow stops - and when that happens flow won't have time to start again so cylinder filling suffers badly.

[naukkis79]

We are not concerned with air acceleration since that is an instant value.

Absolutely wrong. Air signal has speed of sound in tube, that's about first parameter you need to know when you start to tune engines.

[naukkis79]

There have been a number of OEM auto and motorcycle engines produced with 2:1 or greater rod to stroke ratios.

Then we could talk about race engines
1998 v10 3.0l Ferrari F1 engine
91.5 mm bore, 45.6 mm stroke, 110.0 mm rod, 2.41 rod to stroke ratio, 2.01 bore to stroke ratio.

Stan

According to some Ferrari apparently didn't know what they were doing.

Naturally aspirated F1-engines aren't flow constrained, they achieve max VE at max rpm. They instead are pretty much burn time limited, they need to maximize piston TDP time to be able burn at those high rpms with large flat combustion chambers. So they choose as long rod as they can with dimensions they have to have longest possible burn time near TDP.

[ptuomov]

There have been a number of OEM auto and motorcycle engines produced with 2:1 or greater rod to stroke ratios.

Then we could talk about race engines
1998 v10 3.0l Ferrari F1 engine
91.5 mm bore, 45.6 mm stroke, 110.0 mm rod, 2.41 rod to stroke ratio, 2.01 bore to stroke ratio.

Stan

According to some Ferrari apparently didn't know what they were doing.

Naturally aspirated F1-engines aren't flow constrained, they achieve max VE at max rpm. They instead are pretty much burn time limited, they need to maximize piston TDP time to be able burn at those high rpms with large flat combustion chambers. So they choose as long rod as they can with dimensions they have to have longest possible burn time near TDP.

So if they wouldn't be burn time constrained, then they'd choose a rod length that would make the piston hit the crankshaft counterweight?

[Belgian1979]

Thanks Naukkis. I wasn’t aware of that particular aspect of an F1

[naukkis79]

So if they wouldn't be burn time constrained, then they'd choose a rod length that would make the piston hit the crankshaft counterweight?

You have it backwards. For lightest engine mass you drive counterweights effective weight as high as possible, if they need to shorten rod they will shave counterweights and add their mass.

[ptuomov]

So if they wouldn't be burn time constrained, then they'd choose a rod length that would make the piston hit the crankshaft counterweight?

You have it backwards. For lightest engine mass you drive counterweights effective weight as high as possible, if they need to shorten rod they will shave counterweights and add their mass.

As always, I'm doing it wrong...

I am guessing that they are indeed using very heavy crankshafts with a lot of heavy metal. This heavy metal however will go on an inner radius to keep the balance while reducing the polar moment of inertial. I don't see why they wouldn't be at the corner solution already, that is, with the heaviest and smallest radius counterweights that they can make. And once they are at that corner solution, the piston still needs to clear the counterweight and the rod needs to be long enough to do that.

Suppose that they would have any slack in terms of rod length. As long as the heads fit, I'd guess they are driving the deck height down as aggressively as possible. The deck height and the rod length is going to be pushed to the shortest length that still makes the piston clear the counterweight, provided that the V-angle is such that the heads fit.

[David Redszus]

See the following link :
http://web.mit.edu/2.61/www/Lecture%20n ... cesses.pdf

On ram effect one can see that MIT states that this is dependant on the Sp (mean piston speed), runner length and stroke. Why would one want to mention stroke in this relationship if it didn't matter ?
We see in the next element of the dynamic effects that when talking about tuning, more specifically the Helmholtz effect the factors that influence this are runner length and volume.
Thirdly we have the choking effect which is dependant on the pressure differental that exists.

Other factors that contribute are :
- back flowing of gases
- residual gas fraction
- heat transfer losses
- fuel vaporization and air displacement effects
- water vapour displacement effect on air
- pressure drop due to friction in the intake.

When looking upon the multitude of factors involved it will not be easy to see what influences exactly what. But I think it should be clear by now that purely volumetric displacement alone is not enough to discuss the subject, but I bow my head for those with greater knowledge than I have.

Very good post, especially the link. The information is right out of Heywood's text.

One parting shot regarding rod ratio. It means nothing and should be ignored most of the time.
Apparent rod ratio effects are caused by differences in stroke, not rod length.

Engine A
Stroke....Rod ratio....Max Piston speed
3.25".........1.55..........36.2 m/s
3.25..........1.80..........35.8 m/s

Engine B
Stroke....Rod ratio....Max Piston speed
3.75".........1.55..........41.8 m/s
3.75".........1.80..........41.3 m/s

Note that when stroke length is the same, the max piston speed is virtually the same regardless of rod ratio.
Rod ratio makes very, very little difference. Stroke makes a huge difference.

The idea of piston dwell time at TDC is also to be ignored. When stroke is identical, so is dwell time.
Dwell time is a nonsense parameter: what should be considered is squish velocity, which has a
much greater effect on the combustion process.

[ptuomov]

The idea of piston dwell time at TDC is also to be ignored. When stroke is identical, so is dwell time.
Dwell time is a nonsense parameter: what should be considered is squish velocity, which has a
much greater effect on the combustion process.

And your earlier post gave some numbers that indicated that the squish velocity is very similar across different rod lengths (within reason).

[David Redszus]

We are not concerned with air acceleration since that is an instant value.

Absolutely wrong. Air signal has speed of sound in tube, that's about first parameter you need to know when you start to tune engines.

I think you have overlooked a great deal regarding air flow and the gas exchange process.

While finite air pressure waves certainly do travel at sonic velocity, that is not the same as particle velocity.
Particle velocity is what matters.

At higher speeds consideration must be given to the compressibility factor. Air is elastic and any attempt at
rapid change in velocity (in any direction), will cause a change in air density. Cylinder filling is a function of mass flow rate and
available time. Mass flow is a result of particle velocity, (not wave velocity), and air density.

Instant flow rates, or static flow rates, are of little value except to evaluate local conditions. What is necessary is
to understand the total aggregate air mass flow. The gas exchange process must consider changes in density,
available time and flow direction due to changes in pressure ratios.

[Belgian1979]

I was almost thinking you confirmed what i wrote

[Warp Speed]

Instant flow rates, or static flow rates, are of little value except to evaluate local conditions. What is necessary is
to understand the total aggregate air mass flow. The gas exchange process must consider changes in density,
available time and flow direction due to changes in pressure ratios.

I'm no highly educated guy for sure, but IMO, there is allot said right there!

[Belgian1979]

See the following link :
http://web.mit.edu/2.61/www/Lecture%20n ... cesses.pdf

On ram effect one can see that MIT states that this is dependant on the Sp (mean piston speed), runner length and stroke. Why would one want to mention stroke in this relationship if it didn't matter ?
We see in the next element of the dynamic effects that when talking about tuning, more specifically the Helmholtz effect the factors that influence this are runner length and volume.
Thirdly we have the choking effect which is dependant on the pressure differental that exists.

Other factors that contribute are :
- back flowing of gases
- residual gas fraction
- heat transfer losses
- fuel vaporization and air displacement effects
- water vapour displacement effect on air
- pressure drop due to friction in the intake.

When looking upon the multitude of factors involved it will not be easy to see what influences exactly what. But I think it should be clear by now that purely volumetric displacement alone is not enough to discuss the subject, but I bow my head for those with greater knowledge than I have.

Very good post, especially the link. The information is right out of Heywood's text.

One parting shot regarding rod ratio. It means nothing and should be ignored most of the time.
Apparent rod ratio effects are caused by differences in stroke, not rod length.

Engine A
Stroke....Rod ratio....Max Piston speed
3.25".........1.55..........36.2 m/s
3.25..........1.80..........35.8 m/s

Engine B
Stroke....Rod ratio....Max Piston speed
3.75".........1.55..........41.8 m/s
3.75".........1.80..........41.3 m/s

Note that when stroke length is the same, the max piston speed is virtually the same regardless of rod ratio.
Rod ratio makes very, very little difference. Stroke makes a huge difference.

The idea of piston dwell time at TDC is also to be ignored. When stroke is identical, so is dwell time.
Dwell time is a nonsense parameter: what should be considered is squish velocity, which has a
much greater effect on the combustion process.

Agreed on stroke effect, which is what sets the piston in motion in the first place. Taking into account a bore to compensate the loss of stroke that we were talking about earlier, the speed at which the air has to accelerate also changes. However, when you have the same deck height, you need to change other things, either piston total height which increases piston weight or rod length. When the goal is to make power in which rpm factors in I would choose rod length over piston compression height any time. The extra advantage here that the rod length brings is the reduced side force which reduce piston drag.

About dwell this : taking it to extremes and if the above discussion about not sufficient burn time for a fast revving engine, dwell time on TDC could indeed be beneficial.

[David Redszus]

About dwell this : taking it to extremes and if the above discussion about not sufficient burn time for a fast revving engine, dwell time on TDC could indeed be beneficial.

Rod length does not affect TDC dwell time in any meaningful way. It does impact maximum piston speed.