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

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David Redszus
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Re: Rod ratio and dwell time... doesn't make sense

Post by David Redszus »

If there were no limits with regards to engine rpm (not indicated in the above) and airflow as it would perfectly follow the piston, any engine according to you would rev to the moon, which isn't true and you know it.
In a perfect engine, air flow would be directly proportional to piston air demand or theoretical air flow volume.
That is represented by 100% Volumetric Efficiency of the entire engine.

Volumetric efficiency isn't about volume at all; it is about air mass flow and compares actual mass flow to theoretical air mass flow.
The theoretical air mass flow is given by displaced volume x rpm x air density. We do not know the actual air density in a running engine.
The parameters which affect inlet air density are several: fuel type, fuel ratio, % evaporation, temperature, pressure ratio, compression ratio, piston speed, discharge coefficient, valve geometry (size, lift, timing) and compressibility factor.

The formula used to determine VE = Actual air mass / Displaced volume * rpm * air density. Since we do not know the actual air mass, we will ignore those parameters that impact actual flow and focus on theoretical air volume flow to allow a comparison of various engine design factors such as bore, stroke, rods, rpm, valves and ports, etc.
The difference between acceleration created in which I assume 90° being the point with the highest speed would be (and based on the above number)
A. 17545 m/s²
B. 16.227 m/s²
We are not concerned with air acceleration since that is an instant value. We are concerned with
the overall effect of engine parameters which is represented by the aggregate air flow for the entire induction cycle,
including flow reversions. We have included provision for choked air flow whether at the valve or port. Therefore, it is not possible to run thr engines to excessive speeds since the choked flow will reduce air density and limit air mass flow.
Assuming that the force by which the mass is drawn/pushed into the cylinder remains the same for both, the mass of air would have to loose 8.1% in density.
Now we are into required peak energy which does not reveal the correct picture. The flow velocity and air density will change with each piston position , local temperature and pressure change; which we do not know. We can however, summarize the theoretical air volume flow including flow reversions and choked flow at valve and/or port.

Remember the OP was in regard to the effects of rod ratio. The comparison of the two engines shown above is complicated by offsetting design factors such as bore and stroke. If we model an engine with the same bore, stroke, rpms, valves, ports and cams, we can then determine the effects of of ratio independent of other engine factors.
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Re: Rod ratio and dwell time... doesn't make sense

Post by David Redszus »

Two engines with the same bore, stroke, camshaft, valves, cams, port area and rpm:

Engine 1
B = 4.0
S = 3.48
R = 5.7
cv = 717 cc
Mean piston speed = 23.6 m/s
Max piston speed = 38.6 m/s
Piston air demand =663 CFM
Max air flow = 643 CFM
Reversion flow = -80 CFM
Time * volume = 31486 CFM
Piston side force = 5442 N

Engine 2
B = 4.0
S = 3.48
R = 6.25
cv = 717 cc
Mean piston speed = 23.6 m/s
Max piston speed = 38.3 m/s
Piston air demand = 659 CFM
Max air flow = 621 CFM
Reversion flow = -83 CFM
Time * volume = 31491 CFM
Piston side force = 4803 N

The above numbers should be self-explanatory. The last number (T * V) represents the
aggregated flow through the valve for the time the valve is open. It is the area under
the air flow curve measured at the valve/port.

As can be easily seen, when engine designs are identical, the effect of rod ratio is
virtually non-existent except for piston side forces.

All other things being equal, it would take a very large change in rod length (forget rod ratio)
to produce any meaningful effect on performance. Perhaps even the difference in piston
side force could be reduced by piston in offset.
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Re: Rod ratio and dwell time... doesn't make sense

Post by digger »

A short rod exacerbates a bad / wrong port
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Re: Rod ratio and dwell time... doesn't make sense

Post by Belgian1979 »

Stan Weiss wrote: Thu Apr 19, 2018 12:46 pm 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. :shock:
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Re: Rod ratio and dwell time... doesn't make sense

Post by ptuomov »

Belgian1979 wrote: Thu Apr 19, 2018 4:43 pm
Stan Weiss wrote: Thu Apr 19, 2018 12:46 pm 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. :shock:
The Ferrari F1 designers were very skilled in making sure that the pistons didn't hit the crankshaft.
Paradigms often shift without the clutch -- https://www.youtube.com/watch?v=cxn-LxwsrnU
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Re: Rod ratio and dwell time... doesn't make sense

Post by Belgian1979 »

ptuomov wrote: Thu Apr 19, 2018 5:07 pm
Belgian1979 wrote: Thu Apr 19, 2018 4:43 pm
Stan Weiss wrote: Thu Apr 19, 2018 12:46 pm 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. :shock:
The Ferrari F1 designers were very skilled in making sure that the pistons didn't hit the crankshaft.
Don't think this was what they had in mind when employing a 2.41 rod to stroke ratio.
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Re: Rod ratio and dwell time... doesn't make sense

Post by ptuomov »

Belgian1979 wrote: Thu Apr 19, 2018 5:18 pm
ptuomov wrote: Thu Apr 19, 2018 5:07 pm
Belgian1979 wrote: Thu Apr 19, 2018 4:43 pm
According to some Ferrari apparently didn't know what they were doing. :shock:
The Ferrari F1 designers were very skilled in making sure that the pistons didn't hit the crankshaft.
Don't think this was what they had in mind when employing a 2.41 rod to stroke ratio.
I am guessing that this was exactly what they had in mind. When you have bore to stroke ratio of 2, issues come up. Even with the best piston technology that money can buy, you will need to fit in the rings and wrist pin in there, and the wrist pin needs to hold a lot of intertial loads so it can be a pencil. Likewise for the crankpin. Then you need some ratio of piston skirt length to the bore ratio to stabilize the piston assembly. And the piston needs to stay in the bore on the downstroke. Even with minimal counterweights, I think the rod length in those engines is long because the piston needs to clear the crankshaft. I think I've stayed in Holiday Inn once.
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Re: Rod ratio and dwell time... doesn't make sense

Post by digger »

Bmw produced an engine with 1.44 rod ratio with warranty turns over 7000rpm. On another engine 1.53 on it turns 8200rpm. Honda have low rod ratios to.

Sometimes people confuse something that's a byproduct of a certain application with a deliberate design goal

When the stroke is 44mm like on f1 engine it's hard to use 66mm rod that a 1.5 rod ratio gives.
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Re: Rod ratio and dwell time... doesn't make sense

Post by groberts101 »

digger wrote: Thu Apr 19, 2018 7:06 pm Bmw produced an engine with 1.44 rod ratio with warranty turns over 7000rpm. On another engine 1.53 on it turns 8200rpm. Honda have low rod ratios to.

Sometimes people confuse something that's a byproduct of a certain application with a deliberate design goal

When the stroke is 44mm like on f1 engine it's hard to use 66mm rod that a 1.5 rod ratio gives.
This isn't a dig(get it?) towards you specifically.. but you bring up a good point. Some people also blurr the lines of an engines architecture and carry it too far across as a general rule or use it as a benchmark of what's possible for any engine to achieve. Piston size(diameter) and deck height allowance has everything to do with what can be made to live at higher rpm and still carry a decent warranty.

Some good stuff posted here lately. Very informative and gets one to thinkin' about the bigger picture.
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Re: Rod ratio and dwell time... doesn't make sense

Post by swampbuggy »

Belgian 1979---you made the statement that Volumetric Efficiency DECREASES with RPM, did you REALLY mean to enter that statement ???
This is a statement made by one of the most respected cylinder head guys known to man. Above 6500 RPM's one can begin to build wave inertia (inertia i don't think was the word he used but you get the drift) and INCREASE V.E.--below that (6500 RPM) forget it. The complex exhaust systems NASCAR teams use today i guarantee you pull hard across both valves open during the overlap period to help get MORE air into the cylinder to increase V.E. And you can rest assured this is happening at higher engine speeds. Is this a fair statement Warpspeed ?? Mark H.
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Re: Rod ratio and dwell time... doesn't make sense

Post by Belgian1979 »

I’m not sure I want to keep going into this discussion as I obviously do not have the resources nor lab setups to prove anything, so a last comment:

A gas acts as a spring when subjected to a force that pulls it into another place. As it also has mass inertia and initial resistance to change apply as well. This works in both directions as once that mass of gas moves it wants to continue moving.
When you pull harder on a gass when a piston moves downward during the first 90 degrees. There will be resistance to that movement. So it will try to follow but will not follow 100% immediately. In fact I think that force pulls the molecules apart where you’ll have less mass of air left per unit of volume.

To put this in perspective and to illustrate this : on my engine i have 2 airboxes each feeding one bank. Thus the force that pulls air through my 2 mafs is sometimes high sometimes low. What you see is that the air plays a game of trying to catch up with demand withvan initial lag and a later continuing pulse. The same happens in a cylinder but at higher frequency. The difference being that certainly at lower rpm the continuing pulse just gets returned to the intake. At higher rpm this continuing pulse stays in the cyl to create the rsm effect.
Overlap is therebto increase the force to pull the air into the cylinder and this works up to the point where your intake chokes the airstream
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Re: Rod ratio and dwell time... doesn't make sense

Post by 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.
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Re: Rod ratio and dwell time... doesn't make sense

Post by Belgian1979 »

swampbuggy wrote: Thu Apr 19, 2018 10:49 pm Belgian 1979---you made the statement that Volumetric Efficiency DECREASES with RPM, did you REALLY mean to enter that statement ???
This is a statement made by one of the most respected cylinder head guys known to man. Above 6500 RPM's one can begin to build wave inertia (inertia i don't think was the word he used but you get the drift) and INCREASE V.E.--below that (6500 RPM) forget it. The complex exhaust systems NASCAR teams use today i guarantee you pull hard across both valves open during the overlap period to help get MORE air into the cylinder to increase V.E. And you can rest assured this is happening at higher engine speeds. Is this a fair statement Warpspeed ?? Mark H.
All engines have a point at which they deliver their maximum torque. At this point, the airflow is able to follow the air demand by the piston more or less correctly. After that point, VE decreases and HP will continue to rise because rpm is part of the calculation. What my point here is that a long stroke, short rod will have a lower rpm point at which your maximum torque will be produced vs a short stroke long rod engine. But you don't have to take my word for it.

If this would be an easy aspect of an engine to design, probably everyone would be able to build high powered engines without a doubt.
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Re: Rod ratio and dwell time... doesn't make sense

Post by Belgian1979 »

digger wrote: Thu Apr 19, 2018 7:06 pm Bmw produced an engine with 1.44 rod ratio with warranty turns over 7000rpm. On another engine 1.53 on it turns 8200rpm. Honda have low rod ratios to.

Sometimes people confuse something that's a byproduct of a certain application with a deliberate design goal

When the stroke is 44mm like on f1 engine it's hard to use 66mm rod that a 1.5 rod ratio gives.
Power production, even in low rod/stroke ratio engines says nothing as other factors are invovled. Unless specific details are known it is impossible to comment. For instance, Honda works with 4-valve engines. An entirely different beast.

It's also obvious that the specific dimentions imply that you cannot use certain rod combos. However, a 2.50 rod/stroke ratio means that in case of a 44 mm stroke you have a rod of 110 mm. This would leave 66 mm (+- 2.60") for piston height. That's not entirely impossible when I see some pictures of the small height of the pistons involved.

BTW as for a comparison between Nascar (detroit type US engine) and F1 : http://www.epi-eng.com/piston_engine_te ... _to_f1.htm
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Re: Rod ratio and dwell time... doesn't make sense

Post by digger »

a wide variety work perfectly well and often the ratio is just the result of a bunch of other stuff.

The induction changes due to piston speed resulting from different rods are simply much smaller than what you think

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