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

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

Postby nickpohlaandp » Sat Aug 12, 2017 10:09 pm

So I was reading a book tonight and the writer goes into rod ratio and dwell time. I've read this stuff 100 times before, but I always read through a new book all the way... never know when you'll read something new. Well, that's exactly what happened and I'm left confused.

In this book he talks about a larger rod ratio giving more dwell time at TDC. This is how I've always heard, read, and though about it. However, he goes on to say that a longer rod will have less time at BDC and a shorter rod will have a longer time at BDC. This makes no sense to me. I'm viewing piston speed like a sine wave in my head. To me, regardless of your rod ratio or rod length, your piston speed arc from 90 ABDC to TDC to 90 ATDC is going to be an exact mirror image of piston speed form 90 ATDC to BDC to 90 ABDC. Unless I'm mistaken somewhere, this book I'm reading is incorrect. Anyone care to shine some light on this subject?

I'm always open to looking at things in a new way, but I don't see how dwell time could be different at TDC vs BDC without changing the rod length.
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Re: Rod ratio and dwell time... doesn't make sense

Postby In-Tech » Sat Aug 12, 2017 10:28 pm

Play at your hearts content. I believe Stan has part of his graphical stuff to show you time variants vs rod length. All of the math is available if you want to search for it.
http://users.erols.com/srweiss/

Rod ratio crap is generally just that, crap invented by rod and piston manufacturers to sell parts. I am not saying it doesn't play a part, just not as big a part as being claimed by the aftermarket for years. 8)
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Re: Rod ratio and dwell time... doesn't make sense

Postby modok » Sat Aug 12, 2017 10:47 pm

If a short rod makes piston spend less time at TDC, then it has to spend more time somewhere else.
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Re: Rod ratio and dwell time... doesn't make sense

Postby nickpohlaandp » Sat Aug 12, 2017 11:03 pm

modok wrote:If a short rod makes piston spend less time at TDC, then it has to spend more time somewhere else.


Logic would say that the piston speed at any crank angle would be exactly the same as piston speed at 180 degrees opposite of said crank angle. If it dwells x amount at TDC the dwell should be the same at BDC... unless my calculus has really gone downhill.
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Re: Rod ratio and dwell time... doesn't make sense

Postby Walter R. Malik » Sat Aug 12, 2017 11:11 pm

You must be assuming that the rod pendulums in a straight line which certainly is not the LOGICAL case.
The difference in motion is simple geometry of different size connecting circles.

The rod length pendulum is a part of a circle.
The stroke is twice the radius of a circle

The difference between the top and bottom of the crank circle and the pendulum of the rod length, (always the bottom of that circle), will easily show the different motions.
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Re: Rod ratio and dwell time... doesn't make sense

Postby nickpohlaandp » Sat Aug 12, 2017 11:15 pm

Walter R. Malik wrote:You must be assuming that the rod pendulums in a straight line which certainly is not the LOGICAL case.
The difference in motion is simple geometry of different size connecting circles.

The rod length pendulum is a part of a circle.
The stroke is twice the radius of a circle

The difference between the top and bottom of the crank circle and the pendulum of the rod length, (always the bottom of that circle), will easily show the different motions.


Trying to envision this... it's kinda making sense when you put it like that. I'll draw it out tomorrow. Time for bed. Thanks everyone
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Re: Rod ratio and dwell time... doesn't make sense

Postby In-Tech » Sat Aug 12, 2017 11:16 pm

modok wrote:If a short rod makes piston spend less time at TDC, then it has to spend more time somewhere else.


#-o

Yes, this has been discussed ad nauseam. No disrespect, spend some time with the math and see for yourself. Again, there is probably free crap around so you can play.
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Re: Rod ratio and dwell time... doesn't make sense

Postby pdq67 » Sat Aug 12, 2017 11:18 pm

Look at it this way..

If you can make an engine such that the rod spends from 90 to 270 degrees at BDC, you have your answer.

Like this---

270-O--O--O-90
.........l
........O
.......180

The three O's in front of 90, 180, and 270 are the rod journal and the center one is the crank!

It took me a long time to figure this out because I am dense!!

I used to have an old copy of, "Circle Track", mag that explained this in great detail..

And if the rod is infinitely long, it will dwell the same at TDC and BDC!!

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

Postby Circlotron » Sat Aug 12, 2017 11:22 pm

nickpohlaandp wrote:I'm viewing piston speed like a sine wave in my head. To me, regardless of your rod ratio or rod length, your piston speed arc from 90 ABDC to TDC to 90 ATDC is going to be an exact mirror image of piston speed form 90 ATDC to BDC to 90 ABDC.

No, they are not mirror image, surprising as that may seem.
The piston movement is only sinusoidal with an infinite length rod. The shorter the rod gets, the more pointy the sine becomes at the top of the stroke (less dwell near top of stroke) and the more blunt the sine becomes at the bottom of the stroke. This is one of the reasons a 4 cyl engine vibrates in the vertical plane - because the slowing down of the end two pistons at the top of the stroke *doesn't* exactly match the simultaneous slowing down of the centre two pistons at the bottom of their stroke. Same reason a balancer shaft can't perfectly balance a piston engine - it can only make perfectly sinusoidal anit-vibes.

What happens as the crank pin goes from 9 o'clock to 12 o'clock is the pin moves upwards and so the piston does too, but the crank pin also moves toward the centre line and this gives additional upward movement to the piston. (pointy top sine wave). From 3 o'clock to 6 o'clock though the crank pin moves downward and so does the piston of course but as the crank pin moves toward the centre line it adds a bit of *upward* component to the descending piston, same as when it did going from 9 o'clock to 12 o'clock because again the crank pin is moving toward the centre. The piston doesn't actually move up, it just moves down slower than normal.

On an engine with say a 4 inch stroke, rotate the crank to 90 deg ATDC and you will find that the piston is *more* than 2 inches down the bore. The shorter the rod the further than halfway down it will be. That alone proves that piston movement with realistically sized rods is not actually sinusoidal.
Last edited by Circlotron on Sun Aug 13, 2017 12:13 am, edited 3 times in total.

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

Postby modok » Sat Aug 12, 2017 11:30 pm

nickpohlaandp wrote:
modok wrote:If a short rod makes piston spend less time at TDC, then it has to spend more time somewhere else.


Logic would say that the piston speed at any crank angle would be exactly the same as piston speed at 180 degrees opposite of said crank angle.

no,

Stand a 6" ruler straight up, it's end is 6" off the table. Now tilt it, If you tilt it, it gets shorter in the vertical dimension, less than 6, at 45 degrees it's hardly more than 4.2" above the table

The rod does the same thing.
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Re: Rod ratio and dwell time... doesn't make sense

Postby Circlotron » Sun Aug 13, 2017 12:08 am

nickpohlaandp wrote:Logic would say that the piston speed at any crank angle would be exactly the same as piston speed at 180 degrees opposite of said crank angle.

Close. The piston speed at any crank angle is the same as the piston speed at the crank angle that is a mirror image about the centre line of the first angle. e.g it is the same for 50 deg either side of TDC or BDC. This is assuming that the main journal, gudgeon pin and bore axis are all in line, but usually at least the gudgeon is offset a little.

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

Postby RDY4WAR » Sun Aug 13, 2017 1:45 am

nickpohlaandp wrote:
modok wrote:If a short rod makes piston spend less time at TDC, then it has to spend more time somewhere else.


Logic would say that the piston speed at any crank angle would be exactly the same as piston speed at 180 degrees opposite of said crank angle. If it dwells x amount at TDC the dwell should be the same at BDC... unless my calculus has really gone downhill.


Looking at just the rod journal, you'd be correct. However, the rod/crank angle and length of the rod will dictate the piston position at each degree of crank rotation. Make a triangle from the center of the crank main journal to the center of the rod journal to the center of piston pin. With a longer rod, the distance between the piston pin and the crank main journal is longer. If the angle at the main journal is obtuse, changes to the angle at the rod journal have a greater affect on the length between the main journal and piston pin. If the angle at the main journal is acute, the opposite is true. This is true for any rod length but more so the longer the rod.

Let's take a common point. Let's say 30* ATDC and 30* ABDC and the engine is a 302 SBF, with a stock length 5.09" rod vs a 5.4" rod and stock 3" stroke.

5.09" rod
30* ATDC = .256" from TDC
30* ABDC = .145" from BDC

5.4" rod
30* ATDC = .253" from TDC
30* ABDC = .149" from BDC

Calculator I used for this: http://www.torqsoft.net/piston-position.html

You have to convert everything to metric but still useful.

What I like about a long rod engine is the use of a shorter and lighter piston and better geometry for better piston stability and less side loading. The long rod also slows down piston speed at the upper part of the cylinder slightly which reduces inertia on the crank. I like these characteristics more than the minor difference in piston position.

Sometimes the longer dwell at BDC with a shorter rod can be beneficial. If you have a good bit of velocity in the port still trying to fill up the cylinder, dwelling the piston a little longer at the bottom of the cylinder can allow it to fill up slightly more before the valve closes. If the piston starts accelerating up the bore too quickly, you could lose some ramming effect and VE may suffer. This is why rod length and stroke are important to know when spec'ing a cam for the engine.

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

Postby Belgian1979 » Sun Aug 13, 2017 2:11 am

The discussion on rod length has been beaten to death and there is still no concensus on it.

Anyway the calculations around this do in fact confirm that a piston with a longer rod dwells more at TDC. The exact consequences of the trade off of dwelling on TDC rather than on BDC are fiercely debated here.

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

Postby digger » Sun Aug 13, 2017 2:19 am

nickpohlaandp wrote:
modok wrote:If a short rod makes piston spend less time at TDC, then it has to spend more time somewhere else.


Logic would say that the piston speed at any crank angle would be exactly the same as piston speed at 180 degrees opposite of said crank angle. If it dwells x amount at TDC the dwell should be the same at BDC... unless my calculus has really gone downhill.


only if the rod length is infinite

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

Postby The Radius Kid » Sun Aug 13, 2017 4:10 am

nickpohlaandp wrote:
modok wrote:If a short rod makes piston spend less time at TDC, then it has to spend more time somewhere else.


Logic would say that the piston speed at any crank angle would be exactly the same as piston speed at 180 degrees opposite of said crank angle. If it dwells x amount at TDC the dwell should be the same at BDC... unless my calculus has really gone downhill.


No,but you're forgiven.
There's three points to an angle.
In our case,there's the wrist pin,the rod throw and the crank main.
The rod throw is where the angle changes and is measured.
When you're at BDC,the wrist pin is closest to the main.
At TDC,the wrist pin is farthest from the main.
That means as the piston accelerates away from BDC,the wrist pin/rod throw/crank main angle is in line,going to acute as it accelerates away from BDC.
Somewhere past midway up the bore,it transfers over to 90 degrees,than to obtuse angles.
As everything hits TDC,it all lines up again.
The wrist pin is now farthest from the main.
That's the difference.
My bottle of beer said so. :D
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