94eg!

iTrader: (2)

Okay, I found an article that appears to have the information that most of us have been looking for. That info being how r/s ratio changes effect the behavior of an engine. They go into explicit detail about each quarter turn of the crank on all 4 strokes of an engine. It's VERY interesting stuff.

All this info really helps to explain why an engine-designer/head-porter would choose a specific r/s ratio as their "personal ideal".

The thing I find funny is that they consider a short rod motor to be 1.6:1 to 1.8:1, and a long rod motor 1.81:1 to 2:1. They go on to say that anything outside these parameters is considered a design screw-up. This is very odd considering a B18C, with a r/s ratio of 1.58:1, is considered by most to be a rather impressive performance engine. Perhaps they are only considering "race engine" designs...

Here's the link: http://www.stahlheaders.com/Lit_Rod%20Length.htm

Combustion Contraption

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 94eg! &raquo;</TD></TR><TR><TD CLASS="quote">
The thing I find funny is that they consider a short rod motor to be 1.6:1 to 1.8:1, and a long rod motor 1.81:1 to 2:1. They go on to say that anything outside these parameters is considered a design screw-up. This is very odd considering a B18C, with a r/s ratio of 1.58:1, is considered by most to be a rather impressive performance engine. Perhaps they are only considering "race engine" designs...

Here's the link: http://www.stahlheaders.com/Lit_Rod%20Length.htm</TD></TR></TABLE>

The article is probably geared more toward 2V per cyl V8's.

94eg!

iTrader: (2)

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Combustion Contraption &raquo;</TD></TR><TR><TD CLASS="quote">

The article is probably geared more toward 2V per cyl V8's.</TD></TR></TABLE>

Do you mean all the information in general, or just their "design screw-up" comment?

****edit: Nevermind, I just realized that the way you quoted my post answers my question.

94eg!

iTrader: (2)

Here is some more noteworthy information that actually shows piston acceleration vs r/s changes plotted in a graph (in case your mathematically challenged like me). It's also very interesting to note that the infinitely long rod situation effectively represents "mean-piston-speed" acceleration (as mentioned by aquafina earlier in this thread). But as you can tell, it's quite different from actual piston acceleration of a realistic engines represented by the other colors. If you average the acceleration rates of the mean-piston situation, you get zero. But as you can see the average acceleration rates of all the real motors are higher than zero. I guess mean-piston-speed is just an easy way to class motors...

It's also very interesting to note that max piston acceleration is always at TDC. No wonder N/A motor bottom-ends always fail at over-lap.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by e30m3performance.com &raquo;</TD></TR><TR><TD CLASS="quote">

We have asked the question - can a longer rod be used to decrease piston
acceleration and thereby allow the bottom end of an engine to reliably sustain higher
rpms?

The following picture shows how the piston acceleration curve changes as the rod
length is modified (again for an Evo III stroke of 87 mm and a constant engine speed
of 8000 rpm)



This series of curves shows that a longer rod reduces the maximum piston
acceleration. An infinitely long rod (approximated here as one that is 10 meters long)
will reduce the peak acceleration by 23% (relative to a factory Evo III configuration).
But that's as low as the acceleration can be made to go with an 87 mm stroke at
8000 rpm. As the rod gets shorter, on the other hand, the max. piston acceleration is
increased, but only at TDC. At BDC, the piston acceleration is actually reduced by a
shorter rod (at least intially). The piston acceleration curve also begins to form a
characteristic "double-hump" shape. If one were to keep making the rod shorter until
it was only as long as the crank arm radius (a shorter rod than this would prevent
the crank from completing a rotation), then the piston essentially would come to a
"sudden" stop at 90° ATDC and it would "suddenly" start moving upwards again at
90° BTDC. These sudden stops and starts lead to infinite acceleration at 90° after
and before TDC, and this is what the double-hump is starting to show. Of course this
is all pure theory, as in practice the piston and rod consume space which makes the
previous example impossible to achieve. But looking at the theoretical limits of an
engineering problem is always instructive.</TD></TR></TABLE>


Modified by 94eg! at 12:37 PM 5/9/2007

85

looking at the graph you posted, it would be nice on one hand to have a 10M rod but then rod design it self would need to be changed because last time i checked end caps and rod bolts aren't nearly as strong as the center of the rod.

im really liking the shape of that .2M rod


i just thought of something that i think would be cool! i useto have nitro/methanol RC cars that had engines reving up past 35,000 RPMs. i dont have any of them anymore but if someone does, could you measure the R/S ratio?

85

i found an RC boat engine with specs as follows

Specifications
Bore: 24.1mm
Stroke: 24.0mm
Rod Length: 43.2mm
Carburator: 14mm
Displacement : 10.95cc
RPM : 23,000 - 27,000
Horse Power: 5.0-6.0

almost a 2:1 RS

94eg!

iTrader: (2)

The pumping dynamics would be quite different for those two-stroke motors, making them kinda hard to compare to our 4-stroke counterparts...

85

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 94eg! &raquo;</TD></TR><TR><TD CLASS="quote">The pumping dynamics would be quite different for those two-stroke motors, making them kinda hard to compare to our 4-stroke counterparts...</TD></TR></TABLE>

yeah but i was just curious how the raw comparison would be

Pixel_NEM

-

Black R

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 94eg! &raquo;</TD></TR><TR><TD CLASS="quote">Here is some more noteworthy information that actually shows piston acceleration vs r/s changes plotted in a graph (in case your mathematically challenged like me). It's also very interesting to note that the infinitely long rod situation effectively represents "mean-piston-speed" acceleration (as mentioned by aquafina earlier in this thread). But as you can tell, it's quite different from actual piston acceleration of a realistic engines represented by the other colors. If you average the acceleration rates of the mean-piston situation, you get zero. But as you can see the average acceleration rates of all the real motors are higher than zero. I guess mean-piston-speed is just an easy way to class motors...

It's also very interesting to note that max piston acceleration is always at TDC. No wonder N/A motor bottom-ends always fail at over-lap.


Modified by 94eg! at 12:37 PM 5/9/2007</TD></TR></TABLE>


Nice, but..... we do have limitations on stroke and bore with the b,d,h,k motors. So..... what have we learned that is applicable to us?

85

:::BUMP:::

UltimX

iTrader: (1)

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 94eg! &raquo;</TD></TR><TR><TD CLASS="quote">Here is some more noteworthy information that actually shows piston acceleration vs r/s changes plotted in a graph (in case your mathematically challenged like me). It's also very interesting to note that the infinitely long rod situation effectively represents "mean-piston-speed" acceleration (as mentioned by aquafina earlier in this thread). But as you can tell, it's quite different from actual piston acceleration of a realistic engines represented by the other colors. If you average the acceleration rates of the mean-piston situation, you get zero. But as you can see the average acceleration rates of all the real motors are higher than zero. I guess mean-piston-speed is just an easy way to class motors...

It's also very interesting to note that max piston acceleration is always at TDC. No wonder N/A motor bottom-ends always fail at over-lap.
Modified by 94eg! at 12:37 PM 5/9/2007</TD></TR></TABLE>

What is this overlap you are talking about? this indeed is a good topic!

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 85 &raquo;</TD></TR><TR><TD CLASS="quote">:::BUMP::: </TD></TR></TABLE>

94eg!

iTrader: (2)

Overlap is when both intake and exhaust ports are open at the same time. This happens at the end of the exhaust stroke when the piston hits TDC and tends to be much more aggresive on all-motor setups. This valve overlap creates negative pressure in the combustion chamber (in relation to the crank-case) that actually tries to suck on the top of the piston. And as you can see from the graph, TDC is also where the highest piston accelerations are experienced. This is a good lenson to use very strong & properly torqued rod bolts, since this is the ONLY thing holding your bottom end together at this point...

UltimX

iTrader: (1)

thankyou. Couldn't you tune out the over lap with adjustable cam gears?

94eg!

iTrader: (2)

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by UltimX &raquo;</TD></TR><TR><TD CLASS="quote">thankyou. Couldn't you tune out the over lap with adjustable cam gears?</TD></TR></TABLE>

Overlap is a good thing for making power in an all motor applicaiton as it helps scavenge exhuast gasses out of the combustion chamber. This keeps from contaminating the incoming air-fuel charge. You definently wouldn't want to tune it out unless your running OEM rod bolts on an LS or B20...

On a side note, here is a cool picture I made in autocad. On the left is a B16B bottom end to illustrate the 1.6L stroke. The center is an ITR setup with it's amazingly short 1.58:1 ratio. And on the right is an ITR setup with a custom 2:1 rod-ratio. I just wanted to see if there was a good visual difference in how the pistons move when animated side by side. Since the view is rather zoomed out, it's kinda hard to tell but still cool none the less...



The speed of the animation really depends on your computer. It runs smoother on some than others...

UltimX

iTrader: (1)

ahh i see, makes sense although i have nothing to contribute. Nice illistration.

85

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 94eg! &raquo;</TD></TR><TR><TD CLASS="quote">Overlap is when both intake and exhaust ports are open at the same time. This happens at the end of the exhaust stroke when the piston hits TDC and tends to be much more aggresive on all-motor setups. This valve overlap creates negative pressure in the combustion chamber (in relation to the crank-case) that actually tries to suck on the top of the piston. And as you can see from the graph, TDC is also where the highest piston accelerations are experienced. This is a good lenson to use very strong & properly torqued rod bolts, since this is the ONLY thing holding your bottom end together at this point...</TD></TR></TABLE>

"suction on the piston & rod bolt strength"

im not sure if this is what you meant with the rod bolts and suction but just to clarify, even if you had a perfect vacuum in the chamber it would be nothing compared to the force thats pulling down FROM the crank.

94eg!

iTrader: (2)

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 85 &raquo;</TD></TR><TR><TD CLASS="quote">

"suction on the piston & rod bolt strength"

im not sure if this is what you meant with the rod bolts and suction but just to clarify, even if you had a perfect vacuum in the chamber it would be nothing compared to the force thats pulling down FROM the crank.</TD></TR></TABLE>

I'm not saying it's comparable to the downward force from the crank. I'm saying it's in addition to the downward force of the crank. This is always where rod bolts will fail since they are the weak link at overlap...

85

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 94eg! &raquo;</TD></TR><TR><TD CLASS="quote">

I'm not saying it's comparable to the downward force from the crank. I'm saying it's in addition to the downward force of the crank. This is always where rod bolts will fail since they are the weak link at overlap...</TD></TR></TABLE>

i see what you are saying with an addition but i cant see a couple pounds being much of an issue when there are already a few thousand lbs... but every oz counts

94eg!

iTrader: (2)

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 85 &raquo;</TD></TR><TR><TD CLASS="quote">

i see what you are saying with an addition but i cant see a couple pounds being much of an issue when there are already a few thousand lbs... but every oz counts</TD></TR></TABLE>

The important difference (which I failed to mention) is that when TDC occurs on the compression stroke, there is massive cylinder pressure pushing against the piston which counteracts the pull of the crank and the upward force of the piston. This keeps the rod bolts from being stressed at that point...

85

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 94eg! &raquo;</TD></TR><TR><TD CLASS="quote">

The important difference (which I failed to mention) is that when TDC occurs on the compression stroke, there is massive cylinder pressure pushing against the piston which counteracts the pull of the crank and the upward force of the piston. This keeps the rod bolts from being stressed at that point...</TD></TR></TABLE>

i understand that, so the exhaust stroke into the intake stroke as being where the rod bolts get stressed.

The more i think about it i cant help but wonder why the hell we are still using piston engines... they are so inefficient! up down up down up down, so much wasted energy.

there was a new style rotary engine that came out about a year ago (i think) that had 24 combustion chambers and ran on Diesel w/o a turbo. it made somewhere around 50HP but was only about 8" dynamiter and 6 inches tall. having 3 or 4 of these in a car would be so great. sure the production cost is prob a little high and hard to produce but people said the same thing about the mazda rotary.