Engine technology thread.

[Want.FR-S]

Originally Posted by serialk11r

The reason I mention tangent circles is that while the 2 circles are not always tangent, the distance between them (along a perpendicular axis) is equal to the distance between the centers of the tangent circle (circle the rod makes at BDC) and the circle at any other crank angle. Do you understand what I mean?

The point of maximum acceleration should be at TDC always. Max speed depends on the rod:stroke ratio, but it's somewhere around "mid stroke".

OK, I have to re-read the statement several times to get this. If I understand what you are saying, you are saying the distance between those two circles (rod and crank) on the axis is between the centers of those circles, as that is clearly shown in the graph in the wiki page. I got that.

And your second paragraph is correct. The absolute value of the acceleration is at maximum. the graph shows the directional change.

[Want.FR-S]

Originally Posted by SkullWorks

I think the maximum acceleration happens around the 73.xxx degree but max speed is right at mid stroke

Hmmm.. when velocity reaches maximum, its (angular) acceleration reaches 0 and then change direction. In the 6" rod with 2" crank radius example, that is at the crank angle of 73.17615.

[Dimman]

How much of a difference in acceleration forces are there between say a 138mm rod and a 155mm rod, at 7000 rpm and 8500 rpm as an example. (86mm stroke).

[serialk11r]

The peak acceleration is a fixed ratio between 2 different rods.
I don't want to add units to this since it'll have some stupid pi^2 and crap everywhere, but the ratio of peak accelerations is 1.0267.
I think the rod length on the FA is closer to 130mm though, so for fun let's see what the ratio between a 130 and 138 rod are. Okay it's 1.01462.

So it appears that as far as rod strength is concerned, a longer rod doesn't help too much, as peak acceleration and forces are at TDC when the rod is at angle 0 with the piston. So it should be the friction and combustion that a longer rod is good for. I don't know which one is more important though. The effect of combustion is going to affect thermodynamic efficiency, but I don't have any idea of how fast the charge burns.

[bambbrose]

Originally Posted by serialk11r

bambbrose, the velocity at BDC/TDC is 0, but since acceleration is the 2nd derivative of the position, the acceleration is greatest at BDC/TDC.

The point of maximum acceleration should be at TDC always. Max speed depends on the rod:stroke ratio, but it's somewhere around "mid stroke".

I was doing my math wrong. Thanks for checking up. That makes my original Jerk discussion accurate! Funny how your mind's logic knows the right answer.

[Dimman]

Originally Posted by serialk11r

The peak acceleration is a fixed ratio between 2 different rods.
I don't want to add units to this since it'll have some stupid pi^2 and crap everywhere, but the ratio of peak accelerations is 1.0267.
I think the rod length on the FA is closer to 130mm though, so for fun let's see what the ratio between a 130 and 138 rod are. Okay it's 1.01462.

So it appears that as far as rod strength is concerned, a longer rod doesn't help too much, as peak acceleration and forces are at TDC when the rod is at angle 0 with the piston. So it should be the friction and combustion that a longer rod is good for. I don't know which one is more important though. The effect of combustion is going to affect thermodynamic efficiency, but I don't have any idea of how fast the charge burns.

C'mon... give me a g value... please... (le whining) At 7000 and 8500... We can find weights on comparable pistons, pins, and rods, then we can play with imaginary reciprocating components until pricing comes out. Without g, can't figure out the load on the rods and how thin/thick of a cross-section to resist it.

If I remember I'll try to find an article on rod design (stiffness vs cross-section of H or I beam, etc...) I have and post up some of the math stuff that I don't understand. I would imagine as we increase the rod length, stiffness becomes more important...


(I want an 8500 rpm limit on this motor...)

[serialk11r]

Okay so now that I finished my midterm and BSed my stat homework, I'll plug some stuff into Wolfram Alpha for you...

So 138mm rod, 86mm stroke gives 56.3986 mm/t^2 where 2pi t=1 revolution. 7000rpm=700/6 revolutions/second, so t= 6/700s/2pi=0.001364185s. So the acceleration is 30305m/s^2...does that look right? 3000g? :O

155mm rod, 86mm stroke would be 29516m/s^2.
130mm rod, 86mm stroke would be 30749m/s^2.

At 8500rpm, 155mm rod: 43521m/s^2 138mm rod: 44684m/s^2 130mm rod: 45339m/s^2

Just a note though, if you saw the picture of the crank+pistons in the other thread, there is no way the rod:stroke is >=1.6...
What would be ways of lengthening the rod without changing the block? Can you move the wrist pin up relative to the piston?

[serialk11r]

Okay I'm attempting to quantify the differences in combustion time and...well here goes:
http://www.wolframalpha.com/input/?i...rt{9-sin^2x}-3

Blue is 1.5 rod stroke ratio, purple is 2.5(!) rod stroke ratio. As you can see, the difference in cylinder volume vs crank angle are very small! Not to mention, the piston acceleration doesn't actually change that much either... If you reduce the 2.5 ratio to 2, the lines become so close you can barely tell them apart.
http://www.wolframalpha.com/input/?i...{3^2-sin^2x}-3 The scale isn't bad either, it shows about 2 crank revolutions.

A lot of pages back though, I computed a 5% drop in piston skirt friction and force for a somewhat small change in the ratio, so I think this suggests that rod ratio is primarily for reducing the loads on the cylinder bore, friction reduction giving a small increase in power on the order of 1% or so as well.

[SkullWorks]

To improve the rod:stroke it is common to move the wrist pin height (compression height) up into the piston sometimes even into the ring land, special considerations like an oil ring support rail are necessary when this is done but there are no other real drawbacks, besides the price of custom pistons & rods.

[SkullWorks]

From a thermal POV is the reduced friction on the skirt even worth computing, to see if there is an advantage in timing due to lower residual heat in the piston?

My gut laughs,

Is there significantly less bending load on the rod due to decreased angle?


Can you tell us the difference in Max angle VS perpendicular that the rod sees with a 130 and 138mm rods? (it's obvious you can, will you please?)

[Dimman]

Originally Posted by serialk11r

Okay so now that I finished my midterm and BSed my stat homework, I'll plug some stuff into Wolfram Alpha for you...

So 138mm rod, 86mm stroke gives 56.3986 mm/t^2 where 2pi t=1 revolution. 7000rpm=700/6 revolutions/second, so t= 6/700s/2pi=0.001364185s. So the acceleration is 30305m/s^2...does that look right? 3000g? :O

155mm rod, 86mm stroke would be 29516m/s^2.
130mm rod, 86mm stroke would be 30749m/s^2.

At 8500rpm, 155mm rod: 43521m/s^2 138mm rod: 44684m/s^2 130mm rod: 45339m/s^2

Just a note though, if you saw the picture of the crank+pistons in the other thread, there is no way the rod:stroke is >=1.6...
What would be ways of lengthening the rod without changing the block? Can you move the wrist pin up relative to the piston?

I picked 138mm because I could've sworn that is the BEAMS 3SGE rod length. But yeah, I wasn't thinking about the space limits in the H4, with the asymmetric rods, but it could depend on piston design too.

But while looking for confirmation on the 138mm I ran into this page by accident:

http://trdparts.jp/english/parts_engine-3s-ge.html

270 PS (~266 bhp SAE) @ 8600 rpm all motor, all parts formerly available through Toyota Technocraft/TRD Japan.

This was when TRD parts were legit, not re-badge so-so other parts...

[serialk11r]

Holy mother of god, 122.5Nm/L :O !

Skullworks, I think the cylinder walls absorb that heat fast enough for it to not be an issue lol. As for maximum rod angle...I don't see a very simple way yet. Give me a few minutes.

EDIT: Okay, the angle between the rod and the piston is the arcsin of "x position" relative to crank center and rod length, so the max angle is at 90* and 270* crank angle, and the angle is arcsin(crank radius/rod length).
138mm crank has maximum angle of 18.16*, 130mm crank has maximum angle of 19.32*. The angle measurement isn't really what's important though, the sine is the maximum ratio of sideloading force to bearing force, and the tangent is the maximum ratio of sideloading force to net force on the piston.

[Dimman]

Originally Posted by serialk11r

Holy mother of god, 122.5Nm/L :O !

Skullworks, I think the cylinder walls absorb that heat fast enough for it to not be an issue lol. As for maximum rod angle...I don't see a very simple way yet. Give me a few minutes.

EDIT: Okay, the angle between the rod and the piston is the arcsin of "x position" relative to crank center and rod length, so the max angle is at 90* and 270* crank angle, and the angle is arcsin(crank radius/rod length).

Because TRD.


Also they a lot of performance pistons are asymmetric now, too. Lighter and less friction.
Edit:

[Homemade WRX]

Originally Posted by serialk11r

Thanks for the responses...
Can anyone answer the rod length question?

Ok before things get any worse with rod ratio and confusion I really ought to start a thread on this. Guy's that know me or my 3MI Racing stuff on NASIOC and can see my comments on Khiem's long rod article on MotoIQ, which he did a nice job on.

I'll be going through the bottom end combinations and publishing the data when parts come to market...maybe before.