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conrod stress during each stroke and failure
2

conrod stress during each stroke and failure

conrod stress during each stroke and failure

(OP)
Hi, as far as I know conrod is stressed by tensile, compression and bending stress.
I would be interested what stress is imposed on conrod at each particular stroke of 4-stroke engine and when is it most critical.
And at which place of shank (closer to big end or small end) is higher probability for conrod to break and why.

RE: conrod stress during each stroke and failure

highest tensile force is at TDC of the exhaust stroke. At high rpm it can be the highest loading by far.
The Geometry of the con rod determine the stress under a particular load. That includes the basic form plus every detail of how the finished rod was manufactured, also including if there are any material defects.

Small block Chevies of the 70 often showed "indications" in the notch where the rod bolt head seats.
http://remanufactured.com/images/350ChevyReconditi...
Sometimes but much less often "indications" would appear somewhere along the I-beam, usually starting from a notch or deep scratch left over from original manufacturing, or a notch filed into the I-beam as a means to "number" the rod during a previous rebuild by a badly informed mechanic.

Studebaker issued a factory bulletin about 6 cylinder rods that were breaking in service. The break originated in the rod bolt head notch.

Harley 74 con rods often showed indications in 2 or 3 particular regions of the opening machine in the forked rod. Sometimes but much less often "indications" would appear somewhere along the I-beam, usually starting from a notch or deep scratch left over from original manufacturing.

Rods modified "for racing" sometimes end up with severe stress concentrations and may end up "weaker" than well prepared "sotck" rods.
http://www.linkcycles.com/ebaypics/IMG_1087.jpg

The "indications" can range from barely detectable with wet method mag particle inspection, to so deep as to be visible with the naked eye, to one end of an ugly shard among the battered remnants of a rod that finally failed catastrophically due to fatigue.



RE: conrod stress during each stroke and failure

you said four stroke but you didn't mention whether it was turbocharged. rod tensile loading is negligible on some turbocharged 4-stroke engines.

RE: conrod stress during each stroke and failure

(OP)
Thanks for remark, interesting rods for racing...
Regarding stress, yes, that was my assumption - highest tensile stress at TDC of exhaust stroke, but isn't compression stress at TDC of compression stroke higher due to high pressure on piston? What about stress at BDC? And why should be there negligible tensile stress in turbocharged engine?

RE: conrod stress during each stroke and failure

regarding the turbocharged one, because there is always pressure pushing the piston down.

In a gasoline engine at high rpm, tension at TDC-exhaust is a high number. cylinder pressure is not typically very high on a gasoline engine, so lower magnitude stress due to firing at TDC-compression when compared to a turbodiesel which may have double the peak cylinder pressure. The acceleration-related force which causes tension at TDC exhaust also exists at TDC-firing and counteracts the firing load.



RE: conrod stress during each stroke and failure

4 stroke TDC exhaust.
If it was possible for boost to pressurize the cylinder well before TDC (when the intake has not yet opened) and through, during and after TDC when the ex valve is open as well, 15 psi would exert less than 200 lbs force on a 4 inch diameter (12.6 in^2) piston.

According to this online calculator ( http://2.3liter.com/Calc2.htm#PistSpeed ) a stock 1976 350 Chevy (3.48 " stroke, 5.7" long rod) piston at 5000 rpm experiences about 1600 gs.

If a stock piston with pin and rings weighs 650 grams (1.4 lbs) or so then at 5000 rpm the rod has to exert 1600 x 1.4 = 2300 lbs to haul the piston to a stop and make it reverse direction. That force estimate is at the wrist pin centerline. The crank journal/rod big end bearing also has to haul the rest of the con rod back down the hole too, and some portions of the con rod are the load path for that feat.

2300 - 200 = 2100 lbs tension taking full credit for boost pressure.
It would take 185 psi (~ 12 bar) cylinder pressure at TDC to balance the inertia load. That is in the ball park for TDC pressure on the firing stroke, before full cylinder pressure is developed at 10-20 degrees ATDC or so.

i don't have any info for heavy diesel components at 1800 rpm.

RE: conrod stress during each stroke and failure

If it was possible for boost to pressurize the cylinder well before TDC

Don't forget that exhaust pressure can be elevated on a turbocharged engine.
Take a turbodiesel operating at 900rpm (rated), with 280mm bore diameter and a recip mass around 60kg, it only takes 1.52 bar to counteract all of the acceleration. At full speed and 0% engine load you have about 0.95 bar at the end of the exhaust stroke, so not completely balanced (you're left with about 3500N to worry about). For comparison, the rod compressive force due to firing at full load (after you subtract inertia, as if it matters) is 1,130,000 N.

There are some large-ish turbocharged 2-stroke engines wherein the connecting rod large end only supports the bottom of the piston pin - it doesn't go around it - the piston doesn't fly away because there is always enough cylinder pressure to hold it down.



RE: conrod stress during each stroke and failure

ivymike, that does not sound like an auto engine. 60kg of reciprocating mass is huge. The problem is that g forces increase with the square of rpm, being 4 times as great at 1,800 rpm as at 900 rpm and 64 times as great at 7,200 rpm. For auto engines, especially normally aspirated racing engines g forces absolutely dominate the stress profile.
The two areas of greatest strain due to tensile stress in a connecting rod are the neck, just below the piston pin in the typical rod and the big end hoop. Failures due to overload, where there are no particular stress risers, usually occur at the neck where the rod simply snaps from being pulled apart and at the big end where the stretching of the pin eye, along the long axis of the rod, into an oval shape pulls the sides in where they pinch the crank throw, causing high pressure, oil film failure, high friction or seizure of the crank throw, rotation of the big end with the crank and consequent bending of the rod, breakage at the neck... followed by generalized mayhem.

For low rpm engines and for highly supercharged engines or for engines experiencing detonation or hydraulic lock, compressive forces usually dominate. Rod failure is then usually due to column collapse or bending. Buckling forces are 4 times as great in the plane of pin rotation as in the plane of the pin axis (see Euler Column Theory). So, the rod is generally wide in the plane of rotation and narrow in the axial plane. But, collapse in the plane of rotation is preferred as the direction that will cause the least engine damage. Consequently, rods are usually designed to be somewhat less than 4 times stronger in the plane of rotation.

For very high performance rods where strength to weight is more critical, design details are also more critical. This is where you really get into the questions of fatigue life and reliability; bolt notches compared to threaded bolt holes, machined and even polished surfaces vs as-forged surfaces, rod cap pinning or serrated mating surfaces, overall shape as well as contour details with respect to stress distribution, high strength vs light weight materials, cast vs forged vs billet, etc.

RE: conrod stress during each stroke and failure

that does not sound like an auto engine
I didn't see anyone say we were talking about car engines?

RE: conrod stress during each stroke and failure

If you want to see what kind of rod survives at high rpms look at high reving motorcycle engines.

RE: conrod stress during each stroke and failure

(OP)
Yes, I forgot to mention I was thinking of some "better than ordinary" car engine, let's say 4cyl 2liter gasoline, where turbo boost of 1-2bar wouldn't help a lot in decreasing of the tensile stress caused by huge acceleration of reciprocating mass at high rpm, I think.
However, I would be interested to see the progress of compressing and stretching forces (imposed on conrod), when do they swap during whole cycle...

RE: conrod stress during each stroke and failure

(correction to my post above "connecting rod large end" was meant to say " connecting rod SMALL end" - not that it matters)

so if we're talking about high speed gasoline engines, forget the turbo comments, the closed throttle max speed condition will be worse. (I think that was mentioned already)

This ought to be informative for you: https://www.forging.org/system/files/field_documen...


RE: conrod stress during each stroke and failure

Ivymike, the category is automotive and the OP was asking about car engines. That's why I pointed out that what you were describing was not automotive. But, what you said was perfectly true of very big diesels and was useful to illustrate that the balance of stress depends on the rpm range.

This also is informative: http://www.autosteel.org/~/media/Files/Autosteel/G...

In a previous discussion on rods someone posted a video showing FEA analysis of a rod going through the complete 4-stroke cycle. Does anybody have that reference?

RE: conrod stress during each stroke and failure

Here are some Youtube videos of dynamic stress analysis of con rods. In the first you can see floppiness of the rod which is greatly exaggerated, but it illustrates that bending results from inertial forces on the mass of the rod itself. The big end oscillates like a watch balance wheel while the shank whips back and forward. These forces are minor compared to the stretch and compression, but they add to those and perhaps can help initiate a collapse under compression.

http://www.youtube.com/watch?feature=endscreen&...

http://www.youtube.com/watch?v=RMLXnLqvvI8

RE: conrod stress during each stroke and failure

Quote (140airpower)

Ivymike, the category is automotive and the OP was asking about car engines.
140, I missed where the category is narrowed to automotive; and the OP didn't clarify "car engine" until after Mike's informative post about boosted 2-stroke engines. Your subsequent post was very informative also, BTW.thumbsup2

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

RE: conrod stress during each stroke and failure

I really expect designers have enough to worry about keeping the rod together under prolonged service at max power. Catastrophies initiated by rod bearing failure, unless the bearing failure resulted from Big end distortion, are the worry of the lube system group. A 4 stroke rod will accumulate 10,000,000 exhaust cycles in 50 hours at 6000 rpm. For a rod to break in operation is almost certainly a fatigue event, and that requires cyclic tensile stress. Compressive forces just won't do it.

RE: conrod stress during each stroke and failure

"Automotive" includes ships, planes, trains, etc...

RE: conrod stress during each stroke and failure

Quote (Tmoose)

Compressive forces just won't do it
agreed, when the engine is operating within design limits. I've seen rods that have been abused via knock/overboost that have yielded compressively.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

RE: conrod stress during each stroke and failure

For a rod to break in operation is almost certainly a fatigue event, and that requires cyclic tensile stress. Compressive forces just won't do it.

In the end it is not that hard to design a rod to survive either tensile or compressive loading unless you're trying to get very close to the edge and using a non-ferrous material. It has in the past been done very well without the aid of computers - doing hand calculations of stress at a few key areas was/is enough. The funny thing is - you'll have rods that break anyway, once in a long long while ... and that's when the job gets interesting.

Assuming that there was not a failure of another engine system to precipitate the rod failure (like overfueling leading to piston seizure, or a bent cooling jet, etc), manufacturing defects (forging laps in particular) and assembly errors (mostly to do with bolt tightening) have accounted for most of the rod failures I've had to sort out. Rod BOLT failures, on the other hand, can be extremely tricky.



RE: conrod stress during each stroke and failure

Scaling things massively up or down (away from passenger car sizes) is always a lot of fun. When I first started working we were given a top-to-bottom tour of the work that goes on. I recall a failure analysis guy pointing out the obvious cause for failure on a piston. I couldn't see it. I stepped over this huge hunk of metal that was in the way, looking for a mug-sized thing.

- Steve

RE: conrod stress during each stroke and failure

This summer produced some interesting results along the rod failure line in our top fuel funny cars.

One motor severe detonation caused the aluminum rod to shatter like glass but remained lightly stuck together so that the failure was missed....ie the piston still went up and down the bore. However on disassembly the rod simply fell apart into several pieces. No damage to the crank but the bearing was squished severely. Several other pistons were badly burned but nothing was kicked out of the motor. All nine lives used on this one. haha


Our car suffered many burned pistons. The last run of the year severe detonation badly burned holes in all 8 pistons. The worst one burned thru the piston and scarfed about 3/8 inch of the wrist pin away and nearly burned thru the top of rod. Detonation hit this one so hard the top of the rod under the pin showed signs of getting ready to shear or slip from compression. The bottom end got hit so hard it ovaled the big end bad enough that we couldn't get the bolts out as they had been bent severely. We had to sawzall the rod off in several pieces. Ironically it only blackened the crank slightly and a weld repair at the local crankshaft company was all that was required. We were just plain lucky with this one. It still had 150 psi oil pressure as it was shutting down. Good oil pump I guess.

RE: conrod stress during each stroke and failure

bentwings2, I assume that the shattered rod was not discovered on initial inspection, but only after disasembly, not that the engine was run with it in place? Anyway, this is an interesting occurrence. It sounds like it actually shattered in the process of disasembly and suggests some extreme embrittlement. I don't suppose it had been welded?

RE: conrod stress during each stroke and failure

There is a condition where tensile stress is higher than the exhaust stroke on a gasoline engine.

The transition between the exhaust stroke and intake stroke at full engine speed throttle closed operation can produce an even higher stress. A good number of the failed connecting rods I've seen have occurred when the driver closes the throttle at the maximum engine speed, whether to shift gears or slow down, as intake manifold pressure drops, the force required to bring the piston back down again can increase quite a bit.

RE: conrod stress during each stroke and failure

Although the final failure may occur under a particulary high stress circumstance, I'd expect those busted rods were cracked rods for many minutes before actually breaking.
It would be interesting (to me) how long it had been since the rods had last passed a wet method mag particle inspection, and what preparation such as smoothing and shot peening may have been done prior to their racing career.

RE: conrod stress during each stroke and failure

Yes, except for something catastrophic like breakage elsewhere, loss of oil film, detonation or the like, a rod will probably only break after fatigue. Then, flaws are the inevitable sites where the failure began.

RE: conrod stress during each stroke and failure

Sure fatigue builds up over a period including periods of less than maximum stress, however the fatigue weakens the rod, it will let go when the stress required to break it is exceeded. That is most likely to be when at maximum stress unless a lot of fatigue occurs between maximum stress cycles.

Regards
Pat
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RE: conrod stress during each stroke and failure

Pat, agreed, odds are in favour of ultimate failure during a moment of high stress.
Yet, I experienced failure of a cast crankshaft during cruise operation at about 2000 rpm. Examination of the failure location revealed that a fatigue crack had extended over more that 2/3 of the failed cross-section, before the final catastrophic fracture occured!

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

RE: conrod stress during each stroke and failure

I think fatigue is a process that can weaken the rod to the point where normal or even light operation can lead to failure. Used high-stress race engine parts should never be put in a street engine. If they were well engineered and stressed to their max while reliable in racing, they will most likely be subject to continued fatigue even with low stress operation. I don't recall the term used for this. A part can last an unlimited number of cycles when kept below a critical stress, but will progressively fail once the critical stress is exceeded, even at a low stress level.

RE: conrod stress during each stroke and failure

Fatigue limit

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

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