The effect of engine load and vacuum to tensile failure in conrod
The effect of engine load and vacuum to tensile failure in conrod
(OP)
thread71-333643: conrod stress during each stroke and failure
Hey guys,
I need some help here. My group is preparing an engine in a test cell to investigate the effect of engine load and vacuum to tensile failure in conrod. Most of the papers that I found do not include the engine load and vacuum to be contributing to the stress experience near the conrod small end. However, the failures that we found strongly link the engine load and vacuum to be relevant especially when we really push the components to the extreme limits.
Any links to known publications will be helpful.
Hey guys,
I need some help here. My group is preparing an engine in a test cell to investigate the effect of engine load and vacuum to tensile failure in conrod. Most of the papers that I found do not include the engine load and vacuum to be contributing to the stress experience near the conrod small end. However, the failures that we found strongly link the engine load and vacuum to be relevant especially when we really push the components to the extreme limits.
Any links to known publications will be helpful.





RE: The effect of engine load and vacuum to tensile failure in conrod
I would normally think of engine load and vacuum to be counter to each other, where highest vacuum happens at high speed, no load conditions. Tensile failures in the small end might not have anything to do with "load" as load would be lowest when tensile loading was highest...although one might bring it into the discussion w/r/t transient conditions (reducing load suddenly at high speed).
There may be other search terms you could use ... I think some people had specific names for lifting off the throttle at high revs.
Another source of tensile failure in the rod small end is (of course) the piston failing to move smoothly down the cylinder bore (like when it's too hot to fit). This can be made worse by higher engine load, especially during very cold conditions, startup, and rapid+sustained increasing load transients.
RE: The effect of engine load and vacuum to tensile failure in conrod
RE: The effect of engine load and vacuum to tensile failure in conrod
Regarding the third, I expect it will be a small effect compared to inertia (for small gasoline engines at high rpm)
RE: The effect of engine load and vacuum to tensile failure in conrod
RPM is the most stressful influence on a rod and the forces are pretty nuts. Not only that but it is at the moment that the crank is trying to stop the piston and yank it back down. As the piston flies to the top of the cyl there will be very little vacuum because the piston is compressing what little air is in the cylinder into a much smaller volume. As it travels back down the cylinder the vacuum will increase as the acceleration decreases so I really doubt it would be significant at all.
More importantly even a 13 PSI pressure differential (let's say you hit redline and snap off the throttle) on an 82mm piston still only equates to 104 lbs pulling up on that piston. Even if that could be applied at the moment of maximum acceleration on the rod it would be inconsequential to the inertial forces acting on it.
If you are curious about the padding effect of the air and or combustion you need to remember that it only happens every other cycle on a 4 stroke. This means that the rod must take care of it's self on the exhaust stroke anyway.
The padding effect does counter almost every tuners assumption that running more boost or higher cylinder pressure requires stronger rods. This shouldn't be true because if anything that cylinder pressure is taking strain off the tension applied to the rod.
RE: The effect of engine load and vacuum to tensile failure in conrod
Do you have some failed rods whose fracture surfaces you can evaluate?
Attached is an image of the rod journal loading of a mid 60s 426 hemi engine at 7200 rpm, full throttle, with a chunky 800 gram plus piston.
I'd expect the tensile load in the rod up by the wrist pin end to be reduced somewhat since the inertia of most of the conrod is missing. Note there is a big tensile load (16000 lbs) at 0 degrees, the end of the exhaust stroke TDC. Whether the throttle is open or closed that primarily inertia load will be about the same. A bit past 360, the firing TDC, the load goes a little compressive. Closing the throttle would tend to make that load more nearly the zero degree, large tensile load.
So, in my opinion, closing the throttle at full power would about double the rate at which the rod is exposed to the large tensile loads (every stroke) compared to full power operation ( every OTHER stroke). But in the end, the rod is going to fail by propagation of fatigue cracks ( as most do), not from a few hard yanks.
RE: The effect of engine load and vacuum to tensile failure in conrod
RE: The effect of engine load and vacuum to tensile failure in conrod
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: The effect of engine load and vacuum to tensile failure in conrod
RE: The effect of engine load and vacuum to tensile failure in conrod
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: The effect of engine load and vacuum to tensile failure in conrod
RE: The effect of engine load and vacuum to tensile failure in conrod
RE: The effect of engine load and vacuum to tensile failure in conrod
RE: The effect of engine load and vacuum to tensile failure in conrod
Usually someone will run a rod pulsator test at some point though, and that should definitely verify rod tensile performance...but these little engines sometimes use rods with finite fatigue life too.
RE: The effect of engine load and vacuum to tensile failure in conrod
Do those "extreme limits" include the possibility of overspeed, or much increased temperature??
Can rpm be monitored precisely and quickly? If the rpm flashes sky high briefly when load goes to zero then it could be significant.
Are the rods made of metal? Or some composite or polymer?
I think of most any metal part in an engine failing by fatigue, not because 10% higher inertia loading exceeded its ultimate tensile strength.
Was the rod geometry and material chosen based on some expected loading?
If so, what stress levels were predicted in the design phase for normal operation, and when operating at the "extreme limits?"
I'd be working to better understand the nature of the rod failure(s) before looking for the last 5% or 10% of rod tensile loading.
The material properties are likely to vary more than that.
How many hours or cycles does it take to fail a rod?
Are all the failed rods "young" ?
Do any rods survive operating at the "extreme limits" once, or more than once?
Do you have any failed rods whose fracture surfaces can be investigated?
Can you post pictures of some failed rods?
Are the rods forged, or cast by some method, or ever repaired by welding?
What means are available to qualify the rod material is correct?
What
If all the failed rods, but only the failed rods, happen to have a voids or other material defects, or ugly stress raising features due to improper manufacturing right at the point of failure then perhaps the fact they failed at some slight over speed merely reflects the QC needs to cull out the rods with gross material and significant manufacturing defects.
How do the piston, rings, and cylinder look after a rod fails?
Pistons that seize hard in the cylinder sometimes are attributed to rod breakage.
What is the condition of the rod big end, its bearing, and the crankshaft after a rod failure? Big ends seizing hard to the crank journal are also attributed to rod breakage sometimes.