Connecting rod stress 1

[dodge8564]

Can someone please explain how load is transmitted through the drivetrain to the connecting rods?
I am in a discussion in another forum about how an engine needs stronger connecting rods for towing, even if the engine never reaches above 3500 rpm.
Another person is arguing that load is not transmitted through the drivetrain to the engine itself, and the only reason to have stronger rods and bearings is for high rpm's.

 

[patprimmer]

I can't see how anyone can fail to realise that the force is generated by expanding gas in the combustion chamber, then transmitted via the pistons, rods, crank, clutch, gearbox, tailshaft, final drive, axles, wheels, then tyres.

Slightly different for front wheel and automatic transmission drives of course.

However, full power is full power, so the stress on the rods at WOT is the same, towing or not, the only difference is that when towing, more power is required to maintain the same speed when compared to normal.

Regards
pat

 

[dodge8564]

Correct, but what I mean is that towing a load causes more pressure to be exerted on the moveable parts, because of increased resistance to forward motion. Basically I want validation ;D for (load)=(stress)

 

[andyv8]

The load on the con rods will be highest at peak cylinder pressures or maximum rpm. The peak stresses in a conrod are defined by rpm/fueling/ignition and will peak at the engines peak torque figure or max rpm. If more load is put onto the engine (ie by towing) that the engine has available torque, then it will start to slow the engine down and eventually stall it. Adding more load will not increase the peak cylinder pressures and will therefore not increase stress in the con rods.
However......
Once upon a time, BMW tried to put their V12 (car engine) into the Range Rover, but the torsional shocks transmitted through the drivetrain when driving off-road resulted in quite a few twisted cranks...... This wouln't be a problem for the con-rods though as they will only ever see the same pressures as seen during full-load testing on the dyno.

 

[patdaly]

At least from my perspective, the power transmitted is more or less compressive, hence virtually any stock rod made will handle stock type RPMs, HD useage or not. At least in my experience, the only time things come unhinged is when the RPMs climb, and the Tensile stresses exceed the stock rod ( or Rod bolts ) capabilities.

 

[mikikanazawa]

If the engine is stock, peak power is peak power so towing would not normally exert any more compressive force on the conrods than would accelerating at wide-open throttle. Towing would just increase the amount of time peak compressive forces are on the rod.

Unless, of course, the engine is capable of making torque in an RPM range where the oil pump is not providing sufficient pressure in the main and rod bearings. This could possibly cause a shock load on the rod that could buckle it.

 

[dodge8564]

Hmmm, okay, I was wrong. Darn, I hate being wrong.... Thanks for the thoughts guys, I appreciate at least knowing I was wrong.

 

[JayMaechtlen]

what about fatigue life?
in a towing app, you can be running near peak torque for much longer periods of time. (or ranging between peak torque and peak power!)
That means a lot more load cycles. Are con rods typically stressed to a level where that would be a factor- or are they in the "infinite life" part of the curve?

Jay Maechtlen

 

[mikikanazawa]

Actually I think what you mean is more load per cycle, rather than more load cycles, right? Ok unless you've selected a lower gear for towing a load while keeping vehicle speed the same.

Even at that, a compressive force on a conrod does not fatigue it. The tensile force (when the piston is yanked away from TDC) is the primary cause of fatigue. A properly designed and implemented conrod will not experience fatigue to speak of if the reciprocating mass and RPM are kept in check. Ever wonder why a conrod bolt doesn't break even though it can be cycled millions of times? The clamping force is greater than the tensile load.

 

[JayMaechtlen]

No, I mean number of cycles.
I know that the bolts, if properly sized and preloaded, see only a fluctuation in tensile stress.
The rod beam sees a full reversal, from full compression to full tension, repeatedly.
(actually, under load, will only see tension every two revolutions, on the inlet. It sees reversals at the transition from exhaust to intake, then from intake to compression, right?)
comments?
data?

Jay Maechtlen

 

[patprimmer]

Towing does tend to increase rpm by two scenerios

1) Going up hill, it is often necessary to select a lower gear to just maintain road speed.

2) Going down hill it is prudent to use a lower gear to take advantage of engine breaking and preserve the brakes in case they are needed. The rule I was taught was always decend a hill in the same gear you would need for the acent.

This might not apply to large modern cars with light trailers, but in my experience, it starts to come into play as the towed weight gets to within about 3/4 of the tow car weight, and is becomeing critical at about even weight.

Running down hill on a trailing throttle in a lower gear will tend to try to stretch the rod bolts, and the gudgeon pin area of the rod, but most modern cars have huge safety factors in this department, and rods, rod bolts and guedgons last well beyond the life of the rings and bearings and valvetrain, and these mostly outlast the body, trim and suspension.

Back to the original question. It has already been answered to varying degrees at least 3 times, but here goes again. A trailer has no direct effect on the load on the engine components, as full cylinder pressure and maximum rpm produce certain forces within the engine, no matter what. A trailer indirectly effects the engine in that the vehicle requires more power to maintain speed, so on average, the throttle will be opened more and lower gears will be used more often as it is necessary for the engine to produce the extra power to cary the extra load.



Regards
pat

 

[mikikanazawa]

Actually, properly "stretched" rod bolts see zero fluctuation in tensile load.

If they are providing 10,000 lbs of clamping force, and the highest tensile load is 9,999 lbs then the bolts receive no more "stress" than they are already preloaded to.

 

[CoryPad]

mikikanazawa,

You are incorrect. Even properly stretched fasteners will experience pretension variation. Fasteners act as parallel springs, so the amount of the joint force that is added to the pretension is a function of geometry and material properties (load introduction point, screw compliance, and joint compliance). You can learn more at:

http://www.boltscience.com/pages/basics4.htm

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[strokersix]

Rod bolts also experience bending loads, primarily due to deflection of the rod big end under tensile stress at TDC. I understand that this is one reason why bearing inserts are tapered in thickness toward the rod cap parting line, preventing metal-to-metal contact. How large of an effect this is relative to linear clamp loads I don't know. I'm sure others can answer this.

 

[mikikanazawa]

That's true, but the bending forces are "negligible" when discussing a properly designed rod and bolt system. Or so I've been told. Personally I haven't had a rod failure yet, but then I've only built around 30 engines or so. (Four bangers, displacing from 1.6L to 2.2L and all generating at least 150 HP per liter.)

 

[enginesrus]

You mention towing. The important thing happening in towing that isn't happening in a lighter vehicle, especially in mountainous areas. Is constant load. The average auto engine is not designed to put out continuous advertized HP.
Most likely they would melt down from the heat generated. Kinda like the dragster engines. Most have no cooling systems because they only put out the HP for a very short time. As far as the connecting rods needing to be any stronger. As others have said there is no reason for it. The peak pressure load is a compressive load and the con rod is acting as a column when it is applied. I'd say you would blow holes in pistons before you would over stress a stock con rod in that situation.

 

[bettonracing]

I think u guys did a good job collectively of analyzing the topic but alot of the 'basics' were overlooked.

I agree (to a certain extent) that there is more loading on the connecting rod when towing. The fact that the weight of the vehicle tends to resist the engine rotation thru the drivetrain (can be simulated by using some sort of brake to hold the flywheel from turning) and that the same pressure (let's assume peak cyl pressure) is being applied from the piston, there is more compressive load on the connecting rods.

However as mentioned before, the max pressure is at *max* when there is peak cylinder pressure. (actually max should be when the resistance overcomes the peak cyl pressure and the engine is slowing down @ wot.) but since conrods are usually built to handle peak cyl press + some extra, the weakest link would turn up to be the piston (and in few cases the crankshaft), also prev stated.

Conclusion: Towing doesn't justify a new set of rods, but they are put under additional strain (esp obvious when towing is compared to not towing).

my $0.02....make that $0.01...

 

[ShaunT]

I think 2 real factors that have been missed here are Detonation and Fuel grade. These go hand in hand and when wrong will kill anything quickly. Especially something that you are leaning on pretty hard.

Shaun TiedeULTRADYNE/LUNATI Arl,TX(stiede@ev1.net)

 

[bettonracing]

Those two factors more affect the piston than the con rod (even tho they do indirectly affect the rod). But nobody worries about their rods breaking from detonation (and fuel grade). They worry about burning holes in the pistons.


*see original message*

 

[unterhausen]

I don't really see the answer to original question. The answer is that if all that is varied is load, a higer load will cause higher stresses to develop in each part that transmits force from the piston to the wheels. Thus the proper answer to the origninal question is that both people in the argument were wrong.

Consider, if you will, an ideal engine with no possible failure modes and no friction and no mass. The force on the connecting rods is then the force developed by the piston in opposing a load. If there is no load, the engine quickly goes to infinite rpm and leaves it's earthly bounds. On average, the torque required by towing will be more than the torque required by not towing, and thus the average force on the connecting rods will be higher. The reason that people don't worry about this on real engines is that connecting rods are stronger than they need to be to counteract buckling.