...

 

I'm not real sure I can agree with suction being a more powerful force on a piston than reciprocation. Source?

 

Im no expert but here's what common sense tells me.


High RPMs with no load means the pistons are always being pulled back down by the rod instead of pushed down, even on the combustion cycle. that load is transferred from the piston to the rod to the crank to the bearings, meaning more wear on the bearings than if the engine were under load at high RPM. Low rpms, obviously the pulling force is much less significant otherwise idling would damage the bearings. remember the increase of force with RPM is not linear, its exponential. I know for a fact this is the case with 2-stroke engines because every cycle is a combustion cycle - if theres no load on the piston at a high RPM the rod is gonna stretch. my buddy did this to a race bike of his because he was doing entire laps at RPM even when not on the throttle. Im not positive the effect is the same on a 4 cycle engine but i think im making a safe assumption that it is similar. To further support this theory you'll also notice that when most connecting rods fail, the force is a stretching fracture rather than a crushing one.

I know my engine tends to send vibrations through the car a heck of a lot more when the engine is free revved to high rpm with minimal throttle, as opposed to achieving the same RPM with quick WOT stabs. When you free-rev an engine with quick WOT stabs, you are still at least fighting the inertia of the flywheel on the upside, which is at least some form of a load.

going back to what i said earlier, if you maintain these high forces for an extended period of time (burnout) you are going to build heat in the bearings must faster than normal. What does heat eventually equal? Correct a spun bearing.

on top of this ive read things here about engines not being able to generate the same ammount of oil pressure when not under load... im assuming that has to do with the bearings which is admittedly over my head. hopefully this post helps though and im not too full of ****

 

I cant see oil pressure being problem since its just a gear driving the pump.. but perhaps the distribution of oil is affected in some way if free revving?

Personally I think blipping it occasionally is OK so long as your not blipping it into the red... of course not blipping it is "better" just like driving it like driving a car like a grandma is easier overall on a car

Holding a high RPM with no load doesnt really have much practical purpose so I would tend to just err on the side of caution. I don't think engines are designed with holding high rpm & no load situations as a high priority item

 

As I said VACUUM increases the load, given the surface area of a piston vacuum can apply a lot of tension. I am just a lowly carpenter so this math is all beyond me, but it is no small amount of force.

 

The engine would not be running, unless the combustion chamber pressure is sufficient to push the pistons down. Otherwise, where would the energy come from to overcome the frictional losses in the rotating assembly?

 

However on the intake stroke one piston is relying on another cylinder's power to pull it down.

 

big touche there, but thats only going to be for some cases.

if you rev an engine to 6200 you only have load on it until you close the throttle. Once the throttle is closed and the engine is still spinning 6200, you dont have that force, obviously indicated by the engine returning to idle.

Also i think the ammount of load has an impact on this. you can probably make an engine hit its rev limiter with less than half throttle. An engine is only designed to be taken to such RPMs at full throttle.

and this is another good point, however that is also the job of the flywheel.

 

I guess I don't see how centripetal force has to do with the pistons blasting out of the engine being the force felt at the piston is mostly linear. Were you talking about the reaction force between the crank and the rod itself?

Correct me if I am wrong, but the force due to the linear acceleration would cause the highest stress at the rod bolts, not at the thickest part of the material, would it not?

Also, I am confused by you definition of "load". Are you defining it as positive pressure on the piston?

 

Could we just stick to othe tech issues, or some of these posts, with their sarcasm and insults will be deleted, and infractions will be given.

SS RRR - as usual, you are the instigator, and pushing it to the ragged edge.

 

That's true no matter how much load the engine is carrying. The is irrelavant to the discussion.

 

You are pretty much right. The big end of the rod sees not only the inertia loads from the piston assembly but inertia loads from the portion of the rod above the parting line. The stress in the cap area could be considered hoop stress while the bolts see mostly axial loads or stress when the rod is resisting tension loads.

FWIW: We throw around the terms "stress" and "pressure" sometimes interchangeably with "load". Load is a force but stress and pressure are defined as a force per unit area like lbs per square inch (psi). With a valve spring for example the spring has a certain load at it's installed height. Certainly there is stress in the wire, which the spring designer is concerned about, and a certain pressure or load/square inch the spring exerts on the area of the retainer it touches, but the valve as well as the engine builder are concerned with the load placed on the valve thru the retainer, and not pressure. The rod bolts see a load which when divided by the area of the bolt is a stress. The load can remain the same but the stress on a 3/8 diameter bolt is about 36% more for the same load as the stress on a 7/16 dia bolt.

It's only semantics, and most understand what the writer/speaker meant, but it is more useful to use the correct terms. Call it another of my pet peeves. I'll try to keep it straight in my posts to lessen the confusion.

 

Rods are not moving in a linear fasion. You also have to take into consideration the small end of the rod and wrist pin. And yes "load" is being used as postive pressure vs. resistence on the crank. I am talking about the stress on a rod when a piston changes direction.

Not if you are going by the theory that there is more strain on a rod with the intake stroke than when a piston changes direction.

 

Tech:

The intake stroke involves pulling in the air charge from the intake manifold. With max vacuum (~12psi at closed throttle decel) that accounts for about 150# of force pulling up on the piston crown. The tensile forces applied to the rod by accelerating the piston from "0" velocity at the top of the exhaust stroke will make the small forces applied by vacuum to the crown of the piston insignificant. The inertia load and a small vacuum load exists without regard to how the engine was loaded before the throttle was closed. Similarly, at part throttle (since the throttle has to be open to allow the unloaded engine to accelerate to max RPM) you might be looking at about 5psi vacuum, and that means about 60# pulling up on the piston as it drops on the intake stroke. Not much in the big picture, and no differnt than at any time the engine is running at part throttle. Again, once the piston begins its intake stroke, it doesn't make any difference. The dynamics of the intake stroke are not affected materially by the load on the engine. Same can be said about the exhaust and compression strokes..... they are relatively independant of the power stroke.

Moderation:

And, you know that you are not supposed to continue arguments about moderation in a thread. If you have soime sort of problem with anything I said, you are supposed to take it up in a PM, not continue to argue the issue in the thread.

 

Read my responses. I don't believe the intake stroke causes more strain on a rod then when the rod reciprocats. I'm just playing devil's advocate here. You should be directing this to that Lackman character.