How many pounds does the crank pull on a rod and piston after TDC ??

[winr1]

More than a few articles talk about the crank trying to pull the rod in half yanking on the piston after TDC

The piston is trying to exit the block going eleventy billion miles a second

Not exactly how they worded it in the articles though ......




So, I thought since the slug is being slowed down more and more as it approaches TDC ...

Then it dwells at TDC for a few degrees ..... is the piston not stopped ??

I can see forces pulling on the piston downward

But, not as if the piston is moving upward and then suddenly jerked down


Am I missing something ??

( Psstt, Felony... back me up... haz cheeseburgers an schlitz beer )




Ricky.

[machoneman]

More than a few articles talk about the crank trying to pull the rod in half yanking on the piston after TDC.

Actually, it's the piston/pin/ring assembly that's trying to pull the rod in 1/2.

Long ago (and far away!) we ran out of a top-notch Chevy shop (sorry!) in Des Plaines, IL (Pro Motor Engineering) which later moved to N.C. to build NASCAR engines and off-shore race boat engines. Anyway, Jerry Baker, a real legend in the Midwest, advised that when hitting the finish line, to easily let off the gas and NOT snap the throttle closed until the rpms dropped.

More than a theory, he noted that a number of NHRA gasser and Comp/Elim SBC engines they built and serviced (these were 10,000 rpm + engines class-winning engines btw) lost a rod or two not under power but when the driver snapped off at the line. He theorized that the cushion of air above the piston left the building, so to speak, when the throttle plates closed and allowed the rod/piston assembly to free-wheel. 

We and other fellow racers from this shop did follow his advice and to my knowledge, never suffered a blown rod at the finish line (some did drop a rod but while under power).

 

[FrozenMerc]

This is good animation from that shows the rod getting stretched after TDC (and bearing loads)

I spent alot of time at the last company I worked for instrumenting and recording loads / stresses on components within running engines.  It was interesting watching the con rod loads as a function of crank position in real time.  On the dyno's you would miss some of the more interesting things that happen due to rapid throttle opening/closing transient events that often occur in the field (especially on forced induction engines), just because the dyno actuators don't move nearly as fast as ones foot (or thumb).  Of course now with today's drive-by-wire technology, those fast throttle events can be programmed out and not allowed to happen.   The 2 stroke snowmobile engines always added a wrinkle into the picture with the crankcase being pressurized. 

https://www.youtube.com/watch?v=uTpRfREYa9Q

[winr1]

Thanks guys



Ricky.

[wowens]

That animation is pretty. I wish I understood what the hell I was seeing.

[Drew Pojedinec]

To throw a wrench in the works.....
The engines here at work are two strokes.
As such they never really pull vacuum in.
The pistons are not as you normally see them, the piston carrier is attached to the piston pin.
The main part of the piston is only connected to the rod and carrier by a snap ring!

And yeah, these only spin 900 rpms, but when the piston weights 40lbs....

[BattlestarGalactic]

I just cringe when I see guys drive the finish line whapping the throttle.  Ugh.  Then the ones that use the motor to drag the car down after the pass.  Double Ugh.

I use the brake pedal to drive the finish line, not the throttle.  Then I push in the clutch and use only the brakes to slow down.  Just seems easier on parts.  Granted road race guys go ON/OFF the throttle for hours on end, so I suppose it's not as horrible as I think it is.

I don't turn stuff 10K rpm either.

[Joe-JDC]

Folks tend to forget that compression forces the piston down on the compression stroke, and the exhaust is being forced out on the exhaust stroke, so there is not as much of an issue as folks tend to make of it.  However, keeping the weight down is always beneficial and easier on the parts.  The new pistons we saw at JE Pistons are so light, it is scary, and rods are getting thinner all the time, yet handling 6-7-800hp without issues.  Joe-JDC

[RustyCrankshaft]

To throw a wrench in the works.....
The engines here at work are two strokes.
As such they never really pull vacuum in.
The pistons are not as you normally see them, the piston carrier is attached to the piston pin.
The main part of the piston is only connected to the rod and carrier by a snap ring!

And yeah, these only spin 900 rpms, but when the piston weights 40lbs....

New guy at work got in a argument with me about how 2 stroke diesels work. To make a point I walked over to a 16v92 I was getting ready to teardown, pulled all 16 rod caps and fired it off and let it idle there on the stand.

From what you are describing sounds like an engine with type of crosshead piston? EMD? Fairbanks?

If you really want to see odd pistons, look at a 2 stroke double-acting diesel. Never had the chance to work on one, but have walked through one and been able to run through the start-up and shutdown procedures.

[Heo]

Working much like a double acting steam engine?

[RustyCrankshaft]

Yes, similar to a double acting steam cylinder. The notable difference I suppose would be with the diesel both sides of the piston have it's own combustion chamber vs a a double acting steam cylinder that have 2 different sized chambers due to the temperature difference when the steam is pipped from one side to the other. At least I think that is correct with my limited steam locomotive knowledge.

With the double acting 2 stroke diesel, the top piston fires and then as it approaches BDC the opposed piston on the same rod is approaching TDC and fires pushing the upper piston back towards TDC.

I'm not sure there are any modern large displacement opposed double acting diesel's being built. I know there are a number of them still in service around the world. The one I was able to physically play with was a 30's vintage I think. I believe a number of years ago (I can't remember which country it happened in now. I might be able to figure it out from google if anyone is interested) there was a power grid crash that was VERY large. The government had an old powerplant that, while it was upgraded, still had a double acting 2 stroke and they actually used the diesel to bring the grid back online and stabilize the freq. 100 year old technology that would give the EPA (or any other eco-terrorist) a heart attack saves the day again.