Piston Motion Trivia

[Bob Bidonde]

TDC = Top Dead Center, and BDC = Bottom Dead Center.

Have you noticed that each piston in the engine stops for an instant at TDC, and they stop for an instant at BDC because they reverse their direction of motion? This stop and start motion creates substantial inertial impulse loads in the crankshaft.

Also, each piston accelerates from TDC to 1/2 of its stroke and then decelerates until it reaches BDC. This motion also produces inertial loading in the crankshaft, but the loading is not as impulsive as the stop - start motions at TDC and BDC.

The motion of the pistons comes from the geometry of the throws on the crankshaft, and it takes two complete revolutions of the crankshaft (720 degrees) to complete one combustion cycle in a cylinder.

Did you know that the center-line of the crankshaft is offset from the center-line of the cylinders by approximate 0.06"- 0.13" to provide a slight leverage to assure the pistons will reverse their direction at TDC and BDC?

[tinman080]

Very interesting- Theoretically the distance to any finite point can be halved, leading to the conclusion that TDC and BDC would NEVER be reached.......''The college of crackpot knowledge''

[BILL WILLIAMSON]

TRIVIA QUESTION: How many crankshaft revolutions does it take, to fire ALL 12 cylinders, in a JAGUAR V-12?
I'll post the answer on June 17th.
Bill W.

[tbirdtbird]

haha Bill that is a trick question with a very simple answer. You've been up late again, haven't you?! The answer is the same as a 1933 Marmon V-16.

What is the significance of June 17? It isn't Bastille day......

This question is a lot like asking someone, "what direction does the sun rise in YOUR neighborhood..." Oh, let's see, in MY neighborhood, yikes that makes it trickier.....

[C26Pinelake]

How come all the trivia? Did the dog run away or the television quit? ( lol ) Wayne

[Synchro909]

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Originally Posted by tbirdtbird

haha Bill that is a trick question with a very simple answer. You've been up late again, haven't you?! The answer is the same as a 1933 Marmon V-16.

What is the significance of June 17? It isn't Bastille day......

This question is a lot like asking someone, "what direction does the sun rise in YOUR neighborhood..." Oh, let's see, in MY neighborhood, yikes that makes it trickier.....

Maybe without realising it, you have put your finger on a difference between the northern and southern hemispheres. Here, the sun rises in the east just the same but travels to the west via the north. That is, it goes across the sky from right to left where-as "your sun" goes left to right. It's all part of the reason I get all "turned around" when I'm in the wrong hemisphere.

[sphanna]

It is imposible to go straight East or straight West unless you start 5400 nm South of the N pole or 5400 nm N of the S pole. And of course, you can stand on the North Pole and look left or right in any direction and you will be looking straight South.

[Jerry in Shasta]

The sun does not rise or set it is always in the same place in space.

Jus may wag at the question

[d.j. moordigian]

Originally Posted by Bob Bidonde

TDC = Top Dead Center, and BDC = Bottom Dead Center.

Have you noticed that each piston in the engine stops for an instant at TDC, and they stop for an instant at BDC because they reverse their direction of motion? This stop and start motion creates substantial inertial impulse loads in the crankshaft.

Also, each piston accelerates from TDC to 1/2 of its stroke and then decelerates until it reaches BDC. This motion also produces inertial loading in the crankshaft, but the loading is not as impulsive as the stop - start motions at TDC and BDC.

The motion of the pistons comes from the geometry of the throws on the crankshaft, and it takes two complete revolutions of the crankshaft (720 degrees) to complete one combustion cycle in a cylinder.

Did you know that the center-line of the crankshaft is offset from the center-line of the cylinders by approximate 0.06"- 0.13" to provide a slight leverage to assure the pistons will reverse their direction at TDC and BDC?

Well,..since were doing the math for "piston dwell time" and "piston speed",
also "cylinder wall loading". Lets all figure this with a .250" longer rod and
a .250" shorter rod than stock. Does it change the loading on the rod bearing?

[George Miller]

Along with all that, the number 4 rod journal has all the power from the other 3 cylinders going through it. I would say that is a heavy load.

[Fullraceflathead]

The forces placed on the Piston, Rod and Crankshaft are highest during Top Dead Center. It is during that direction change the forces far exceed the power stroke forces.
This is why Rods break at high RPM.

[40 Deluxe]

Originally Posted by Bob Bidonde

TDC = Top Dead Center, and BDC = Bottom Dead Center.

Did you know that the center-line of the crankshaft is offset from the center-line of the cylinders by approximate 0.06"- 0.13" to provide a slight leverage to assure the pistons will reverse their direction at TDC and BDC?

What about those engines that do not have any offset?

[Patrick L.]

The wrist pin is also offset, at least in modern engines.

Ever think about piston speeds between short and long stroke engines ?