Piston speed considerations!` Ouch!

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Robin
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Piston speed considerations!` Ouch!

Post by Robin » 13 Jul 2004, 02:10

I am a bit too long out of college to remember exactly how to calculate mass due to deceleration but I recall a particular exercise on a 'small' high speed marine engine at 1000 rpm when an alloy piston at the point of reversing direction at the end of the stroke weighed in at just over 1 ton imperial!
Now the 1.9 diesel has a stroke of 88mm and produces max hp at 4000 rpm. This equates to 66 strokes per second, a linear piston speed of 13.12 miles/hour (?) and an induced mass at the point of reversal at stroke end of **** and that is where my maths falls apart. Plus I do not know what a 1.9 piston actually weighs.
So next time we hit the governor limit think about those poor pistons belting up and down plus the inertial weight of the car bearing down on the four wheel nuts as we howl round the corner and the one<u></u> bolt holding the lower suspension link together.
I am ever thankful that the designers at Citroen are better at maths then me!
R

Dave Burns
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Post by Dave Burns » 15 Jul 2004, 00:59

First of all piston speed is not linear, because they are conected to the crankshaft the pistons are traveling at their highest speed exactly half way through the stroke, for the rest of the stroke they are either accelerating or decelerating, the piston rapidly decelerates to zero at the end of each stroke and doesn't simply start back in the other direction at whatever speed the engine is turning at, that would be physically impossible.
One stroke is one working length of the bore travelled i.e in one direction only, or the distance between the big end journal centres measured at 180 degrees of rotation.
The four cycles of the fourstroke engine, (1)induction, (2)compression, (3)power and (4)exhaust is two complete engine revolutions, so thats approx. 133 strokes (up and down) per second @ 4000 rpm.
At 4000 rpm the piston travels at its fastest at around 60 feet per second or a lowly 41 mph by my reckoning.
Dave

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uhn113x
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Post by uhn113x » 15 Jul 2004, 12:50

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">The four cycles of the fourstroke engine, (1)induction, (2)compression, (3)power and (4)exhaust is two complete engine revolutions, so thats approx. 133 strokes (up and down) per second @ 4000 rpm.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Or, <b>suck-squash-bang-blow !</b>

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batwad
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Post by batwad » 15 Jul 2004, 16:34

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by uhn113x</i>
<b>suck-squash-bang-blow !</b>
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
What you get up to in your own time is your own private business, and I'd rather you kept it that way [:o)]

Robin
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Post by Robin » 16 Jul 2004, 21:55

So Dave what does a piston weigh at the point where it changes direction? I accept your principle but inertia does affect the piston even though as you rightly say it is accelerating and decelerating during the stroke. Plus we have the weight of the con rod too! We would have to take the average linear speed and I am not saying my maths are correct by any stretch but I just recalled the example from college after hearing of an engine being worked on, a marine unit, losing it's piston and head after a governer failure. Mind you the piston weighs in about 14 lbs so 1500 rpm was a bit much for it!!
Any further thoughts? R

tomsheppard
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Post by tomsheppard » 17 Jul 2004, 02:31

BMW's new V10 has steel coated pistons running in an alloy bore without rings at 20 M/s.
As the piston is changing direction, its weight will be static.
If the con rod broke at half way down, this would give the biggest bang.

Dave Burns
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Post by Dave Burns » 17 Jul 2004, 16:33

If the pistons weight was 1Lb, traveling at 60 ft.sec. its kinetic energy would be 56ft.Lbs, that theoreticaly would be enough to push a 1Lb weight a distance of 56 feet, or a 56Lb weight a distance of 1 foot, so if it hit you it would be like being hit with a bag of spuds at 41 mph, painful but still not astronomic figures.
I doubt they weigh as much as that anyway.
As for the 14Lb marine piston, without knowing the stroke you can't calculate what its energy level would be, but a 6in stroke would give it about 335 ft.Lbs, so unless it had a very much larger stroke than that its a lot short of a ton.
Dave

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Panjandrum
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Post by Panjandrum » 20 Jul 2004, 18:08

It's been a long time but my sums agree with Dave on the maximum speed which is reassuring - I can't have forgotten all of it.
The force on the piston is proportional to the acceleration, which is maximum at the top and bottom of the stroke when the speed is zero.
By my rusty calculations in Robin's 1.9 deisel the piston is pulling something like 790g - 8000 times a minute.

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uhn113x
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Post by uhn113x » 21 Jul 2004, 12:46

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">quote:
--------------------------------------------------------------------------------
Originally posted by uhn113x
suck-squash-bang-blow !
--------------------------------------------------------------------------------
What you get up to in your own time is your own private business, and I'd rather you kept it that way <hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
LOL, Alex - only do this in other people's time, and it is useful for clearing blocked jets! [:D]

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fastandfurryous
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Post by fastandfurryous » 05 Aug 2004, 21:45

<quote> So Dave what does a piston weigh at the point where it changes direction? </quote>
Exactly the same as it weighs when it is stationary. It will have a level of acceleration, which is highest when it is stationary, at the end of it's stroke. It's almost SHM (simple harmonic motion) but not quite, as the angle of the conrod matters in this case.

Robin
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Post by Robin » 05 Aug 2004, 23:13

OK then, fun this one. If it still weighs the same why does a tennis ball in the face hurt so much?
Kinetic energy or momentum?

arry_b
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Post by arry_b » 06 Aug 2004, 02:43

The mass is constant, "weight" is a more esoteric concept.
The tennis ball hurts because of f=ma
The force on your face is directly proportional to the decelleration the ball experiences as it hits you.
Then Newtons laws kick in with equal and opposite forces, and the force your face feels is the same as the ball experiences (less a little bit lost from the accelleration away from the ball that your (much heavier than the ball) head experiences.
Either way - it's [B)] for you!

Robin
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Post by Robin » 06 Aug 2004, 15:08

Right, so back to my original point. The conrod fails so we now have a disconnected mass of piston and rod travelling at a speed up the bore. This then impacts with the head which is 'removed' by the forces involved. How much force is there in the travelling piston and rod? The one I am aware of weighed in at about 14lbs (cast iron) plus rod which would have been around 5lbs. So the disconnected bits would have weighed around 19lbs, the stroke would be about 12 inches and the estimated speed at failure was 1500 rpm. I think that is a bit low mind you for this engine. Traditionally built engines of this nature were massively over engineered but it was several decades old so metal fatigue would have played it's part. It was rebuilt with modern alloy pistons and helicoils in the block.
So can we calculate the forces generated by the piston and conrod? R.

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Panjandrum
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Post by Panjandrum » 06 Aug 2004, 16:48

Righty-ho then. The bits presumably came apart somewhere on the upward stroke. Not while being accelerated upwards but at, or after, the point of max speed - just after half-way up. If it came apart at that moment the bits would have been travelling at 53mph.
Ignoring the possible energy loss in the flying bits due to the energy needed to actually complete the break, and the energy loss due to compression in the cylinder, and the roughness of my calculations; the flying bits weighing 19lb hit the head at 53mph.
Technically speaking, you can't work out the force without knowing the rate of deceleration of the flying bits at the point of impact, which means knowing stuff like the plasticity of the piston and the head.
But the effect is like having a 19lb weight drop on the head from a height of 100ft.
Alternatively, as this piston would have been pulling 378g at 1500rpm, it's "weight" at that point was 3.2tons.

Robin
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Post by Robin » 06 Aug 2004, 22:00

Panjandrum, your a star. I was beginning to think my memory was playing tricks, mind you at my age it probably is anyway[:(]
The head was retained by four 1/2 inch bolts, not studs, into a cast cylinder so at 3.2 tons plus some fatigue and the general condition of old cast iron, I am glad I wasn't near the thing when it happened!
If you have watched Das Boat or seen the way marine engines are built up of singles on a longer and longer crankshaft, I have to say that head failures are far from unknown as is the parting of conrods from their crankshaft. The difference is that we can usually take a whole cylinder or piston and rod out and carry on running the engine. A jubilee clip and pad of rubber around the crank journal sorts the oil squirt problems.
Cheers all, R.