Piston speed considerations!` Ouch!

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Dave Burns
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Post by Dave Burns » 08 Aug 2004, 03:27

Hmmm...a 12" stroke @ 1500rpm gives a maximum piston speed of 78.5fps or 53.5mph, and at 19 Lbs would have a kinetic energy value of 1820 Lbs, so nearly a ton by my calculations.
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Panjandrum
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Post by Panjandrum » 09 Aug 2004, 18:32

If it weren't for the nostalgia value of recalling all this stuff I would have given up by now. But I haven't.
Dave: agree: kinetic energy about 1820 ft.lb when piston and conrod are at max velocity about half-way up the cylinder, and they are weightless.
Before the fatal fracture, when the bits are at top of stroke, their acceleration is 385g and their weight is 3.2 tons. The conrod strain is maximum and the kinetic energy is zero.
The question of weight is interesting, but once things go wrong its energy that matters. The point is not the weight, but what you do with it.
When the bits have come apart and are flying free up the cylinder they weigh 19lb, and the damage they do on the cylinder head is all about what happens to their 1820 ft.lb or so of kinetic energy.
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Post by John F » 04 Sep 2004, 05:12

Gentlemen, do you consider that if the conrod broke on the compression stroke of a diesel engine the piston deceleration could be far greater than you have used in your calculations; possibly less so on a petrol engine but still significant?
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Post by John F » 04 Sep 2004, 05:21

Plus the fact it wouldn't be connected to the crankshaft/flywheel effect; perhaps I am having a senior moment[:I]
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Panjandrum
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Post by Panjandrum » 07 Sep 2004, 15:39

The calculation of weight and deceleration works whether or not the conrod breaks. In either case, the bits (piston and so on) have a vertical velocity of zero at the top of the stroke. If they remain intact, the forces of deceleration appear in the conrod, bearings, crankshaft and to an extent the cylinder head. If they have come apart, the forces appear in the piston, valves and cylinder head on impact. This ignores the stresses in the vehicle's owner and the future downward acceleration of their bank balance.
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John F
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Post by John F » 08 Sep 2004, 04:56

Panjandrum, whether or not the conrod breaks at TDC there is no velocity and the mass is the weight of the piston which is in stasis, though theoretically weightless; however, the piston is subject to a high downward pressure of the burning fuel/air mixture. If the piston is attached to the conrod it will never hit the cylinder head. If the conrod breaks but does not disintegrate before TDC the piston will still continue to be pushed upwards to a greater or lesser degree depending on the angle of the fracture and it is still subject to the downward force of the compressed/exploding air/fuel mixture. In this latter case the piston could possibly impact with the cylinder head. This ignores the vector of forces, the theory of falling bodies and the liquification of gasses in a perfect vacuum.
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Panjandrum
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Post by Panjandrum » 08 Sep 2004, 19:45

Mass is constant.
Weight is product of mass and acceleration.
Forces acting on the piston are gravity, pressure in the cylinder (including combustion pressure on alternate strokes) and forces due to approximately harmonic motion.
The piston is weightless only momentarily at the point where the net forces acting on it are zero, somewhere about mid-stroke.
At TDC, forces on the piston are effectively acting vertically (both the compression/combustion force and the force due to max downward acceleration). In the circumstances, acceleration due to gravity may be ignored as marginal.
There is no gas in a perfect vacuum.
The above assumes the cylinder to be vertical, and also ignores the fact that I have no personal experience of snapping conrods.
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Post by John F » 08 Sep 2004, 22:46

panjandrum, I'm glad you noticed, its the same as my bank account - empty[:D]
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Robin
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Post by Robin » 09 Sep 2004, 00:26

Heavens Gentlemen, this has turned into quite a thesis. What I can tell you is that most marine engine conrod failures occur on the exhaust stroke when they are in free flight without any braking or damping effect from compression. Except in very rare instances, the failure occurs when the engine overspeeds due to governor failure or in some cases the engine running in reverse when there is no governor operation at all.
Inspection of failed conrods usually points to reused bolts on the big end, little end fatigue at the pinch bolt or a failed piston leading to the rod exiting via the crankcase. This is because the piston generally fails under compressive load on the power stroke, wedges in the cylinder on the exhaust stroke and then the next cylinder to fire and upto 500 lbs of flywheel forces the conrod out of the cylinder wall and block with a great deal of noise and smoke. Not pleasant in the confines of a ships engine room!
The really big engines, not in general service these days were too ponderous to have this problem. It really only occured on 'high speed' diesels - upto 1500 rpm max!!!
I have really enjoyed the responses and discussions. Robin
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Post by John F » 09 Sep 2004, 05:26

Robin, with your experience of large diesel (ships)engines you might be able to answer a question that has plagued me for at least 40 years; in the early '60's I sold a very large radial drill to a company called Newbury Diesels who manufactured ships diesel/multi fuel engines and on the engine governance system they had a critical vibration feature that prevented the engine running at a certain narrow band of speeds. Obviously, the engine passed thro' this speed band as it speeded up or slowed down but I always assumed that this wasn't a problem as the transition was relatively quick and that the damage to the engine would take place only if the engine ran at this critical speed for a reasonable time. Would you care to surmise what would have been the end result of running the engine for a long period at the critical revs? I worked at Petter Engines for 12 years but even the slow speed engines ran at 1500 rpm so we never had the critical vibration problem.
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Panjandrum
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Post by Panjandrum » 09 Sep 2004, 14:34

Robin,
It has been fun.
Speaking of large marine engines, the biggest were on the Titanic (built by my grandfather and his brother). OK they were steam engines, but with cylinders of 52", 84" and two by 97" diameter and a stroke of 75" they were quite awesome. Delivering 15,000 bhp at 75 rpm.....
Gerry
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Post by Robin » 09 Sep 2004, 23:58

John, the problem of critical vibration plagues every rotational machine to a greater or lesser degree and the larger the rotating or oscillating masses the greater the consequential effect, damage or failure. It is in effect why an engine has it's sweet spot, my Xant has it about 3100 rpm whereas another one may be a hundred or two either side.
Most large 'diesel' marine engines are built around a huge and rigid bed frame that houses the crankshaft. In the earlier engines the reciprocating masses were not fully counterbalanced. The sheer mass of the components and relatively low rotational speeds enabled the unit to absorb the alternating upward and downward forces. Quite acceptable if the engine was mounted on a concrete foundation and driving a pump or a dynamo/alternator. This was most probably the case with the Newbury engine which was fairly conventional in construction. Three parts. A top, a middle and a bottom. They would have been suited to variable speed installations and thier smaller units were popular in everything from small river tugs to narrow boats. Indeed they are now much sought after.
However in some instances if installed in a ship, this would cause harmonic or sympathetic vibrations to build up in the hull and the whole ship would start to vibrate. This could become critical to the structure and if this was allowed to continue the whole installation in theory could become unstable and cause fractures in the hull.
Several methods were adopted to reduce harmonic risk but mostly engines installed in ships were tested to ensure they did not cause this problem. Differing firing orders, counterbalance shafts and opposing piston designs were but a few.
It is also why many steam piston engines were installed horizontally. Forces would then act along the bed instead of vertically. This being the more rigid installation. It was why the engines in the Titanic were reputedly so smooth too Panjandrum.
Going back to my early training I was introduced to large engines which were awe inspiring. A decoke was being undertaken and the cylinder heads were off, scaffolding was in place and the engineers were removing the carbon with shovels and wheelbarrows!
In short John, if left to vibrate they would have simply broken up or jumped off their mounts.
To add further complications the crankshafts were constructed from several parts all either bolted or interferance fits. I have seen shafts that have sheared and continued to rotate and if it is the part that is not driving the governor then it could just speed up until failure. In these engines each cylinder has it's own dedicated injection pump. Hope that helps solve a forty year mystery
[^] Robin.
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Post by John F » 11 Sep 2004, 01:15

Thanks Robin, the reason I asked was because I had worked on quite large Mirrlees engines and this problem never arose; mainly because (I now think) they were used in industrial applications, pumping & generating where they were in very large foundations and tended to run at one speed whereas a ships engine has to be capable of a speed range. The size of a bed plate in a ship may well secure the engine but the vessel itsself is floating in a liquid and can vibrate far more than if it were in concrete. All the Mirrlees had separate injector pumps and in fact some engines had two per cylinder as they were dual fuel. Even the smaller Nationals and Petters had a pump per cylinder. Its a bl**dy long time ago but I seem to remember each Mirrlees had a diesel donkey engine running a compressor to charge the air receiver to start the main engine and we had to "bar" the flywheel round to the correct position to start.
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Post by Robin » 11 Sep 2004, 02:45

John, the donkey engine arrangement for compressors was a common approach and it was surprising how often these engines failed. The bar in the flywheel, Yes, I remember it well! Some of the engines had the compressor on themselves so to speak so if the engineer forgot to recharge the cylinders then it was repaid with an hour or two on the hand pump!!! Oh, how wonderful these old engines are. Have a look at http://www.internalfire.com/
Did you ever come across the explosive charge? I cannot imagine how much strain it put on the engine. More popular on the continental engines I think.
You mention the idea of the boat bouncing in the water. I once had to set up a cruiser to Lloyds specs with twin Perkins installed. When set to the idle speed Perkins data the sides of the cruiser moved in and out so much it caused waves with white tops! I got the certificate signed and reset the idle speed up by 30 rpm. All was then peaceful. Robin
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Post by John F » 11 Sep 2004, 04:04

Robin, I used explosive charges (blank cartriges) on tractor engines but in a Senior Moment can't remember whether it was on Oliver or Field Marshall or the Oliver Field Marshall; never used them on any thing larger; from your question I gather you did.
Petters had a diesel engine starter called the Handraulic in which you manually pumped up pressure in a hydraulic reservoir attached to the engine and when you pulled the control lever it shot a rack across a pinion attached to the crankshaft a fearsom rate of knots to start the engine; I think it gave you 4 impulses before the pressure was exhausted. Mind you, if you were drifting onto some rocks and the engine was sulky it made for quick,strong, right arms. The pressure reservoir was awesome, if I remember it was about 8" diameter with about 3/4" walls and had a screwed cover with a very fine thread, about 15 tpi x 1.5 long. As well as this it had 4 equispaced safety bolts. They had this set up in the Experimental area test house on an engine and one night Staines was alerted by a loud bang but from whence no one knew. Come the morning there was a neat round hole through the 3" thick perforated zinc covered soundproofing, same in the roof and I don't think the cover was ever found. The cover must have weighed about 3lbs.
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