Linear force generated by a piston of varying diameter & stroke

[timclegg54]

Hi, I'm not an engineer but I do dabble in working out solutions to engineering problems in the manufacturing environment. I am working on a project/idea at present and I need to be able to calculate the height that a single piston and cylinder, internally combusting, can project a 100kg weight into the air. The conrod would remain aligned with the pistons linear movement, just like a ram, in other words, the reciprocating motion of the piston would not be converted to rotation. I am guessing that the distance would vary on the efficiency of the combustion system but there must be a sort of average that I could use to work from. The 2 variables that I can imagine effecting this calculation are the piston diameter and the length of stroke,but I'm not an expert as I already stated. I'm sure that someone out there can get the ball rolling for me and once it is, I can ask more questions. Thanks for reading and if you do, participating.
Tim.

 

[Baluncore]

You are proposing a cannon or mortar. In short, the height will be determined by the type of propellant, the length of the barrel and the aerodynamic shape of the projectile. Any piston would follow the projectile.

 

[timclegg54]

Hi Baluncore, when you say cannon or mortar is this how you refer to the system I described as this is not what I'm working on? The propellant would be petrol or similar (LPG), just like inside a car engine, and to start off, a cylinder length of 150mm and dia. of 100mm would be used to get things off the ground so to speak. Simplified, the reciprocating motion would be generated by the weight being propelled into the air, then falling back onto the piston (not literally you understand), creating another compression cycle. Does this help you get your head around what I'm looking for?
Thanks for getting the ball rolling!

 

[timclegg54]

Just an addition to the above, The piston stroke would be limited and so could not fly out of the end of the cylinder, it would be the weight that would "leave" the system.

 

[Baluncore]

If an object is thrown in the air as a result of an explosion, as you suggest, then this forum needs to make sure it is not helping you to build a dangerous device or a weapon launcher.

The design of the device you propose is really analogous to that of a grenade launcher or mortar.
What is the 100 kg payload and why do you want to throw it up ?

I think you will have real problems retaining the piston while still being able to throw such a mass. You will need something like an air cushion to decelerate and catch the piston above exhaust ports in the cylinder wall. That makes it quite difficult to launch the mass. If the projectile was the “piston” you would be building a gun.

There are some excellent “bouncing cylinder” pile drivers that have a piston on the pile and a cylinder that is dropped onto the piston. Diesel fuel is injected by the fall of the cylinder so it will compress and ignite just before contact. That prevents metal to metal contact. The mass of the cylinder has it's momentum reversed each time which doubles the force applied to the pile.
For an inverted form, a bit like what you describe see; http://en.wikipedia.org/wiki/Pile_driver#Diesel_hammer

Then again, maybe you are considering something like a free piston engine.
http://en.wikipedia.org/wiki/Free_piston_engine

There have been grenade launchers powered by liquid fuels and even by steam.
http://en.wikipedia.org/wiki/Holman_Projector

 

[timclegg54]

Well, that raised a chuckle. No I'm not looking to build a grenade launcher. I don't know how old you are or where you live but I am 59 yrs old and live in the UK. When I was about 15, the road workers would sometimes use a device that was filled with fuel. This device was used for compacting the road surface by means of a cylinder and piston. When the device was manually pushed down, on reaching the bottom of the stroke, a trigger would ignite the compressed gas and the whole device would leave the ground. It would only rise up a few inches but would then fall to the ground and start the cycle over again. As long as you pulled the trigger at the right time, it would continue to launch itself into the air. I know I shouldn't have but I did have a go with one. This is the sort of thing I want to build. I'm looking to raise the weight about 300 to 500mm into the air. I suppose you could look at it as a suicide grenade launcher. That last bit was supposed to give you a giggle. It did me. I haven't looked at the links you've posted,I'm going to do that now. I just wanted to assure you there is nothing sinister about what I'm looking to do, I just don't want to spill the beans just yet. Thanks for your reply and information, I'm going to look it up right now

 

[Baluncore]

I think the type of petrol powered jack hammer / compactor you are referring to had two opposed pistons in the same cylinder. An internal flywheel with the magneto in the top of the machine was coupled to the upper piston, which also uncovered the inlet ports. The lower piston was coupled to the tool and covered the exhaust ports. Once the body falls to where compression was again available between the upper piston and against the lower piston head, air and fuel induction from above, then ignition, then exhaust below can occur. The machine was started by a permanently attached crank handle that drove the flywheel through a one way clutch.

 

[Baluncore]

The attached image is of a free piston jumping hammer without a flywheel. It was used to compact refilled trenches in the middle of the last century.

"It is, in effect, an internal combustion engine in which the work of lifting is done by power of burning fuel. The initial processes in the cycle of operations are taken by the operator, who lifts the casing, then the piston is raised and dropped, thereby drawing in a charge of fuel and air. If the casing is now lowered, the charge will be compressed, and operation of a trigger twitches the armature of a magneto to produce a spark at the plug points. This fires the charge, the casing is thrown up, the burnt charge escapes from the exhaust port and the sequence re-commences as shown. The whole thing weighs about 230 lbs."

 

[Baluncore]

Attached here is a jumping pile driver hammer, again from the middle of last century, but still in use today.

Whilst on the subject of these hammers, we should note another type seen in action some time ago, based on the internal combustion engine of compression ignition variety. Sliding on two steel guide rods is a heavy block in which a nicely machined cylinder is formed. Projecting from the anvil block, at the base of the guide bars is the piston, so placed that it will enter the cylinder nicely. Drilled through its head is a fine bore hole, and this connects with a fuel-oil pump, the lever of which is pushed over by a cam-plate as the cylinder block falls.
What happens in action is this : to start up, the cylinder block is hauled to the top of its travel by tackle, and then allowed to fall. It is, of course, full of air, and this is compressed by the piston to about one-fifteenth of its original volume, sufficient to bring it up to almost red heat. This is hot enough to ignite the fine spray of fuel oil squirted in the piston head by the action of the pump. The oil fires, and the pressure of the expanding gases drives the cylinder block up the guides, permitting the burnt gases to escape and the cylinder to fill with air again.
The actual blow is, in effect, a double one, being partly due to the falling weight, and also to the reaction from the throwing up of that weight. The jumping height can be varied by altering the amount of fuel injected by the pump, and this obviously mitigates the blow. It must be noted that no physical blow of metal on metal is struck, for there is always a small cushion of highly compressed air in between the cylinder and piston heads, enough to keep the base of the cylinder block away from the anvil block.
Jumping height ranges between 2ft. 6ins. and 6ft. and the falling block may weigh from 6 cwt. up to 35 cwt. according to size.

 

[timclegg54]

Yes indeed, the device in #8 looks like the thing that nearly smashed my rib cage when I was 15. The one below it I have to say looks impressive. to raise those sorts of weight, I can only imagine the piston must have been enormous. Hats off to the guy that machined the cylinder in the block and lined it all up.
So...How do we start to calculate the relationship between the piston size and the height it will throw a 100kg weight?
The more I study the latter system, the more I think I could use this as a basis for my problem. Maybe not have the piston and cylinder actually part company but definitely food for thought.
Thanks again!

 

[Baluncore]

You have specified that the piston must remain captive.
That will require an air? cushion to catch the piston, or the piston will spread or shatter.
Will the piston be free but retained like in the jumping compactor?
Will a piston rod be attached to the piston?

Your cylinder has to be long enough to both accelerate and decelerate the piston, (& rod ?).

The mass that can be thrown will be dependent on the cylinder area.

You have only specified a mass parameter of 100 kg.
By making a long cylinder you can throw it up maybe one mile.
By making a wide cylinder you can use a low grade fuel with thinner cylinder walls.

Also, consider inversion, is it the piston or the cylinder that moves.
Is there a safer and lower running cost solution, maybe a pneumatic cylinder or conveyor?You say jump. I ask “how high” ? How high do you really need to throw the 100 kg?
How fragile is the 100 kg? Will it be damaged with a high g acceleration?
How much space is available for the cylinder and stroke?
Is this supposed to be mobile or is it installed on a concrete base?
How heavy can the base / foundation be to prevent it burrowing underground?
How often is it required to jump?
Is it an autonomous repetition or paused cycle?
Does it have to be economic to operate?

Design involves seeing how to trade off the cost constraints against benefits.
For every element size or capability feature of the design there are three competing curves.
1. The value or final cost advantage of that feature to the operator.
2. The cost of manufacturing that sized element.
3. The cost of operating that sized element.

If you cannot proceed to constrain or evaluate those trade-offs yourself, then you will need to better identify your application.

 

[timclegg54]

I don't bloody believe it...I just spent the best part of two hours answering your questions above and was about to post and the bloody text has vanished. I'll have to repeat it all again but not til tomorrow now.
SO INFURIATING

 

[Baluncore]

Deep breathing . . . relax . . .
If you write your posts in a local document, you can copy and post it all when done. It will always be there, even when your network does notwork, or behaves like a yoyo. I'm on the end of 40 km of radio links. I don't close my replies file, so ctrl-Z always works.

 

[timclegg54]

Thanks for that. I'll post tomorrow now. I need a beer and a chill. Or is it a chilled beer I need?