# Double Acting Steam Engine



## Foozer (Jul 2, 2009)

Looking at steam chest designs, as many a variety as one could ask for yet all do the same basic function. Steam in, steam out. What I cant seem to find is the answer to, is how the difference in piston surface area is compensated for. Given the same allowance of steam, one side with full face should exert more force than the other which is reduced in surface area by the piston rod. In a model this is not an issue.

In full size applications is the amount of steam differentiated between the two sides of the piston to equalize the power stroke over the 360 degree rotation of the crank.

Making models is only half the adventure, how they truly operate is also part of the travel.

Thanks

Robert


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## GailInNM (Jul 2, 2009)

Robert,
The short answer is that they did not try to compensate for the piston rod area.
If you listen to a video of a steam locomotive working hard on a grade, you can hear the beat. Since the cylinders are 90 degrees out of phase with each other, there are two strong beats followed by two weaker beats. CHUFF - CHUFF - chuff chuff. You can nor really hear it unless they are working hard. 

If you want to really mess your mind up also consider that on a conventional double acting engine with the piston rod - cross head - connecting rod - crank pin configuration, the crank pin rotates less than 180 degrees as the piston moves from mid stroke through top dead center and back to mid stroke. Then on the other portion of the rotation through bottom dead center it moves more than 180 degrees. 

Back to the piston rod area. Steam pumps often used this difference in force by making the piston rod much larger than would be normal for an engine. The weak stroke was used for the suction side of the pump where not as much force was needed, and the strong stroke used to force the water against the high pressure load. Many designs used the piston rod as the ram for the water pump end. 

Gail in NM,USA

Edit: here is a video clip where you can hear the uneven beat. You have to know what you are listening for, but then it is obvious.
[ame]http://www.youtube.com/watch?v=ymCOfjiKgyQ[/ame]


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## Foozer (Jul 2, 2009)

GailInNM  said:
			
		

> Robert,
> The short answer is that they did not try to compensate for the piston rod area.
> If you listen to a video of a steam locomotive working hard on a grade, you can hear the beat. Since the cylinders are 90 degrees out of phase with each other, there are two strong beats followed by two weaker beats. CHUFF - CHUFF - chuff chuff. You can nor really hear it unless they are working hard.
> 
> If you want to really mess your mind up also consider that on a conventional double acting engine with the piston rod - cross head - connecting rod - crank pin configuration, the crank pin rotates less than 180 degrees as the piston moves from mid stroke through top dead center and back to mid stroke. Then on the other portion of the rotation through bottom dead center it moves more than 180 degrees.



diminishing crank angle vs expanding crank angle, Ok my mind is tweaked



> Back to the piston rod area. Steam pumps often used this difference in force by making the piston rod much larger than would be normal for an engine. The weak stroke was used for the suction side of the pump where not as much force was needed, and the strong stroke used to force the water against the high pressure load. Many designs used the piston rod as the ram for the water pump end.
> 
> Gail in NM,USA



Actually clever idea. little power loss pulling the water into the pump and the higher power cycle to the driver gets some what balanced by the bleed off used to pump the water, not a lot granted but someone was a thinking.

Thanks, I like to know how things work at least to the point of brain explosion. Yup the uneven harmonics come through, can see the overlapping sine waves. The out of phase higher one add support to the lower, yup it'll mess with the mind alright 

Robert


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## Blogwitch (Jul 2, 2009)

To get a totally balanced cylinder, where differential areas are balanced out, sometimes the piston rod was extended thru the opposite end of the cylinder, complete with stuffing gland, but it was not connected to anything, just there so that you had the same surface area in use on both sides of the piston.

You will also find this feature on steam pumps, so that you get a continuous steady flow rather than an unbalanced pulsing flow, where the speed and pressure in both directions are the same.

So if you see a cylinder with a rod going in and out on the opposite end of the crank end, that will be a balanced cylinder, sometimes the blank rod end is covered by a long tubular casting.


Blogs


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## tel (Jul 2, 2009)

I think Alan might touch on the subject *HERE*. You might have to hunt around to find it tho'


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## kvom (Jul 2, 2009)

Fascinating. I was wondering about this very subject myself.


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## Foozer (Jul 2, 2009)

Blogwitch  said:
			
		

> To get a totally balanced cylinder, where differential areas are balanced out, sometimes the piston rod was extended thru the opposite end of the cylinder, complete with stuffing gland, but it was not connected to anything, just there so that you had the same surface area in use on both sides of the piston.
> 
> You will also find this feature on steam pumps, so that you get a continuous steady flow rather than an unbalanced pulsing flow, where the speed and pressure in both directions are the same.
> 
> ...



If I got it right, stationary engine like a pump, the closer to a balanced condition the better. Wouldn't want the pump to try and walk off the stand, get too many "who designed this thing" queries.The additional weight is made up for with the increase in fuel efficiency. 

Interesting mover steam is, lots of oomph in those little bits of water particles. Now to hunt up and see if any designs have the cylinder off center of the crank axis. Alter the duration the steam has to work upon each side of the piston, more to the cons of such a means. I think that is what GailInNM referenced to in his post

 Tel reading this now. If that doesnt explode my brain nothing will, Thanks for the lead 


Robert


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