Newbee question about engines.

[Peter.]

Hi folks.

I have a fairly basic question about steam/air-driven engines:

Is it possible to build a self-starting, and reversing engine that will start from any crank position and start consistantly in the desired direction, using just one cylinder? If so, how is it achieved?

Pete.

 

[bearcar1]

Welcome Peter. The answer to your question(s) is yes, more or less. ??? What I mean by that is single cylinder engines utilize only one crankpin, ergo there is the possibility that the engine would come to a stop with this pin in either a top-dead-center or bottom-dead-center position. In this state the engine would be unable to self start. It would require that some sort of intervention be enacted upon the crankshaft to initiate a start. Most of the time however single cylinder engines will self start and all can be run in the reverse direction. Slide valve engines for the most part require that the timing be changed by either rotating the eccentric 90* relative on the crankshaft or by altering the valve gear geometry. There are a many variations of this as well. Oscillating types of steam engines where the cylinder itself moves in a sweeping motion on a fixed pin can be reversed by simply 'swapping' the intake and exhaust portals, either manually or through some sort of valve arrangement. I hope I have answered your questions and do not hesitate to make further inquiries, there is a lot of combined knowledge available here on HMEM and we are more than happy to share.

BC1
Jim

 

[bearcar1]

Peter, I got to thinking some more about your self starting question last evening and it occurred to me that what I said was only applicable to the double acting engines. That is steam or air admission is alternately timed to be present on both sides of the piston in the cylinder via the valve mechanism. Single acting engines only have steam or air being supplied to one side of the piston and the other gets vented to atmosphere. These types of engines as well as the oscillating types I mentioned previously, may or may not self start but will usually require a small manual assist to get them going unless they too are double acting. I am sorry for any confusion I may have caused, it is not complicated but sometimes I take for granted that others know what I am talking about and forget that sometimes I get way ahead of the learning curve in answering questions. I'm going to go and stand in the corner now. :hDe:

BC1
Jim

 

[speedhound]

Would a dake engine not be capible of this? But i surpose it does not count as a single, having two pistons.

 

[Peter.]

Well I did ask for 'single cylinder' speedhound, so I guess it would apply though I didn't consider multiple pistons in a single cylinder when I asked the question.

Thank you for the replies. I guess I should put a proper introduction in the welcome forum, but to continue this thread, although I have no interest in live steam, building an air-operated small stationary is something that appeals to me very much.

Jim: I do understand a bit about steam operation as my late father was into steam railway preservation. I did understand already that a single-acting cylinder would never self-start, I just wondered if there was some trickery by which a double-acting cylinder would manage it consistantly.

How about some kind of mechanincal restriction to stop an engine ever coming to rest at the end of a stroke?

 

[itowbig]

if the flywheel was weighted so that the piston is in the right spot on a single it would start every time but...
you would have a balance problem then

 

[speedhound]

would a spring at each end of the stroke work pushing the piston to a half way point? If anyone understands what i mean :big:

 

[cidrontmg]

It could be done in lots of electro-mechanical ways, but I guess that would be cheating :big:
Seriously, if there were some easy way to do it purely mechanically, I´d bet that such a scheme would have been invented long ago, and it would have been in use, and thus common knowledge. But the big steam engines of yesteryear needed a donkey engine, or holes in the flywheel, or some other "external" means of positioning the piston suitably before starting.

 

[Peter.]

I was thinking more of a pawl and detent on the crankshaft, such as is used to locate shift drums on motorcycles, it could have a ramp across the dead-centers so that it could never rest there - in one direction it would be pushed past, the other it would be pushed back.

For a small engine with low friction, would this not work?

 

[Dan Rowe]

Peter,
Interesting what is the reason always starting is needed? I can think of a couple of ways to accomplish it with more parts which means more maintenance.

You could use a simple cam that operates a small bypass around the supply valve. The cam would be set to open the bypass at both dead centers to give a shot of air to pass the center point.

The shaft could also be used as the bypass valve attached to the end of the shaft. A hole in the shaft or attached valve lines up with the bypass pipe at dead center to give a shove to the piston at dead center. Like a rotary ball valve that only opens at the dead centers.

I can sketch the last idea if needed.

Dan

 

[Peter.]

Hi Dan.

Good idea - and more in keeping with the engine theme.

No particular reason other than my own perception, that an ideally engine should be able to start, stop and reverse itself using just the controls. I had a Mamod engine as a child, and that required a flick to get running, I always wished it wasn't necessary.

 

[Tin Falcon]

In order for a steam air engine to self start you need a piston at or just past top dead center and the intake valve open. In order for this to happen at any point in a 360 degrees travel of the flywheel you need either 4 cylinders or two double acting cylinders so in effect each one is 90 degrees out of phase. then you will always have an open valve and a piston for the air or steam to push on.
I am not saying it is impossible to do with a singe cylinder but not likely practical.
Tin

 

[kf2qd]

A possible way to make a single cylinder steam engine self starting would be to introduce a slight imbalance into the system - like an out of balance flywheel - which would pull the piston away from TDC when at rest. This would limit speed of the engine to keep it from shaking itself to death. A low speed engine might work, much speed and it is going to shake.

3 single acting cylinders would be self starting, or 2 double acting cylinders out of phase by 90 degrees.

 

[cidrontmg]

A possible way to make a single cylinder steam engine self starting would be to introduce a slight imbalance into the system - like an out of balance flywheel - which would pull the piston away from TDC when at rest. This would limit speed of the engine to keep it from shaking itself to death. A low speed engine might work, much speed and it is going to shake.

3 single acting cylinders would be self starting, or 2 double acting cylinders out of phase by 90 degrees.

Yes, and it would also need the engine to always remain at the same (vertical) angle. If you tilt it suitably, it won´t start again... ;D

 

[Peter.]

Hmm, so there are still some possibilities. Running an engine out of balance doesn't appeal to me but there's no reason why an engine could not be built to be out of balance whilst stationary yet balanced when running.

 

[steamer]

A steam engine is a very simple machine....because of that it tends to be reliable. Be careful... for if lots of "clap trap" mechanism is installed, you may compromise the reliability and end up with something more complicated than a self starting twin.....which works..... ;D

A double acting single can not be counted on to self start....though it may, but never when you really want it to. 8)

A 2 cylinder double acting fore and aft compound with cranks at 90 has the same problem. ...though not as bad as a single...... trust me here I can vouch for this......

A twin double acting simple with cranks at 90 will self start reliably if timed correctly.

If you have a single cylinder and you want reverse, ( RC application) you may want to consider a gear box reverse and just leave the engine running in one direction.....that would be better than swimming out to get it...

JMHO

Dave

 

[Dan Rowe]

If a double acting cylinder has a valve that gives 100% admission then the D slide valve is exactly covers the steam ports at dead center, and this in theory will be the only position that will not self start.

In practice the pressure most likely will not start the engine a few degrees before dead center because of friction and the angle of the connecting rod is not favorable. A scotch yoke would help with the connection rod issue, but there is still friction.

As Peter said that this will be an air engine there is no need of valve lap for steam expansion and no need for lead on a tiny engine.

So now we have an air engine that will not start only a few degrees before dead center. I still think that the cam operated supply valve bypass idea will work with a little trial and error to keep the engine from coming to rest on the few degrees that it will not be a self starter.

I agree with Dave that it adds complication to a simple machine and other options might be better but thinking out side the box sometimes is an interesting exercise.

Dan

Edit: To make my valve bypass sugestion work the reverse sequence is a bit like a diesel ship engine. The supply valve has to be shut then the engine comes to a stop the reverse gear is switched and the supply valve is opened and the engine rotates the opisite way.

 

[Peter.]

You're quite correct Dan I'm working on the idea of a small, air-driven engine which has no load other than it's own inernal friction. I do understand that an engine running on steam or with a load would be a different kettle (sic) of fish.

Thank you for the replies so far, and for the warm and welcoming responses.

 

[steamer]

Expansion engines will favor the fluid that expands the most with the least pressure drop for a given increase in volume.

Most steam engines will run MUCH better on steam than they will on air........for this very reason.

However....if its your the president of your steamboat/locomotive company...you can do what ever "floats your boat".....and you can have all kinds of fun doing jsut that..... ;D

Dave