Self starting small steam engines

I recently purchased a small marine engine with 2 x 5/8" single acting pistons with 11/16" stroke. The cranks are at 180 degrees so that when one piston is at tdc the other is at bdc. The motor turns over quite sweetly with low pressure air. If the air is turned off then on again it will sometimes self start......probably about 25% of the time, but as the engine is destined for a boat it will be mighty inconvenient if the engine stalls midstream in the river.

Other than remaking the crankshaft and setting the cranks at say 90 degrees are there any other simple wheezes to promote self starting.

Mick

Not really.

On holiday in France many years ago I bought a French-language model boating magazine, it included a design for a steam engine that had exactly this design fault. A double acting steam engine with a 180 degree crankshaft is virtually pointless (aside from reducing vibration compared to a single cylinder design).

The good thing is that a new crankshaft shoudl transform the engine, giving smoother power and needing a much smaller flywheel as well as becoming self starting.

Neil

Wouldn't a single acting twin with a 90deg crank still have a dead spot? There would be a point where both cylinders were on the exhaust stroke so won't start.

Edited By JasonB on 26/07/2018 16:26:55

single acting 90 degree twin would be worse than 180. Little you can do except adding an electric starter motor driving through a sprag clutch, but that might be getting silly.

Edited By duncan webster on 26/07/2018 17:09:11

You can add a cam and light spring perhaps acting through a roller that prevents it stopping at dead centre. It doesn't have to be a strong spring that moves it when it is stationary just a light bias that discourages the two end positions.

Is there any angle other than 90 or 180 which although not guaranteed, might provide most likelihood of self starting?

The engine has a rotary valve by the way.

Mick

My confusion - I blame the heat.

The French engine was single acting with a 90 degree crank.

180 is best for single acting, but 90 for double acting.

Even three cylinder locomotives (which were invariably double-acting) would occasionally stop at a dead spot. The usual method to get away way to put it in back-gear to move it 1/4 or more of a turn. It should remembered that few locomotives had a cut-off or more than about 80%.

Duncan and Bazyle......would you kindly expand on what you have said above. I will post a picture of the engine in the morning which might assist on the cam/spring and sprag clutch ideas.

Mick

Edit:- I was tinkering with the idea of a solenoid operated spring mounted rod which would "flick" the flywheel if necessary. Unnecessary complication agai I suppose.

Edited By mick H on 26/07/2018 21:37:40

One technique is to provide a variable pitch prop so that the engine can be kept running. The prop is then used to reverse. Any twist on the blades will be wrong for one direction, but that does not matter too much for reversing.

I think the small Stuart Twins like the Sirius had a 180 crank. I have one of that style here that does, although I am not sure if it is a Stuart or a copy.

John

sprag clutch idea was to use an electric motor to turn the steam engine for a very short time so that it could get going. Then stop the electric motor and the clutch allows the steam engine to keep running. Only works in one direction of course. You could use the same scheme with your solenoid and lever.

I suppose you could try leaving the motor engaged, no sprag clutch, then you could have slip eccentric valve gear but you'd have to make sure you started the motor to slip the eccentric before turning the steam on, and you'd have to be sure the steam engine couldn't the overspeed the electric motor. An electric clutch would definitely be overkill

The best you can do with single acting cylinders is with 3 cylinders at 120 degrees. Malcolm Beak has a design on Modelboatmayhem I think. Not entirely helpful for you unless you can make another part.

Of course this problem was not unknown in full size ships and was called 'barring' where the flywheel had notches or holes into which a bar could be put to move the engine over. In luxury installations a small engine was fitted called a 'barring engine'.

This is the engine. How would your cam/spring idea work Bazyle?

Mick

I have been playing with it this morning and I have found that I can get 100% self starting on very little air (5-10psi) if I position No.1 cylinder at approximately 45 deg. after tdc. I can find no other position which will give me repeatable results. Perhaps a small cam....such as a round headed rivet....on the flywheel circumference with a light spring loaded plunger playing on the flywheel circumference might encourage it to stop in the required position. Would this be the basis of the idea that Bazyle refers to? Experiments continue.

Mick

Basically add a disc to the end of the crankshaft with a shallow recess in its edge. Then arrange a spring and plunger with a rounded end to match the one in the disc. Set the disc so that the plunger is in its recess at the point where the engine self starts the best.

With luck when you come to stop the ending the crank will carry on rotating until the plunger engages with the recess and will then hold the engine at the correct position so that it self starts next time.

I think Jason's idea which is almost the opposite of mine is the best, Getting it to stop in the best position exactly rather than just discouraged it from bad positions.

Thanks Jason / Bazyle, That sounds good. Gauging the depth of the recess will obviously be the subject of trial and error. I was wondering about the inevitable "click, click, click...." as the plunger engages the recess but a plunger with a teflon tip might solve that problem. You never know, this could become an essential add-on for non-selfstarting engines. Thanks for the assistance everybody but any other suggestions still welcome.

Mick

Crikey, that's a coincidence! This is the engine I'm converting to metric and also wondered about the 180° opposed crank pins.

Ed.

What are you intending the engine for Ed?

Mick