simple oscillating engine theory

[geoff walker 1Participant @geoffwalker1]

[geoff walker 1Participant @geoffwalker1]

Hi All

When the piston in a single acting simple oscillating engine is at tdc is there any guidance I can use as to where the top of the piston should be relative to the inlet/outlet port in the cylinder.

When should the upward travel of the piston stop, just below, over, or just beyond the port hole.

I of course understand that the piston must not block the port hole when the hole is aligned with either the inlet or exhaust on the standard.

Geoff

[JasonBModerator @jasonb]

Geoff, Muncasters drawing (not Westbury's) shows the end of the piston half way across the port when at TDC.

Do you have his drawing or are you trying to go by Westbury's which does not show as many dimensions. Julius De Waal has also drawn a slightly larger version though most of his drawings have not been proved by anyone actuallly building the engine but may be of use.

[geoff walker 1Participant @geoffwalker1]

Hi Jason

Geoff, Muncasters drawing (not Westbury's) shows the end of the piston half way across the port when at TDC.

Yes I do have Muncasters book and I can see he has the piston half way across the port hole. The problem I have is that if you take his piston rod length of 1 and 15/16" from the centre of the crank pin to the end of the piston you can't make it fit unless it overlaps the port hole, that being off a 0.486" throw.

JDW also has the piston lapping half way over the port hole.

I may well just shorten the con rod and have it overlapping half way. I was keen to stick as much as possible to the Muncaster sizes but may have to compromise.

thanks for your help Geoff

[RedsetterParticipant @redsetter]

Surely the port will be closed at TDC, so it doesn't matter whether the piston overlaps it or not.

[JasonBModerator @jasonb]

But at 1degree either side it will be open so you don't want the piston in the way.

I just drew it out roughly to Muncasters dimensions and the piston comes half way if you use the 7/16" crank throw but if you go to the larger one I suggested in the other thread then that will cover the piston so pivot to crankshaft distance would need to be adjusted to compensate.

Edited By JasonB on 18/06/2018 19:46:24

[geoff walker 1Participant @geoffwalker1]

But at 1degree either side it will be open so you don't want the piston in the way.

Yes I think you're right there Jason, just checking myself and it's a good 30 degrees before the piston fully clears the port on the down stroke.

Thankfully the solution is simple just shorten the con rod.

thanks again, great simulation, and I'll keep you posted Geoff

[BazyleParticipant @bazyle]

It doesn't matter if the hole is covered and the piston can reach to half a thou of the end, just file a notch in the piston to allow the steam into the end of the cylinder. A bit more than half a thou would be better for fast running to allow free flow but all the end space is wasted volume, it has to be filled every stroke. This is not quite the same as for a 'properly' valved engine that expands the steam.

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