Valve design, timing

[Captain Jerry]

Hi Y'all

I need a little help with valve timeing for compressed air engine. There have been a number of discussions here and elswhere and I think I understand such concepts as lead, cutoff, expansion and how to modify the dimensions to produce change, but I don't think I have ever seen any discussion on the overlap of timing between adjacent cylinders in a multi-cylinder engine. For example, on the three cylinder swash plate engine that I am building, if steam of HPA is admitted to each cyl for 160deg in the cycle then during 40 deg of each cylinders's power stroke, the source of air or steam is shared with another cylinder. Is it better to limit each cylinder's share of the power to 120 degrees or less so that there is no overlap or does it really matter?

Thanks for your thoughts or experience.

Jerry

 

[Maryak]

CJ,

In a multi cylinder simple steam engine i.e. one that is not compound or triple expansion, the steam is supplied from common source and admitted to each cylinder as required by the position of the valve relative to the piston, (90 deg before or after DC which means no lap or lead of the valve). The exhaust is again a common line from the cylinders. Probably the angle of obliquity of the crank is the determining factor normally around 47 deg before/after DC at which point any more admission of steam/air does little to enhance the power from the cylinder. What I'm trying to say is there is little downwards/upwards push and a lot of sideways push on the crank.

IMHO better to remove the overlap unless you need cushioning at the end of the stroke.

Hope this helps in some small way for your wonderful design so far.

Best Regards
Bob

 

[Captain Jerry]

CJ,

IMHO better to remove the overlap unless you need cushioning at the end of the stroke.

Thanks Bob, thats exactly what I was looking for. I had not noticed the overlap until I began to refine the geometry of the valve. It looks like the only way to remove the overlap is to modify the shape of the cylinder port. It will have to be more of a slot than a simple round hole which is probably better anyway.


Jerry

 

[gbritnell]

Jerry, what Bob stated is correct but maybe a little further explanation might help. Miniature steam engines have been around for a very long time. Some run on steam and some don't. For the enjoyment of watching the engine tick over it was easy to just hook some compressed air to it and there was also the safety issues of having the engine run on steam in front of a crowd of people. Although the engines were running on air they still had the timing specifications of a steam engine, lap, lead, advance etc. An engine designed to run on steam and running on steam uses the steam expansively, that is to say it's not necessary to admit steam for more than 60% of the stroke. Now when you run it on compressed air and cut the air supply off at 60% the engine only uses the inertia of the flywheel and rotating parts to carry it through to the next pressure admission. The only negative is that on a very tiny engine they won't run extremely slow because they don't have enough flywheel action. Now that builders are creating engines made to run strictly on compressed air the admission time could be increased somewhat to get this slower running feature. How much I can't say but I would think that 75% of stroke would be more than ample. To get back to your question, one of the design intents of a multicylinder machine, whether locomotive or stationary, is to make it self starting. As long as there is pressure on crankshaft from one cylinder it's not really necessary to have another cylinder working at the same time so in your case a small amount of pressure overlap is all that would be needed to self start the engine.
gbritnell

 

[Captain Jerry]

Aha! Overlap is necessary for self starting. I new that but it got lost in the fuzz. So on a three cylinder engine each cylinder should be open for at least 120 deg. So maybe I should aim for slightly more than that? Maybe 130?

Jerry

 

[SandyC]

;D

Jerry,

To add a bit more to what Bob an GB have posted...

Think more in terms of each individual cylinder... the time the STEAM/AIR valve remains open (% cutoff) should relate directly to the full stroke for the cylinder in question... not a number of degrees of rotation dictated by how many cylinders there are.

I.E. 75% cutoff would mean that the steam/air valve would close after 75% of the stroke had been achieved... the remainder of the stroke would be accomplished by steam/air expansion... up to the max of 180Deg crank rotation, relative to that particular cylinder,... at which point the piston direction would reverse.

At this point in the cycle... on a single acting cylinder the EXHAUST PORT would be opened by the valve and would remain open for the entire return stroke... again 180 deg.

On a double acting cylinder the EXHAUST valve would be opened and the OPPOSITE STEAM port would be opened and the piston would be driven back up the cylinder.
The waste STEAM/AIR from the previous stroke would be expelled through the OPEN EXHAUST PORT whilst fresh STEAM/AIR is being INPUT on the opposite side of the piston.

The angular relationship between the different cylinders only means that one or more cylinder is providing thrust power to the crank, and by virtue of the angular difference making starting easy.

Since COMPRESSED AIR has far less expansion capability than does STEAM at the same pressure, I would suggest you make the time the inlet valve is open something closer to 90% - 95% of FULL STROKE/per cylinder.

Hope that helps make it more clear.

Best regards.

SandyC : ;D ;D

 

[Captain Jerry]

Thanks Sandy. Its beginning to make sense now. I have three good opinions from experienced engineers whose input I respect. It would be great if we could get together over a Guiness and discuss this. Oh well, I'm opening mine now. Wish you were here.

To put it all in the same terms, since stroke is taken as 180 degrees, then 75% of stroke is 135 degrees (GB). 92.5 % of stroke is 166 degrees ( Sandy ). And no overlap at all is 120 degrees for a three cyl single acting (Bob. Lot of room for discussion. The average seems to be about 140 degrees or 80% of stroke, which provides about 20% of overlap. That should give good self starting. That seems about right for air. If I were designing for steam, it would seem that any overlap would detract from the use of expansion because before one valve closes to make use of the expansion, the next valve is opening and the expansive volume is just bled back into the common supply.

It seems like there is plenty of room to experiment.

Jerry

 

[gbritnell]

OK, a little more clarification. Staying with simple engines as opposed to compounds the layout for a 2 cylinder engine, again locomotive or stationary, is that the cranks are at 90 degrees to each other. With a double acting engine what better design could there be? You now have power every quarter revolution plus the engine is self starting. When designing an engine for steam usage there are so many factors to be considered but there's enough information there to start a completely new thread. In my previous reply I used the word overlap to explain what was needed to keep the engine running when in fact the word lap for steam usage doesn't mean that at all but rather the amount the sealing face of the valve overlaps the port. When one steam valve closes (cutoff) and another opens there is no bleeding back of steam. It all comes from one supply and feeds both valve chests when required. I would say that for your engine you could use any amount of overlap successfully the only thing is when you use too much you're not really moving the piston that much because the rod has already started to swing and the stroke motion is reduced. To explain what I mean, draw a circle and put your pencil at the top center of the circle. now start tracing the circle clockwise. In the first 25 degrees of arc you haven't moved very far in the down direction but quite a bit to the right (swing). Now from 25 degrees to 155 degrees you move more in the stroke dimension and not much in the swing dimension. The last 25 degrees is like the first 25 degrees so you aren't putting that much down movement into the piston so leaving the pressure supply on too long won't provide that much work, some yes but not enough to account for anything.
gbritnell