Next Weekend II

[Lopez Mike]

That's a good question, Wes. I've never sorted out the 'right' thing to do on a double acting engine like that. Hmm.

Let's assume that the bare crank is close to being in balance. That would mean that each cylinder, standing alone, is balanced at zero percent except for the weight of the lower 'half' of the connecting rod.

The parts that are moving vertically will cause that cylinder to vibrate up and down. That part of the mass of the connecting rod that is below it's center of mass will make the engine shake from side to side. So mostly up and down for that one cylinder.

Now the same conditions exist for the other cylinder. Mostly up and down with a bit of side to side.

Now the two effects happen at 90 degrees from each other. So we have two rather eccentric ellipses at right angles to each other. Add them up (beyond my math skills. Bart??) and you will have the vibration diagram.

EXCEPT!! The two cylinders are one in front of each other. Thus one is wiggling around a few inches fore or aft of the other. That, my dears, is probably why we don't let these beasts rev up very far. It woudl shake the fasteners right out of our boats!

In those few cases where some lunatic has designed an engine with this crank arrangement and want so rev it up, he/she has probably added in some double speed balance weights on an auxiliary shaft.

Too much for me. "My head smells, my feet hurt and I don't love Jesus."

As an old German mechanic once told when I was obsessing about some tiny problem on a marine engine, "Put it together. It will run fine."

Mike

P.S. It's worse than I made it out. The pistons don't go up and down in a simple sinusoidal pattern because to connecting rod angularity. Yaaaah!

[Mike Rometer]

Mike, all this would seem to be an arguement to keep all reciprocating parts as light as possible.


Any one for aluminIum?

[Lopez Mike]

Of course it is. And I'm happy to see aluminum used in places where it might help a lot. Oddly enough, I'd like to have the total weight of the engine be less as my boat is small and weight slows it down. I would recast my engine frame in aluminum if I were to start peeling off weight. But I would paint it so no one could tell what I had done.

The obvious route to weight reduction is to dispense with this silly notion of having the fuel burned external to the cylinder. Light if off with a spark or by compression heat right in there where it can expand without so much energy loss in pipes and such. Use some light fluid mix like gasoline. Aargh!

Now that all of the hopeless romantics (including me!) have fainted, I'll go back to my hole.

Mike

[87gn@tahoe]

Mike(s)- invisible reciprocating parts that is... I would like to retain the original beauty of the engine. Just helping the old gal out a little. Opening a door or walking her across the street, rather than driving her around town and doing all her chores.

I agree that if I really wanted efficiency, I'd throw in a die'sel; Throw the old gal out and hang out at the local community college.

[artemis]

Is the weight of each piston a derivative of the bore ratio between the cylinders? That is to say: The quantity and quality of the steam including the size of the accumulator (HP exhaust pressure to LP inlet pressure) is very carefully worked out based on many things. It probably isn't significant in our little boats, but I wonder in larger naval plants if the weight of each piston, etc. wasn't carefully calculated so that all the many variables would work out in such a way that the HP, LP, IP, etc. would deliver the same horsepower from each piston stroke (difference in weight would make a difference in horsepower delivered). Don't know if it was done, but would be a real nightmare in the pre PC days.

Anybody up to designing a program that will take all this into account. Objective to tweak the engine to greatest possible efficiency.

[barts]

Actually, the weight of the piston doesn't affect the delivered horsepower; any work done on the upstroke
is returned on the downstroke; the same is true for any losses due to friction.

- Bart

[Mike Rometer]

Yeah, but that doesn't take into account inertia. The piston and piston rod has to stop and start twice each revolution, likewise a small proportion of the con-rod. That takes enegy too, so a lighter piston etc. will use less.

[Lopez Mike]

Actually, it doesn't work that way. Unless there is a lot of friction, the stopping and starting of reciprocating masses is pretty much loss free. There is more energy lost to piston ring friction than to cross heads and connecting rod bearings.

In fact, now that I think of it, I would speculate that an unbalanced slide valve might well be the prime culprit in small steam engine frictional losses. A lot of bearing area and a lot of pressure at higher throttle settings.

Mike

[87gn@tahoe]

Trying to make a bloody post in here but keep getting error 80 messages and "server has timed out" messages on three different computers with separate connections. VERY frustrating!

[barts]

seems to work ok.

- Bart