Sea Lion Engine build

[Ironman]

I think that would not be much ofa sine wave as the tops of the eccentrics would be only 30 degrees apart, but it was a way to shift from using the 11 degree BTDC eccentric to the 19ATDC eccentric without interruption.


Your idea of a cam at right angles to the crank plane will drop off a pile of weight. It is an interesting way to do it and leaves you free to develop any profile you want.

[barts]

I think that would not be much ofa sine wave as the tops of the eccentrics would be only 30 degrees apart, but it was a way to shift from using the 11 degree BTDC eccentric to the 19ATDC eccentric without interruption.


Your idea of a cam at right angles to the crank plane will drop off a pile of weight. It is an interesting way to do it and leaves you free to develop any profile you want.

Here's a link that describes the math involved in adding sinusoids:
https://www.cs.sfu.ca/~tamaras/sinusoid ... soids.html

I'm now looking for a suitable right angle gearbox... The amount of torque involved won't be that high; I'm pretty sure it's mostly inertia loading from the the engine speeding up and slowing down every cycle; I'm looking for a spiral bevel unit to reduce noise.

- Bart

[fredrosse]

"a small high pressure cylinder with a piston valve and regular eccentric linkage of some sort, exhausting to the uniflow low pressure cylinder; this was also a popular setup in Europe on stationary engines and on some British marine engines."

Was that arrangement basically used for the several WWll aircraft carriers that had Skinner Uniflow engine? I think the Skinner engines used poppet valves, but the proven gains with both compounding plus uniflow for the LP cylinder were the peak of reciprocating steam engine efficiency. Skinner built these engines well into the 1950s, and I know of one large coal fired power plant (1980s) that used a new Skinner Uniflow engine to drive the exciter for a several hundred megawatt generator.

[barts]

"a small high pressure cylinder with a piston valve and regular eccentric linkage of some sort, exhausting to the uniflow low pressure cylinder; this was also a popular setup in Europe on stationary engines and on some British marine engines."

Was that arrangement basically used for the several WWll aircraft carriers that had Skinner Uniflow engine? I think the Skinner engines used poppet valves, but the proven gains with both compounding plus uniflow for the LP cylinder were the peak of reciprocating steam engine efficiency. Skinner built these engines well into the 1950s, and I know of one large coal fired power plant (1980s) that used a new Skinner Uniflow engine to drive the exciter for a several hundred megawatt generator.

I've not looked at this in detail; I've not seen a image for a compound Skinner uniflow - all the ones I've seen pictures of were single expansion. I've seen compound uniflows in a pumping station in the Netherlands; that installation is both a museum and a working installation, seeing service a few times a year when floods threaten.

https://en.wikipedia.org/wiki/Ir.D.F._Woudagemaal

A double acting compound uniflow is a very long cylinder indeed. The marine version I've seen in a book was single acting with poppet valves acting directly between the high pressure and low pressure cylinders..

http://kimmelsteam.com/ajaxcompound.html

Aha! And here are some pictures of a Skinne4r compound uniflow:

https://www.asme.org/getmedia/8999fb61- ... -1952.aspx

- Bart

[Oilking]

Yes, the Skinner UnaFlows on the escort carriers were five cylinder single expansion type. From a discussion with someone that worked for Skinner in their later days, I learned that the four compound engines on the car ferries were the only ones that they built. There is legend that a dominant personality at Skinner had some strange views on how to balance these engines and prevailed. As a result these engines gave the ferries a distinct side to side motion. The person I talked to tended to support the legend.

The hope was that the compound could compete with larger deisels of the day?

Dave

[Ironman]

Bart
Some where past you mentioned the load on the mains as 2600lb
That got me thinking, and as I escaped school at grade 10 and went to the U of Life, I don't have your math strength.

I did some pathetic figuring with Piston area(25.96 sq in) X Mean pressure 180psi(4635) X Stroke (.48ft) =2224.8Ft LB Torque
I chose an arbitrary Mean pressure of half what I assumed operating pressure would be.

HP=FxRxRPM/63025 says this is an 17.6 Horsepower engine at 500rpm
Which with those gobs of torque should scoot that boat along.

[marinesteam]

The PLAN formula is a useful tool but the numbers input must be good. That is why the engine must be indicated or input data be taken from a reliable source, I believe there are steam tables that exist to allow the selection of MEP in a design context. If the MEP is just a guess the result will be suspect as well. The boiler pressure isn't what the PLAN formula uses and to do so will give a vastly inflated HP rating. The mean effective cylinder pressure (MEP) is the average pressure of the steam as it expands during the power stroke and is determined factors including; pressure at the steam port (different from boiler pressure) and the cutoff.

This was a response to a prior thread but valid here as well.

50% of valve chest pressure might be a better place to start, Maybe your not far off but you really can't know until the MEP is measured or the engine put on the brake.

Ken

[barts]

I tend to be a pretty cautious engineer so I designed the crank, bearings, etc to withstand a locked engine at full cylinder pressure. Note that while the BMEP of a uniflow at 200 psi boiler pressure is typically somewhere between 50 and 60 psi according to Mark's Engineering Handbook, the peak loads occur while the intake valve is open and the cylinder is assumed to be at full boiler pressure. The piston, rod and bearings see this peak load; the crank will never see the peak torque unless the engine is modified to have a lot bigger cut-off than I have planned, but just in case, it's designed to handle it.

BTW, PLAN applied to this engine using measured BMEP values from other (larger) uniflows yields:

55 psi * (5.75/12) * 3.1416 * .*(5.75/2) ^2 * 400rpm * 2 power strokes/rev /33000 = 16.6 hp.

Like most uniflows, the engine is capable of at least 50% more at overload, and will readily throttle down to a fraction
of this value by reducing the cut-off.

This flexibility and efficiency does come at a cost; the bearings and other components need to be large to cope with high peak loads compared to the average loads.

- Bart

[marinesteam]

Bart,

I know you would be using good numbers. I had just meant to warn against Ironman's use of 50% of 180psi as being more than likely, a bit high. It is interesting that Ironman's estimate was so close seeing that the difference between the MEPs used is so large. I haven't looked at the math closely enough but is seems odd.

Ken

[barts]

Bart,

I know you would be using good numbers. I had just meant to warn against Ironman's use of 50% of 180psi as being more than likely, a bit high. It is interesting that Ironman's estimate was so close seeing that the difference between the MEPs used is so large. I haven't looked at the math closely enough but is seems odd.

Ken

Note that PLAN/33000 has N meaning power strokes per minute, not RPM... so double acting engines
need to multiple RPM * 2 - which neatly compensated for a 2x too high BMEP in Ironman's example calculation.

I used 400 rpm here; the prop I have actually will let the engine spin somewhat faster, so the power goes up somewhat.

We'll see what happens in a year or three when I get this engine running - one test is worth a thousand expert opinions.

- Bart