4 cyl. steam engine

[paul gough]

I have begun to think about a double acting 4 cylinder oscillating engine with the cranks set at 135 degrees. This provides a power impulse every 45 degrees and in theory provides an even torque application. I have tried to work out whether there might be some utility in regards to vibration by arranging the cylinder "timing" other than sequential  eg. 1-2-3-4 to something along the lines of an internal combustion engine to achieve better balancing. I am afraid I have tied my mind in knots over this and end up with a blanked out brain and headache. Anybody with a clear head or thoughts on this matter and who would be kind enough to advise will be thanked profusely. Regards, Paul Gough.

[crueby]

One consideration is whether this is a compound engine, with the exhaust from the first feeding the steam chest of the second, or will all cylinders take input steam directly?

[gipetto]

If a 4 stroke v8 engine is balanced, then a 2 stroke v4 is balanced(i think i read on the wikipedia page that it is). one would think that a v4 steam engine would be ideal. i would look at those.

[paul gough]

I thought everybody in the Northern hemisphere would be in bed by now!? Anyway, sorry for not being more specific in the description so as to provide a clearer picture. The engine would be 4 cylinder, inline, simple and reversable. I am inclined to make the  flywheel and power take off central, ie between cyl. 2 & 3. Steam supply will be through a port-face and cylinder block as is common with models of this sort. Regards, Paul Gough.

[crueby]

Its not even 8pm here - Northern hemisphere but around the side in my own pocket universe...   

Um, the 135 degrees is not working out when I draw where the cranks would be... That would put them at 0, 135, 270, and 405 (45 past the first '0')
Either I misunderstood something in your description, or you are using circles with something other than 360 degrees?   
Usually the cranks for a 4-cylinder would be in one of two layouts: every 90 for a 'simplex' engine, or possibly every 120 for a compound with split LP cylinders.

[paul gough]

In a sense that is correct, but as we only have a single circle it equates to positions of 45 and 225. This is not a new idea. Mr Maunsell of the Southern railway in the UK designed a 4 cyl. locomotive with this arrangement of cranks, albeit with centre cyls. driving an axle forward of the outer ones. Regards, Paul Gough

[paul gough]

P.S. If you draw a single circle with the positions of the pistons/cranks at the eight positions you can see the relationship.

[crueby]

Was there supposed to be a picture in your  last post? Just a big blank spot?


And your list of numbers has 270 twice?  I still dont get it. Why show pistons in both locations, the cranks are not evenly spaced out...

[paul gough]

I have edited the previous post and will try to get across the crank dispositions as best I can. I don't have a camera anymore to take a photo of a drawing. Draw a circle, say around the outside of a circular protractor and mark off 0 degrees and then 180 for the position of the first piston/crank at the each  end of the stroke,or T.D.C. Then the same for 135 degrees and its opposition at 315 degrees, then add another 135 degrees and so on for a total of four times. You will end up with eight positions 45 degrees apart. You should end up with a circle showing 0 & 180, 135 & 315 270 & 450 (or in reality corresponding to 90), 405 (corresponding to 45) & 585 (corresponding to 315). In other words a circle with every 45 degrees marked off from a zero position.

 Another way is to mark in four points that progress at 135 degrees intervals from zero. Then place another set of points diametrically opposed to these, ie. 180 degrees from them. The same outcome will be achieved. Regards Paul Gough.

See Photo.

[paul gough]

I added a sketch of the crank positions to the last post, hope this makes things clearer. Regards, Paul Gough.

[crueby]

Gotcha!  That helps my understanding a lot.


Okay, that looks like it will give you a balanced thrust from each of the pistons in each direction, which is good. The crank itself will be unbalanced for mass unless you have counterweights on each web, since one of the webs will be closer between two of the others. The counterbalanced webs were common, and should look good.

[Charles Lamont]

Another interesting 4-cylinder loco with eight beats per revolution was the experimental 0-6-0 designed by Hookham for the North Staffordshire railway. It was intended to provide rapid acceleration for commuter traffic with closely spaced stations and steep gradients.

There is no way to make an intrinsically balanced crankshaft for this arrangement. If you put the cranks 1, 2, 3, 4 at 0, 90, 135, 225 it will sequence 1T, 3B, 2B, 4T, 1B, 3T, 2T, 4B which seems to me reasonably well distributed.

If balance is important, perhaps you should go for a straight six!
(Or three, of course).   

[paul gough]

Helo Chris, Glad I got my meaning across. Charles thank you for your mention of the Hookham loco, I was not aware of it and will certainly look for some information on it. If you have a couple of references handy that would be most helpful.

As to the balacing, it was primarily to do with power application to the shaft and thus the admission sequencing which you have indicated. I had thought if there was any advise on it pointing towards better balancing due to this then  I could re-arrange things by inter-changing the crank settings of particular cylinders. I am inclined to go with your assessment of it being "reasonably well distributed". I am probably 'over-thinking' the issue but was curious about it.  Again, thank you for your input. 

[crueby]

Unless you are going to run at very high speed or build it very large the balance should be fine!

[paul gough]

Hi Chris, This engine is to provide  something of an experimental platform. Initially it will be for comparing a reversable in-line 4 cylinder oscilating engine with 90 degree crank setting to one with 135 degree crank setting in terms of its behaviour. The first iteration will likely be as a low pressure, low speed engine, e.g. 2 to 3 bar and 40 rpm approx. This reflects the sort of operation of a full size engine of this type. Partly the 4 cylinders and 135 degree crank setting is to provide improved self starting and hopefully less vibration. I am also interested to discover what the steam demand and therefore size boiler is optimal under these output conditions. The second iteration is to split the engine in two and place them together (side by side) and integrate their output through gear drive, again comparing 90 and 135 crank settings. The third itteration after determining which arrangement is better, my intention is to explore the highest pressure and speed (rpm) this engine can deliver at a level that could be sustained without detrimental effects. Finally, I may proceed with a test to see just how far one can push this type with pressure and speed up to and including some sort of failure, which I expect will be in the trunnion retainer and seals area. To be sure, a rather lengthy exploration, though this is the sort of thing that interests me now. Regards, Paul Gough.