Opposed piston steam engine

[Kingsley]

I have been thinking recently about the design of an opposed piston steam engine which uses one piston as an occulating inlet valve and the other as an occulating exhaust valve.

I have done some simulation of it in Excel and made a rough animation:


The port on the left is the steam inlet and the port on the right is the exhaust. You will see that the stroke of the right hand piston is longer than the left and that the two pistons are not quite in phase. By doing this it is possible to have most of the steam admission taking place after the point of minimum separation between the pistons (think of it as after top dead center) and have most of the exhaust happening after the maximum seperation between the pistons (think of it as after bottom dead center). This gives it an advantage over standard uniflow engines where the exhaust timing is semetrical on either side of the end of the stroke.

I am also investigating if the engine can be further improved by changing the proportions of the connecting rod length to crank throw and by offsetting the crankshafts to adjust the motion of the two pistions.

I have had a bit of a search for other people doing this kind of thing, but the only similar discussion of this idea was the first post on the following page http://stanleysteamers.com/phorum-5.1/r ... 239,page=3 which does not appear to have received any further comments.

Does anyone here know about anyone else attempting such a steam engine?

[Mike Rometer]

Offsetting the crank from the cylinder centre line is often done on shorter stroked I.C. engines as a way of gaining more direct effort on the power stroke, whilst reducing piston side-thrust. I think it would be inappropriate in a steam engine as it would act against it's purpose in reverse, and create heavy side-thrusts on the piston and cylinder wall on the power stroke.

Though interesting, I can't see a lot of benefit from your thoughts above, as the 'inlet' piston is effectively shortening the stroke of the 'exhaust' piston so reducing the expansion volume in the cylinder. To correct that the stroke would need to be lengthened, with the disadvantage of the increased crank throw as well. Also it is only single acting so is at a mechanical disadvantage in that direction as well.

Sorry to be so negative.

[gondolier88]

Agree with above, unfortunately, very interesting idea, and I can see where you could think there would be a major saving in thermal inefficiencies as you have both the valve chest and cylinder in one.

The valve movement would also create a violent steam entry onto the piston, as there is no compensation for compression by pre-entry for steam before TDC- but that could me eyes deceiving me!

Greg

[Kingsley]

Thanks for those comments, I agree that there are a lot of disadvantages of the design like that it is not very compact and that the timing is almost imposible to change once it has been built, however I am thinking about the engine more as an interesting oddity rather than it being the solution to all the problems of steam engines that would create a steam revival.

On the advantage side I think there is the possiblility of good thermal efficiency due to the arrangement setting up a thermal gradient down the length of the cylinder. The uniflow engine is often described as being more efficient because the exhaust leaves the cylinder by different ports to the ones it enters by thereby reducing the heating and cooling of the cylinder during each cycle. This engine would also do that and would maintain the rest of the cylinder wall at a more constant temperature because the left hand piston would cover the hottest part of the cylinder wall from the exhaust steam and the right hand piston would shield the coldest part of the cylinder wall from the hottest steam.

On the topic of the pre-entry of steam into the cylinder, this is possible by selecting the correct phase difference between the pistons for the selected throw of both cranks.

Regards

[Mike Rometer]

It is an interesting concept; similar in some respects to the two-stroke dismals produced by Commer in the 1960's. Another problem I see is how will the engine reverse with a fixed valve timing, as effectively that is all that happens in a normal design, also notching up will be a bit difficult. My thoughts run more to the use of poppet valves, or sleave valves, but that like most things, has already been tried, with varying degrees of success.

[barts]

For a uniflow, it seems that minimizing the clearance, using 750 F steam, jacketed heads, poppet valves with variable timing is the right approach. The superheat seems like a lot, but at 15 or 20 expansions little but the cylinder head will reach that temperature. I'm designing an engine along these lines - w/ 225 psi working pressure.

- Bart

[fredrosse]

Two pistons with uniflow ports has been tried many times in history. The concept looks very good, especially with both pistons completing a relatively long stroke in a very long cylinder, the thought was that the steam space between the two pistons can be kept essentially at the same temperature as the cylinder walls along the expansion path. However, the two piston faces will have a large temperature gradient during the process, so initial condensation rears its ugly head again on the piston tops. Also during the exhaust (return) stroke, there will be cylinder and piston top cooling, further contributing to initial condensation when live steam is admitted.

[Kingsley]

Two pistons with uniflow ports has been tried many times in history.

Do you have anymore information, such as who tried it or the names of any books or links to any websites where I can find out more.

Thanks
Kingsley