single expansion crank angle

[wsmcycle]

I am looking for help in understanding the mechanical reason behind 90 deg crank angle on a single expansion engine. I already see the self starting issue. The difficulty i see is the imposed unidirectional bent the engine would have. Which cylinder(primary or secondary) should lead in the normal "forward" direction". Whether that is CW or CCW, one of these it seems to me would be the better steam expansion direction for the leading cylinder.
Thanks

[DetroiTug]

I'm running a twin simple or single expansion. The 90 degree throw orientation provides self starting at any degree of rotation and also provides a smoother torque output. The top of a piston has a larger surface area (more force), the bottom of a piston with rod connected omits that area equal to the cross section area of the rod. All steam engine pistons have more force going down, unless it's a Pearl engine - rule for orienting hydraulic cylinders in application - they push more than they pull. With 90 degree throws, the pistons meet one another in the top quarter travel and again in the bottom quarter of travel. Yes the twin engines do lope, but small price to pay for self starting and smoother power at the crank. One could set them at 180, the torque output would essentially be identical to a large single cylinder.

Hope I've understood your question and answered accurately.

-Ron

[Lopez Mike]

I agree that in hydraulic practice the difference in area between the two sides of the piston is significant but when you do the arithmetic for our engines with our spindly little piston rods, the differences are probably not worth worrying about.

An odd fact about phasing is that on almost every locomotive every built in every nation, the right side of the engine leads the left side in forward motion. It makes absolutely no difference in practice. The one exception I am aware of was the Pennsylvania R.R. in the USA where the left side led the right. Probably some curmudgeon of a chief engineer with a bee in his bonnet.

There were some three cylinder locomotives built with the center cylinder inclined to the outer two to provide clearance for its connecting rod past an intervening axle. That axle still needed a small amount of 'crank' to clear the center connecting rod. The sound of these atmospheric exhaust engines was unique. An odd stagger at low speeds and then a most pleasing waltz rhythm at higher speeds.

I would be interested to know if there is/was any standard in this matter with paddle wheel steamers.

Fred??

Mike

[gondolier88]

When you say 'single expansion' do you mean a high pressure double acting engine of one or multiple cylinders, or do you mean a 'single acting' engine, with steam on one side of the piston only?

Greg

[wsmcycle]

When you say 'single expansion' do you mean a high pressure double acting engine of one or multiple cylinders, or do you mean a 'single acting' engine, with steam on one side of the piston only?
response
I meant a double acting, single expansion. two cylinders.

lead and lag
in one instance the primary cylinder (HP) leads the secondary cylinder (lp). When you reverse, you have the opposite. How can the lp cylinder be supplied lp if it has to move before the HP cylinder has any "exhaust"? I understand it will start, but it seems there would be less power in that direction.

[fredrosse]

Almost all USA sidewheel steamers had one engine, single cylinder double acting, over 99% of them I would say. The USA stern wheelers were all two cylinder double acting, or steeple compounds, but all had cranks set at 90 degrees, same as railroad practice. In Europe the sidewheelers I have seen were all triple expansion, with cranks always set at 120 degrees. Have not seen sternwheelers over there, although there may indeed be some.

[gondolier88]

When you say 'single expansion' do you mean a high pressure double acting engine of one or multiple cylinders, or do you mean a 'single acting' engine, with steam on one side of the piston only?
response
I meant a double acting, single expansion. two cylinders.

lead and lag
in one instance the primary cylinder (HP) leads the secondary cylinder (lp). When you reverse, you have the opposite. How can the lp cylinder be supplied lp if it has to move before the HP cylinder has any "exhaust"? I understand it will start, but it seems there would be less power in that direction.

Thanks for the clarification.

Firstly, the HP cylinder is not there as a valve to supply steam to a LP cylinder, it is an engine in it's own right, and easily has enough power to turn the crank until the LP has it's own working pressure from the HP exhaust. In large engines the LP cylinder pressure/temperature is very important to maintain a good efficiency, and won't attain full working Pr./Temp. until a few hundred rev.s have gone by. An impulse/starting valve may also be used which temporarily injects HP steam into the LP receiver, creating a twin simple engine and warming the cylinder through quicker.

It is a fallacy that the LP cylinder is there to make more power, it is there to eke out the last remnants of energy from the HP exhaust only in an effort to increase efficiency, in reality in 90% of their applications compounds are often as efficient as a simple engine, or even less efficient.

Greg

[wsmcycle]

Yes it is true, the hp is not a valve for the LP BUT. If the LP cylinder and valve are not positioned to receive the exhaust of the HP, the differential pressure across the HP cylinder is diminished and that means power loss.

[gondolier88]

I see what you mean, but in practice upon start-up the LP cylinder drains would be open, and the cylinder would be at atmospheric pressure on both sides of the piston. However, as stated above, the impulse/starting valve would be used if the HP stopped at TDC/BDC without steam passing to the LP receiver, but, with a decent valve gear this likelihood can be lessened, or overcome by a quick burst ahead and then astern.

Greg

[wsmcycle]

I understand the start-up will be accomplished. I am probing an understanding of running power when the HP is leading the LP versus the HP following the LP. Is there a best running direction for this 90 deg crank. Leading? or following?