Steam Engine Clearance

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deeferdog

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Can anyone explain to me the effect of increasing or decreasing cylinder clearance in a steam engine? If I increase the clearance, and by this I mean the distance from the top of the piston (at TDC) to the cylinder head, will the engine consume more steam? If it does, is more power the result? Conversely, does reducing the clearance have the opposite effect? Finally, how does a designer of a steam engine determine what the clearance should be? At the moment I'm building a twin cylinder steam engine and I've had to fiddle around with the plans to such an extent that I'm starting to wonder if its going to work. Probably not the best time to ponder these things as it is over half finished. Help would be much appreciated. Cheers, Peter
 
Clearance volume is basically dead space. You have to fill it with steam before you start to put pressure on the piston. At the end of the exhaust stroke the clearance volume is at best full of cold steam / water which cools down the in coming steam for the next power stroke. For efficiency clearance volumes should be kept as small as possible and the pistons and insides of the cylinder covers should be as smooth as possible to help prevent condensation in the cylinder.

Hope that helps.
 
As I understand it, the power that a steam engine produces is a function of rpm, steam pressure, and the timing of the valve events.

The force generated on the piston rod is a function of the steam pressure acting on the top of the piston, and the area of the piston (pressure/area).

The critical valve timing events are admission, cutoff, release, and compression.
For small model engines, I have seen all sorts of bad valve timing work sufficiently for the engine to run well without load.

For locomotives, getting the valve timing correct may mean the difference between pulling the passengers and cars up a grade, or not having sufficient power for that work.

Also important is equalizing the valve timing events on both the instroke and the outstroke.
Some odd situations like large (full size) vertical engines use asymmetrical valve events to take into consideration gravitational forces.

For engines with Stephenson's links, the valve timing needs to be checked to make sure that the events are the same in forward and reverse.

I don't make the clearance of my model steam engines too tight, since they are not generally designed to pull a load (my future designs will pull a load, but even those will not be too tight).
The problem with clearances that are too tight is that the bearing surfaces will wear, especially at the connecting rod bearings, and if the take-up adjustment is not done correctly, the connecting rod will get shorter or longer as the wear is taken up, and this can cause the piston to crash into the upper or lower head.

The ideal connecting rod bearing arrangement is one that results in zero change in length as the bearing wear is taken up.

Efficiency may come into play for a locomotive, but for a model engine running without load, I don't think you will notice any efficiency changes with clearance changes. There may be efficiency changes due to clearance at this scale, but they would probably be difficult to measure.

Launch engines are operated under load, and so the launch folks tend to pay much more attention to engine design and sequence of valve events than model engine builders. Efficiency could be important for a launch engine, but I have seen people opt for a two cylinder non-compound over a compound design, just because the don't care about the fuel efficiency of a small launch, and they want the simplicity of a non-compound engine.
Others insist on a compound arrangement since they may be limited in how much fuel they can carry.

As Richard mentions above, excess clearance is just wasted space that has to be filled and emptied with each stroke, and the extra steam required to fill this space does not provide any additional pressure on the piston, and does not provide any more power to the engine as far as I understand it.

For full sized engine designs I have seen, the piston does not generally intrude into the passage space at TDC and BDC, else piston slap can occur.
The interior of the heads can be adjusted during the machining phase to protrude more down into the cylinder (assuming you are making cylinder covers from scratch), but the protrusion space on the cylinder head at the passage area (for tight clearances) is generally milled to a round shape, to provide a ramp entry of steam into the cylinder, and to prevent the protrusion from blocking the passage.

For steam engines that do not use a D-valve, sometimes a relief valve is fitted to the upper and lower cylinder heads.
A D-valve will lift off of its seat and relieve a hydraulic situation where you may get an unintentional charge of water into the cylinder.
For a non-D-valve, you can bend/break things if you get condensate in the cylinder, especially if you are running very little clearance in the engine, since some valve types will not automatically relieve a hydraulic lock situation.

As a side note, the piston rings on large steam engines generally overruns the end of the bore slightly to prevent ridge buildup.

For maximum efficiency, an early cutoff is used.
For maximum power, a late cutoff is used, such as a locomotive trying to start on a grade from a standstill with a heavy load.
The Stephenson link allows the cutoff to be varied dynamically to suit the speed of the engine, and greatly improve efficiency.
The Stephenson link also has the critical advantage of advancing the timing (advancing the admission of steam), which is similar to what an automotive engine ignition does.

.
 
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Excessive headspace will use more steam everytime the piston reverses direction. The cylinder goes to very low pressure on exhaust and the cylinder must come up to pressure to move the piston the opposite direction. Any steam that fills the dead space is not doing work, it is just filling space. And when the piston reverses all that extra steam must still be exhausted, wasting the energy in that steam.
 
Engineers could use clay or plasticine to physically measure the clearance dimension, but not always best practice as there could be high spots... Better to jack the piston up to bump and mark the crosshead, then lower to bump for other bottom - Then work out best piston installed height. With vertical engines give a little more at the bottom for wear down. (With traction engines need to allow similar for boiler expansion with heat and pressure!) Shim between bottom end and connecting rod foot if needed. I've never seen these shims as manufacturers usually got it right. Vertical engines do wear down though, and if not corrected with re-metalled main bearings, shims or packers might be a temporary fit. Clearance volume is really a piston to cover issue, but there is also the port volume to fill and empty on each stroke. Correct to say this space is wasted. A big improvement achieved with poppet valves and Corliss valves.
 
Oops and P.S. - Forgot that with one port for steam and exhaust, this single port has to be sized to get the exhaust out and not the steam in, so there is more port volume to add to the clearance volume.
 
I remember reading somewhere in the dim distant past that some dead space in full size reciprocating engines was beneficial in producing a cushioning effect to protect the bearing loads from the sudden change in piston direction.
How much/little volume and what effect on efficiency I know not.

Dave
The Emerald Isle
 
The cushioning effect is more to do with the timing of the exhaust valve so that the residual steam in the clearance volume is compressed meaning less live steam is needed to fill the clearance volume at the start of the new stroke.
 
Large engines MUST have cushioning. There will be a point where the exhaust edge of the valve closes and compression starts. It can clearly be seen in an indicator diagram. Little engines will not decelerating these large masses, but a theoretical diagram is possible. Perhaps one of the simulation programs can provide a theoretical indicator diagram.
 
Here is an indicator diagram which shows the four events of a steam cycle (admission "A", cutoff "B", release :c", and compression "D" on the bottom of the curved shape).

You can see that the pressure inside the cylinder returns to the same pressure as supplied by the boiler by the time the next steam admission cycle begins.

This sketch is from "Verbal Notes and Sketches for Marine Engineers" by J.W.M Sothern.

The "card" is the diagram that is traced by an indicator.
An indicator was a device that recorded the pressure inside of a steam cylinder, and a top and bottom card were needed for complete information about what was happening inside the cylinder.
Indicators were mechanical versions of pressure transducers.

This sketch is very helpful because it shows the valve position that is associated with each of the four events.

The clearance is also shown in the upper diagram at either end of the cylinder.

As I recall, the work performed by an engine is equal to the area of the diagram ?
And the efficiency of the engine can be determined by the indicator diagram.

The curved shape is a trace of the pressure inside the cylinder.
The bottom of the shape is at atmospheric pressure, and the top of the curve is at the pressure supplied by the boiler after it makes it way through the steam piping.
The pressure at the engine is a bit lower than boiler pressure due to pressure drop in the piping.

I will have to brush up on my indicator diagrams.


rIMG_3107.jpg
 
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I am not keen on the bottom diagram, which is a polar plot.
I find the sinusoidal diagrams used by Dalby to be much easier to understand.
Both diagrams represent the same information.

The bottom diagram is a rectangular valve diagram that I plotted from an spreadsheet, and it is for a 20 hp Stanley auto engine.
The diagram can be envisions as if one were looking into the steam seat, with the curved lines tracing the path of the D-valve.
This diagram is very useful in that is shows both the piston position and the valve position for any give crankshaft position.

You can see that during admission of steam, the D-valve does not completely open the passage, which is typical on many steam engines I have seen.

DALBY-Diagram-01.jpg



rDisplacement-Diagram-Stanley.jpg
 
Engineers could use clay or plasticine to physically measure the clearance dimension, but not always best practice as there could be high spots... Better to jack the piston up to bump and mark the crosshead, then lower to bump for other bottom - Then work out best piston installed height. With vertical engines give a little more at the bottom for wear down. (With traction engines need to allow similar for boiler expansion with heat and pressure!) Shim between bottom end and connecting rod foot if needed. I've never seen these shims as manufacturers usually got it right. Vertical engines do wear down though, and if not corrected with re-metalled main bearings, shims or packers might be a temporary fit. Clearance volume is really a piston to cover issue, but there is also the port volume to fill and empty on each stroke. Correct to say this space is wasted. A big improvement achieved with poppet valves and Corliss valves.
Here in the States, all the hot rodders use a product called plastigage, which accurately measures clearance.
 
Many years ago I built an engines so piston top dead center was flush with the end of the cylinder. Head gasket provided about .010" gap between the piston and head. Engine would not run well steam could not get into the cylinder quick with that small gap. Thicker head gasket made the engine run much better so I made changes to the engine. There is probably a gap spacing that works best but not sure how to calculate it other than trial & error. You don't want gap too large it will waste steam.

I use to have a 100 year old factory built 4x4 steam engine piston was dish shape on both ends. That engine would run 60 rpm on 5 psi steam. I sold that factory engine to the man that owns the Memphis Queen steam boat in Memphis TN. If you ever take a ride on that boat you might see the 4x4 engine in the room where they make popcorn and candy he used it to run the old steam power kitchen equipment.
 
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I know that guy, or perhaps it is one of his sons/relatives.
I will ask him if he still has that engine on his boat.

He did mention the other day that he still had the Stanley 20 hp engine that was on his steam launch from the 1800's.

.
 
A small amount of dead space also helps prevent hydraulic locking against condensate in the cylinder which can be very damaging.
Regards, Ken
 

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