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ModelWorks Pride
of Penrhyn
Overtype Steam Lorry Valve Gear Setup |
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June 2005: When I originally assembled the POP I was confused by the kit 7 instructions. When the valve gear is assembled according to the text the engine would not run - one cylinder operated in the opposite direction to the other! To solve the problem I swapped one pair off eccentrics so that both cylinder ran in the same direction but in my ignorance did not realise that there was a right and wrong choice. October
2008: I have recently been investigating the valve
gear on my 4 1/2" Foden
and I have realised that I did not properly understand what was going
on when I "fixed" the POP. The POP like the Foden is a 2 shaft
engine so in order to get the reverser to work in the accepted sense
the
eccentrics need to be crossed. Another source of confusion was my
belief that crossing the eccentrics affected the angle of advance.
When running in full gear the configuration of the links makes little
difference so the angle of advance and direction of operation of a
given eccentrics is not affected by the configuration of the rods. The
forward eccentric is determined by the eccentrics relationship to the
crank. I now realise that the configuration of the rods (open/crossed) alters the way the engine notches up. I seems to be a fact of life that engines with crossed rods won't notch up as much as an engine with open rods. The effect will be particularly pronounced where the connecting rods are short in relation to the throw. Had the POP been a true compound as opposed to double high an option for early cutoff would not have been as important. Even though my thinking was flawed there is no doubt that my changes to the POP's valve gear made a tremendous improvement. There is however a downside to my modification in that the reverser has to be pulled back to go forward. Since operation of the reverser is a major part of my emergency stop routine I now would not be willing to drive a "backwards" engine. A better solution is therefore desirable. It may be possible to change the reverser linkage to achieve forward travel with the reverser forward or to optimise the POP's valve gear to work satisfactorily with crossed rods. I have left to account below to record what I did. I have crossed out statements I now know to be incorrect. I stumbled across this error whilst I was investigating the Burrell's valve gear - it hit me in a blinding flash. When I originally set up the POP gear I was frustrated that what ever I did the cutoff was occurring at the end of the stroke and I found than only one reverser notch in either direction produced even running. The Burrell investigation had taught me that the eccentrics are advanced beyond 90 degree of the crank so that the admission event occurs near top/bottom dead centre allowing the different reverser setting to alter the point of cutoff. Reversing the angle of advance (computer simulation) had shown me this makes the cut of point constant whilst the reverser alters the admission point - exactly the problem I was experiencing with the POP. The salient point to note is that not only does crossing the eccentrics cause the engine to run in the opposite direction it also reverses the angle of advance. Given that ModelWorks preset the angle of the eccentrics on the crank it follows that there is only one right way to assemble the valve gear. Whilst the instruction text is incorrect it turns out that the isometric drawing ISOKIT7B is correct. Warning not all POP batches are the same - The eccentric orientation for Winson POP's is different and, if in doubt, I recommend checking from first principles. As the Modelworks instruction are confusing I have produced a diagram that is applicable to POP Kit 7 instruction dated 01/07/03.
Each cylinder needs to be setup/checked in turn. Rotate the crankshaft until the piston is closest to the crank and the crank pin is towards the back of the POP - I call this Bottom Dead Centre (BDC). As shown in the diagram above the eccentric links should be open and not crossed. To avoid any possibility of confusion I have included a picture below of this setting.
With this setup the POP goes forward with the reverser backwards. I have exaggerated the angle of advance in the figure but it is possible to observe this on the POP. Set the engine reverser to mid gear and rotate to BDC. Looking vertically down on the crankshaft note the position of the rear of the eccentrics. Now turn the engine through 180 degrees to TDC and observe again the position of the rear of the eccentrics - it should have moved backwards.
Having sorted out the angle of advance I was disappointed to not to discover a dramatic improvement so I decided to measure the valve displacement in order to produce a valve timing diagram for the POP.
I have reproduced my results table
for both forwards setting in
the table below. All dimension are in mm. The same
data is plotted
on the right; the red ruled list represent the POP's valve lap which on
my POP
is 1.08 mm. I am currently pondering whether the POP would
work better
with an increased lap.
April 2006: I have recently looked back on this article expecting to find details of the replacement side valves I had made. I replaced the valve soon after producing the above indicator diagram and the improvement in performance was dramatic. Not only did have to increase the inlet lap I found that I had to decrease the exhaust lap. In simple terms with the old valves steam was being admitted before the completion of the previous stroke and being exhausted before the stokes completion completion. Subsequent to modifying my POP I did the same modification on Brian's POP and in 24 hours transformed a jerky runner into poetry in motion. I have found this short video I took of Brian's POP after replacing the valve and fitting cast iron piston rings. This is particularly good example as I know that the only modification performed were to the pistons and valves. Before the modifications there is no way Brian's POP would have run this slow. October 2008: In relation to crossed rods, open rods, result in a greater valve movement hence my need to increase lap. |
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