Hackworth valve gear etc.

As the build notes for the engine mentioned the inherent inaccuracy of the 'modified Hackworth' valve gear, I thought it would be interesting to simulate the gear - here are the results.

This first clip shows how the motion works going forwards, as seen from the right hand side of the loco.  (I am using my own terms in the description, and I have made some assumptions about the dimensions of various parts).  The 'connecting rod' from the crank to the piston slide is shown in blue.  The video clip starts with the piston at bottom dead centre with the eccentric at top dead centre. As the eccentric goes round, it drives the motion with the 'eccentric rod' linked to the 'motion arm' .  The motion arm is attached to the 'reversing arm', which is as far back as it will go for the engine to go forwards.   The 'valve rod' is attached to the eccentric rod not far from the motion arm, so that the resulting movement at the valve slide is a combination of the vertical position of the eccentric (matching the crank position) and the movement resulting from the swing of the motion arm (out of phase with the crank).  The movement at the valve slide is such that when it is up, the piston will be forced up by steam from the slide valve; when the valve slide is down, the exhaust is open to let the steam out as the piston goes down.  In between, the steam expands to provide force for the engine.  

The next clip shows the engine in reverse, with the reversing arm as far forward as possible.

The graph shows:

- in mid blue, the position of the piston slide;

- in red, the valve slide positions when going forwards;

- in light green, the valve slide positions in 'neutral'; 

- in purple, the valve slide positions when reversing (look at the chart from right to left).

The horizontal lines show possible positions for inlet and exhaust port positions - the exact dimensions of the slide valve will determine if my assumptions are realistic.

(the valve slide positions are not to the same scale as the piston slide, and are adjusted to fit on the same area of the chart)

I think I can now see why Steve at STW was talking about inherent inaccuracies of the valve gear.  I think that designers aim for symmetry to enable the best control and efficiency.  This can be seen to not be the case here.  The red line shows that when going forwards, the valve is being pushed much further than in all the other cases.  This seems to arise from the eccentric rod over-reaching and forcing the motion arm too far up.  This could be relieved by rotating the Crank Bearing Housing by about 15 degrees (anti-clockwise in the clips).  I am not sure if I have the confidence, knowledge or skills to do this successfully.  (If it works don't fix it!).  I have seen images of Hackworth valve gear in which the slide valve is positioned at an angle to the cylinders to align it directly with the centre of the valve gear - to achieve this would mean inserting a wedge across the slide valve mating surface (not my idea of a risk free improvement).
I have been worrying about the effects of condensed steam forming an hydraulic lock in the cylinders, but I have been told that slide valves are able to cope with this situation.  If (incompressible) water forms inside the cylinder, the excess pressure lifts the valve which is normally held down by steam pressure.  The excess water is supposed to be released and pushed to the exhaust by the steam which rushes through.  I would like to run the engine really slowly if this situation occurs.

I was also wondering about putting the cylinders in a wooden casing or box so that heat losses can be reduced and the risk of burns be lessened.

Engine in assembly

Before I started assembling the engine, I tried to make sure everything so far was able to be assembled. I am in two minds about the benefits of good daylight: you can see what you're doing, but you can see what has gone wrong with the paintwork.  On examination, I find the paintwork needs to be improved for complete coverage, and then another coat is likely to be needed to make a good presentation: I even have a better brush with which to apply a final coat. I shall need to look critically at all the details to make sure that potential dirt traps are suitably filled. The domed nuts for fixing on the sides can be seen.  The central tube of the engine has been put in place temporarily - it seems to be a good fit, but the upper fixing may need to be filed out so that the mid-way support fits the superstructure comfortably.  Although the tanks are stainless I thought they would look better with  the same finish as the rest of the car.

...and a view of the rear of the car so far.  Some paintwork is not yet finished.

Now for the newly arrived parts:  This shows the arrangement of the steams ports in the cylinder. Not shown is the work I have done to widen the central hole to 6.5 mm diameter and tapered the cylinder ends of the holes to improve the steam flow (without any experiment I don't really know how effective this work will be)

This last picture shows that I have started on re-shaping and polishing the central exhaust port to hopefully improve the outgoing steam flow.

The next task will be to fit and assemble the crank shaft and bearings - I have little experience of fitting parts which have an interference fit, so I am hoping that my efforts will be successful without needing to use too much hammer aided persuasion.  The next instalment is expected imminently, as long as FedEx don't lose it!  So I am assuming that I shall soon have the pistons and connecting parts in place, and then some more painting.  Some of the engine parts are likely to get warm, so I shall look out for some higher temperature paints for them.

Engine Coming

FedEx lost part of my engine. The latest kit contained 2 packages; one was delivered.  They have now said that the 2nd package has been traced. The missing part was the frame - I now have the cylinder blocks and crankshaft pieces.  It is no real hardship to be delayed as I have some extra time to finish some more painting.  I am painting over any bolts which form part of the main assembly and which I do not intend to undo again.  I have gained some bumps and chips to the paintwork, so another coat will go on when the re-assembly has been done.
Thinking about the next stage. ... In principle , I believe that all steam channels should offer the least resistance to the passage of steam.  The routes should be free from any unnecessary constriction and should be smooth with no sharp edges.  An opening with sharp edges has a discharge coefficient of about 0.6, but that for a smooth opening is about 0.98.  I think that any "fitting" activities will involve smoothing sharp edges and irregularities, polishing the sides and maximising the opening area - all to reduce pressure losses.  I was surprised to see that the steam route from the port to the cylinder end comprised three drilled holes next to each other.  To reduce pressure loss along the way, I shall taper (/streamline) the ends of the holes so that they more resemble a venturi,  the holes might allow for some widening, and the holes could be smoothed, probably with a needle file.  I am not sure if there would be any undesirable side effects from combining the 3 holes into a single slot - could it reduce the strength of the cylinder, or cause distortion, or cause a stress concentration/ fatigue cracks?  I shan't be touching the steam ports as I don't want to upset any valve timings.  The GA of the engine gives an indication as to the dimensions of the valve gear - it should be possible to make a simulation.to better understand any problems with timing set-up.
The exhaust port is not centred on the exhaust pipe fitting - I shall try to reduce any irregularities by filing and smoothing. 
Pictures to come.

Trial assembly of superstructure

The superstructure is now assembled and suspended ready to trial fit onto the lower chassis.

There was a certain amount of filing needed to enable the bolts to align with their holes.  The plywood sides fit nicely and add some rigidity.  

The next job after the final trial fitting will be to dismantle it partly and do some painting.

Trial assembly completed, front view.

Rear view - the inside looks empty now!

shows how the battery box is too far to the left - I shall need to touch up some paint where the box jammed against existing paintwork. I have sorted out the positioning by using some longer bolts and spacing washers to attach the pedal mounting to the box. (The packet of new bolts can be seen in the box, next to the sealant to be used for keeping out dirt and wet)

The rear left spring mounting was welded to the chassis with not quite enough clearance for the nut.  After filing a little bit, the nut fitted snugly, as shown.  Now I can start to dismantle and paint.  I must remember to use WaxOyl on parts that the paintbrush won't reach, and to use some silicone sealant wherever there seems to be a risk of dirt and moisture getting in.  The chassis side members will need a piece of cord, so that wires can be fed through at a later stage.