For about a year now I have been saying “Yes, the engine is done, apart from a few bits and bobs” – so the other week I found myself waiting for some timber to arrive and decided to just “knock those bits and bobs on the head” – 10 minutes inspecting the engine revealed that there were in fact (at least) 23 jobs still to be done!!! So so far we have spent two weeks reducing this list to 7…. (and deciding to do some much later!) Herewith some notes on some of this activity…
Finishing the drive train
Firstly I completed the prop-shaft drive train which we discussed in the last post – only the prop-shaft itself now needs machining, and I have purchased the 7 foot of inch and a quarter 316 stainless needed and found a nice man with machines big enough to tackle this job (which he will lend me !!!)
Here’s a pic:
While we have had the engine running on air for some time, I really wanted to finalise the valve setting, and cut the keyways into the eccentrics to complete this bit of the engine.
LP Valve Chest Errors and Lap measurement
This meant that the open questions about errors in the drawings of the LP valve chest re-emerged… You may recall in the Hints and Tips section I highlighted some issues with the dimension of the ports in LP valve chest – these are shown as two different sizes on two different drawings 😦 – stripping and inspection showed I had machined them to the larger size (3/8″) which should have been bad news as this would affect the Exhaust Lap. However, when searching the words in the book produced the answer seemed to be that the Exhaust Lap should be 1/32″ (check this again Mal) but inspection of the drawings show this was actually going to be 1/4″ if the smaller port size was used or or 1/8″ if the larger size was used! <<baffled mode on>> – so I was expecting a problem in setting the valve events… (as Luck would have it I have a spare LP valve casting, so come the worst we can remake it, but lord knows to what dimensions!).
HP Valve leak
Also while playing with the engine on air it was clear that there was quite a leak (blowby) on the piston valve at the bottom of it’s travel. I knew that there was a small amount of taper on the valve and liner (my skills are not good enough to overcome the wear/inaccuracy on my 70-year old lathe) – but I thought I had compensated for this when building and hoped the SIX (6) piston rings would solve this, but clearly not 😦 ….. after a day or so’s playing adjusting and checking I finally realised that the piston valve was assembled upside down <what!> (because I stamped the wrong end “top”) thus my “compensation” was actively worsening not improving the situation! Assembling it the other way up dramatically improved things 🙂 .
Leak between Valve Chest and Cylinder – Doh!
I had also noticed a leak between these two castings – “The Book” said they were meant to be joined with a layer of Araldite as a sealer. I had fretted about this a lot, as it is well outside the operating temperature range of this type of Epoxy, and here it was leaking when it was stone cold!!! So I decided to strip and refit with some Rocol Steamseal (AKA Foliac). Stripping it down (and not looking forward to having to clean off all the Araldite) imagine my surprise to find the joints perfectly clean…. I had forgotten to put any sealant in at all!!!!! Idiot!
Setting the Valve Timing
“The Book” suggests a sequence of processes to set the timing involving templates marked out to position the eccentrics, and these require the use of the Lap and Lead measurements to construct. So given the above discussion about the doubts on the actual Lap and Lead shown and the fact that there were only a single set of dimensions for the two entirely different valves, and a set of words that seemed to leave a lot of unanswered questions, this was clearly going to be a “challenge”.
Listening to the valves
After DAYS of “faffing about” trying to interpret the words in the book and pages of description in the SBA Steamboat Handbook and an hour or so talking to John Maltby of the SBA, I decided to throw the theory to the wind and try a practical approach. So I connected a low pressure air supply to each cylinder, and attaching a digital angle gauge to the crank, I removed the relief valves from the cylinders, and barred the engine over by hand while listening for the hiss of air signifying the opening and closing points of the inlet side of the valves. This demonstrated that things were FAR from right. Following the same approach to find the exhaust port events (this time feeding the air into the exhaust ports and listening for the air flowing back into the cylinders) just made things more worrying…
So I decided the right answer was to document this initial state of affairs, and then adjust one thing at a time to try to improve the situation. Initially I started with scrappy bits of paper, but soon became overwhelmed by the number of numbers, so I designed a “worksheet” in Open Office to let me record the numbers (remember you get 4 events for each end of each of 2 cylinders and then a whole other set for astern timing – so 32 in total!) – Here’s an impression of some of the pages…
This was not a quick process and took several days and pages of notes to get a set of events that were as good as I could manage.
The game is to try to let the steam in just before TDC and BDC (like spark timing on a petrol engine) – then stop the steam part of the way down the stroke and let thermodynamics provide some of the push at the end for the sake of efficiency, and then open the exhaust (hopefully AFTER you have closed the inlet), and then shut the Exhaust just before TDC/BDC to provide some “compression” to cushion/slow the pistons at the end of the stroke and reduce the bearing loadings) – at one time or another the engine violated every one of these goals!
One of the key pieces of understanding was that by putting shims under the foot of the eccentric rods one could raise and lower the valve in ahead or astern and this demonstrated that about 2mm raising was needed on each valve, and significantly improved the timing of the valve events. (Later I removed the shims and raised the valves on their stems as the same changes seemed to work on ahead and astern on both cylinders, which suggests some in-built issues with the geometery of the valve gear, but this is a very arcane subject. Some when I will put all the data into Docstader’s or Ashton’s design simulators and see what they suggests ).
There were moments of deep despair but in the end I got something I felt good about, and as we would see later seem very effective….See video at end of page…
Having gotten the valve timing to a point I was happy with I then needed to cut the keyways in the eccentrics. Nigel explained this could be achieved by hammering a broach through the eccentric <gasp>, but I decided to push my trusty shaper into play.
I have tried this before but had trouble with tools digging in with semi-disastrous results. Consulting some of my old 1900 – 1940 books suggested some better tool holder shapes and designs, and what you see below works … I had a brainwave to make the holder to take 1/4″ end-mill and slot drill shafts… I had a number of broken small end-mills of this size (the 1/16″ ones just break soooo easily) and could simply insert these into the tool holder and then grind them to shape on the quorn, I just gave them about 3-degrees of top and side relief and about 10-degrees on the front and all was well…. see following pics (and shaper porn videos)…. Note how we cut the slots UPWARDS, this has two effects, 1) it stops the clapperbox coming into play (as I can’t lock it on my machine) and 2) it ensures that the backlash in the vertical slide does not result in the tool “falling” into the slot and cutting too deep… You might think this is knowledge born of a profound training in the subject – whereas actually it results from getting it wrong for the first attempt!!!
A bit of retro-techno-techno!
This is the pipe that transfers the exhaust from the HP cylinder to the intake of the LP cylinder and uses an external steam jacket (steam on it’s way to power ancillaries) to reheat (dry) this steam. Quite a simple construction – importantly it’s installation allows us to run the engine as a true compound for the first time….
The results I am VERY pleased with – the engine runs much more smoothly, and runs ahead and astern really well – and surprisingly “notches up” very well (adjusting the reversing lever to trade power for steam efficiency) – (editor’s note: The commentary on the video says there is a ‘simpling valve’ fitted, this is wrong, it’s an ‘impulse valve’ which provides a shot of HP steam to the LP cylinder to help starting if the HP cylinder is on TDC or BDC).