Having decided to change the lubrication system to take oil directly from the pump and direct it to important parts of the engine (see here) it was now time to fill in the details. A few decisions needed to be made to be able to proceed:
1) I decided not to introduce any kind of overall flow control or pressure regulation – the entire unrestricted output of the pump would be used.
2) 4 parts of the engine would be targeted directly: (1) the crankshaft (and therefore everything accessible in the inside of the crankcases – main bearings, piston/cylinder); (2) the upper bevel and (3) & (4) both cams. Oil from the feed to the upper bevel chamber would flow down to the lower bevel chamber and then, via a drain, to the crankcase. Oil from the cambox would be scavenged via an internal tube the height of which has been set at the bottom of the gears and returned to the oil tank by gravity.
3) The relative flow rates to each part of the engine would be controlled by the diameter of quills. Now, a quill is essentially a small diameter pipe rather than an orifice but, in both cases, the flow rate is related to the square root of the pressure drop. The upstream pressure is the same for all of the quills – it will be whatever the pump delivers, but the greatest unknown is the downstream pressure which will be different for each and, for the crankshaft and upper bevel feeds, an oscillating pressure resulting from the crankcase pumping. The effective area of the crankshaft oil drilling/big-end bearing assembly downstream of the quill in the mainshaft (which is where the oil ends up before it gets thrown out into the crankcase) provides an additional flow restriction and is also unknown.
4) Bearing these considerations and uncertainties in mind it is difficult to do anything else but to assume the downstream pressure for all of the quills to be similar and to base the flow rates solely on the area of each quill.
The sizes that I have chosen for each quill and the relative flow rates using these assumptions are given in the table below.
|
Targeted Engine Part |
Quill diameter (mm) |
Flow Area (mm2) |
Relative Flow Rate (%) |
|
Crankshaft |
3.25 |
8.30 |
47 |
|
Upper Bevel |
2 |
3.14 |
18 |
|
Inlet Cam |
2 |
3.14 |
18 |
|
Exhaust Cam |
2 |
3.14 |
18 |
(Yes, I know they add up to 101%!)
If all of the flow areas are added together and, hypothetically, the oil exited from a single quill then the diameter of that quill would be 4.75mm (3/16”).
Lubrication of the K-12 phosphor bronze bush and shaft supporting the K-18 bevel gear is also a bit uncertain. These will have to rely on oil finding its way behind the bevel gear and being transported by pressure pulsations from the crankcase and any pressurisation that the upper bevel quill feed provides. In practice, I don’t think there will be a problem here.
It took a couple of days of machining to make all the brass bits and pieces for connecting the ¼” OD copper pipes and making the quills etc. The T pieces are bought-in items. I would also normally buy the fittings that are commercially available but as the post is usually a bit slow over the Christmas period and it can get quite expensive with the sheer number of fittings required it was more expeditious to make them.
I showed a picture of the crankshaft quill arrangement in a previous blog (which has a LH thread to prevent it unscrewing) and is made of aluminium
but the bevel gear quill
and cambox quills
are made of brass and are silver soldered into the screw caps.
After bending a few copper pipes, the arrangement looks like this
….and is starting to take on the appearance of a model steam engine.
When I made the fittings, I didn’t appreciate how far those for the cambox quills would protrude and end up interfering with the frame down tube at the front and the carburettor at the rear. I therefore decided to remake these for a more compact assembly using 900 brass elbows.
The oil feed to the engine and the scavenge back to the oil tank will be plumbed in when the engine is in the chassis. The oil tank that I have will also need another entry to accept the oil return from the cambox.
And so, the completed network of external piping looks like this.
The RN carb has been loosely fitted to check that the pipework avoids contact.
There are a few small jobs to complete the engine build but is nearly time to start putting engine and chassis together.
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