Maltelec
Site Admin
Joined: 23 Sep 2007
Posts: 156
Location: Cumbria, UK
Posted: Mon Oct 08, 2007 4:03 pm Post subject: My new engine design
For quite a while now (about 1 1/2 years) I've been designing an engine. It is quite different to nearly all other steam engines, though it is a uniflow design.
I have put 2 pictures on here, one showing the engine without the valvegear and one with as it's almost impossible to see what is going on.
The valve gear enables the engine to run from 0 to 100% cutoff and also alter the timing from TDC backwards to BCD (for reverse, this also alters the cutoff start time). This does mean that the cutoff won't work going astern, and both the inlet valve and exhaust ports may be open at the same time, but I don't plan to go astern too much.
Its 100mm bore, 80mm stroke per piston. The idea is to run it with a very large cutoff to get the power out of the expanding steam, hence the 160mm stroke, but the valvegear will also enable me to use it nearing 0% cutoff for starting and manuvering until the vacuum builds up.
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elpenor
Just Starting Out
Joined: 04 Oct 2007
Posts: 5
Location: Newport, South Wales
Posted: Tue Oct 09, 2007 3:38 am Post subject:
Simon,
I like opposed piston engines, the original Elpenor had one, a 7 cylinder diesel, but there is a lot of work here for no real advantage other than being a bit of fun. A few things do come to mind though.
First, the top piston and its rods are much heavier than the bottom so the stroke should be reduced to about 2/3 of the bottom. This will help with the balance.
Second, lubrication of the bottom piston will be a problem as there is no crosshead and there will be a lot of side thrust on the piston. Too much oil slopping around will get into the exhaust and need to be removed in the hotwell. The top piston, on the other hand is a long way from the crankshaft and a single "M" shaped conrod with a gudgeon pin in the piston would be simpler and lighter.
Third, you could get away with only one set of exhaust ports in each cylinder, although this would lead to cooler exhaust steam sweeping along the cylinder and you loosing the advantage of the uniflow, which keeps the hot inlet away from the cool exhaust.
I would like to see what plans you have for the valve gear.
David
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Maltelec
Site Admin
Joined: 23 Sep 2007
Posts: 156
Location: Cumbria, UK
Posted: Wed Oct 10, 2007 12:09 pm Post subject:
I've got a cunning plan for the weight problem, make the top piston out of aluminium and the bottom from cast iron or suchlike. Essentially keeping the top piston and bar thingy the same wright as the bottom piston, and the bottom conrod and crank pin bearing the same wight as the 2 long con rods and bearing.
It won't be perfectally balanced, but it should be quite reasonable.
I did think about an M shaped conrod, and I've been wondering if it would be any advantage over the current design.
Quote:
you could get away with only one set of exhaust ports in each cylinder
Almost certainly, but as its no great effort to have 2, and there is quite a large volume in the cylinder where all the steam which may have taken nearly the full stroke to fill then has to be removed within a much shorter ammount of time. That would be the worst case situation of course, but I designed it with my mind set on only wanting 1 bottleneck in the whole system, that being the controlling factor (the valves).
Lubrication is always a problem, but I'll hopefully of thought of the most ideal system before the end.
The valve gear is the tricky bit.
Ignoring the rear shaft, which simply rotates the front one by 180 degrees in relation to the crank for reversing, it uses the same principle as Elpenor to move the cams but it uses the cams in a completely different way.
The central driven shaft has a keyway in it. A larger shaft is ps placed over the central one, and this has a helical keyway in it. Using a ring with fingers pointing inwards so they go through the outershaft helical keyway and into the central shaft's straight keyway, the outer shaft is rotated from 0 to 180 degrees (ish) with respect to the central shaft.
Where the central shaft comes out of the outer shaft (the outer shaft being shorter), there is a cam fixed to the central shaft each end. There are 2 more cams one fixed at each end of the outer shaft. So just taking 1 side into account, you have a fixed central shaft cam on the left (blue), and the fixed outer shaft cam on the right (orange), with a 60mm gap in between:
You should be able to see the mechanism in the middle above the 2 cams. Each cam lifts by 10mm. The shaft to the valve is levered so a 10mm lift causes a 10mm movement in the shaft. The shaft also has a 5mm sprung gap in the middle so it takes up the first 5mm of movement before it starts to open the valve.
The mechanism in the middle of the 2 cams is a double-roller bearing link in a horizontal Z format. So the bearing which runs on the orange cam is towards the back and the bearing which runs on the blue cam is towards the front, and they are pivited in the middle.
When 1 of the cams becomes high (by 10mm), 1 cam follower (the bearings) which pushes the middle up by 5mm so no movement is transfered to the valve.
When 2 cams are high, the 2nd cam follower is raised by 10mm, which raises the middle by a further 5mm, there is no more space to be taken up by the valve rod and so the valve is pushed open. The valve has a lever on it to make the 5mm of movement push it open by 10mm.
So basically, the vale only opens when both cams are high.
The outer shaft can be turned through 180 degrees (ish) which moves the innter 2 cams round while it is running so instead of both cams being high together, the cam followers will rise on one cam for so many degrees before the 2nd cam becomes high, and the 1st follower will lower before the 2nd one does.
This will enable me to alter the cutoff of the valves from 0% to 100% so the maximum ammount of steam is let into the cylinders before the valve cuts off and it will then expand as much as it can do, especially due to its relatively long stroke.
The valves are balanced poppet valves.
_________________
I've got the vehicle , just need the boat.