Conversion of 4 Cycle Utility Engine to Steam

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vtsteam:
After testing the valve location, I enlarged the valve hole and tapped it to 1/2" pipe thread to fit the valve body.

I also chipped out steam passages in the head using a chisel and continued shaping with an electric die grinder. I later polished the grinding marks out and assembled the valve on the head.

Getting close now, I thought. All I need to do is braze a couple lobes on the cam shaft opposite the present ones, and I'm done.

Not.

vtsteam:
So with valve and head pretty much completed it was time to simply add some braze metal to the camshaft, as secondary lobes, file them off to profile and I was good to go.

Just had to figure what that profile was. I marked the cam gear with the existing valve events after turning the crankshaft through 2 revolutions, expecting the timing to be reasonably close to steam timings. I figured it must be in the ballpark because so many people on YouTube and elsewhere have simply duplicated the existing exhaust lobe at 180 degrees, hooked the engine to their shop air compressor and spun up their new steam engine.

But after looking at the timing marks on my gear, I realized things were very different. 4 cycle timing is MUCH broader (naturally) than 2 cycle timing. And those cam lobes were a lot wider than what I needed. To figure the actual dwell for the crankshaft you have to double the dwell you get from the camshaft.

On my engine this meant that the existing cam would open the inlet for about 200 degrees, instead of the desired 100 degree dwell, and the exhaust would be open for about 300 degrees instead of the desired 180. Not only that but the two events would overlap by about 60 degrees. No good.

The photo below shows the measured valve events on the stock cam.

vtsteam:
This seemed to mean I'd have to modify both the old cam lobes and the new ones. Well modify how much? I spent a lot f time sketching out cam lobes, and found a cam coordinate program on the web.

I wanted at least one eighth inch of lift on the inlet cam (for the ball valve) and I wanted about a quarter inch of lift on the exhaust cam for the poppet valve -- roughly the same as it had been on the IC engine. When I tried to sketch this out, or enter it into the program, the inlet cam wouldn't work out unless I made its "base circle" much larger than the original cam. I could not get .125" lift for 100 degrees of dwell on anything smaller than about a 2" diameter base circle. The original cam lobe base circle was 1".

I learned a lot through this exercise. The main reason the base circle had to be so large was that the camshaft was running at half the speed of the crankshaft. And the "mushroom" tappets will bridge any hollow in a cam profile. They will always maintain a tangent to whatever two points they rest on -- in a proper cam they will only rest on one point at any time, but it is possible to design a new cam profile that isn't correct -- in that case, the tappet will "correct" it by ignoring the dip and bridging it.

A small dwell angle like the 100 degrees we need on the inlet of a steam engine (a short cutoff for steam economy) means you need a very big cam to get even a small amount of lift. Particularly on a half speed camshaft.

Anyway, I eventually drew out both an inlet and an exhaust cam that seemed like they might just work, assuming I didn't try to round off much of that profile. Pretty much straight sides to the top of the bump, a sharp angle there and a straight side back down. Not an ideal profile as far as wear goes, or for valve train acceleration. But the best I could do within the constraints of the size of the valve chamber, which I measured carefully. The inlet cam would just fit in. The exhaust cam was somewhat smaller.

So I drew these out as full round cam blanks. I would have to turn off the old cam blanks with the lathe (also rescued from storage) and then braze on the new cam "donuts".

I figured I'd make the exhaust cam bore a little bigger than the inlet cam's so it would slip on the camshaft easier. The camshaft was a solid piece of cast iron, including the cam gear.

Below are drawings of the cam blanks I decided to use. The method of construction was to to turn the blanks to base circle diameter, and then braze on pieces of .125" dia. and .25 dia drill rod. These would set the max lift, and I would fillet them with braze and file to profile. When the file touched both the rod, and the base circle blank, that would be the profile.

(Both show only one lobe for clarity, though each has two. The inlet cam doesn't illustrate the .125 drill rod pin, the exhaust cam does show it and the intended braze fillets)

vtsteam:
The coordinate program I found on the web gave me a bunch of coordinates that looked good as a table, but were totally wrong when I plotted it out. My pin and braze method was the one I chose to make the cams.

I turned down the camshaft lobes, made the cam blanks and bored them, and then brazed the pins in position after carefully marking out. Then I spent an afternoon carefully filing the braze down until I had my new cams.

Below are the cam blanks from scrap 1/2" plate -- interesting patterns on the face from exposure.

And the filed cams ready for mounting on the camshaft. The big one is the inlet cam.

vtsteam:
A chance reference on an internet account of another conversion (the usual weld blob camshaft conversion job) mentioned having to narrow the width of the new cam lobes, because other wise they would interfere with the crankshaft counterweights when they swung up to the cam area.

When I read that I ran out to the work shed even though it was a chilly night, assembled the cams onto the shaft and installed them into the cam space in the engine. Then I rotated the crank.

:doh:

Uh...........no go.

Big time no go.

Not even "trim a little here, slide this over a bit" go.

I mean NO GO. The space for the camshaft in the crankcase was obviously sized to the crankshaft throw. So unless the cams were original diameter, they would interfere.

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