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Radial engine build


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2300cc. I'm aiming for about 100BHP at 3500rpm.

The heads are from a motorcycle engine which develops 18bhp per cylinder, so I think that 100 total (approx. 14 per cylinder) is not unrealistic.

What happens when the supercharger is engaged is anyones' guess. I've had a swipe at the boost pressure calculation, but there are so many assumptions that I can't have any faith in the results.

There may be some fettling of supercharger gear ratios when it's up and running.


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Performance is not really my goal here. The supercharger exists primarily as a means of improving mixture disribution between the cylinders. The ability to add some modest boost is a bonus.

This week, I have mostly been marking out and machining heptagons. I have also learnt to post the pictures in reverse order so they show up in the intended order. :D




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  • 2 months later...

Between a new job, starting NPPL training and life in general, I have made some progress with the engine:

The first picture shows one of the barrel base plates. 7 of these make up the outside of the crankcase, with a cylinder barrel mounted to each one. In this pic, the plate is bored and drilled but has yet to be machined at the angled ends.

The second pic shows the setup for machining the angled ends. These are individually matched so that when everything is bolted together, there is equal compression in all the joints.

The third pic shows the crankcase partly assembled with 3 of the barrel base plates machined and one part machined.




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  • 3 months later...

More swarf has been created, as I have pocketed the inside faces of the crankcase front and rear plates.

Switching into production mode, I made 28 Aluminium Bronze inserts for the locations where the cylinder studs screw into the crankcase.

By the Fly-In, I should have it assembled into something that looks like a radial engine.



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The crankcase assembly is now fitted with 28 dowels to ensure alignment after dismantling and reassembly.

The first picture shows the assembly set up on the mill to finish the main bearing bores.

The second picture shows the crankshaft, rods and pistons assembled into the crankcase.

This week: Trial fit barrels and heads, fabricate a test stand.



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  • 4 weeks later...
  • 4 weeks later...

One of the more challenging parts to manufacture is the cam ring.

The first picture shows the blank being turned. This a an 8kg billet of EN8 steel which will be a 1.5kg part when finished.

The second picture shows the first stage of machining the cam profiles. In this operation, the base circle is being created. I have left the inside diameter of the blank smaller than finished size so that I could drill some small holes at the centres of the various radii which form the cam profile.

The cam profile is based on the timings of the Dnepr cam (from the same engine as the heads). I have modified the profile to take advantage of the large radius cam ring and generate near constant acceleration valve motion. This reduces stress on the valve train components and permits the use of softer valve springs.

The followers will be roller type, eliminating the need to harden the cam ring beyond the properties of the EN8 material.



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It all looks sooooo much smaller in the pictures that it actually is !!! :-)

I feel a little silly being one of the people who asked in the early days if it was for a Paramotor LOLOLOL

fkin thing could power a spitfire!! lolol


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Unfortunately the power / weight ratio won't be adequate for flying anything modern.

At a projected 100BHP output and 120Kg dry weight, it would just about drag a small WW1 biplane into the air.

I wouldn't be confident that it wouldn't shit out at 50ft.


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  • 2 weeks later...

Machining the crest radii on the cam profiles and finally boring out the surplus material left in for fixturing.

On the wall next to my mill is a sheet of paper with a long list of angles and offsets required to generate the relatively simple cam profiles.

I am amazed that I managed to do all of it without a single phuquup! I had planned in some detail how I could recover the part in the almost inevitable event of a major machining error.

Next: timing gears. A relatively simple compound planetary arrangement. For complex timing gears, look up the Bristol Centaurus - described as a 2500 horsepower Swiss watch!



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  • 1 month later...
  • 2 months later...

At the limit of the machines capacity, the holes for the pushrod tubes are bored into the tappet ring. Each hole is counterbored to house an O ring seal at the base of the pushrod tube and similar features will be machined into the cylinder heads. Anyone with experience of Meriden Triumph engines will understand!

Each of the 7 lugs on the inside of the ring supports a pair of the swinging tappets, whose forked ends overlap. Inlet and exhaust tappets are identical, it is the design of the tappet and the offset created by the overlapped ends which places each tappet in line with the appropriate cam.


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