Radial engines

[Parksy]

Hi all

I was wondering how people go about machining the cam ring in four stroke radial engines. A conventional cam is relatively straight forwards to machine using a mill and rotary table. Can a cam ring be done in a similar fashion?
Things I've learnt so far are that a 9cylinder radial has four lobes on each cam ring and the cam ring rotates at 1/8th of the crankshaft.
Does a 7cylinder also have four lobes on the cam ring and again rotate at 1/8th the speed of the crank?
I have a basic idea on radials but would like to expand my knowledge somewhat.

Thanks all

Andrew

 

[petertha]

Hi Andrew. My head has been in 5-cyl mode for so long, I cant think about more cylinders I think the procedure goes something like this. Hopefully someone will correct if I'm wrong.

Number of inlet or outlet cams = (Y-1)/2 where Y=number of cylinders
so 5 cyl=2 lobes, 7cyl = 3 lobes, 9 cyl = 4 lobes

Ratio of crankshaft speed to cam ring speed = (Y-1)
so 5cyl = 4:1, 7cyl = 6:1, 9cyl = 8:1

Now you need to figure out a gear cluster that a) yields this reduction ratio and b) satisfies that the pitch diameters line up exactly because its constrained between CS centerline & ring gear ID and c) the small tooth crankshaft gear accommodates the CS diameter itself.... and probably a few more factors I forgot about. So only certain gear combinations will work. Here is a post I contributed regarding gear layout that might help visualize.
http://www.homemodelenginemachinist.com/showthread.php?t=25583

Different gear arrangements will translate into larger/smaller overall cam ring assembly ODs if that's an issue (ie. this must all be housed within the crankcase volume). So a few design trade offs here & there.

For example if you had the gear arrangement like the link but designing for a 7-cyl radial, one option might be 40 tooth (crankshaft) > 4o tooth (idler) > 16 tooth (on common idler shaft) > 96 tooth internal tooth ring gear. So the ultimate ratio is 96/16=6:1. Another option could be 45 > 45 > 18 > 108, ratio = 108/18=6:1.

In terms of making the cam ring, there's a few builds on the forum to reference. Some lucky fellows utilized CNC but others I believe used rotary table & graduated angle/radius cut routine over the cam lump profile. At least.. that's my plan

 

[gbritnell]

Hi Andrew,
Just to clarify a little more, there are no 4 cylinder radials. All are odd numbered. I have only built one which is a 5 cylinder based on the Morton M-5. When I redesigned the engine I discarded the original design and went with the more traditional internal gear cam ring. My cam ring has 3 lobes on one ring which operates all the valves. I used a commercially available gear which had enough material on it so that I could machine the lobes into the O.D. I then added to the gear to provide for more thickness and to create a center hub.
Peter, I'm not all that well versed on radial cam design but in your explanation you state number of inlet and outlet cams and then calculate lobes. Cams, lobes? As I said my 5 cylinder has 3 lobes and one cam ring.
gbritnell

View attachment ENGINE RADIAL SHT 4f.pdf

 

[petertha]

Thanks George. I was actually digging for your nice build post to specifically reference but it was getting late. I got my formulas from Aircraft Engine Design by Liston (1942!)

I think what's going on is rooted exactly as you stated, even though the gear cluster arrangement is same as referenced, the difference is your inlet & outlet share the same lobes. So 22 > 22 > 10 > 60 yields 6:1 ratio. But 6/3 lobes = 2 CS turns per 1 cam event, so it all works out.

I haven't thought this through fully so maybe blowing smoke. But I think sharing the same lobes locks the inlet & outlet timing the same. That's not necessarily bad in fact if the engine is happy, its simpler mechanically. The example I mention assumes 2 separate dedicated cam rings, one for inlet the other for exhaust. The cams can be phased relative to one another & also lobe lift duration can be altered to affect overall timing (inlet may be A degrees, exhaust may be B degrees). Maybe these factors provide more design flexibility when required?

Having said that, I have to go back & check. I think my 5-cyl cam lobe profiles are identical even though the I/E followers will occur in different plane orientations & so move pushrods > rockers a bit different in 3d space. Must not be enough to fret about.

Did you do your cam lobe profiling on rotary table? Any chance you can locate your post? If I can keep plugging at my snails pace, I might actually be making mine this winter (fingers crossed).

 

[Parksy]

Thanks guys, I really appreciate this information.

Gbritnell, I meant to say four stroke, not four cylinder. I seem to do that a lot...

 

[Parksy]

What sort of bore to stroke ratio do radials generally have? Do they run a flywheel?

I assume they run a dry sum system. What are the critical oiling points inside?

 

[petertha]

Here are some engine specs of engines I was interested in and/or had access to data. It was intended for a different exercise but I threw bore:stroke ratio in there for you. You might be able to obtain more figures from RC manufacturer websites like OS, Saito etc.

Flywheel? Going to say mostly not because radials are mostly aircraft engines so look best with propellers I'm not up to speed on flywheels myself, I guess its doable but you probably want cooling.

Lubrication varies by engine. The glowplug radial I'm building has premix oil with methanol fuel, but the timing gears run in a segregated oil bath. The induction comes in the rear of crankcase & then up into induction pipes, so that path mists the crankpin & rod ends in the process. The Edwards 5-cyl glow has a mechanical oil pump, it runs straight methanol. The gasoline 9-cyl radials like Hodgson have oil pump & I believe zero/minimal oil in fuel. I've always wondered about he rocker/valve faces for lube but some of the RC engines I've seen are pretty wet under the valve cover so I guess there's enough blow-by? I cant speak other engines in this regard.

 

[gbritnell]

Hi Peter,
Yes my profiling was done on a rotary table. As I stated the internal gear that I purchased had a lot of stock on the outside. This allowed me to mill the lobes onto the ring gear. The valve timing is symmetrical but for modeling purposes it's not a big deal.
Here are 2 links on this forum to the build and running the engine. The Youtube video is in the second posting.
gbritnell
http://www.homemodelenginemachinist.com/showthread.php?t=15122
http://www.homemodelenginemachinist.com/showthread.php?t=20963

 

[mcostello]

Why must radial engines be an odd number of cylinders?

 

[IceFyre13th]

Why must radial engines be an odd number of cylinders?

I Googled your question and got.......

Four-stroke radials have an odd number of cylinders per row, so that a consistent every-other-piston firing order can be maintained, providing smooth operation. For example, on a five-cylinder engine the firing order is 1, 3, 5, 2, 4 and back to cylinder 1.

 

[gbritnell]

From what I gather there are several reasons. One being that the engine runs smoother due to the angle of firing between cylinders and the shorter stiffer crankshaft. Another is the way the cams are made (ring type) they wouldn't lend themselves to even numbered, odd firing engines. I'm sure that when they were first designed even to odd numbers were looked at.
gbritnell

 

[petertha]

... the internal gear that I purchased had a lot of stock on the outside. This allowed me to mill the lobes onto the ring gear.

One detail I was looking for & sorry if I missed it - did you case harden the machined cam lobe surface once profiled, or leave it stock hardness vs. risk of warping? I recall you used rollers on the cam followers vs. 'sliders' which probably helped the situation I assume.

I'll have to cross this bridge myself. Plans call for cam ring hardening although its confined to plates which are then attached to the internal gear. I guess the only way is to try.

 

[Parksy]

Thank you all for your responses. I would love to make a radial engine one day and in my mind, the cam ring and gears to drive it would be the hardest part of the project.

 

[gbritnell]

Hi Peter,
The cam gear as supplied from Stock Drive Products is made from 303 stainless steel. The hub flange that I added is the same material. Even if I had thoughts of making a hardened ring at the beginning I wouldn't have been able to do it with this material. You correctly stated that I have rollers on my lifters. These are made from drill rod and hardened. To keep the lifters from rotating in the guides I milled small flats on them and then tapped the crankcase for tiny set screws which were ground flat on the end. I put a dab of Loctite on each screw and adjusted them so there was about .002 clearance. I use hardened lifters, or roller lifters, running against unhardened drill rod cams in my other engines. For the amount of time the engines run I have never had any wear issues, even on my 302 V-8 which probably has more than 300 hours on it.
gbritnell