Valve timing 4 stroke glow

[petertha]

I put together a little comparison of valve timing figures among some engine specs I had access to (all 4-stroke, glow plug ignition, methanol fueled). I was interested in how they lined up more as a guide to something I’m drawing up now, so excluded gasoline/ignition.

The commercial engines were referenced in Clarence Lee’s book: The R/C 4-stroke Engine. He mentions the factory specs & also his own measured timing, sometimes they don't correspond. The other 'shop construction' engines I had to try & figure out from SIC–type drawings. (Hmm.. I think I got it close, but I’m still scratching my head on some). I’m still getting my head around ‘cam math’ so hopefully its accurate.

What stands out is a surprising variation when they are lined up despite what I'd think to be similar requirements: easy starting, good idle, nothing really racey or high performance? I’d appreciate any comments on this. Maybe it just doesn’t matter that much? Or maybe any home-shop simplification trick trumps ‘ideal settings’?

Also, does anyone have comparable 'Intake Open BTDC' type numbers on methanol radials like the Edwards 5? I haven’t evolved from the primordial ooze to understanding ‘bumpy cam ring planetary gear’ type engines yet. ;D For example, the Rob Sigler pdf plans show a tabular cam profile chart: Intake Lobe 1: Lift = 0.0 at 355 deg, Lift(max) = 0.0750 at 25 deg, Lift = 0.0 at 55 deg. How do I turn that into comparable numbers?

 

[gbritnell]

I can't say I know nor have I studied 4 stroke, glow plug methanol fueled engines but I have studied 4 stroke, spark fired, gasoline fueled engines. I do know that glow engines have a higher compression ratio (not compression pressure) than spark engines. That being said by looking at the cam specs you posted I see where generally the intake is open is open much later in it's cycle, ABDC, and the exhaust opens much earlier in it's cycle, BBDC. When you leave the intake open later on an engine you reduce the compression pressure (not ratio, that's fixed) so with a generally higher ratio you would have to limit the amount of cylinder fill otherwise the compression pressure would become extremely high. Probably the reason for opening the exhaust earlier is to make sure that by the end of cycle that the exhaust gases are completely evacuated. I don't know for sure but I would think if you look at the specs for 2 cycle, glow plug engines you might see similar numbers. As has been discussed in other threads about cam timing specs the raw numbers can't necessarily be applied to every engine. A cam manufacturer will want to know what type of compression ratio your engine has as well as rpm range, exhaust manifolds/headers etc. All of these figure into getting the best performance out of a specific cam profile. If a cam is designed for certain features and your induction system won't flow what it needs to then the cam won't operate optimally.
With higher rpm engines like small glow plug engines the scavenge/fill characteristics would be quite a bit different from larger slower spinning engines.
gbritnell

 

[mu38&Bg#]

Peter, what is your goal?

 

[petertha]

Hi Greg. The (very lofty!) goal is to make this Kinner B-5... and hopefully witness it run reasonably well... one day! So far, its a still a W.I.P dream in my mind, but slowly coming along. I have no misguided notions that constructing it will be a personal challenge, but I'm commited starting the journey. I get plenty of inspiration from the awesome projects & accomplishments on this website. And useful answers to my never-ending stream of questions! ;D

 

[Swede]

I have a wonderful old book from the 1930's called "aircraft engine maintenance", back when almost all airplane engines were round. It has an appendix with all of the timing numbers for the various engines of the day. If you want, I can look up a particular engine.

I've found with models that it is simply not all that critical unless you are building a race engine. The common goal is easy starting and reliable running, and if the valves open roughly where they are supposed to, then the engine will run.

It is very easy to get all worked up over cam profiles. Again, the reality is that if the cam lifts the valves open anywhere close to theoretical, the engine will run fine. Hand-filed cams work, as do milled cams. One need not pursue cosmic cam grinders and the like.

 

[mu38&Bg#]

Model aircraft glow four stroke engines are designed to be powerful and there are several manufacturers competing for profit. So, compared to some of what you see in model engineering, valve timing looks a bit wild. Enya in particular have long duration, but really don't run appreciably different than any other brand. Many factors come into play and cam timing is just one. I can say that all of these engine use "real" cams. That is to say they are not four arc, or flat flank. None of them have ever updated their cam profile, either. 20 year old cam part numbers appear in recently released engines. However, the rest of the system has changed, valve and induction tract sizes, possibly rocker ratios. Small diameter flat followers do limit how aggressive the cams can be.

If you're looking at building something of comparable power to weight ratio as a model airplane engine, some serious research is required, or just borrow the cam profiles. Complicated cam profiles are not easy to design or produce. The profile (actual valve time-area) is what matters not really the timing figures themselves. Those that can design good engine cams get paid well for it and demand they get paid.

Differences in cam profile are not very obvious in smaller displacements, so I wouldn't worry about it all that much. I see Swede posted before I did. For an easy to start engine I'd go with timing of the OS engines before the surpass series.

Greg

 

[petertha]

I have a wonderful old book from the 1930's called "aircraft engine maintenance"...., . If you want, I can look up a particular engine.

Yes please, that would be interesting. Because the 1:5 model will be methanol/glow vs aviation gasoline on the 1:1 banger, probably different timing but I'm curious anyway. I was able to order the the 1942 overhaul instructions which has some good info. There are some pics of the cams & gears, rebuild specs, parts details etc. but unless I missed it, didnt see timing specs.

This Kinner had some model variations but of anything pops up in your book under Kinner 'B-5', 'R-440-1' or R-440-3' thats the one.

 

[petertha]

... or just borrow the cam profiles....Differences in cam profile are not very obvious in smaller displacements, so I wouldn't worry about it all that much. ...For an easy to start engine I'd go with timing of the OS engines before the surpass series.

Yeah I was able to take apart my dad's old rusty OS FS 61 for a looksee. It is comparable cylinder displacement, shorter stroke though. Visually the cam has a very slight curve, but when I sketched it out with it's 0.315" base circle, 0.070" lift, timing tangent line from the quoted specs & nose radius, there just isnt a lot of difference between a gentle curve & straight line connecting the tanget lines to the nose to satisfy the geometry. For now I have decided to just clone the timing, adopt a straight line for machining simplicity & move on with the drawing. I'm familiar with the RC stuff & how they run & seems like many constructed engine have similar flats over hyper-watcha-ma-call-it profiles.

It just struck me that the timing was quite different among the constructed engines, meaning how the cams were phased relative to each other & TDC, BDC etc. But who knows, maybe they had a different idea or less nitro or...

 

[Swede]

Yes please, that would be interesting. Because the 1:5 model will be methanol/glow vs aviation gasoline on the 1:1 banger, probably different timing but I'm curious anyway. I was able to order the the 1942 overhaul instructions which has some good info. There are some pics of the cams & gears, rebuild specs, parts details etc. but unless I missed it, didnt see timing specs.

This Kinner had some model variations but of anything pops up in your book under Kinner 'B-5', 'R-440-1' or R-440-3' thats the one.

Kinner B-5 specifications:
125 HP, 1925 TO RPM, 295 lb, 5.25:1 compression, dual Scintilla magnetos
Ignition timing, BTC 26 degrees
Intake opens 25 BTC, closes 82 ABC
Exhaust opens 65 BBC, closes 42 ATC

 

[mu38&Bg#]

The main issue you'll see with a flat flank is wear. The edge of the follower will ride on the cam. With good materials and low run time it will still last forever. Occasionally these cams in model engines still ear away completely when they blow the heat treat on the cam. Some modelers put a lot of time on engines, so they do have to last.

Greg

 

[petertha]

The main issue you'll see with a flat flank is wear. The edge of the follower will ride on the cam.Greg

Thats an interesting point you bring up, I was wondering about this too. Here are some pics of the typical OS valve assembly. I think Im seeing ever so slight curved cam profile between base circle & nose radius. But a perfectly flat bottomed tappet. Its not obvious on this pic because its showing the pushrod ball depression end, but Im holding one now from this gutted engine & its flat. Actually I think its even free to rotate around.

On this OS engine cam, its the opposite wear pattern. The flatish part is stock looking like the base circle but the noses are shiny wear. (Not the picture, thats from a parts catalog, mine looks like it was resting with the Titanic). Might have something to do with my dad's lack of valve gap setting maintenance! Anyway, it has a lot of hard miles on it. If I could get that I'd be happy.

Im also showing my drawing idea. Right now the cam has a flat profile, along the lines as how Ive seen others make them, presumably because thats easier. It has a 0.080" lift (probably more than I need, 0.070 is what I want the valves to move but the I have to figure out the pushrod angles still). The cam radius is 0.100" which leaves a tiny witness flat on top. With this geometry I measured the flat segment to be 0.136" long. But I have a curved bottom on the tappet. So I wonder, would a [curve tappet bottom + flat segment cam] combo wear about the same as the OS [slightly curved cam, but flat bottom tappet] combo?

 

[petertha]

I have a wonderful old book from the 1930's called "aircraft engine maintenance",

Thanks for the specs Swede. Is this the book you have by chance? Or maybe you can provide the author etc

http://www.amazon.com/dp/B000NQ539E/?tag=skimlinks_replacement-20

 

[petertha]

I stumbled on a review article for a YS 140 FZ. I used to run those engines back in the day, but I had no appreciation for their cam specs. These were the psuedo-supercharged type, but look at how different the timing is vs the other commercial 4S's. And YS's weren't really revy, they were propped best at being grunters.

IO = 62 deg BTDC
IC = 68 deg ABDC
EO = 100 deg BDC
EC = 32 ATDC
overlap = 94 deg!
intake valve dia = 15mm (0.590"), cam lift = 0.160" (=27% of cam dia). Exhaust valve = 13mm (0.512)

I think the OS lift works out to about 21% of valve dia by comparison, thats what I assumed for mine.

 

[gbritnell]

Peter,
The one thing you have to realize with the rounded base lifters is that they will need some type of appliance to keep them lined up with the cam. With roller lifters there is a link between pairs of lifters to keep them from rotating. With flat tappets this is a non issue.
gbritnell

 

[Swede]

Thanks for the specs Swede. Is this the book you have by chance? Or maybe you can provide the author etc

http://www.amazon.com/dp/B000NQ539E/?tag=skimlinks_replacement-20

Yeah that's it exactly. At $5 for a used copy, I'd jump on it. It's got a lot of excellent information.

 

[mu38&Bg#]

Peter, that first pic of the OS cam is one of mine ;D. I posted it in a forum when discussion came up of OS cams shedding their nickel plating. Tha I once found a site or software online somewhere that did some cam math. I came to the conclusion that OS followers are just barely large enough to avoid riding the edge. As soon as I get my 4th axis running in the mill I'm going to plot the profile for the cams that I have for some deeper research.

If you look at the OS cam and note the narrow contact area, that's where you'd have the follower edge dig in if you aren't careful about the cam design. With flat flanks you'd want to design so the follower is large enough that the nose radius is under the follower when it reaches that point. Think OHC with buckets on the valves. That will be a hard jerk when running. I've spent some time reading about cam design a couple years back and decided that I had much to learn if I wanted to produce RC engines. BTW those followers in your pic are Saito and they are smaller in diameter than OS, some are "top hat" with a larger diameter head that contacts the cam. If space in an issue I would choose that route first in your design.

The YS cams do look quite a bit different than OS. Other interesting engines that are harder to find are the car series OS four strokes, FS26S-C and FS-40S-C. These had larger valves, heavier springs and much different cams. A tuned FS-26S-C could run up to 25kRPM, making peak power near 22k or so! The 40 was a bit slower.

Greg

 

[petertha]

...with the rounded base lifters is that they will need some type of appliance to keep them lined up with the cam.

Yes, I know exactly what you mean. My original thinking here was to have flat faces between the tappet 'shoes' & positioned with a very close slip fit between them. So the idea was the faces allowed them to slip up & down, always sharing some contact area with one another, but not be able to rotate (much) because the stem part is constrained concentric to their holes.

But I can already see that any necessary running gap, even a couple thou, means they will rotate to some degree. I was aimig to see how much. I think I've already moved off this idea in favour of a cylindrical, free-to-rotate tappet....which ideally favours a more profiled cam shape. Seems like its a chicken & egg thing!

 

[petertha]

Yeah that's it exactly. At $5 for a used copy, I'd jump on it. It's got a lot of excellent information.

All domestic (USA) shipping. Grrr... you guys get ALL the good stuff!

 

[petertha]

...Peter, that first pic of the OS cam is one of mine....
... found a site or software online somewhere that did some cam math.

Uh-oh! What are the odds. Hopefully no copyright lawyers in your family. I'm an honest guy & usually always quote references... I swear! ;D

Ive seen references made to Camcalc? Java? program. I think it generates those hypo-?? (curvy) cam profiles & a table of cut depths at degree increments once you input valve timing & some geometry. I rememebr it worked for me once when I was just poking around, but then not again. Might have been a short term glitch, I'll earch again.

Seems like model designers go down one of 3 paths: simple tappet & more involved cam, simpler cam & more complicated tappet, simple tappet AND cam & live with any consequences!

 

[mu38&Bg#]

I mess around a lot with OS four strokes. I have an FS-40S on the bench right now that will soon be diesel.

Camcalc from www.modelenginenews.org is a modelers type of design. I haven't checked it in a while, but a lot of people here and elsewhere have used it with good success and long lasting results. I don't remember the one I found, I only know that it was some kind of demo. It let you design and analyze all kinds of wonderful cams, but you couldn't get the data out to build one.

Because cams and followers could easily be replaced if you find the means to create better parts I'd just go with what you can build now. if you get to the point where you can do better, it will be easy to make another set.

Greg