Hobbsy

My Feedback: (102)

Maybe I have had the only one failure because I tend to prop my Diesels in the 9,500 rpm range. Higher rpm requires higher compression. Ray's calling experienced Diesel guys, "internet fliers" because they haven't broken anything isn't being fair. The truth of the matter is that that their experience may be the reason the experienced guys don't have failures. I have not broken my LA .10 nor my Fox .15 and it is one of my oldest and most used conversions. It spent a lot of time flying a TeleMaster 25 ARF, one of Hobby-Lobby's earliest ARFs. The .15 replaced a Fox .25 quite nicely. I dumb thumbed the Tele 25 into a fence post.

buzzard bait

Here's something I don't get: there seems to be an assumption that a larger prop is more likely to result in a broken crank...the 13x5 vs a 10x6 for example.

But you will run LOWER compression (and lower rpm) using the big prop, and HIGHER compression (and higher rpm) using the small prop. So why would a big prop be more likely to break the crank? Other than prop strikes of course.

Jim

gkamysz

My Feedback: (19)

There is less margin of error to get the actual ignition timing right. When the engine is turning slower it's much easier for the pressure rise in the cylinder to be very fast and this leads to high forces on parts. If it were running faster the piston is already traveling down and expanding the gasses and reducing pressures. The amount of time for combustion is almost fixed, while the the pressure curves are very different and different RPM(non throttled). If the compression ratio and mixture settings are just right you shouldn't have any problems. But, with a two stroke it's hard to tell that you have them right. My four stroke diesel conversions make it obvious they're not happy very quickly, especially when loaded with large props. There is a very audible knock, the get progressively worse. Two strokes and smaller engines are not so obvious, when ignition timing is advanced beyond ideal.

Greg

cessna098

The only diesel engine I've run was a McCoy .09 back in the '60's. I was an inexperienced teenager and it was my first diesel engine. I had a couple of runs on a test stand fiddling with the needle valve and compression screw. I was impressed by how the engine would speed up by turning in the compression screw. Just when I thought I had it adjusted just right it quit suddenly. I tried to turn it over but it was locked up. Taking the backplate off I saw that the crank web had broken. I figured that my crank failure and that of many diesels was due to dialing in too much compression.
DennisV

Hobbsy

My Feedback: (102)

Greg, I think you have that theory exactly backwards, the over compressed sound, referred to as the, "egg frying" sound by most is closer to the peak rpm with a smaller prop. When peak rpm is reached with a larger prop the over compressed sound requires quite a few degrees of extra turning on the compression screw. That's the reason I favor larger props because its easier on the engine. At lower rpm and lower compression the combustion pressure is more of a piston pusher than a piston knocker.

gkamysz

My Feedback: (19)

Dave, if the engine is set correctly, I agree. Our diesels are usually set near detonation on the side of normal combustion. If you're conservative on settings, the engine will be fine. Not every engine is obvious about being over-compressed at low RPM, because it doesn't always result in knock.

Hobbsy

My Feedback: (102)

It may not sound like a knock but it is still there as a secondary rattle at the very least. Our hearing is our best tool when setting up Diesels. It is a learned skill to, as the old saying goes, "listen louder."

N99JH

My Feedback: (66)

I agree with Greg to a degree that tuning the engine "Just Right" is a little tricky. After a warm up I tune the compression to where the engine peaks in RPM (measuring actual RPM) and than lean the mixture until it starts missinig. Back it off a bit, reduce compression and try to optimize RPM again. When I reach that point, I back the compression slightly to the point where it still does not lose speed. For all I recall, my engine was running pretty cool and rock solid - never missed one beat.
Joshua

buzzard bait

That's interesting. I've never tried maximizing the rpms. I try to begin with the lowest compression that will get it to start, and then increase compression just to the point where it stops missing. I don't increase it further to maximize rpm. Maybe I'm not getting the max out of my engine, but maybe my settings are quite safe.

Jim

123Cat

Good question , is that the point of max revs or can you squeeze the compression down more ????

earlwb

Yeah one has to watch out for that as, yes you can increase the compression more, than is needed when you get to max RPMs.
As a engine heats up it needs less compression, but some people may not readjust the compression to account for that. So they increase compression when the engine is more cool when starting, but don't back off when the engine heats up.

I normally adjust the compression to be just a touch under compressed when hot or maybe right on the money, there isn't much difference at that point. I don't remember needing to increase compression to get my engines to start when cold, so I only needed to dial in the compression for a particular prop size and then I seldom need to tweak it after that.  But I know of guys with some more cantankerous little engines that are like that though, where after starting they back off the compression.

Mr Cox

It is indeed the other way around...

Recycled Flyer

The assumption is that over propping an engine will shorten its life.

Any machine that is operated beyond its limits is bound to suffer wear that is accelerated from normal.

Problem here is that what defines the machines limits and what is acceptable wear?

Something rarely touched on is the greater flywheel affect that is gained from a 'larger' prop, the greater the residual inertia left over from a previous power stroke, the less that is needed to create the next one and this smooths out the impulses in the engine somewhat.

But there is a point of dimishing return where the extra load starts to erode any flywheel benefits.

earlwb

It might be that someone over props the engine, but also over compresses the engine as well. Thus it puts a lot of stress onto the rod and on down to the crankshaft then too.  I remember guys getting stronger rods to correct a rod breaking problem on a engine, then the piston would be a problem, so they got better pistons, and then the next weak spot was the crankshaft and then it would break on them.
.

Hobbsy

My Feedback: (102)

Optimizing it to max rpm with a prop in the recommended range is not the same as overloading it. Softening it down to lower rpm blows away one of Diesels greatest attributes and that is fuel economy and also causes poor midrange throttling. It also leads to more rapid cooling off near idle speeds leading to slow spool up. Optimised = crisp throttling.

N99JH

My Feedback: (66)

As I promised, I had my metallurgist friend examine the fracture. Looking at it under a magnifying glass he said that he could not confirm a fatigue fracture and he thought that the failure was due to a sudden impact. I recall that after the engine quit running, I found the spinner and prop on the floor. It may be that the lock nut worked itself lose, allowing the prop to fly off and causing the engine to overspeed and destruct? A theory anyway. Monday I will have another crankshaft and will resume testing.
Joshua

Mr Cox

It sounds more like a possible "hydroloc" to me. Too much fuel and oil above the piston and it cannot go over TDC, so it goes to a dead stop. This shears the crank pin and trows off the prop...

Regarding internet fliers... I have not ran many conversions (just one), only dedicated diesel engines and have not yet broken a crank. To me a "large" prop and correct fuel means that you can run it with a low compression and a little rich to avoid missing. The colour of the exhaust can also be used as an indication of the temperature and load on the engine.

Another important thing is the fuel, if you have lost Ether (can happen during storage) or don't use any ignition improver you'll also need a higher compression.

123Cat

Thats the good thing about Diesel you can get it all wrong with the settings and ruin some gear , I take out 2 planes with Diesel , and mostly they go OK , I am surprised at the compression they run at , its metal tearing stuff

Mr Cox

Well set up on good fuel, the compression is only slightly higher than on a glow engine. There should not be any metal tearing...

N99JH

My Feedback: (66)

Hydroloc in engine that has been purring steadily for about 10 minutes? I am not sure how that can happen. The exhaust color was a very light brown and there was no sound of tearing metal.

Hobbsy

My Feedback: (102)

If the fuel is right there is no metal tearing compression. The flipping effort on Diesels is the same as for glow if not lower. If the flipping effort is higher, then something is wrong, contra is screwed in too far, not enough ether in the fuel etc.

qazimoto

My Feedback: (1)

Purpose made diesels do have "metal tearing compression" and the "diesel flick" is by necessity much more strenuous than with a glow.

There is also such thing as a "diesel fit" for pistons and liners. Otherwise hand flicked hot starts just aren't possible.

Perhaps the answer to the dilemma is that the successful conversion heads are quite "leaky" due to the o-ring seal
and the better sealing ones increase compression and a shaft failure results.

Ray

AMB

Locktite 401 If the contra O ring leaked that much you would have I think a lot of oil exiting by the compression screw or sort of a hydro lock above the contra
in other words it could not raise do to the oil/fuel mix taking up that free space, again just a guess on my part regards martin
along with a large compression loss if the former

also a while back some one posted drilling small hole in the head of a PAW , no O ring on those, to prevent a charge build up above the contra from blowby to avoid it from igniting again, theory do not know if it every happened
i

qazimoto

My Feedback: (1)

That's my point Martin.

Yes the pressure relieving holes in the outer head are a common modification with conventional
diesels where there may be some slight leakage while the engine reaches operating temperature..

If conventional diesels with lapped Cast Iron contra pistons leak slightly then an o-ring must to a greater extent.

Contra's only leak oil when they have very poor sealing, we're talking just poor sealing.

People above are reporting low compression with the conversion heads, and the design is never used in competition engines,
where the push/pull head is almost universal except for vintage engines.

The notion of ignition above the contra in a PAW is fanciful, an engine would be clapped well before that stage.

Ray

Hobbsy

My Feedback: (102)

Ray, I have about 25 davis Diesel heads and a couple of AJCoholic heads and have never seen one leak aroung the contra, not even a tiny bit. Nor have I seen an MVVS or an Irvine factory head leak aroung the contra, only the Paws leaked around the contra hence this drain hole an this PAW .60 and .19. I don't know where you get your info but it sure isn't from hands on experience.