Me and a friend of mine have been experimenting running diesel in our engines for a short while.
I have made my own conversion-head for my .61 MAC engine and my friend has a Davis-head for his 1.08 OS FSR.

Since then I've got my engine running quite well, but we're still having trouble getting the OS to perform.

What I thought strange though, was that both of our engines tend to get really hot when we run them. With my friends engine it got so bad, that he decided to convert his engine back to methanol because he was afraid he would ruin it.

I was always made to believe that diesels should run much cooler than methanol engines, so I suspect we're doing something wrong. Are we perhaps overcompressing the engines? Any help would be really appreciated!

Thanks in advance,

Cyril

Cyril,

Overcompression would be my first guess. Be sure to make final adjustments after it has heated up for a couple of minutes. Perhaps a little more ether or lube might help also. What fuel formula are you running?

Good luck with it.

George

Yep, sounds like overcompressing and thereby too early ignition.

I'm not quite sure about the formula of the fuel I use, because I bought it premixed.
On the container it only says that the oil-percentage is 16%.
I'm guessing that the mixture resembles the PAW mixture for racing (15% oil, 50% paraffin, 32-33% ether, 2-3% IPN)

I'm not quite sure what the compression was when I had the engine running, I'd have to take of the head and measure how far the contra is screwed in. I'll see if I can do that tomorrow. On the OS with the Davis-head we had the compression screw turned out about 1.5-1.75 turns from fully in.

How long would it normally take for a 0.61 diesel to warm up? Mine gets to hot to touch after about 1 minute!

overcompressed--- turn screw out till you get a miss then a little in till it stops missing overcompressed and too rich it will get hot
and as stated maybe low ether in the fuel and you are increasing compresion to get it to run tkes a minute to get up to temp
and when it does compression can be slacked off-- on my enya 25 diesel I and hold my fingers on the muffler and not get burned

How do you tell if it is too Lean?
I take it pinching the line as with glow isn't the answer
Or does the engine just stop

Pete

Forger the pinch test the diesel does just the reverse if it too lean it will slow down and stop as it leans not speed up, richen slowly
while adjusting the compression a half turn or less in or out on the compression screw and you will get the right balance of running
after doing this a few times you will have the sweet spot on the money you may need a yery slight touch up on the needle too
a little triva here also one the guys I fly with has a friend that "went electric" on one of his 60 size planes spent $1200 on the conversion no that is not a typo but he does have 2 lith. flight batteries

Do I understand correctly that, contrary to methanol burning engines, a diesel will run hot if the mixture is set too rich?
And if the mixture is too lean it will just die and not run hot?

It will slow and quit if too lean it will not get hot-- it will get hot if overcompressed most problems come from overcompressing and trying to compensate by running rich

On my experience diesels run hotter than comparable glow engines. Why is this? the exhaust from a diesel is relatively cool. You can stick your finger very close to the exhaust to observe the color of the exhaust oil on your finger, whereas the same action with a glow will burn your finger badly. So most of the heat of combustion in a diesel is staying in the engine. Yes the muffler will be relatively cool, but the head will be hot. If a diesel is overcompressed it will heat up and slow down, while running a black exhaust. As you lean a diesel out it will start to miss and then stop. I think you may be off on your settings, but I expect a diesel to get hot in comparison to a glow. Others may not agree.

Jim

Leaning will cause the engine to run on-off-on-off. The leaner it is the longer te on-off periods.
Diesels do get hot yes, its not that they run cold or so.
I agree with most of Jim 's cooment, as these are my experiences as well. (although my diesels dont run hotter)

Ok, here are the engine settings when I last had it running:

- I removed the head and measured the position of the contra-piston. I calculated the compression to be 1:20,44 (assuming that compression is calculated from the point where the exhaust/intake ports are just closed)
- Both the full-throttle and the idle needle were opened about 2.5 turns.
- The carb is a 10mm OPS slider, which opens fully at full throttle.

Assuming I calculated the compression correctly, I don't think this is the problem. Maybe I should try leaning it out more?

Remember, I'm not looking for fuel-efficiency, but for maximum horsepower, as this engine is being used in radio-controlled tractor pulling.

Ran irvine 53 davis head wood zinger 14x4 at 8200 temp ranged between 240 and 250 at junction of head and cylinder
at lower speed dropped to 190 this is an infrared unit and may be a couple of degrees off but t was repeatable within 5 degrees
martin--fuel davisABC

Being as I have had a K&B 4011 glow engine run fine on Davis 1/2A fuel once it warmed up, I even more so suspect you are overcompressed.

Jim

Cyril,

I have no experience in setting up for a tractor pull. My guess is that you are setting compression for the load of a flywheel alone and when you put a load on it (via clutch?) the setting will be off. If that is the case remember when you adjust compression you are setting ignition timing. If you greatly change the load, you will need another setting.
You might try lowering compression a bit and see if it runs cooler but I doubt that you will get a compression setting that is optimum for both flywheel and max load. My guess is that you would set it for load and try to just keep it lit while on flywheel alone.

George

Micropuller,

Measuring the chamber and calculating compression ratio is not the way you tune a naturally asperated 2 stroke cycle diesel. Tuning mus be done as a combination between mixture and compression ratio. Normally a diesel will burp when under-compressed. Screwing the contra-piston in will increase the CR and the burping will cease. Mixture then can be leaned until you begin to here a slow-down. Also remember that your initial starting compression setting that will make the engine run steadily will be too high for running at operating temperature so that without touching the compression you have the engine overcompressed. With my diesels I find the happy spot after it is up to operating temerature and very seldom change the compression after that. When I first start it, it will burp a bit then smooth out when up to operating temperature.

It is a common mistake to try to compensate for over rich condition by over compressing. So initially start rich, lean until is slows a bit. Back off the compression and see if it picks up. If it does you're headed in the right direction. The idea tuning is as lean as possible with as little compression as possible. I then richen a quarter turn or so for no other reason then it makes me feel better.

Randy

dup post martin

Hi Randy and the rest of group too--You are on the money on your advice-- had a conversation with Bob Davis today and he said the same thing you stated most of the problem of over compression and than running rich is the result of too LITTLE prop load br running
a glow size prop or one just slightly larger it requires more compression then richer and the temp goes up. with the increased load
you then DECREASE compression and lean it out it then sounds right less labored and much smoother and cooler . my norvel 061 runs an 8x4MAS
and my TT10 an 8x4 MAS the norvel instructions read a 6x3 on glow the OS 10 reads 7x4 so sure the TT10 the same and the TT is still
a little under propped I may go a 9x4 which will bring it down to the 9000 range and the compression screws are at least 2 turns out
from the bottom the TT61 had so much compression as a glow it needed 3 head shims before I could get the screw out far enough
to get the diesel miss and swinging a 14" prop (do not remember the pitch)maybe 6 0r 7 martin
you can increase the prop load 30% over glow

GCB is closest to correct for a tractor pull setup. Before you go off and run a diesel on a tractor, first learn how to run it on a propeller and the fuel it was made to run on. As for making your own head that only makes the learning problem worse as things are never as simple as one assumes if one does not no enough about the details. When you make a Chinese copy without the Chinaman there may be problems.

Forgive this silly question, but it just occured to me. Doe this setup have a cooling fan? And what about the muff (head) does it have oversized fins.

Thanks for all the input so far!

Just to answer a couple of questions:
- We don't run cooling fans on any of our our tractors. (we use both 2-stroke and 4-stroke engines, either burning methanol or gas, ranging from .21 cubic inch car-engines to 3 cubic inch aeroplane engines)
- My home-made head is equipped with extra cooling fins.
- It is impossible to buy pre-mixed diesel fuel, like the DD-fuels in Holland. I bought my fuel from a long-time diesel enthousiast, who mixed it for me after I had told him what I was planning to use it for.
- When adjusting the engine, it does indeed only turn the fly-wheel (and the clutch, which is a fly-wheel on it's own, being made out of a piece of nearly solid steel 10mm thick and 40mm in diameter)

I suspect the theory that the engine is overcompressed while under load is correct. I think the best way to go would be to take it out of the puller and run it with a propeller. Any idea on what prop I should use for testing? According to the manufacorer's specifications, the engine should make 2,2 HP @ 22.000 RPM

Cyril,

The plot thickens...Cooling fins assume that there is some air blowing past them. If you are running with a flywheel without anything to stir the air, there is no way the cooling fins will be effective. Racing cars use heat sink heads successfully because they are moving.

My guess is that had you been using an iron/steel diesel instead of an ABC conversion, you would have burned up your engine by now because of thermal runaway. Aren't the other guys having problems?

You might consider setting it up with a prop, as suggested, then run it in your tractor by starting, making the pull, then shutting down before it can overheat (if that's possible).

Good luck in any case.

George

As I stated in my earlier message, none of our models use forced cooling, simply because we run them for such short periods that they don't even get a chance to overheat.
The average pull takes 3 seconds. Including starting and hooking the tractor to the sled that would be about 15-20 seconds running time.
Most of our members let their pullers warm up for a short while before pulling, just to get the engines up to operating temperatures.
We even use inboard marine engines without any extra cooling, without problems.

Setting the mixture / compression with the engine on a test stand using a propeller seems like a good solution, but what size proppeller should I use?

Cyril

What your not getting here is that a glow engine must turn high Rpm to produce high power and torque the diesel on the other hand does not. You must change your drive ratio by at least 25%. The diesel has a flat torque curve. What is the deck hight in your engine? What is the squish area percent and what is it's angle? What is the percentage amount of cetane in the fuel?

Engine bore: 24mm
Effective stroke (from closing of exhaust port): 13mm
Total capacity (excluding combustion chamber and squishband volume): 5.88cc

Squisband: 0.2mm
Squishband volume: 0.09cc

Bore contra-piston: 15mm

Cetane%: 2-3

All help is greatly appreciated!

Cyril