brokenenglish

Like Steve above, I never disassemble engines that have nothing wrong.

For a stuck contra-piston, I've used the following technique successfully many times.

If the contra-piston is simply "stuck", then the first problem is to "unstick" it, any way you can.
I do this by screwing it down slightly, i.e. actually increasing compression, just to unstick the c/p.
Then slacken off the compression screw "a lot" (unscrew it several turns).
Then put plenty of fuel in, and make sure you can still turn the engine over gently (if not, drain off some fuel!).
Then give the propeller a good whack!
This usually knocks the c/p back, no problem.
If not, repeat until it does.

IMO, there are two important points in this:
Increasing the compression means that you HAVE moved the c/p (i.e. it's no longer stuck).
The higher compression gives more force to knock the c/p well up the bore...

It works for me...

DaveyMo

Greetings All:

Great advice coming here. It takes time for each of you to peck this stuff out on the computer keyboard, so your generosity really shows!

Tonight I reassembled the engine without removing the contra-piston as suggested. The tight nature of its fit still shows, but you've all wisely convinced me that this is probably for the best. Bravo! I did use a depth gauge to set it at about a fifteen to one compression ratio for starting (some Algebra came in handy to reverse calculate the ratio suggested by motorboy in another thread). With the proper fuel and fuel line on hand, all that's needed is some warm weather and I'll have it on the outdoor test stand in short order. Will get back to this thread then and provide a progress report.

Again, thank-you all!

DaveyMo

Greetings All:

With warm weather, I'm finally making some strides on the KMD 2.5, thanks to the good advice here. It's most gratifying (and a bit amazing) to start an engine without any form of electrical assistance - I think I'm really going to like this diesel thing!

A progress report: It took a lot of flipping and some outside heat to find the right starting compression setting, but it will now start predictably after an exhaust port prime and 10 flips. So far I've had two, one-minute runs at a slow to medium speed with a bit of sputtering and white exhaust smoke. After slightly reducing compression, it slowed down a bit (very gratified that the contra piston is responsive both ways). Haven't got it to run at peak yet.

So I suspect my next move is to the heat cycling process. In order to peak, I take it that small, alternate steps involving higher compression and richer mixture would be the way to go?

At any rate, the engine is a beauty, and I really like the diesel smell! Here is my break-in setup. Thanks!

telmore

I use a reasonably thick garden glove.

Meranda lost my old favourite at Old Warden the other week so I have a new one.

Annette

gcb

As you increase compression you need to lean it out. When the engine has been running for a minute or so it will be at operating temperature, that is the time to make final adjustments. Once you have the needle set for peak mixture, decrease compression slightly until it starts to decrease RPM. Now increase the compression until it is at peak (no RPM increase with more compression). This is your peak setting. Pull the fuel line and let the engine cool. Try to restart without changing compression or needle.

Looking at your setup, you might want to lower your tank to just below the intake or you may get flooding from the fuel in the line siphoning into the engine when you choke it to get fuel in the line.

Good luck.

George

Recycled Flyer

Hmm, I am still shy on the use of large diameter draggy props for running in purposes since they do not allow the engine to rev out freely at its most critical time, or in other words - resistance to acceleration puts strain on engine components when they are at their most abrasive time.

I can see the logic behind a larger prop that provides extra flywheel weight for ease of staring but that where the advantage of extra inches of prop blade ends.

Better to use a mechanical weight like heavy hub, lead washer or similar and ditch the aerodynamic drag because the weight stays a constant whereas the drag increases as a square of the speed.

gcb

I don't believe I've heard of a large prop damaging a diesel while I have heard of them flying apart by over revving.

Kerosene provides good lubrication (compared to alcohol and gasoline) to wearing surfaces.

I believe the conrod lower end breaks-in during the first couple of minutes. After that you are concentrating on the piston/cylinder on an iron/steel setup. Some add a little extra castor during the first runnings.

I break-in all my 2.5cc engines on a 9x4 prop. So far, no problems. Of course...your engine, your choices.

George

Recycled Flyer

I don't advocate over revving but simply 'free' revving.

And its the same advice with any IC engine, when its new you do not labour it in the lowest gears but let it spin freely with a light load.

Getting it to start undoubtedly leads to the large prop/heavy flywheel thinking but what you really want is the fly wheel effect with less of the dynamic braking of the prop.

Overly large props do damage diesels, just ask the free flight boys for examples.

DaveyMo

Annette:

Too bad about the loss of the friendly old glove. I too use a rather thick glove and am quite happy with it.

Dave Mo

DaveyMo

Thank-you, George, for a clear and elegant explanation of my next steps. I'll print them off and follow them "religiously"!

I take it that under ideal circumstances, peak compression setting would also work for cold starts. Failing that, would I then revert to lower compression for starting and then after a short run time, bring it up to peak? I'm also taking your other advice and am dropping the fuel tank a quarter inch. I think you're on to something there given a bit of fuel pooling I noticed just inside the intake.

Dave Mo

DaveyMo

Recycled Flyer

My apologies to you and George for having put you both at loggerheads. Your point and his counterpoint have made a fascinating discussion, and I've picked up a lot more knowledge as a result. Accordingly, you both have my gratitude!

Now by your lights, the 9x6 is too aggressive. In reviewing this thread, I think it was Warren who suggested an 8x4. Is this what you would have in mind?

Thanks again.

Dave Mo

Recycled Flyer

Usually you would have more compression on a cold engine for starting and less on a hot one but sometimes priming and the wet mix this gives will give artificially give you more comp anyway due to the less compressible nature of the wetter fuel/air mix so I basically agree with George.

http://www.eifflaender.com/instruct.htm

And dropping the tank below the spray bar is fine as long as your model never goes inverted!

gcb

That advice was pertaining to the test stand he is using (shown above). On my CL airplanes I usually center (plus shims) the pickup as needed to achieve the same setting whether upright or inverted.

My planes are mostly sport or stunt so I tend to use either an 8x6 or 9x4.

George

DaveyMo

Yes indeed, George, I'll not be flying with that mustard jar shown on the test stand! And thanks, Recycled Flyer, for the PAW link; that set of instructions is well worth saving.

Dave Mo...

Recycled Flyer

Hey Dave,
its all good here and honestly these things are hard to prove in the real world.
But I do think that a 2.5cc engine that thrives at 22,000 rpm would not like the load of a 9x6..........ever.

8x4 sounds good.

Cheers.

DaveyMo

Recycled Flyer:

In understanding reality, two things grab my attention: real-world experience, and abstract models showing non-linear relationships between forces! Thanks again!

Dave

gcb

Dave,

Here is a good site that explains the care and feeding of diesels. It was written by an engine guru named Adrian Duncan. His explanation of how and why to break-in an engine is excellent.

http://www3.telus.net/dieselcombat/about_diesels.htm

The site itself has LOTS of good information. That is where I found the break-in procedure that I use.

Bear in mind that there is seldom only one way to do something. :-)

Enjoy.

George

Recycled Flyer

Hi Dave,
I am not sure which side of the line you rest on, but I will sign off with this -

I have a friend who has flown F2B since before I was born, and he directs me to use a prop load lighter than heavier (typically in pitch),
And another who has hand built hundreds of diesels and fixed probably thousands in his life time who tells me that more engines have been ruined by high prop loads than any other cause.

Also every engine manual that I have ever read directs me to use a low load and vary that during the break in time.

Now how does one prove that one way may be better than the other? You would have to invest thousands of man hours and compare like to like in order to gain any valid comparison and even then its either at the end of an engines life, where longevity becomes apparent, or outright power figures that need considering.

And temper this with the urgent need to simply get that engine initially running and its not hard to see that a large diameter prop is chosen.

So its easy to see why some recommend a diameter and pitch within the engines operating range but within that range you have a choice - choose wisely.

As a final endorsement please checkout - http://www.eifflaender.com/instruct.htm and note well the smaller prop sizes used for run in.

gcb

Unless I am missing something those instructions say for a .15 (2.5cc) to break in on a 9x4 then use anything between a 7x4 and 9x6.

As I stated before I do not race so I feel no need to get up on the peak of the power curve. I have no experience in the 22K range. I will bow to you and your friend's experience and expertise.

George

raglafart

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George
I'm not so sure that Recycled's experiences will necessarily be first hand and is much more likely to be a recycled quote of someone else's!

Recycled Flyer

You are right in many aspects John, I have not flown for about 35 years but do have good memories of when I did.

And I also have an excellent memory for all things technical and a great appetite for leaning about model engines.

These days all I do to correlate what I consider to be correct, lean on my experiences from the past and pass it on giving my train of logic for judgement every time.

Am I wrong to do so? If I am then I am sure that you are more than welcome to enter this discussion, its a publlc forum so what is your take on all of this then?

raglafart

My Feedback: (10)

Chris
I'm just interested in your own personal experiences with model engine building and modifying. I'm simply curious if you've built any engines or customized engines with new pistons, longer rods, different heads, modified timings, different materials, different fits or other parts that lead you to some of your conclusions?
I see in other forums you say you are no racer so I'd suspect you may be from a stunt background and wonder what special stunt engines you may have set up?
Just inquisitive, that's all

Cheers John

Recycled Flyer

Hi John,
you are a moderator elsewhere so you more than anyone should know that the above is waaay off topic and personal but if you want to pursue this the perhaps a PM would be appropriate?

But yes my own building experience stems from stunt, combat and smattering of scale in control line, and towline gliders with the odd Jetex thrown in when I was really young.

raglafart

My Feedback: (10)

Hi Chris

You seem to have such a deep and wide knowledge of engine set ups and I merely wondered how you'd accrued such expertise.

As such, I'd have thought my question about your engine building history quite appropriate.

No need for pm's. Continue with your advice please. It's always interesting and informative to read your posts. None of us are beyond learning.

Cheers John

maxtenet

Best info on running a diesel for noobs!

http://carlsonengineimports.net/FAQ.html

Max