ranchpig

Hi all,

I'm VERY interested in a diesle for a 1/4 scale Citabria and AT-6. I figure i would have to go w/ a 4stroke for the "sound" value and torque but they're pricey and you all are saying the diesels have a better sound etc.

If diesel's are so much better... why isn't OS, Magnum, etc. manufacturing them?

Also... couldn't you use a glow plug with the model "diesel fuel"?
obviously you'd lose that timing thing but how bad?

Last Question: If Davis Diesel rep is here, are there heads for a fox78 and/or kraft .61 (original not RJL)
and is the price list in pdf form the entire selection for the replacement heads?

Hobbsy

My Feedback: (102)

The Diesel fuel would not work with a glow plug because the glow plug depends on a catalytic reaction with the methanol to keep it hot.

ddd

No we do not make heads for old style baffle piston engines. Those on the price list is a buch.

maxtenet

The reason diesels aren't manufactured by everyone is that the interest is not there from American fliers (who are the bulk of engine purchasers). Plain and simple, most people can't adjust a needle valve correctly and then you throw in compression adjustment and everyone is scared away by their lack of confidence in their ability to tune them! It ain't all that hard! Try it you'll like it!

Max

ranchpig

makes sense... the ol' supply & demand thing, eh?

DDD, what is an old style baffle engine? and those on the list you wrote "buch" do you mean bunch? Which is to say that's it? Also is it true that the larger size diesels are not as efficient or easy to "tune"?

If so... is the 60 and undersize the best?

Hobbsy

My Feedback: (102)

Ranch, the baffled piston engines were the ones that had a baffle or ridge on the left side of the piston top, its purpose was to direct the intake charge upward as it filled the cylinder while the exhaust flowed down and out. They had a single large intake port across from the exhaust port. They were also referred to as loop scavenged engines. The largest conversion I have is a K&B 100 and the effeciency is the same as any other. It is actually better as a Diesel because as a glow a Saito 100 can kick its butt power wise but as a Diesel it turns a 16x8 almost as well as a Saito 150. If you are looking to try one, the Fox .50, Fox .74 and Super Tigre .51 are my favs. Have you seen Mr. Davis' list of heads, if not just go to his site and click price list.

Jim Thomerson

I have run a K&B 4011 on Davis 1/2A diesel fuel, needle turned in two turns. Had to leave the plug connected for about 30 seconds until it warmed up. Ran fine. Also ran a Tower 40 with a low compression head on diesel fuel. It would undercompress without the glow plug, but ran fine with the plug connected. I ran the K&B because someone said it couldn't be done. TheTower was just getting diesel fuel out of the teststand tank. This suggests that the compression ratio for 40-size glow and diesel engines is pretty similar.

The '51 or '52 Aeromodeller Annual published a batch of fuel economy tests on 2.5 cc engines. Fount the best economy with a glow engine running on diesel.

So far as I know the OS 15 diesel was the only baffle piston diesel produced. I understand the contrapiston was pinned so it could not rotate.

Jim

Hobbsy

My Feedback: (102)

I suspect that in neither case is the glow plug staying lit, the 4011 has enough compression pressure to fire the Diesel fuel when warmed up the Tower engine does not. The glow plug adds enough heat to the combustion chamber on the Tower engine when energized to cause combustion. What did the glow plugs look like afterward?

Jim Thomerson

I also doubt the glowplug stayed lit. I never thought to look at them afterward, but just went ahead and ran the engines glowplug. So I don't think the plugs were hurt. No idea what repeated running with diesel would do to a glowplug. Some of the South African guys flying CL stunt at 6000 ft elevation are running glow engines on diesel fuel, and claiming much more power than with glow.

Jim

Hobbsy

My Feedback: (102)

That's interesting, I wonder if my High compression Saito .80 would run on it. Food for thought huh.

maxtenet

Hey,
Try the Saito, I've been thinking about dieselizing some of the four strokes, but it may be too complex. I'm thinking maybe shave the head down for compression and put a small relief screw in to back off compression. Obviously not the best for combustion chamber shape! The problem is that these four strokes are too expensive to just randomly screw around with!

Max

Jim Thomerson

I understand Bob Davis did a 4-stroke diesel conversion some years ago. I've been told that our model diesels are actually detonation engines. When the compression temperature is high enough the ether fires, igniting the kerosene. This happens all throughout the combustion chamber, so combustion chamber shape makes little difference. (May not be true for 4-strokes. No idea.) This is why diesel exhaust is relatively cool; it is not burning, but is rather just expanding, and therefore cooling, gas.

In a glow plug engine the burn is initiated at the glowplug and proceeds outward. I often stick my finger in diesel exhaust to see what color it is. If I should do this with a glowplug engine, my finger would turn red and blistered, because the exhaust gas is still burning and much hotter.

Jim

David Cutler

I could be to do with a diesel has to be made to closer tolerances and there is an extra piston to fit into the cylinder (the contra piston) all of which means higher costs, and, if keeping competitive price with glow, less or no profit.

-DC

gcb

I think Eric Clutton touched on this in his "Dr. Diesel's Diary". He suggested using glow fuel to prime and have diesel fuel in the tank. (Hope I'm not quoting the wrong source.)
If you try this remember that diesel fuel attacks (swells) silicone tubing so I would suggest a metal tank so you only have to throw away the line going to the needle valve. I hope everyone knows that you should not use silicone tubing with diesels. You need neoprene or tygon. Perhaps plastic will work also.
Although your engine may run this way, you would have to play with head shims, etc. to get it to run well. Remember that the compression screw is adjusting timing.
That would be a good way to get someone to launch your plane that normally wouldn't launch a diesel because of smell.

George

Hobbsy

My Feedback: (102)

I believe Mr. Davis was working on a conversion for the Webra rotary valve fourstrokes and they abruptly quit making them. They are still sought after as a novel engine.

Hobbsy

My Feedback: (102)

OK, I ran the Saito .80 on the Diesel fuel and it will definitely run on it. However it has too much compression, I could not go beyond half throttle without it sounding like the cylinder might become the next Voyager Spacecraft. It had a fantastic idle, 1,600 rpm with a 13x6 Scimitar, at one half the throttle arm travel it was turning the 13x6 at 7,600. The exhaust was much louder and melted the Tygon fuel tube off of the muffler tap. I ran a whole 12 oz. tank out at half throttle and less. I did not harm the engine at all, I ran about 6 oz. of glow through it to make sure and pulled the backplate. It required no glow power, in fact I removed the element from an old Fox non idle bar plug and used it. For Kicks I tried a 13x8 Scimitar, I couldn't go beyond 1/3 throttle with it. So, what next? It's difficult to believe that a compression adjustment is not do-able. Got any ideas Max.

ranchpig

Hey why not take a 1/4" bolt and tap it out w/ a 2-56 and bolt it in and you could make your own compression adjustment You know if it worked... you could put Davis out of business by selling them... lol

Dan don't give me a hard time

Hobbsy

My Feedback: (102)

The problem with working with such a small space for the adjustment is that the small space could easily become an auxiliary combustion chamber and have a blow torch effect, scorching the center of the piston. Something more open would be required. The volume would need to be carefully calculated. No danger of putting Mr. Davis out of business, his heads are simply the best, bar none.

sigrun

Study a little aeromodelling history and the answer to that should be as obvious as to why VHS defeated Beta in the VCR wars and why Beta is longer a home user format.
The diesel has however managed to survive...just.

Diesels have their place and I'm certainly an admirer, but pro/con arguments are just as meaningless as the usually stupidly emotive 4 vs 2 stroke argument.

Graeme Marion

Greetings from Sunny Queensland[8D]

Regarding adjusting the compression on a glo motor running on diesel fuel.

There is an engine marketed in Australia as the "Saturn Falcon" which has a machined brass screw in place of the normal glo plug.

Go to [link=http://homepages.tig.com.au/~saturn/index.htm]Tony Cincotta's Saturn Hobbies.[/link] and click on ENGINES.

An allen key is used to make the adjustment.

All very simple.

Regards from Graeme Marion.

Hobbsy

My Feedback: (102)

The head appears to be thick enough to easily contain a contra piston which I suspect it does.

gcb

Hobbsy,
A little food for thought: Since compression adjustment is basically for timing, you might try different size props on the Saito that COULD allow higher throttle settings.
Since most normal compression ignition engines allow both compression and fuel/air mix adjustment to get proper timing for a particular load (prop), perhaps attacking the problem by adjusting fuel/air and load to get the proper timing for a particular compression setting is feasable.
Another factor is the fuel mix. Perhaps adjusting the ratio of ether to kerosene will help.
Good luck,
George

ddd

Regarding your question of If its so good why don't the major engine producers make them. The answer is quit simply, why should they your buying what they make know. The engine market today produces engines no better or worse than say 25 years ago. The last big breakthroughs were ABC by Supertigre in the early 70's and schnurle porting HP .61 Blue head in the 70's. By the way schnurle porting goes way back into the 20's the boost port was designed into the two port system by Bill Wesnewski Of K&B in the early 70's.

I'm glad everyone is having a free spirted discusion, it great.

P.S the guy that goes diesel is the guy that chalenges the status quo is more daring and inquesative a person.

Graeme Marion

Greetings from Sunny Queensland.[8D]

I just happen to have one of the "Saturn Falcon" engines.
I took a few photos for you to see.

The compression is adjusted by screwing a brass screw in or out with an allen key.
There is no separate contra piston. The brass screw occupies the place where a glo plug would normally go.


Regards from Graeme Marion.

Dr. Wine

My Feedback: (2)

The brass screw looks a little bigger than a glow plug and and on that small engine it is the contra piston. On a large displacement engine, if one would replace the glow plug with a screw, it would create a "hot spot" atop the piston. Eventually it might burn a hole. This happens on REAL diesel engines when other than normal combustion is present. I know the model engines run cooler/wetter and maybe it would be enough to keep this from happening.

Hobbsy,

A Saito 80 diesel at full throttle would be nice. Keep experimenting, I'm very interested in your results.