Hi all,
I have in my possession a new PAW 40 diesel that is a purpose built stunt engine and after talking with the engine's builder and designer, one of his team mates that flew with him and a fellow PAW 40 user here in Australia, here are a few tips that got passed along to me -

1. Run with an 11x7 prop, this gives about 9500 rpm on the ground (this coupled with a thinner than normal wing section will give good airspeed and wind penetration. Run in on a slightly smaller prop (I know, it is controversial but I have my reasons),
2. After running in on the higher % oil fuel use 20% castor oil, 30% ether, 50% kero and add to that about 1.5% IPN (the amount of IPN needed will be less as the days temperature rises),
3. Use 1/2 x1/2" engine bearers and generally solidly mount the engine, this is to spread the vibes out all the better,
4. Warm the engine up and then quit it prior to use, this will get you to the correct run setting a whole lot easier,
5. Use a chicken hopper tank, as it seems to prevent a lot of the end of run stuttering when the tank runs low,
6. Always side mount the engine as this stops the fuel charge in the lower crankcase from being thrown into the head during manoeuvres.
7. Don’t count on any type of a 4/2 break with a diesel, simply use the flat torque curve to keep the airspeed constant through the entire pattern – much like a four stroke does,
8. Diesels like to be heat soaked and do not need anywhere the amount of cooling air than a glow does so its OK to cowl them in tightly,
9. Diesels do not need anywhere the amount of muffler volume that a glow does so the classic collector ring and small cast in expansion chamber is just fine, and this formula can be carried over to converted engines also.

Now I know full well that diesels are fairly rare in stunt but they are refreshingly different and can be of benefit at high altitudes due to their larger prop sizes biting better in the thinner air.

And they are a talking point despite them being labelled as ‘smelly!’

Anybody wish to add something?

Not apropos to anything, but I've been wondering for years:

What do they smell like when you mix the fuel with biodiesel?

No idea as I have never used it but I can find out if you wish.

It'd be interesting to know if it makes them smell any better.

I own a Davis Diesel head, and like the way the engine works with it, but I keep going through head gaskets.

Tim,
Which engine does your DD head fit onto? And 'going through head gaskets' is definitely not the norm here mate so perhaps a call to Bob Davis will be fix the situation for you in the quickest possible manner.

But I would encourage you to participate in this forum as it is a mine of information about everything diesel.

http://www.rcuniverse.com/forum/forumid_87/p_1/tmode_1/smode_1/tt.htm

Perhaps I will see you on the other side soon?

I fly diesels when I can.

The pointers for the PAW 40 BB Stunt should do very well. The Eiffelaenders have been making PAW engines since the late 50's at the latest. Most were .15 and .19 cu in (2.5 and about 3.0 cc for the metric set.) They developed and campaigned a .29 when the flier-member of the company felt a larger stunter presented better. Several time winner of the UK Gold Trophy; quite decent flying! The original .29 "FREEBIRD' was designed for their then new .29 (5cc) engine mounted inverted. Even the factory guys went to profile/sidewinder for the final version. Their .35 is a stretch of the .29 in the same case.The 40 BB (which they call TBR -twin ball-bearing races) came later. Their smaller diesels always flew trouble-free sidewinder, too.

The fuel 'stink' might be from either of two ingredients - ether or kerosene. The ether smell disappears when it is burned during running, but might bother you before that. Good CLEAN kerosene has little smell before and after burning. I've used camping supply stove and lantern kerosene fuel, which are both low odor before and after. (They can make the exhaust oil pretty sooty, though.) Lately, I'm trying #2 Auto diesel from the local gas station. Very nearly odorless, and the exhaust oils seem to stay MUCH cleaner. Kerosene's power output doesn't seem related to odor. Can't beat the price, either.

For ether, the best source seems to be John Deere Starting Ether, available from any John Deere sales or service shop. That is, anywhere people farm or do serious gardening. Can't guarantee that it is available everywhere, the EPA (Enjoyment Prevention Agency) may have hit it in certain very ecologically ga-ga West Coast States... Capturing the ether from the JD spray cans is a trick, though.

On the RCUniverse website, there is an entire forum dedicated to "Everything Diesel." There's much more about diesel engine operation in there, of course. Some of it is even solid and useful. ...Particuarly methods of milking the JD spray cans.

Model diesels run at compression ratios above 18:1 - there isn't room in the combustion chamber for just about ANY excess 'liquid.' Liquids don't compress - try it and you can break metal. So, no 4-stroking; the engine will enforce that, and you will pay a hefty fine... The fuel is also quite a bit more fussy about fuel:air mixture range, than methanol-based fuels. That partly explains their greater 'economy' for similar power.

You may have more trouble finding a glow-engine stunt flier to launch your diesel-powered stunter, than problems with the engine...

Bestaluk!

Jim,

I sure would like to see actual measured numbers, too! Designed&built-as diesels all seem to be more strongly built than most glow engines...

It might be that your 4011 that DID run on diesel fuel had a superior sleeve/piston(Dykes-ring) fit, compared to the other that wouldn't. It (unlikely for K&B) might have had production tolerances stacked so that compression ratio was a bit above spec. There may also have been some catalytic action from the glow plug - possibly on the ether or DII - which leaves the question of why the second 4011 DIDN'T.

The  numbers I cited were mentioned by guys on RCU: Pe Reivers, Jens Eirik, GKAMYZ, possibly even Bob Davis... You've seen the same numbers there, too, right?

Another odd combination? I managed to run an old 4-bolt backplate, baffle-piston Fox 19 on sparker fuel - poorly, and VERY hot - and only as long as the glow plug was hooked to the battery. Not a promising result, just felt a need to try it.

Lou, if model diesels need 18+ to one compression ratios, how come a K&B 4011 glow engine, with a much lower compression ratio, will run just fine on diesel fuel?  I'd like to see some actual running compression ratios on some diesels, as I am inclined to think they run at lower compression ratios than what people say. 

Oh no, not this one again!

There are two measurements used in regards to compression ratios -
Goemetric generally is the industry standard and simply uses the volume of bore and stroke plus the head volume divided into the head volume,
And Relative, which also takes into account the stroke only after the ports close and is the more precise measurement to use but is harder to determine accurately, hence the adopted use of geometric.

Now the geometrically measured ratio will always be much higher than the relatively measured one and although the standard is the former it may not always be.
And there is no way of knowing which ratio has been quoted in any engine review.

As to an unmodified comp ratio glow engine running 'fine' as a diesel, please define fine?
And what was the comp ratio to start with? Around 12:1 I am willing to bet and there is no way an 8:1 glow engine will fire unless the fuel is pure ether in a warmed up engine.

And I believe that Clarence Lee once did a test on a Saito four stroke that used 18:1 comp ratio, so that level of compression seems to be well within engine design manufacturing limits.

Chris,

To your reply #10 - THANK YOU for reminding us all in this discussion of the difference between Geometric and Relative Compression Ratios! On very little reflection, I feel sure that the higher numbers often cited are for Geometric CR. The lead time before BDC (Bottom Dead Center)  after the exhaust port(s) open DOES seem to approximate the difference (after having run many port onset time calculations over the past few decades).

If anyone is having trouble grasping the difference between the two methods, try this:

The cylinder volume through the full stroke is simply the area of the piston times the stroke distance. Whatever space is left above the piston at TDC (Top Dead Center) is the combustion chamber. The Geometric CR is the full stroke volume divided by that combustion chamber volume.

HOWEVER, the piston cannot start to compress the fuel/air gases and droplets in the cylinder until the last sleeve port (the exhaust) is closed by the top of the piston. The distance the piston rises to TDC, then, divided by the combustion chamber volume, is the Relative CR. Obviously, the height of the exhaust port above BDC means that the volume of the cylinder below that is not relevant to actual, effective compression ratio applied to the fuel/air charge before combustion.

Now, diesel users have that trick variable combustion chamber to work with. Whatever works out well for a particular engine defines the narrow range of "correct" compression ratios - i.e., RELATIVE compression ratios.

NB to Andrew T:

There's a discussion over on RCUniverse "Everything Diesel" forum which introduces the idea of a difference between 'droplet' combustion and point-triggered ignition of combustibles.

Say, WHAT???

Glow and spark ignited engines have a specific, fixed point, in both time AND space, that triggers the burn of the fuel/air charge in the upper cylinder. Our diesels (properly, compression-heat ignited) engines do not have that.

What they do have is compression heating of the fuel/air mix to the temperature flash-point of the ether molecules present at/about TDC. The ignition may or may not develop from a single point. ...It probably doesn't... It may be, in effect, spontaneous (under the conditions) at numerous points around the area of the piston head. Flame propagation rate is not so much involved if ether 'flashes' at a hundred or more different points around on the piston head within a few degrees of shaft rotation - IOW, within a few hundred-thousandths of a second.

Heat loss to surrounding metal, cooler and "wetter" droplets of oil and fuel mixed, and perhaps other factors, appear to give the " "diesel" " engines a longer duration "push" than the flame-travel speed controlled gasoline and methanol engines experience.

(Info: Years ago, SAE released a paper comparing flame-propagation speeds for several fuel combustibles. But not for "alcohols." Near stoichiometric conditions - in which all the oxygen present is consumed in burning all the combustibles present - "alcohols" (usually methanol or ethanol) had flame speeds too high to measure. Gasoline and Kerosene (principal combustibles in, respectively, spark and "diesel" operation) WERE measurable. We don't operate "alcohol" burners anywhere near stoichiometric conditions. We do use a side-benefit of the "alcohols:" the chilling they yield on vaporizing i.e., converting to gas from liquid state, as part of our cooling package.)

(Info, continued: Obviously, our combination of richer (than stoichiometric) fuel mixture settings, in the presence of a large amount of non-combustible oil, damps the rate of burn for both alcohol- and kerosene-/gasoline-based fuels, and affects the cooling "package." In effect, glow engines may more easily reach an explosive release of combustion impulse, across a smaller range of shaft rotation°, than an oil and mixture strength damped 'diesel.'  Kerosene's slower, inherent flame-front propagation rate, may trigger from 'flashing' of ether molecules in a more random manner than the specifically-placed spark in a spark-ignited engine, or the combination of hot-point, catalytic and compression-pressure heating that lights off combustion in a glow engine. Different temperatures, across the ether molecules present, should mean a spread of time over which they do flash and ignite adjacent kerosene. The higher torque output of 'diesels' is largely due to the greater heat yield possible from kerosene, but may also include the spread of time over which the kerosene starts to burn, to release its power... In that sense, I see things differently, Andrew.

(Remember, 'diesels" may seem to "soak" hotter over the entire engine than glows, due to the absence of the high heat of vaporization cooling of methanol vs: kerosene -BUT, they run cooler in their hottest zones across the entire engine! Early British diesels, reportedly, running about 50% ether/50% oil fuel, often chilled too much to continue running. The inclusion of kerosene (paraffin, anyone?) in the fuel blend added potential power yield, and reduced the chilling from vaporizing ether. NB: Kerosene does not vaporize easily...)

Well, hey, everything about our model engines and planes still impresses me as a miracle allowed for the few to enjoy, and many fewer to try to understand to any degree. If we ever DO learn it all, we have topped Everest, and all that's left is the fatiguing, oxygen-starved climb back down to the "real world."

I hope never to go THAT far...