Foaming Diesel?

I've noticed I'm getting a lot of bubbles in the fuel line. Is diesel fuel subject to foaming like glow fuel does sometimes? If so, will a few drops of Armorall help? Is there an "anti foaming agent" for diesel fuel?

Also, I've got a standard Brodak uniflow tank with the uniflow port vented to atmospheric pressure (no muffler pressure). When I cap the overflow vent I'm getting fuel comming out the uniflow vent when the engine is running. What could be pressurizing the tank? The fuel and tank are at ambent temperature when fueling and running. Could foaming in the tank do this?

Any tips on preventing fuel foaming?

Oh, by the way: The airplane is an ARF Flight Streak. The engine is a PAW .19 The prop has been balanced (and the balance checked...)

Ron,
Are you sure you don't have a pinhole in your fuel line? Also, what kind of line are you using? Remember, diesel fuel causes silicone to swell. Neoprene or tygon are good.

I don't think diesel fuel foams (but I don't know for sure). You may be getting some vibration because of the rather light stucture of the ARF Flitestreak front end. One thing you can try is putting a piece of foam between the tank and fuselage to cut down on transmission of vibration.

Diesel fuel seems to me to expand and contract more than glow fuel with changes in temperature, so that is something to keep in mind. Sounds like you are getting siphoning of fuel out the uniflow. This can be fixed, maybe, by bending the uniflow around in the airstream until it stops. I have seen uniflows which did not siphon. I use a one-way valve on mine to stop siphoning and to cut down on affects of outside air pressure changes. Even making a cap for the uniflow, with just a large pinhole to let air in, may cure your problem.

Ron,
Assuming that you don't have a leak somewhere in the tank, your observation of fuel coming OUT the uniflow vent while the engine is running sure seems to indicate foaming to me. I am no diesel expert, but you might try to put a little fuel into a test jar, and shake it to see if it foams. If so add a smidgen of ArmorAll, and see if the foaming stops. Before I would potentially trash (= contaminate with ArmorAll) a large container of diesel fuel, I would try to see if the engine will start and run on what is in the test jar.

Instead of using ArmorAll, another possibility to prevent foaming is to see whether you can add some vibration damping material between the tank and the fuse.

My suspicion is that even with a relatively smooth-running PAW, the front end of an ARF Flite Streak isn't sufficiently robust to tame vibration. If you can, try installing an OS .20FP or .25LA and see if the foaming goes away.

Of course, a fuel-line (or even a tank) pin-hole could be the cause.

And please let us know . . .

Ron:

I have a PAW 19 plain bearing on my flitestreak with the stock tank hard mounted. Haven't had any problem as long as I can set the compression correctly. These engines seem to set up an harmonic frequency at certain RPM's that could be causing the foaming. Have you tried different props, if so is the foaming the same with them?

Since You have the twin ball race 19 how much oil are you running?

I didn't like the noise my ARF FS made (even with an FP20) and so added a 2" X 2" ply triangle between the nose and the LE of the wing. Plane still sounds a little cheesy, but much better. Wonder if this might help the fuel issue.

>Ron:
>
>I have a PAW 19 plain bearing on my flitestreak with the
>stock tank hard mounted. Haven't had any problem as long as
>I can set the compression correctly. These engines seem to
>set up an harmonic frequency at certain RPM's that could be
>causing the foaming. Have you tried different props, if so
>is the foaming the same with them?
>
>Since You have the twin ball race 19 how much oil are you
>running?
>
>Carl
>
>

Great help guys. Thanks!
I'm using Eric Clutton's (Doctor Diesel) premixed fuel. I think it is equal parts by volume, ether/kerosene/oil. The oil is castor..

I'll try different props... I'd planned to do that anyway to see how the engine/airplane performed. I haven't done the "shake the can and see if the fuel foams" test yet. I'll try that tonight.

The thing that foams in fuel is the castor oil. Armorall does not mix with diesel fuel (oil and water).

If fuel is coming out the balance tube then air is getting in somehow.

Paw is not made with balance in mind, good luck, maybe you can find the magic rpm in the air. The crank doesn't even balance itself let alone the heavy piston rod assy.

>Great help guys. Thanks!
>I'm using Eric Clutton's (Doctor Diesel) premixed fuel. I
>think it is equal parts by volume, ether/kerosene/oil. The
>oil is castor..
>
>
>I'll try different props... I'd planned to do that anyway to
>see how the engine/airplane performed. I haven't done the
>"shake the can and see if the fuel foams" test yet. I'll try
>that tonight.

Hi Ron, concerning diesel fuel,if you are running a .19TBR, you
can use 20% Castor for Sport and Stunt flying, for Racing and
Combat you should run 18% Castor. the plain bearing .19 Paw can
run 20-25% Castor. If you are running the 33% fuel, that is for
running in only...or the very timid....Eric Clutton has several
appropriate fuel blends...so does Red Max. try this formula for
Sport or Stunt: 20% Castor,2% Amylnitrate,35% Ether and 43% kerosine
Red Max has, or will blend this fuel for you.
We have been using this
blend for over twenty years...Also in all those twenty years; I've
never experienced diesel fuel foaming.
Best Regards; Tony

Hi,

Yesterday I tried a new/old mod that looks like it works! Still a few prop and setting issues to lock in, but...

I've seen diesels spit fuel out the uniflow tube, too. Finally figured that ether's volatility is so great that some vaporizes inside the tank, particularly at Arizona summer temperatures and with running vibes.

With uniflow venting, vaporizing ether varies pressure inside the tank and pumps raw fuel out the vent tube ... the spitting we see.

Overflow tubes "uncover" into dry space inside the tank first. A 'Tenny tank' (pinhole vented overflow tube, ONLY) allows any vaporizing ether to vent --dry-- out the overflow tube. It also resists fuel loss as that is the only path vent air has to enter the tank interior.

Results: dead steady run in maneuvers and level flight! No warning before cut-off, however. ...Not even a few laps of surging diesels sometimes give us. Just slight further leaning for ONE second then silence! And the only goop on the model was obviously BURNT fuel -- no raw fuel.

My mod is removable -- a piece that's easily soldered up. The tank's overflow tube is 1/8" OD. (All tubes copper, of course.) I sweated a short length of 1/8 OD onto 3/32 OD, and a length of 1/16 OD inside the 3/32. Lengths? -- the 3/32 and 1/16 tubes, about 1/2" to 5/8"; The 1/8 OD 'collar', about 1/4".

File, deburr, neutralize, etc., and make sure the 1/32" ID pinhole opening IS unobstructed. The piece looks like a Lite'n'Brite peg. Slip its stem into the overflow tube and hold it there with a short length of fuel tubing.

The 1/32" ID has no effect on vent air flow at the rate needed - say 6 minutes for 2.5 oz fuel. (Engine: 19PAW plain bearing) It DOES, apparently, allow the vaporized ether to vent. Yes, the tank was originally plumbed uniflow, but now I use the uniflow tip for fueling, not venting, and recap IT instead of the overflow.

I don't expect this to work as well for glow -- nothing in that fuel has ether's high volatility. A 'Tenny tank' for glow would gradually lean out like the old over/under vent tanks, but wouldn't likely spew fuel like they did (single path for vent air, again.)

sounds reasonable to me that vibration and heat may be causing the eather to evaporate in the tank pressurizing it... There is certainly some pressure build up in the storage can until I remove the lid. I'll certainly give this a try. It's easy and it sure can't hurt

What is the vapor pressure of eather? how does it compare to alcohol/nitromethane?

LAST EDITED ON Aug-11-04 AT 09:14 AM (CDT)
 
Ron,
I can't give you the exact vapor pressure, but it is very low, and if you don't take care, you can find the % of ether in your can of fuel drops significantly over the time you are using the fuel (why the darn engine can get so hard to start). I know of some guys keeping the diesel fuel in a cooler.

This is all very interesting. My cans of diesel fuel are in my heated/ac shop. When I come in and turn the machine on to change the environment, in a few minutes the diesel fuel cans will start to pop loudly as they expand or contract. Scared me the first couple of times until I figured out what it was.

At one hot day contest, I forgot and left my fuel can in the sun. I couldn't get a long enough run to finish the OTS pattern. I took a bulb (Sullivan black 4 oz, with a pinchoff) of fuel and put it in my icechest. Solved the problem in a fairly short time, maybe 5 minutes.

In recent years, my diesels have flown only on uniflow tanks, either with a one-way valve, or with muffler pressure, on the uniflow. So I wonder if it is essentially a closed tank pressurized by the partial pressure of the ether. I havn't seen any need to vent off the ether pressure, and I fly fairly regularly on 90+ days and occasionally on 100+ days. I don't think I change the needle valve setting to compensate for temperature. Maybe the system goes to equilibrium fairly quickly?

Incidentally, when I fill a tank, I can see the either outgassing out the overflow. The refractive index of gasious either is not the same as for air.

A couple of interesting observations: diesels seem to bare some similarities to the popular 4 strokes: large prop, favorable torque curve, etc. Also comparing a couple of OEM advertised specs on the PAW 35TBR and a SAITO FA-40a. Both weigh 300g. The PAW puts out .75hp at 11500rpm, the Saito puts out .65hp at unspecified rpm. The PAW has a recomended prop range of 11-6 to 14-6 the Saito has a recommended prop range of 10-7 to 11-6. Both are user reported to have a very constant flight speed in c/l stunt application. Yet diesels aren't popular at the top levels of competition. I didn't see any in the top 20 list at the Worlds that was posted here on this forum. There must be a good reason: if there was any advantage the top guys would be using them. Some say it's the inconsistancy from run to run (I never did learn how to get a Fox 35 to be consistant ). Yet there is anecdotal evidence from folks right here on this forum that they get consistant runs time after time. Is it tecnically possible to get World Class runs time after time from a diesel? or is it just learning curve?
I'm fascinated by the "tweaks" the two stroke folks go through: head shims, venturi size, props, glow plug heat range, partial blocking of bypass ports, different fuel mixtures (nitro) for different days, not to mention all the tuned pipe stuff, etc., etc. So this is not dead simple either.

Ya know, I think we're just "tinkerers" at heart. How many of us would take a specialized stunt motor like the RO-Jett or the Rustler-Merco out of the box and tinker with it?

Tweaking engine parameters is just part of the fun.

The good news: great weather at the flying site today.
the bad news: started an out side loop with insuffecient altitude
The good news: damage is not too sever.

The real good news is the big improvement in engine run.

I made a simple version of your restrictor. I put a pin in the uniflow vent and filled the vent tube with solder. Then heated the pin with the soldering gun until it pulled out leaving a just small hole.

I have now made the following mods to the setup: restrictor in the uniflow vent tube, muffler pressure to the uniflow vent, plugged over flow vent, tank mounted on neoprene wet suit material, and the aft end of the tank is wedged out a little. The run is much, much improved. It still surges a couple of time before shutting down not but for several laps like it did before. Still lots of learning curve on prop sizes and compression/needle valve settings. But this is progress.

Ron,

Just one caution on muffler pressure for diesels...

I did a series of engine test and setup for a guy in Tucson a while back. All DDD converted mid-size throttled engines...

I noted that on DDD fuels and most of my homebrews, there was a considerable accumulation of black, granular something in the exhaust oil. ... most of us have also seen the gunk smears on our diesel models at times, no?

I made a flow-through trap from a standard orange transparent plastic pill bottle -- simple: one tube in (from muffler) one tube out (to tank). After a few minutes running, this granular matter collected in the trap.

NOT in the fuel going to the engine! The clear tube from trap to tank remained clear. It wasn't, before.

Kerosene, the power fraction in diesel fuel, burns sooty. Soot is carbon. Diamonds and graphite are also carbon. Which of those did the trap collect? I'm not going to take a chance that it is the hard, cutting one.

I'll stay with suction feed, or some CLEAN external pressure source.

I noticed some of that when I was running my Tower 40 with RJL head on the bench. On the bench, I could not adjust the engine to clear up the exhaust. However, when I started flying it,I quickly got to the adjustment point where I was getting clear exhaust oil. This rather surprised me, and I do not understand the difference. I used a piece of silicon tubing for the muffler pressure tube and flew it a couple of hundred flights. After I went through 4 cranks (due to running a heavy hub, I hope) I converted the engine back to glow and have put several hundred flights on it (with heavy hub). I'm still using the same piece of silicon tubing. One of the RC guys posted that his tubing and tank were both black, but he saw no problems.

Running on the bench, the muffler was above the fuel tank, and the pressure nipple was toward the side of the muffler. Flying, the engine is side mounted, the nipple is more to the top, and the muffler is well below the tank. I have not noticed oil in the muffler line, but I havn't really looked too hard. I did see that, after filling the tank, when I put the vent cap on, a little fuel would squirt out the muffler tube. If I put the muffler line on before the vent cap, fuel would siphon big time down the muffler line.

I don't see any positive argument for running black oil back into the tank, but I doubt we are making anything close to diamonds at our pressures and temperatures.

Ether evaporation pressure in the tank has been mentioned, and is a real phenomenon, however it is clearly less than muffler pressure.

Yeah, I gotta agree that black gook pumped back into the fuel tank is a bad plan. I ran the engine a bit on the bench with the pressure line simply dumped into a very small jar (looking for "stuff" coming out). I didn't see anythng in a few runs so I suspect that the flow rate is very low and the oil residue is being routed out the exhaust for the most part. I ran it this way on the plane because it was simple to hook up for experimental purposes. For a permanent set up the trap is a good idea.

My next experiment is a fuel bladder made from a "Zip Lock" baggie. The plastic seems to not be affected by raw diesel and the fuel bladder can be made any size by heat sealing the edges. The badder goes inside a "clunk" type tank and the muffler pressure is plumbed into the tank outside the bladder. That way none of the exhaust crap contaminates the fuel. We'll see how that works... Perhaps you've already tried it?