Exhaust is black, black,black...
 

Hi All:

Today when I started up my MVVS D7 with 9x4 wooden prop. the exhaust was very black! I think that it should be vey light brown at worst. No matter how little compression I used, or how the needle valve was set, the exhaust was very very black. I remember that this is a sign of over compression but that was not the case today. Could it b a fu problem?

The engine started very easily as usual and ran great! Perhaps there is some aluminum, back plate, rubbing somewhere, I think that in a glow engine, black exhaust is usually a sign of aluminum being rubbed of a surface.

Any ideas of what is causing this?.


Tia,

Franchi

It means we.don't have a new Pope yet ��

This has to be the best reply to a forum question that I've ever seen!

Hi Franchi,

Some engines naturally run darker than others, likewise with some fuel blends.

So while an over compressed engine will always have a dark exhaust some under compressed or correctly compressed engines may also be quite dark.

You could try dripping some of the exhaust residue on a paper towel and leave it for a day or two. The oil will spread out leaving the carbon in the middle. If the oil at the edges of the puddle is a normal colour (dark honey or thereabouts) then you are probably fine, the dark colour is due to the carbon. You might see some shiny particles which may indicate aluminium rubbing.

I've found some kerosene produces a lot more carbon than others. Jet fuel runs a lot cleaner with less carbon on the piston and in the exhaust compared to hardware store kero over here, but Tim (quicksport2.5) hasn't found that to be the case in the States.

Maybe it'll clear up when the new pope is elected.

Dave H

MVVS diesel's are notorious for having black exhaust residue. On the other had when I first run an engine that's been in storage with ATF as an afterrun oil, it usually has black exhaust for a little while.

Reading your posts it seems to me that you're sending your diesels off to fly with too much compression. I'm guessing that you're flying them in control line models? If so it's ok to release them with a slight under compressed miss. It'll warm up in a few laps. Both compression and needle setting can change as a cold engine warms. You usually tune a c/l diesel over a number of flights.

Otherwise it could be your tank pickup position, fuel too old, DII percentage, kero quality, venturi hole too big, etc etc.

Any pictures of your setup?

Could it be that when you wind back the comp screw that the contrapiston doesn't actually move? It can be hard to tell sometimes. They often get stuck with carbon in the liner after a period in storage. The solution is to disconnect the fuel line, wind back the screw a few turns, prime the exhaust against a closed piston and then flick the prop really hard. It should give a quick burst and stop. This usually moves the contra upwards.

If not take the liner out, and using a piece of wooden dowel on the inside to tap the contra up a bit. Thoroughly clean the liner inside, reassemble and try again.

Sounds like a fuel problem to me, do you have any fresh commercial fuel to try with?
That would quickly tell you if there is something wrong with the fuel you have now.

"The byproducts in the exhaust of a running alcohol (glow) are water vapor and CO2. The exhaust residue on your glow powered plane is loaded with unburned fuel thus it is clear. The exhaust of gasoline or diesel are unburned Hydrocarbons and carbon and if a 2 cycle, engine oil. That's why the exhaust you see coming out of a glow engine is white clouds just like what you see in the sky (water vapor) whereas the gasoline or diesel is gray smoke. The oily residue in gasoline engines is a function of low oil content in the fuel. The black oily exhaust residue from the diesel is normal and is nothing more than carbon mixed with oil but not cut by unburned kerosene. The lighter the color say brown or golden brown in a very rich diesel thus diluting up the exhaust with unburned fuel."

http://www.rcuniverse.com/forum/ever...t-qustion.html

I've been blending my own fuel for a while, and find that gas station #2 diesel (USA terminology) burns cleaner than kerosene from other sources.

There seems to be no loss in power, economy or setting, as well. Hope this works in other areas as well...

Well, that doesn't really answer much though.

In previous discussions here the general consensus has been that the color comes from temperature of the combustion. Larger engines (above .15) then tend to have a darker residue compared to smaller engines. A larger prop also means a lower load (and less developed power) and these then have less colouring of the exhaust residue compared to a smaller prop on the same engine.

A 9x4 prop on a .15 engine sounds about right, although one can use a little larger prop than that too (eg. 9x5 or 9x6).
So if this gives a very dark, black, exhaust residue then I would still say that the fuel could be the problem. I mainly use engines below .15 and when these are propped for around 10krpm at full throttle, the exhaust is only lightly coloured. It will get darker on smaller props and higher rpms but not black.

I don't have the MMVS D7 engine myself, was that produced at the same time as the Modela series?

I regularly use .15 size diesels (R250, Parra, Fora, PAW GTS2, MVVS etc) on 7x6 and 7 x5 APC and JfX (Hobbyking) props. They usually run somewhere between 17K-19K. The exhaust is usually honey coloured. The MVVS do run a bit on the black side. Out friend Frank is a bit sparse on descriptions and pics of his actual engine setup. From his posts on other forums it seems that he's flying control line models. Perhaps he'd like to elaborate a little?

I run converted glow engines as diesels and regularly get dark exhaust oil on all of them. .25 up to .45 size engines. The last engines I ran were K&B Sportster .20 and .28s on a 10x6 APC prop. Both engines ran dark (very dark brown - almost black) when set to peak mixture and compression settings. RPM for the engines was 10,800 and 11,300rpm respectively. Higher prop loads gave a lighter color residue, but I am not sure those bigger props would fly the model as well. I haven't tried it, so I'm not sure. My ST .45 conversion turns a 13x6 APC at 10,800rpm and runs rather dark residue as well. The fuel I use is 31% ether, 22% castor, 47% Jet A, and 2-3% Amsoil Cetane boost for ignition improver.

Perhaps too low of an ether content requires more compression which makes the residue run darker?

Do you really need that much Amsoil? We're running half that, between 1-1.5%. A few years ago I spoke to a senior chemist from one of our oil refineries. He said that all modern Kero was basically Jet A-1.

It was recommended to me to use 2-3% Amsoil because a) it's not exactly the same chemical as the "standard" ignition improver for model diesel fuel (doesn't work quite as well perhaps?) and b) it is supposed to make needle setting easier... I did find the engines hand started easier when using 2% versus 1% - I used 1% Amsoil C.B. Initially and found 2% to be better.

The Jet A that I have is clear as water and seems a little less viscous than the heating oil kerosene I have used which has a light brown color to it. The engines run no differently on the two different distillates - no difference in power or exhaust oil color.

I follow these guidelines found towards the bottom of the page. There's a good little ditty about the Amsoil Cetane booster and the normal amyl nitrate used in diesel fuels elsewhere in the world. It seems 2-3% is good and more than that can cause erratic running and sagging...


http://adriansmodelaeroengines.com/c....php?cat_id=72

Perhaps Frank could come back and tell us what has changed to make the engine change its character.
did the fuel change in any way?
did the engine have any changes?
was it last run in a different season?
As Quasimoto says there's a shortage of info, so it's hard to make a real assessment of the situation.

"Could it b a fu problem?"

I want to know what a "F.U." problem is ?

I know the bikie bar version ............

It was a typo! It should have been FUEL! Lol

franchi

Dirty running diesels
 

My MOVO D2 Normale runs dirty even when under compressed, like some of my early MVVS diesels.

The medical castor oil will produce more black exhaust than castor oil. But nothing to be worried when the engine is running without problems or nothing to be hurt by fuel since the engine is in fact overlubricated.