earlwb

OS .40LA diesel engine conversion.
I had bought this NIB OS .40LA engine a long time ago with a DDD head fromDavis DieselDevelopment.
So since I was trying to determine if my old diesel fuel was any good. Sort of a sanity check from the last weekend when I couldn't get a couple of Elfin .09 diesels to run. I thought I would give the OS engine a go and see what happened.
I first ran a couple of tanks through it as a glow engine, to ensure it was working OK. It ran well and idled really well too.
So I let it cool off some and swapped out the glow head for the diesel head.

It was fairly easy to get running on diesel fuel, which was nice. So my old diesel fuel was still useable.

After running it and tweaking the compression and needle a little, I was getting about 11,500 rpms using a Master Airscrew 10x6 prop. I used the MA 10x6 prop to get a idea of how the engine performed as I remember the .40 engines did years ago with 8x6 props.
A small frame Fox engine turns a 10x6 at about 12,800 RPMS. Other .40 engines do about the same too. The OS .40LA is a improved version of the old OS classic the .40FP. Although both the FP and LA ran quite well, they were a little anemic compared to other ball bearing equipped .40 engines at the time, one reason is the bushing supported crankshaft has more drag to it so it loses some RPMs from that too.

One thing I noticed was that I couldn't get it to idle as low as it did with glow fuel. It didn't want to idle below the low 4,000rpm range. I'll have to experiment with it some, maybe the idle mixture screw is set too rich for model diesel fuel. Or maybe it needs a larger prop and maybe a spinner nut to help keep up the momentum or inertia to overcome the higher compression of the diesel head.

AMB

Earl. One thing I did notice, you do not have any pressure line hooked up to the tank, also you mention the fuel is old, you may have ether loss and the mix on the idle may be off. It should achieve a much lower idle 1200-1500, also may be low time on a new engine, lots of variables looks like medium and high speed is ok
best to you martin

earlwb

That is a good thought.  Thanks.
   Can one hook up muffler pressure on a model diesel engine? I didn't as I have never seen anyone use it yet on a model diesel engine. I assumed that all of the ether in the fuel might be more flammable than normal.

The fuel is a gallon can of Davis Diesel Development Fuel that was still tightly sealed from the factory, I bought it 2 or 3 years ago.  I had never opened it until a week ago. I poured about a quart out into a smaller can for usage. The can would puff up in the summer and squash in during the winter. So I assume the ether hadn't leaked out of it. But I don't know if some seepage had occurred though. The can seemed pretty full when I opened it.

NM2K

My Feedback: (14)

Just my guess, but I think that the engine needs more run-time as a glow engine, then all will be well.

I wonder if my RJL Diesel conversion head will fit my OS .40 LA? I know it was designed for the OS FP.


Ed Cregger

JohnAV8R

Elweb, nice pictures. Looks like you are having fun.
The diesel doesn't need a pressure hook up.
A .40 (7cc) diesel on a 10 x6 prop is not doing "its thing", let it lug a little. As you showed the Atom 1.8 cc, it turns a 12x5 at 6500 rpm and it is less than 1/3 the size.
Revving the diesel up may require a higher compression and the glow .40 rod may break or wear out a lot sooner.
Also remember the carb. If you are using the same carb for both glow and diesel, the diesel does not require the same volume of air, so perhaps do not open the carb as "wide open" as you would use on glow.
Lastly, the larger size diesel will not require the same ether content in the fuel as the smaller diesel.
Bring your info over to the vintage diesel thread.
John

NM2K

My Feedback: (14)

Actually, it is my guess that the Dieselized OS .40LA would benefit more from the exhaust pressure hook-up than the glow engine. After all, it will be running at lower rpm with less air moving through the engine to pull fuel from the tank. A gas version of the Perry VP-20 oscillating fuel pump (vibration actuated) wouldn't hurt either.

The Magnum FS-30 carburetor would probably be a perfect match for a .40 two-stroke Diesel. Remember, I'm just guessing.


Ed Cregger

controlliner

Actually an OS .20 FP carb will fit the .40 LA perfect and the small throttle bore will improve the engine's performance. I took an OS .40 LA carb and fitted it to my OS .20 FP engine to get more power as a glow.

peterburford

Have to like that test stand!
Peter

Recycled Flyer

This has puzzled me for the longest time since the fuel/air mix for a diesel demands a far higher air ratio than a glow does.

The smaller throat diameter to me is all about fuel atomisation and air flow speed past the spraybar so I conclude that a model diesel really does like great draughts of air if it could only use that to properly mix the fuel.
I suspect that the poor throttling at low engines speeds relates to the lower air speed past the spraybar giving you globs of fuel instead of a fine mist and of course when you reduce the throat diameter of the venturi the airspeed increases and this reduces the fuel particle size.

Part of why injection systems rule in full sized diesels.

Cheers.

AMB

Earl just noticed your prop size in the Davis instructions he suggests and 11x8 for a 40 MORE PROP , this will drop your top end to maybe 10500// A 12X6 OR 7 will work too too,but should bring down the the bottom end. If you are using the gallon size davis fuel you are fine the 1/2A stuff he only had in qts > your fuel is most likely fine the way you handled it , when you say will not run below 4000 and if it just slows down and stops you are not getting enough fuel changing the carb to a smaller bore or sleeving down the original will increase your fuel draw at lower rpms , try the prop and pressure thing first before changing the carb first and yes that is one of the nicest test stands I have seen good job as peter mentioned, my guess with the larger prop and tank pressure you will see an idle drop, try it before more glow time martin
also full out will be about 3/4 open on the barrel it might bog down full open

The LA 40, 46 are a well mannered sport engines not a powerhouse on glow but really comes on with a Davis conversion

Recyled flyer is totally on with the air velocity thing on the carb

One thing I noticed that it appears your compression screw is almost all the way in, with the larger prop, bigger load, I think you will be a turn or so out( compression lowered) for that nice
diesel hum

earlwb

Thanks for the information.
I figured the prop was too small, but it was the first one I found in my prop box that I hadn't reamed out already for other engines.
Anyway, it was interesting to see how it did with a 10x6 prop anyway.
I'll try it again with a bigger prop after I buy one or two bigger ones.

I like the idea of the OS 20FP carb. I'll have to locate one.
Thanks again.

Recycled Flyer

Just wondering what a filter over the intake would do for the idle, it should create a more chaotic airflow over the spraybar (this works well with control line fixed venturi engines that can use a larger than normal intake diameter and get away with it) but since I don't fly RC variable throttled engines I am guessing here.

Just might be a cheap fix instead of paying out for another carby.

Thoughts anyone?

Jim Thomerson

I've flown a Tower 40 both diesel and glow on an Old Time Stunt control line airplane. I used muffler pressure in both configurations. Ran an 11 x 5 glow and 12 x 6 diesel.

Hobbsy

My Feedback: (102)

OS LA .46 Graupner 12x7 DDD Plane Fuel==9,450
OS LA .65 Bolly 13.5x8 DDD Plane Fuel===9,300

Earl, here the numbers for my LA .46 and .65, your .40 should turn a 12x6 at about the same numbers as the .46 turns the 12x7.

earlwb

Thanks for the information.

So we can run muffler pressure using model diesel fuel then?
I saw that some people are using muffler pressure. I just wanted to confirm it.
Thanks

Hobbsy

My Feedback: (102)

I like the look of the new heads with the annular fins, I have one on my Webra .50

hsukaria

Hobbsy, the picture of your Webra shows some sort of ring inside the carb. Is that to reduce the airflow for diesel use? Or is that just a normal Webra carb?

Does anyone know if the muffler pressure in a diesel is lower than a glow since the diesel fuel combustion is more complete inside the cylinder, and therefore does not require high flow exhaust? (Of course, I'm theorizing here - i.e. BS-ing).

Hobbsy

My Feedback: (102)

H-man, several guys who post here have run rather long exhaust pipes with no detrimental effect. The carb is actually a 7mm MVVS carb, I got about three of the 7mm carbs from Dar Zeelon. Back to the axhaust pressure question I believe the exhaust pressure is less because as you stated the combustion process is complete at the point the blowdown starts, hence cooler.

gmedson

On occasion, I have gotten an rpm increase with an exhaust extension. Guess it must be a resonant-tuning thing, although a straight tube does not conform to tuned pipe shapes.

Regards. -Gary

spaceworm

That happens also with glow K&B Sportsters, as well as possibly others.

Richard

earlwb

In the old Quarter Midget Pylon .15 engine racing planes. Everyone had to cut a slot in the short straight exhaust tube to help prevent a tuned pipe effect. So yes you can get a bit of a resonance going on with a straight pipe.

Recycled Flyer

There is no theoretical reason why you can't run any form of pressure on a diesel but there is a practical one for exhausts - getting all that muck back into your tank, do you really want that when most diesels suck like a vacuum cleaner on steroids anyway?

I have seen RC clunk tanks after a couple of runs that looked decidedly black inside when putting exhaust fumes back through what should be a clean system.

spaceworm

Maybe you could put a Porex filter on the inlet side of the tank from the mufller pressure line?

Recycled Flyer

I doubt that the total pressure would be lower in a diesel's muffler volume but the amplitude and expansion of the shock wave should be less since the combustion takes place entirely before the exhaust port opens. In other words the total flow should be similar to any engine that size running at the same speed but it would pulse or 'ring' less than a glow would.

The swept volume inside a diesel acts as an internal expansion chamber so the engine needs less in the way of an external one.

Glow engines can actually have their flame front emerging from the exhaust and need all the external muffling they can get! (Its possible to see the flame front from a glow with an unmuffled glow on a dark night, its an eerie sight indeed.)

Cheers.

Recycled Flyer

efore it
How long would you give a medical grade filter before it clogs up?