Diesel or glow? Is diesel better?
 

Just picked up my latest copy of Model Aviation this evening and noticed an article on diesel engines. The article listed several advantages to diesel and sparked my curiosity. I know absolutely nothing about diesels. But I have been searching for an alternative to a problem I have.

I have been flying glow for several years and have acquired several plans for 60 size warbirds and have a TF kit for an FW190. My problem is the rising cost of glow fuel and my aggravation with finicky glow engines. Lately, I have been converting weedeater motors to run on electronic ignition with good success and I like running gas engines. The reliability of the gas engine and the availability of cheap fuel appeals to me and fits my budget. Problem is that there aren't any gas engines small enough to fit the 60 size warbirds that I like to build. True the Zenoah g20 is small and will fit some of the Hanger 9 ARFs. It is a great engine and I have one in a GP Tiger Moth. But it is too large to fit in the cowl of the FW190 and too heavy to fit the 62" ws models I have plans for.

When I read the article the light bulb went off and I thought, Hey maybe I could convert one of my glow engines to run on diesel. I have a ST G90 with a Pitts muffler that would bit perfectly in this model. But would it work and would it be economical? Looks like the fuel is rather high at 30 bucks a gallon and only available from Tower. But how far will that 30 bucks fly? Is there a head out there somewhere for a ST G90? How do you run and tune a diesel engine? And most importantly, is it economical?

The article in MA made it sound very appealing. Lately, I have been leaving my glow planes at home because of all the field equipment needed to run them. I like being able to just carry the plane, the fuel, my transmitter, and a battery charger to the field and being able to just walk up and flip the prop to start it. Will operating a diesel engine be just as easy? Will the G90 have the same power to fly warbirds?

Lots of questions I know. I looked through several pages of this forum for info but most threads seem to be very specific in nature and not very helpful to a newbie. I even did asearch for a conversion on the G90 with no success.

I would appreciate a link or site that might offer a tutorial for a newbie like me.


Thanks
Jim

RE: Diesel or glow? Is diesel better?
 

There are many posts on this all good if you do a search on this Site will come up also it will only burn about about half the amount of glow as model diesel fuel hence cost equiv is about $15 a gallon'for run time check out the davis diesel site, this is a powerful engine on diesel and has been for years
ALSO JUST CLICK BACK ON PREVIOUS POSTS I have one convereted with a Davis head one of the best out there martin

RE: Diesel or glow? Is diesel better?
 

Jim, I am a Diesel enthusiast having about 20 conversions including a SuperTigre .90. They reduce your field equipment to no glow battery, a starter if you wish, I use a starter. It is difficult but not impossible to have a dead stick with a Diesel, running out of fuel is just about the only way to achieve one. If memory serves the ST .90 turns a 14x7 three blade at about 9,000 rpm.

RE: Diesel or glow? Is diesel better?
 

As a kid, I used to run webra mach one, taifun and all kind of diesels that came my way on line control (UC) planes. The .15 typically ran a 8x6 three bladed prop or 9x6 two blade.
A fellow modeller in my village was crazy about american glow engines. When a plane refused to hand launch (remember the 1950ties), he used to add some nitro, and then just made it.
I then went to sea, and aquired some McCoy and OS engines, so I decided to replace my webra .15 diesel with an OS .15 glow engine. BIG MISTAKE! My Mew Gull racing plane would hardly taxi, let alone take-off. After adding a lot of hard to get nitro, I managed a flight which ended my MewGull. Tjose were the days.
After the 60ies, glow engines improved a lot, and diesels pretty much became a thing of the past, yet they still provide the ability to turn large props, and provide a lot of thrust without the need of much cooling. They run and run, lean or rich, hot or cold, burping and struggling maybe, but running just the same without sustaining damage.
It is had not to just love them.
Switching to gas in larger planes turned my attention away from these little wonders, albeit with a lot of nostalgia.

RE: Diesel or glow? Is diesel better?
 

Jim,

There are some significant differences between glow and "diesel" (in quotes because they are not *true* diesels in the large-engine sense. True(sic) diesels compress the air THEN inject the fuel. Our "diesels" compress the fuel already mixed of ether to start ignition, kerosene for the main power output, and oil (natch) for lubrication. It's a technicality, but sorta nice to know. One gent posting (here, I think) thought he could hit a gas station truck diesel pump and go flying. Joe W's article told it pretty much like it is.)

There are several good basic references on how to operate our diesels. Dr Diesel (Eric Clutton) is in effect the USA rep for PAW diesels from the UK. Eric has published Dr Diesel's Diary, an excellent history and manual, and an interesting read. Bob Davis, of DDD conversion heads and much else, provides great information. Carlsen Engine Imports, in Arizona, also has a fine outline of the things you should know. The threads in, and mentioned in, this forum provide a lot of great information. Worth checking...

If Joe Wagner didn't stress it very strongly - you'll notice the fuel smells different from glow fuel. Your flying buddies may razz you about that...

Learning the (fairly simple, but not always intuitive for an experienced glow engine user) combination of adjustable fuel mixture setting AND compression will take a positive attitude to mastering it. Using a DDD head, you'll get a truly excellent instruction sheet not for just mounting the part, but for how to get started using it with confidence and comfort. The information is VERY good.

Welcome to the compression-ignition side! Enjoy!

RE: Diesel or glow? Is diesel better?
 

As Lou has stated the Davis instructions are first rate, he also has an additional publication called Diesel 101 which you don't actually need but has great info on why hobby Diesels work the way they do. Ask him for it if you talk to him.

RE: Diesel or glow? Is diesel better?
 

Here's a good link: [link=http://www3.telus.net/dieselcombat/about_diesels.htm]Operating Diesels[/link] The site is about PAW .15 powered CL combat, but the basics of starting, break in, and mixing your own fuel should help you out.

RE: Diesel or glow? Is diesel better?
 

G'day Is diesel better? I would say it is different. It has some advantages over glow and it has some downsides.

Most of my engines now are four strokes. Like you I got sick of finicky, noisy glow two strokes and I find four strokes easier to live with - particularly the smaller Saitos. I have them inverted, sideways and upright and they all run well so long as the tank is in the right place and the engine has been run in. Initially they don't idle all that well and so I run them in in a trainer to get them running right before I use them in anything important.

I have a couple of diesel powered planes at the moment. One is powered by a PAW 1.5cc engine (a half size Dalaire Sportster oldtimer) and the other is powered by a Magnum 46 converted to diesel but with a carby from a GMS 32. The original carby was simply too large for operation as a diesel. The smaller carby tunes well although I probably loose a little top end performance but it has plenty anyway.

The diesels use fuel at about half the rate that a similar glow engine does but the fuel costs more. The big advantage is that once a diesel is running, it will rarely stop until it is out of fuel. When I was racing diesels years ago I used to curse them for not stopping when I had got the tune totally wrong. They just plodded on until the fuel ran out. The downside with fuel is it is more expensive and harder to get although in the US you can buy it far easier than I can here in Australia.

If you want to try diesels I would suggest you start with a smaller engine before trying to get your ST90 to run as a diesel. A small PAW or an OS 40 or 46 LA with a Davis head would be good starting points. The LA engines seem to make very good diesels. A bloke turns up at our Diesel Day every year with his LA46 conversion and it works really well. Then I would put it in a plane that does not matter too much and do some experimenting. If you like what you find, then go ahead with the big ST. Hobbsy seems very happy with his but he has a lot of diesel experience. A chat with Mr Davis would be a good idea too. I hear he is very helpful.

But if you simply want to spend less on fuel, your ST 90 will probably run quite happily and reliably on less nitro than you are using. Most European motors (the ST was designed there even if it was made in China) will run will very little nitro and low nitro fuel is much less expensive than high nitro stuff. Moki and Webra engines need little or no nitro. I have been making fuel here in Oz lately and FAI fuel (no nitro) costs me about AU$15 per four litres (about a gallon) where as 15% nitro costs about AU$27 or almost twice as much. But we have found that the FAI fuel lasts almost twice as long as 10% nitro fuel which makes it extremely cheap to use. 5% lasts almost as long and makes the engines far easier to tune.

So ... after long winded rant.

I would suggest that if you just want to save money, then using less nitro will probably save you heaps without any great loss of performance. If you want to try diesels, then they do have some real advantages - no glow plugs, rare flame outs, great torque, lower fuel use but the down sides are higher initial cost, higher fuel cost/gallon and a learning curve. If you still want to have a go (and I hope you do), then a smaller cheaper engine and a cheap plane would be a good place to start but your ST90 will run well as a diesel with a Davis head if you just want to jump straight into the deep end of the pool. If you do this, I still would suggest a cheap plane like a large trainer to test things out on.

The photo is my diesel converted Magnum 46 SL in a Great Planes Big Stick 40 with Bolly 11.5 x 5 prop. Even badly tuned it hauls the stick round really well and never looks like quitting.

All the best

Michael from Oz

RE: Diesel or glow? Is diesel better?
 

I had 2 diesel engines and enjoyed them. Both PAWs, an .033 and a .15. I found two characteristics of diesels I didn't like. Most importantly they use a high percentage of oil and that oil is castor. It can cover an airplane and is a sticky mess. Ive read that you can use a long exhaust extension but I couldn't get mine to start with a long extension. Guess be great on a pusher. Also Im not fond of the odor- some folks love it! Personal preference I guess. Even though they just sipped fuel, and starting was easy after you learned how, and no problems with dead starting batteries or blown plugs. The two problems were two much for me. To each his own.

RE: Diesel or glow? Is diesel better?
 

Ah Hutch, It's an aquired taste.....;)

RE: Diesel or glow? Is diesel better?
 

G'day oily handers. Well I took the Stick in the photo a couple of posts back out today and gave it a fly with its diesel converted very second hand Magnum 46XL. As I had not run it for some months I tried a slightly different approach to starting.

Firstly I filled the tank but I did not connect the fuel line to the carby. I then gave the engine a few drops of fuel down the carby wide open. Next I applied a "port prime" to the side of the piston (I have no muffler on it at the moment) and with my finger protector on, gave it a "smart" flick. It fired first flick. I primed again and it fired again after a couple of flicks and this time ran on the prime in the crank case for about two seconds. Time to fit the fuel line.

Then, with the radio on, fuel tank connected and throttle wide open (because I had forgotten) a prime and one flick had it roaring away. Some minor adjustment to the compression and we went flying.

The flight was fine but I did discover that the low end needle is too rich and I need to sort that out next time.

The other two blokes with me were impressed at the power, relative lack of noise (for an engine with no muffler) and relative lack of mess. No worse than any other two stroke but probably stickier with all that castor.

RE: Diesel or glow? Is diesel better?
 

Good info guys. I might be curious enough to give a diesel a try. I have Pilatus PC-6 and H9 Hellcat I haven't chosen power for yet. I've been doing electric so whether I have to learn Nitro or Diesel, it's all the same to me.

I think I understand the basics... It's not really a "diesel" engine, it's not really regular "diesel" fuel, it's not really cheaper, and it can be just as finicky. It probabley needs special fuel lines and a special fuel tank. I can get my head around that stuff.

Please correct me if I've misunderstood. These are some questions that have been rolling around in my head concerning mufflers on "diesel" engines:

How does one pressurize the fuel tank without a muffler?
Is it true that one can just use a header and a silicone hose and route the exhaust back through the fuse and out the back?
Does using diesel make using a "tuned pipe" pointless?
Can one still use a smoke system? Does the pipe get hot enough to burn smoke oil?
If not, does the "diesel" leave more or less smoke than nitro? I keep picturing my tractor's puff of black smoke on startup (even with bio-diesel).

I've been reading a bit about this but these questions never come up, and I'm sure there are questions I haven't even considered yet. Can someone please fill in the holes for me?

Thanks

RE: Diesel or glow? Is diesel better?
 

Also, has anyone seen a conversion for a moki/mark 1.35. There is one collecting dust at my LHS and it has been marked down considerably. I think the FAI fuel is recommended so, if I understand things correctly, it may be an easy conversion.

Thanks

RE: Diesel or glow? Is diesel better?
 

G'day

I have used lots of diesels but mostly in control line racing back in the 1970s. The couple I run now are just to amuse my self and keep the smell of diesel around the place. Hobbsy, on the other hand, uses many and may come to your aid.

However I can have a stab at answering your questions but remember, I am not an expert in this field.

With a diesel, it is better to use a smaller carby. I use a 32 size carby on my 46 and a 25 might even be better. You can sleeve the carby to achieve this. Super Tigre and Enya do this. With the smaller carby throat, you do not need to use muffler pressure unless you have a really bad tank setup and in this case it will probably be just bad anyway and not work at all.

I am not using a muffler at present but I have in the past and will in the future. You probably should use a muffler. This makes the old "port prime" difficult but I am going to install a tube into the muffler so I can do a port prime (ie, wet the side of the piston with fuel - has the advantage that you don't easily flood the engine this way)

I think that a tuned pipe for diesel would be "different" because of the way diesel fuel burns. Experts will tell you more.

The exhaust gas from a diesel is cooler than from a glow. How this would affect smoke I don't know but I suspect it would make it less effective.

Diesels make little smoke. What you see is unburned oil mostly and there is not a great deal if that. A Saito four stroke makes a lot more.

I would suggest that your first experiments be with something relatively common like an OS LA 46 which makes a good conversion. Then, more people can help you and also you are more likely to have success. If you like the results then get something more exotic. You are lucky to have Davis Diesel readily at hand. I understand their products are excellent and their help is great too. And they have fuel available which is a great help. Where I am fuel is a real problem.

All the best.

Michael

RE: Diesel or glow? Is diesel better?
 

Muffler pressure works well with diesels. However we did not use mufflers in the 1950's and 60's at least not in the US. Some engines would lean out at the end of the fuel tank flying U/C. Someone in the early 1960's came up with a solution to this problem. Known as the Uniflow fuel tank system. A Uniflow fuel tank would get a consistan run the whole tank. No leaning out at the end. A couple years back I was flying a Playboy with a GB diesel and stock 4 ounce fuel tank. It would lean out at the end. On the test stand with a standard one ounce tank this was not a problem. See photo. So I had to go to the Uniflow set up for the four ounce fuel tank. Or design and build a muffler so I could use muffler pressure. I went the Uniflow tank solution. The other photo shows a Uniflow fuel tank used with an Enya .06 glow on the test stand. Cap the overflow after filling the tank. The other copper tube faces forward and goes to the bottom of the fuel tank. If you go to the SAM web site Dave Harding has an article with drawings on the Uniflow principal. Can be used with plastic clunk fuel tanks also. Jack

RE: Diesel or glow? Is diesel better?
 

Photos did not make it on last post. Try again. Jack

RE: Diesel or glow? Is diesel better?
 

My favorite diesel from the 1950's was the Super Tigre G-30 2.5 cc rear induction. Favorite diesel from the 1960's was the Webra Mach II 2.5 cc front induction. Both user friendly. Since no mufflers ran well with Uniflow fuel tanks. I stopped flying U/C in 1969. Got rid of my engines from the 1950's and 1960's some time in the 1970's when in got into R/C sailplanes. I should have held on to those engines. Jim Dunkin photos lifted from the web. Jack

RE: Diesel or glow? Is diesel better?
 

I had one of those ST G-30's but mine had a red muff. Had a lot of fun with that engine. Sold it to a guy who wanted to try diesels.

Never had the Webra Mk-2, but still have a Webra Mk-1. Found it to be an excellent engine for CL profiles because I could reverse the needle assembly and have the needle run through the fuselage while the fuel nipple would be pointed toward the tank wedge for easy connection. Only down side is having to drill props to fit the thrust washer.

Opinions. This is mostly old-hat for diesel users but may help a diesel newby:

Generally, most use a smaller intake size with diesels because they use larger props running fewer RPMs which decreases intake velocity. Smaller intakes increase velocity to maintain fuel draw.

One of the advantages of diesels is using the contra to adjust timing for different props. To accomplish this with a glow requires adjusting head shims, plug heat range, and fuel mix. That said, some diesel newbies tend to adjust compression WAY too much. Perhaps that stems from making final adjustments before the engine has reached normal running temp. If it is running a little under compressed, it still retains most of its pulling power.

Diesels are excellent for scale ships where large props at lower RPMs are needed. Nothing sounds worse (to me) than a high wing scale ship with a glow turning a small prop at a gazillion RPM (sorry, NORVEL lovers). In this application, make SURE you let the engine heat-up before final compression setting or you may greatly overheat your engine by running it over-compressed.

You never have to worry about a dead booster battery.

Avoid silicone. Diesel fuel swells silicone and will cause the tubing to slip off of fittings. Neoprene and Tygon are two good fuel tubings to use on diesels.

I believe tuned pipes are not as effective on diesels because initial combustion is more complete. The energy in the reflective wave would be already expended. This is merely opinion, I have not tried it.

Please correct or add opinions...

George

RE: Diesel or glow? Is diesel better?
 

Thanks for the tips GCB. Good catch on the silicone tubing thing. I'm affraid I might have otherwise learned the hard way.

About your statement regarding "lower rpm"; Is it normal for the rpm to be lower in general, or is it because you typically bump the prop size up that you get lower rpm? I was under the impression that Davis Diesel claimed a slight rpm increase.

Thanks.

RE: Diesel or glow? Is diesel better?
 

I ran a K&B 4011 glow on diesel fuel. It ran fine; turned same prop exactly the same RPM it did as a glow engine, but needle turned in one and a half turns. Ran first model Drone diesel, then put Drone glow head on. Exact same RPM. Needle leaner on glow.

RE: Diesel or glow? Is diesel better?
 

George, the red head G-30 was a couple years later. Attached photo. I agree with your thoughts on diesels. Except I have no experience with tunned pipes. I am just a sport flyer. Jack

RE: Diesel or glow? Is diesel better?
 

Others have already answered the lower RPM question, but one additional thought: Some older long stroke diesel designs run better on large props. They were designed to peak at a lower RPM and will break if run way past their designed RPM. Famous engine man George Aldrich used to say, "Never under-prop a diesel".

When running an old diesel, I would consult data that suggests RPM range and prop sizes. As a rule, I like an 8x6 or 9x4 for a .15 diesel. I have run my old David Andersen twin stack (~1952) with a 9x6.

Diesels (purpose-built or conversions) of modern construction can usually handle a wide range of props. If in doubt, use what the manufacturer of the engine or conversion suggests.

George

RE: Diesel or glow? Is diesel better?
 

Break-in

Is it better to do breakin before or after the conversion? I am assuming after, but would like some opinions.

RE: Diesel or glow? Is diesel better?
 

Mr. Davis recommends before then you'll have your needles close to their Diesel sseetings. They will need to be leaned a little for Diesel operation.

RE: Diesel or glow? Is diesel better?
 

Thanks, You guys have about covered everything I can think to ask. I'm going to get that motor on the stand an get her going. I'll chime in when I have some results.

FYI- I emailed Davis Diesel concerning the smoke system question. He responded within 24 hours (which in my opinion, is quite gentlemanly) and stated that the mufflers don't really get hot enough for a smoke system. Good to know.