ddd

I think it time to address theconfusion between a purpose built diesel and diesel conversion of a glow engine. Diesel engines were developed in the late 30's but didn't come into their own until lthe post WW IIperiod. The domminant engine was of course the gasoline engine as glow doesn't come on the scene untill 1949 just as diesel was starting to make some inroads in the American market. At the time gasoline engines were very lightly built and many of them didn't do well on the glow plug and some companies like O&R were put out of business since they could not stand up to the added power and thus stress brought about by the Glow conversion. At that time diesel engines were more heavily built.
Fast forward to the 1970's and the advent of the schnurle ported and the ABC model engine. Very beefy designs for the added power and that is when we came along with diesel conversion.If you take the back off a model engine from then to now andconpare it to a post WWIIdiesel design you will see the added beef residesin the Gow schnule ported engine. If we were doing it wrong then why did the following companies follow our leed , Irvine, MVVS, Enya dieselizing their glow 20's through .61's.

I trust this will put this matter to rest for awhile.

BoB Davis

Lou Crane

Bob,

An excellent summary of the timeline! Thanks!

There are a few other differences, between designed-as and your excellent diesel-conversion types.

I go on and on, occasionally, here and other sites, about the (?) "port volume" (?) conditions for glow or diesel fuel use. In short, as I see it, glow fuels, based on methanol and glow ignition, are extremely tolerant of rich mixtures.

1) Methanol can - usually does - vaporize well in the intake flow. Kerosene-based diesel fuelsprobably donot do that as well, and kerosene has a much smaller tolerance for rich mixtures. These ideas allow glow engines to have relatively LARGE bypass volumes - the vaporized methanol stays vaporized. And it will still burn at very rich fuel/air mixtures.

2) Because of 1.), above, most moderate RPM, designed-as, diesels I've been inside of have quite small bypass channels. They are large enough to do two things - first, they CAN supply enough air/fuel mix to the combustion side at the moderate RPMthe engines seem happy at, and second, the small channels keep the flow speed up to where the kerosene, whether vaporized or in the form of extremely small droplets, doesn't get a chance to slow down and collect into larger droplets. Vaporized fuel, or very-fine-droplet mixed fuel burns better than a smaller number of large globs...

3) Again, referring back to my opinions in 1.) and 2.), above, all our model engines are essentially air pumps. For methanol-based fuels, we can get away with, and gain from, overly rich settings. Methanol will still burn productively at very rich settings. We can rev out, gaining horsepower, quite a way past torque peak RPM. Kerosene-based fuels, for one thing, cannot burn well, if at all, at excessively rich settings, and for another- with the higher compression ratios needed for "diesel" combustion, there isn't room in the combustion chamber for large globs of liquid fuel. Can you say: hydraulic lock?
____SO, the ability of the engine to pump air works with methanol for more horsepower at higher RPM, but limits the amount of kerosene-based fuel it can burn...

4.) All this aside, the "diesel" system is tolerant enough of wierd conditions that the designed-as diesel runs well, economically and strong at around torque peak RPM (where the volumetric efficiency of the engine is at max.) The diesel ignition is also tolerant enough of things that your conversions of the over-breathing designed-as-glow engines do fantastically well - running well, economically and strong at torque peak RPMand somewhat above. (And, with appropriate props, can pretty well match any glow engine performance as long as we don't go too rich with kerosene.)

ASIDE: In CL Stunt, engines like the classic Fox 35, have been "improved" by blocking-off part of the bypass volume. That keeps the bypass flow speed up, for more turbulence (i.e., better mixing of the fuel-air charge), and makes it less sensitive to g-loading effects (remember- fixed throttle, no carb, g-loads from 1.0 to perhaps 30g.) I wonder if there would be any effective gain for one of your converted engines to do something similar to an over-generously ported designed-as glow with your conversion head? If so, I'd guess it would mean more stump-pulling torque around torque peak RPM. rather than an extended RPMrange.

They are a bit different, but both approaches work - as-designed and head-converted diesels, for one, and glow and diesel engines, for another.

Glad you're still making it easy, simple and successful for so many glow-engine users to learn that diesels don't smell bad, just different...

gkamysz

My Feedback: (19)

The only thing that I feel falls short in conversion of some engines is that the carb is too large. I know the solution is often not to open the carb fully. Appropriately sized carbs make all the difference in my four stroke conversions.

AMB

Greg ENYA sleeved the carb on the 25CXD conversion ; Davis suggested a quick fix use a air intake filter on the carb and just add a thin plastic disc'with a hole in it to reduce the air intake of course not running the carb wide open around 75% or so works too on MY Irvine 53s I run 70 -75%
open and run fine either way is does show the reduced fuel consumption of diesel martin

gkamysz

My Feedback: (19)

An engine run on diesel fuel flows less air than an engine run on methanol. I haven't attempted to tune a carb when it's partially open. The main needle circuit is influenced by the idle needle and this makes things tricky.

Lou Crane

Greg and Martin,

Sounds like we're all pretty much agreed. I just rambled on about the internal processes. You BOTHidentified the practical side - that converted glow engines' throttles match the rich mix tolerance of methanol glow fuel better than the more finicky mixture tolerance of kerosene-based diesel fuels. Whether it means no gain from throttle above 2/3 or 3/4 - which I've also observed - or it suggests it may be a good idea to replace the stock OEM carb with one for a smalller engine.

How much smaller? Hmmm. Perhaps the torque performance increase we see with our diesels would indicate that. If a .25 diesel can turn a larger prop the same RPMas a .36 or .40, would that suggest a better fit for diesel-converted .36 to .40 engines to use a .25-size carb? This example would match the favorable torque around torque peak RPMfrom the more potent kerosene heat-yield. Remember - torque peak RPMfor any engine is very likely to be where the volumetric efficiency is best. ...Where the engine can take-in and exhaust fuel-air mixture most efficiently.

Both the ENYA restricted carb, and Bob D's suggestion of a 'NASCAR-plate' type restrictor seem to address this.

Whatever, diesels are very gratifying to run. I thank any and all who keep it possible for us to get and use them!

AMB

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gkamysz

My Feedback: (19)

Volumetric efficiency when talking about a two stroke engine is a tricky subject. You must refer to trapping efficiency. The amount of fresh charge actually available to the combustion process. An engine could post fantastic volumetric efficiency figures only to have much of the fresh charge go straight out the exhaust.

Lou, what is the heat yield of glow fuel compared to diesel fuel?

Jim Thomerson

As I understand it, the heat yield of diesel fuel is higher than for glow fuel, but the air-mix ratio is higher for glow fuel so that  glow fuel  produces slightly more power than diesel fuel.  Maybe As I understand it  . . . is an overstatement!  Has anyone ever published a comparison of power curves for a glow engine and the same engine converted to diesel? 

gkamysz

My Feedback: (19)

I hope to be able to do a dyno plot before and after of a converted engine this summer. I've never seen more than some prop figures. And we know not all engines respond the same way to conversion.

Lou Crane

Greg, sorry for the tag team delay: just got back in here....

I don't have the heat yield numbers to hand, if they exist. The matter is a bit more complicated than lets us easily find the yield of our fuels as mixed. It is the KEROSENE and METHANOL contents of the respective fuels that differ. ASIR, methanol - ideally - yields somewhere around 10,000 BTU per pound, and both kerosene and pump gasoline are in the 19,000-21,000 BTU per pound range. (NOTE: the fuel yield testing is by pounds of fuel per pounds of (either air or) oxygen. Not sure which, but for chemistry is would seem oxygen, as nothing else in our ambient air takes part in combustion. A complication, then, as there's only around 20% oxygen in air...)

Some other complications: Kerosene in many diesel fuels goes a bit over 50% by volume; methanol in most glow fuels is reduced by the oil fraction and the nitro fraction. Oils, say 22%, nitro 15%. That's 37% accounted for, leaving 63% for the methanol. ...By volume, most often.

Nitro yields combustion heat, too. Haven't come across its ideal yield in BTU/lb, but it may not be as high as methanol, since it is much denser than methanol - something like a Specific Gravity of 1.14 for nitro and around 0.8 for methanol. Since most fuel makers mix by volume %, the weight fraction of nitro is higher than its volume fraction.

Then there's the 'damping' effect on combustion from having (we hope!) non-burning oil mixed with the flammables...

AND, the reason I stressed the word ideally each time, so far, is that we don't burn these fluids in the most ideal way, the way the heat yield values were determined. That hides behind that weird word "stoichiometric", which as I understand it means all the combustible fluid is burned in all the oxygen available for combustion. ...No unburned fuel left, and no un-consumed oxygen, in the exhaust gases...

We may come closer to stoichiometric with kerosene, since it does not tolerate as wide a range of fuel/air mixtures as methanol. But remember all those other factors, including that the ether in a diesel blend also contributes some heat during combustion. Again, I haven't seen a number for ether's yield in BTU/lb, but expect it is much lower than kerosene's. Ether is also much less dense than kerosene, so its weight fraction % is even less when the fuel is mixed by volume %. And, since nitro brings its own oxygen to the "fire," the glow engine's breathing capacity is assisted directly.

Also, we run glow engines richer, as a rule, than diesels. We use the evaporating chill of methanol to help cool the glow engine. Kerosene doesn't chill as it sprays into the intake, most likely because it does not evaporate as easily. In both modes, the oil does help to cool things some. When we see visible exhaust vapors, we know that some fuel or fuel and oil passed through the engine without burning away, leaving the metal uncovered. (Actually, that fear may be a fallacy, even if the oil burns. When the oil is exposed to the combustion "flame" the piston is on the way down to BDC. Through BDC the fresh charge is sprayed into the cylinder, ready to lube the piston on its way up to TDC.)

IOW, we use the fuels according to the ignition type involved. We can pass quite a bit of unburned methanol and oil through a glow engine, even when running fairly "lean." "Diesels" need to be leaner because of the narrower tolerance to fuel/air ratio and because the required high compression ratio leaves little room for globs of pressure-precipitated fuel...

If everything were simple and easy, our hobby wouldn't be much fun or challenge, would it?

Lou Crane

Jim,

We need to refresh our thoughts again? In our case: Power is the rate of doing work. Torque is the force that does the work. Agreed?

A weaker torque, applied more times a second, say, could make more power than a stronger torque applied less often. Whether that is true depends on the, in effect, Force times Rate at which it is applied. A smallish diesel may be able to turn a largish prop at the same RPM as a larger glow engine. That only says that both engines are producing the same torque at the same rate (RPM) on that prop. Recent example: ENYA's 25BB diesel cites 9,000 RPM on a 12-5 or 12-6 prop. Many stunt-suitable CL 40's might not be able to do that.

The difference is that on a lighter prop load, the same 40's can run higher RPM, produce more POWER while the ENYA 25BB-D may need destructively high compression settings to try to match them, on those smaller props.

And, relevant to Greg's comments: 1.) yes, of course, volumetric efficiency, as a practical thing, must relfect the trapped volume,

and, 2.) We CAN rev-out glow engines because the methanol-based fuel WILL burn at less than ideal conditions far longer than a kerosene-based fuel can. The torque curves we used to see in the P.G.F. Chinn engine tests (or those by Billinton, or way back, Ron Moulton and Ron Warring) showed torque and horsepower by RPM. Diesel torque curves reached high values around 9,000- 10,000 RPM and fell away gradually. Glow engines torque curves fell more rapidly from peak, but the methanol made the result productive of higher horsepower for a lot more RPM range.

We could probably wind up typical "built-as" diesels if we insisted, but the transfer channel volumes in so many of them are too small to keep it positively productive, however well they serve around torque peak. And the excessive compression settings, to advance the ignition point suitably, would be very punishing on the engine.

gkamysz

My Feedback: (19)

Lou, if you haven't found the lower heating value of fuel components you haven't looked enough. The www is a huge place these days, but you have to be careful about where you find your information. We really don't care what volumetric efficiency is as a user. The most useful figure will be BSFC if consumption is even important to the user. While tests of 50 years ago are still interesting, they don't really mean much when we talk about modern schnuerle ported engines.

If you are going to run a built as diesel fast, it should be designed to run fast. Some just aren't capable. Just how much difference is there in compression ratio between 10k and 20k? How much different is a plain PAW .15 and a tuned combat or team race version?

An Enya .25SS Diesel can't turn a 12x6 at 9k. That's just wishful thinking. Unless somebody has one that actually makes that kind of power.

http://www.modelflight.com.au/manual..._SS_Diesel.pdf

http://www.dkd.net/clmodels/acln/acln130.pdf

Lou Crane

Greg,

Agreed, I haven't chased through the web looking for those specific values. Even Brake Specific Fuel Consumption is pretty arcane for our uses. We can measure fuel consumption per unit time, but in-air RPM is almost always an estimate, as is the prop load at the relevant airspeed. How, then, calculate brake horsepower?

The odd condition of having such blended fuels, IMHO, makes it more difficult to state reliable specifics. So much depends on the complex blend of fuel components, user factors, engine, setting, prop, altitude, humidity, temperature, phase of the moon, etc.

Not so sure I can buy the idea that "modern" schneurle engines are fundamentally different/better than engines of 10 to 15 years ago, when Mike Billinton ran engine tests for AeroModeller(RIP), and I think I'd seen some of his work in an occasional MAN since. He tested ABC, ABN, AAC schneurles more than any retro crossflow engines. Nice work. He didn't reduce torque and HP curves to idealized, smooth approximations. Still, invariably, his peformance graphs indicated torque peak RPM at moderate values, however high the engine could wind out before HP peak RPM... Exceptions, too. He also ran some tests with tuned pipe boost, and graphed the torque and horsepower consequences for several different max boost and max degradation conditions from the pipes...

So far as "effective" volumetric efficiency - it seems to me that in a given engine there is one optimal "gas-flow" zone (air and vaporized fuel being considered gaseous) where the volume transferred is optimal, where pumping losses are best countered. THIS is what I expect more rigorous, complex testing and analysis would find to coincide closely with the engine's torque peak RPM. Seems to me to be a matter tied to the internal sculpturing, paths and instantaneous flow velocities in several small zones... which we generally don't mess with... Pipe resonance effects superimpose on this 'natural' flow.

The actual compression ratios at 10K and 20K are more rather unrealistic numbers for the average user. They aren't very far apart on the compression lever, anyway. The loads involved are more concern to the engineers who develop engines, not to most of us at the user-end.

Of course, any method of ignition can serve engines designed to whatever RPM conditions are sought for. Design from blank paper (monitor screen?) works best, if your demands are very far off 'the usual'. PAW engines are pretty sturdy, to begin with, so re-engineering to withstand loads may not be a factor. Fit, clearances, sleeve and shaft timing, assembly alignment and most suitable part weights within the factory tolerance spread are easier to optimize when you have the entire batches of a production run to cull through. In addition, PAW factory surely knows what further touches to porting best serve combat and racing. The performance differences are definitely there. But PAW is not a best example of purpose-built high-RPM diesel engines. For combat, perhaps, with the local popularity in some areas of Nostalgia Diesel FAI Combat, but all first tier current FAI Combat engines are glow these days. TR diesels turn in the vicinity of 30K, and get plenty of laps per tank - now, that's amazing!

The ENYA 25 SS-D I'm breaking in shows ~10,000 RPM with about a half-hour on it, on an 11-6. I expect the factory numbers are possible, eventually, but are not ordinary flight settings. BTW, I live at ~5,000' above sea level, but diesels are less susceptible to altitude effects, or better said, the adjustments allow compensating for those better.

ADDED: Went back and read your links. John Modistach's report on the 25 SS-DTV is interesting and well written. The 25D I have is the non-TV version, although the effective choke areas are similar. Also, after the first 10 minutes on 1-1-1 fuel, and a few minutes on the 3-3-2 blend cited, I'm running 5-3-2 blend, with the 50% being kero. The VicStunt report on the FORA is interesting, too. I'd expect higher RPM... Haven't spent much time browsing VicSTUNT lately... Thanx!

gkamysz

My Feedback: (19)

Nobody cares about consumption except the F2C guys.

Loading the engine on the ground to the in-flight RPM allows one to measure and calculate in flight HP. It's not rocket science and there are several ways to measure in-flight RPM accurately these days.

Maximum torque does not always occurs at peak volumetric efficiency. It is dependent on many other factors. After research I have found that such generalities aren't the rule, especially, two stroke engines can be all over the map.

If the compression isn't significantly different between 10-20k why was it a problem for the Enya a few posts ago which wouldn't even be run that fast?

Modern team race diesels are closer to the typical glow engine than any old diesel.

Yes, I think that diesels have the advantage of compensating for elevation quite easily. It's not that one couldn't with a glow engine, just that nobody does.

AMB

Just a thought guys, I think the site has become too technical for the average or new modeler, we may need a site just for theory and design of IC engines regardless of fuel gas,diesel,or glow. the *hard core here* does comprehend these very informative theories and principals
But at least I think the original site was to help with running and setup issues, show our engines in applications, compare engines suggest an engine for an a particular use, solve running issues ( the bench of course for break in, set up)
Fuel, start, fly, enjoy. The newcomer will be overwhelmed with all this data and an other electric flyer emerges martin

gkamysz

My Feedback: (19)

The site is not too technical. There is nothing making anyone participate in technical threads. Most of the threads here are not technical and is refreshing to those of us who are not newbs or actually have interest in how these things work, to see anything other than "where to I get diesel fuel" questions. If technical discussion was relegated to another forum chances are I would stop visiting here. After all we are discussing diesels. I know that Davis doesn't like technical discussion. Yeah maybe he has the only line of modern diesel products, but is not the only one that can figure it out and discuss it. The Russian model engine forums talk about engine manufacture and theory all the time. I've never seen any complaints that technical discussion was brought up. If we disregard the technical reasons of why we like diesel the leaves the awful smell and hard to come by fuel to discuss.

Lou Crane

Greg,

...selectively, so as not to annoy you more than necessary...

I believe I wrote the compression setting was not much different. The space in the combustion chamber, between the power piston and the contra-piston, is small. The contra-piston screw thread is not variable. At high RPM compression settings, a quarter turn MORE compression changes the compression ratio more than the same quarter turn toward LESS compression. Is that agreeable to you?

peterburford

I, for one, enjoy any technical comment and discussion.
Without it, this thread's question of "purpose-built or conversion" really becomes only "warranty or not".
Peter

[email protected]

I look foreward to technical information on this site. Even though I have been running diesels since the mid 1950's I am rather ignorant of the technical aspects of our model diesel engines. I learn good info from technical comments on this site. Keep it going. Jack

AMB

OK all too technical may have been a poor choice of words, in retrospect it would be nice if we had a little more input and the actual use of our engines in use and in flight to show what they can do Rocket Bob has done a great job for one of "show and Tell" and the cult thing on Davis conversions ?? people will use what works and works wel we have 100s of posts under the heading of Club Saito on the glow site
for one I guess we have a Saito cult too
and others .I do have an issue with a statement that DDD heads destroy engines, which is an absurd untrue statement
I would like to pick up when I can afford it one of Burfords little diesel which from the reports I have read are far superior to PAW
MY first diesel in 1950 was a drone 29 which was in a home built slab wing U/c on 50 ft lines, as a kid, I then went to college and dropped out till about 15 years ago and got back this time in RC my first engine was irvine 40 diesel ( a modern design) then it grew to 5, thats when I found Davis and bought modern schnerle ported engines from 10 to 90 maybe 20?? and great results
a few PAWS were added too but the Davis conversions and his fuel is the mainstay
yes the irvine 40s are still here and updated with his heads to avoid the chance of dropping that big brass contra

I just ordered more fuel from davis and his sales of fuel are increasing, I guess diesel is not dying off that quick. My guess electrics have brought s lot of people into RC and it appears they are going to try IC engines as the next step glow or diesel, OS seems to be bringing out new engines every few months, so has TT economics dictate you are not going to bring a product to market that does not sell martin

AndyW

My Feedback: (1)

I like to think that some 70 postings to YouTube http://www.youtube.com/profile?user=hopeso&view=videos on diesel and diesel conversions has helped,,,, at least a little, to take some of the mystery out of diesel and widen the interest. But maybe not. In any case, the hobby expanded exponentially for me when I discovered diesel.

In my opinion, any engine built for glow will do well as a diesel. Some exceptions might be those with light conrods or weak cranks. Throttling can be an issue but most times a reduction in the bore of the throttle barrel helps a lot. Muffler pressure helps as well.

Lou Crane

Greg,

To yours of June 26, I know that it is possible to measure in-air RPM, as CL speed and racing fliers do. It is not simple, nor usual in the gear most of us haul to the field... Last I saw it done (in CL Speed), it involved a fairly high-fidelity tape recording, from the flier's center location (to reduce Doppler frequency shifts) and a program in a laptop computer to convert the recorded sound frequencies to RPM.

Now, that's too technical, for me at least... [8D]

Andy, to yours of July 6:

You wrote: "...most times a reduction in bore helps..." Am I correct that the 'bore' you refer to is the choke ID?

Also, agreed, some mass-produced glow engines may not be sturdy enough for dependable conversion, without needing to be nursed along like a feeble old maid... THIS is probably where the charge that DDD heads break engines started. Bob D's first commercial product was the Cox .049 conversion head - teflon seal, loose "contra-piston," large thumb-screw comp adjuster... It wasn't Bob's head that broke shafts. It was the users who expected the "usual" 16,000 to 18,000 RPM on 5" props, instead of the more suitable 10,000 to 12,000 on 7" props. Bob's stronger shafts have about ended that problem, no?

However, most of the schneurle ported, taper-cut ABC/AAC/ABN engines that are almost universal now, are much more sturdy than the reed valve Cox .049s of the 1970's, when Bob first released his head for them. Even so, one of our regulars in this forum has managed to suffer shaft breakage on a highly reputable glow .40 converted to diesel. I know JT didn't intentionally set up any conditions that would cause the breakage, so I'll write that off as an "off label use" (as they say about medicines) that the engine could not handle.

Exceptions prove???

To some others in here, allow me to paraphrase Jack Nicholson in a movie: "The TECHNICAL? You can't handle the TECHNICAL!"

AndyW

My Feedback: (1)

Lou,

Yes, that's what I said.

Also, it's not running diesel on glow sized props that breaks things, necessarily. Just the higher compression ratio required to light off the fuel in a diesel engine, regardless of prop size is what can over stress some glow designed parts. Plus, the stronger punch when that kerosene lights off adds to the loads. Larger props have more mass and inertia for the combustion event to overcome. It all adds up.

The larger Norvels, .15 and up, have pretty massive cranks in comparison to some others. No way you'd break one of those cranks. BUT, those skinny, dogbone rods are prone to bending unless you're very careful. More care is needed, compared to when you run them on glow.

I did break the crank pin off of one of the .15s. But that happened to be on glow and it broke while it was running. Just a fluke, a bad part that got past the QC guys.

I just got one of the stronger, heavy duty, TD cranks from Stewart, from Downunder and I'm very much impressed. LOTS of counterweight with a drilled out pin and it looks more like a crank should. I've broken TD cranks on diesel running 8 x 4s. This one looks like it'll hold up just fine. It has a hair larger shaft diameter than the Brodak/CS and this will be ideal for turning one of those into a light and powerful diesel. The stock cranks kept breaking when run on diesel. No reflection on the engine per se as just like Cox, they were designed to run on glow.

So there. [X(]

RocketRob

My Feedback: (1)

Lou, This is a sport engine designed for training kids. It was originally named the Fora Junior but I guess they decided that sounded a bit to juvenile. It is also available for $80-100.
It is very well mannered and an easy handler. Mine in stock configuration spins an apc 7x6 at 15,100, but take a #1 sized drill to the venturi and it spins the same prop to 16,900 @ just under .5hp. And the threaded push/pull contra adjustment is silky smooth. This betters my PAW GTS 2.5 by 400 rpm.
We need more purpose built diesels like this.