Recycled Flyer

Yes thats right Jim and kinda my point here, there is so much waste fuel coming out the back of the exhaust that you are going to need to compensate for it at the front end with a bigger carby.

Just witness how much fuel drag cars flow through their engines and the size of the blower/ carby/ injector that is needed to keep up with the running of it - and just for laughs here I have an image in my head of imagine Davis Diesel heads on a Top Fuel car with smaller carby's on it!!

Gotta stop thinking about these things.

Recycled Flyer

And my best guess with the above question (and it's the core of this thread I suppose) is that too large a throat size on a diesel will cause it to fail atomising the fuel correctly, and will cause a rough run regardless of the needle or comp setting..

Diesel fuel air mixtures seem to contain larger droplets than methanol mixtures do ( the mix just seems heavier) and you will need the gas flow at a high speed in order to get the best out it - hence avoiding the 'mad squirrel with hiccups' syndrome.

And the fuel air charge is very prone to high 'G' forces (at least in control line stunt) and is the main reason why Ilike to side mount them, it prevents the charge from being thrown into or out from the crank to the head volume causing a momentary stutter from flooding.

So smaller carby's and to a lesser extent smaller transfer ports all seem to provide this much needed high speed gas flow.

Increase the carby size beyond what is needed and there goes your gas speed, but you can get away with less restrictions with transfer ports because the atomisation has already taken place prior to reaching them, hence why you see some ducted fan diesel conversions and racing diesels use humongeous transfer ports and still run well.

Again, its just the way I see it.

Hobbsy

My Feedback: (102)

To my mind many glow engines have too large a carb on them so the mfg can claim those outrageous 17,000 rpm numbers with a popsickle stick propeller and win the paper HP race. We would never use the engine that way. A case in point would the new Webra P-5 .91, the recommended prop range is 14x12 to 16x8 with 8,500 to 9,000 being its useful range. But here's the kicker, the specs say its rpm range is 2,500 to 16,000. Many have tried to run a 14x6 on the P-5 and have been disappointed, some guys are trying the same with the new series OS's designed for larger props and lower rpm. My P-5 would turn a 14x12 Graupner at just under 9,000 on 5% Fox fuel with 20% lube.

AndyW

My Feedback: (1)

Some criticism of glow to diesel conversions has been that most glow engines are timed to run small props real fast. So when you switch to diesel, and use a bigger prop, you're not getting the best out of the engine that you could. I've never been disappointed in my conversions but then again, I don't have any experience with larger, traditional, diesels.

So I decided to run glow sized props on some of my smaller conversions and I found that I had to use higher compression to get a run and the result was an engine that ran "gnarly". It sounded overcompressed and ran rough, but would not run otherwise. The fuel was not changed, using a 39/39/20/2 mix.

AMB

Ok Group if you look up Audi formula 1 race car diesel V-10 (amazing 510 hp )and they state this with RESTRICTED air intake. looks like little or big same 'deal'
and a lot less revs than its gas counterparts and about 1/2 the pit stops for fuel martin

Recycled Flyer

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Andy, did you reduce the carby throat size down at all here?<o>
</o>

Because if you didn’t then the fuel atomization may have been compromised by the lower speed gas flow around the jet. This would give a lumpy mix with bigger fuel droplets and give the same report as your ‘gnarly’ run.<o>
</o>

Interestingly some racing diesels that use small props also use larger throat sizes but resort to peripheral jets around the inside of the throat diameter to encourage a finer emulsion and this gives better range and power but with the trade off of poor response during maneuvers, but I don’t see many racing models doing patterns so it doesn’t really matter.<o></o>

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qazimoto

My Feedback: (1)

Andy it sounds like you were using too much DII.

When you increase the rpms of your diesel the fuel does become more important, particularly the Cetane "level". Your mix at 39% ether is much higher than normal, you might find that you could get better performance with less Cetane booster. This is easily determined by experiment. Normal team race practice is to mix your fuel without it, ie 2:2:1 ratios for a litre. Decant say 100 ml and carefully measure and add small volumes of your booster and test the mix. The right % for the day becomes obvious very quickly.

Racing diesels use much less oil than others. For example Nelsons run happily on 10% or even less and like degummed or other “enhanced� castor. Hence the fuel is noticeably less viscous and atomised better. Contrast this with a plain bearing PAW using 25% castor oil.

About 2.5cc (.15 ci) seems to be about the optimum size for a diesel. There have been very few really successful production engines ones larger than that. I mean that there have been hundreds of 2.5cc diesels made but only a handful of say 5 cc ones.


Ray

AndyW

My Feedback: (1)

Some interesting points here, thanks guys.

No, no change in venturi size but the glow sized prop, 6 x 3 did run very fast so there was good airflow into the engine. Note that the engines ran/run great on big props, even an 8 x 4 on a Norvel .06,,, and in fact, ALL my 1/2A conversions run really well on that size prop. That's two sizes up from glow.

About less oil for racing diesels. That makes sense, less viscosity, less drag. But what of longevity? But racing has all to do with winning. One win per engine is perhaps not out of the norm?

About cetane. Well, I assume that because I had to increase the compression ratio, my cetane rating was too low. But an ignition improver isn't the same as cetane rating, maybe. All of this does beg for more experimentation, your tips are appreciated.

About big diesels,,, come to think of it, that may explain my problems getting a 26cc gasser to diesel conversion going. Having said that, my Norvel .25 and .40 conversions run great. They DO need an electric starter but they light off nicely and sound solid and reliable on a big prop. Then there are all those big, Davis conversions. Would it be fair to say that traditional diesels, built with traditional materials and timing, holds them back from being successful in larger sizes?

I recently ran both a ferrous and an ABC Elfin, 1.5cc, and was intrigued by the timing and the bypass versus exhaust timing. What if modern timing principles were applied to these engines? What if a larger clone were built with modern timing?

Have synthetic oils been used in diesel racing?

Mr Cox

I think it is only natural that the compression has to be increased for smaller props. I have always thought of that as being a "timing" thing, higher revs need higher compression in order to fire just before TDC. Glow engines are a little like that too but we don't usually have an adjustable compression to tune on.

I don't really like small props on diesels anyway, they just sound labored to me. This is true even for double ball-bearing ABC engines like the Irvine 20D. But then I have never really played with the fuel for the purpose of running small props.
DDD has some conversion formula for the prop size but I just go by the top revs (aiming for around 10krpm) and how they sound. At least "two" prop sizes up I would think. The Irvine 20D I like with a 10x6 (while a 20 glow would run something 8x5-9x4) and I have a PeeWee that happily swings a 6x3 (compared to 4.5x2 for the glow) etc.

AndyW

My Feedback: (1)

At the time I ran the small props on diesel, I was just curious, having just read about racing diesels. The information just posted has prompted me to experiment in this area. Racing diesel, because of the nature of the best, appears to be a challenge that a number enjoy tackling. I understand that the UK has long standing racing events for diesels.

Wonder how well diesels would do in those foamie, ducted fans. I have several foamie DFs that I want to install real engines into. These were going to be glow, but why not diesel? Well, one reason is that diesel fuel melts foam but there are fuel proofers for that.

lildiesel

The notion of a diesel needing a smaller carb is what got me to buy the simple little air choke carb for the Sure Start .049 offered by Bernie at Xenalook with the idea of putting it on the rear venturi of my MK 17. It if works at all I will finally get to take it out of the knock off Nobler I built with a 36" wing span and put it in a good RC trainer. Time for another overpowered Whizard.

Mr Cox

Just remember that it is not a carb, it is only a choke mechanism.
So while it should reduce the rpm somewhat it does not really offer any "throttling" and more fuel might actually be spent at the lower rpm compared to a higher rpm.

lildiesel

I'd be satisfied if it choked the MK 17 enough to make a powered landing possible. I fly in a pretty tight field next to a local county park and forcing a landing on the little paved strip is preferable to running out of fuel over the scrubby prairie grasses.

Jim Thomerson

When I flew my Tower 40 on diesel, I used the same venturi that I use when running it glow. I ran a 12 x 6 diesel and an 11 x 5 glow. My impression is that Super Tigre G20-15's run better upright than side mounted in CL stunt airplanes. The Tower 40 was side mounted and ran fine in both modes.

AndyW

My Feedback: (1)

I've installed a proper carb with an adjustable airbleed to Mills diesels and they ran great, and throttling was very good. I also converted one of the Mills engies to glow and it ran fine and in fact, was a one flip start, most times. The long stroke appeared to the reason for that.

lildiesel

Thia aounds fascinating. Can you tell us all about it?

AndyW

My Feedback: (1)

That was done some ten years ago, very early in my diesel awakening. This was before I was Internet savvy enough and before YouTube became what it is. And, I had no digital camera. But the glow version used a modified, Cox head. I struggle to remember the details but it wasn't that hard, a head clamp was made and the glow insert was installed much like is done on the MP Jets engine. I ran it on a diesel sized prop and it started easily, ran as strongly as the diesel version and throttled well.

This was done for a correspondent but I don't even remember his name.

Recycled Flyer

Jim, with the Tower 40 that might simply mean that the choke size in the original glow form was not maxed out and so suited many an application.

Interesting about the mounting direction with the Super Tigre G20-15, that tells me that the crankcase volume could have been perfectly minimised and so avoiding the thrown around fuel charge.

I have heard about many stunt diesels 'changing gears' on sharp corners and although Ican't detect any air speed loss it does sound disconcerting!

Recycled Flyer

Here is a link to a site with specs and pics of Team Race diesels -

http://www.go-cl.se/trh/f2c-eng_hist.html

Note that in all the more modern examples that they use an annular ring with peripheral jets for the carburation.

And yet the choke size remains somewhat small. These diesels thrive on small props though and commonly use 6.5x 6.5 "

fiery

My Feedback: (2)

L'il Diesel

I had assumed that the MK-17 (And the Marz 2.5) could not be properly throttled due to sub-piston induction.

Has anyone successfully throttled one of these russian beasties?

AndyW

My Feedback: (1)

That notion had to do with Cox engines and doesn't apply across the board. Early VA .049s and the Norvel .049 and .06 series has SPI and throttling isn't a problem. In fact, I had turned down a Norvel .06 piston down to fit a VA and that gave far more SPI than you normally see and throttling wasn't a problem. That can be seen here. http://www.youtube.com/user/hopeso#p/u/4/mRPgkYjReTA


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VA .049 MK1 Wasp piston and SPI

From: hopeso | April 17, 2010 | 236 views

The VA .049 MK 1 was the first, MODERN, new 1/2A (.049) engine to come along in quite some time. Earlier, the G- Mark .06 out of Japan was the first new engine to compete with the already, well established, Cox series of engines. But the G-Mark used the same steel piston and iron cylinder design as Cox, along with a ball socket connecting rod,,, again, just like Cox.

Enter the VA .049 using modern metallurgy, a high silicon aluminum piston running inside a nickle plated cylinder. This would be designated an AAN engine. Besides having modern metallurgy it also utilized some quite unique features including a split case and a connecting rod of unusual design. The top end of the rod was supported by a threaded carrier which was screwed into the inner threads of the piston. This design has one prime advantage over others. You could add and delete shims atop the carrier to affect timing changes.

VA provided a variety of shim thicknesses allowing you to fine tune the timing for your particular application.

Also unique to VA was the availability of numbered pistons. These would run from 1 to 6 and had a range of diameters according to the cylinders they rode in. You could use this feature to advantage, as you could select exactly the degree of pinch at TDC that you wanted for your application. As well, when the bore of your cylinder wore down, you could restore compression seal by selecting the next size up piston. Quite a handy arrangement for the serious competitor.

HOWEVER, they're not making these anymore, so what do you do, in that case. What I did was to take a Wasp, .06 piston and rod and simply turn down the OD and lap it to suit the VA I wanted to restore.

The result can be seen. A decent, almost 22K, along with good throttling.

It must be remembered that the VA is a rear exhaust engine and as such, the usual, half moon cut into the bottom side of the piston is not a good idea and so VA made the rod long enough to clear the crank web at BDC. However, the Wasp piston has that cut out already installed and when adapted to the VA, that produces considerable SPI, or sub piston induction.

That feature can give you more power on a 1/2A engine and IS utilized on most, notably Cox. But that feature is detrimental to performance when installing a muffler on a COX engine. On a Cox engine, that is, but as shown, certainly not on the VA.

This particular VA was run with its stock piston and it delivered a good, solid 22K on an APC 5.5 x 2 prop and 25%, Castor based fuel. This is with the muffler installed. It must be noted that the stock VA does include sub-piston induction and was originally developed to run wide open and without a muffler. But adding a muffler didn't hurt the top end, we've found, and it also didn't hurt throttling,, as can be seen.

In fact, using the Wasp piston introduced some huge amount of SPI and with the muffler we lost only a few hundred RPM.
... (more info)

AndyW

My Feedback: (1)

And here, http://www.youtube.com/user/hopeso#p/u/3/IECeeHJLSDo we see the same engine, (with gobs of SPI) run as a diesel on a huge, 8 x 4 prop and as can be seen, even with a muffler, it throttles just fine.

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VA .049 Wasp piston SPI on 8 x 4 diesel
From: hopeso | April 17, 2010 | 372 views

Here we have the same, VA .049 MK 1 with the Wasp piston which sports huge amounts of SPI. As can be seen, the engine runs and throttles fine. The fuel is a home brew of 39% ether, 39% kerosene, 20% Castor oil and 2% Amsoil, Cetane Booster. While the video shows some hesitation on acceleration, further tweaking produce a solid 8K and clean transition.

The prop is a Cox, 8 x 4 grey. Smaller props were tried and the extra compression required made them fussy, gnarly and un-cooperative.

As can be seen, SPI hasn't hurt throttling, even with the muffler installed.

AndyW

My Feedback: (1)

But that's not to say that the MK 17 or the Marz 2.5 will throttle well and the problem just might be due to SPI. But only because of circumferential porting OR unique timing, along with SPI. But SPI alone isn't responsible for poor throttling.

In fact, all things being equal, port DESIGN can hinder throttling. The perfect example is the Norvel AME .06 (three large bypass ports) compared to the Norvel Big Mig .06, (five smaller bypass ports). The latter throttles virtually perfectly as long as you have a means to adjust the mixture at idle. A simple, adjustable air bleed, is all that's required. But the AME, three port design, is a much tougher nut to crack. Taking out the SPI doesn't help, nor is the elimination of the muffler. The five port design atomizes the fuel better at lower RPMs. The AME was intended for all out power and as such, at high RPMs, bypass efficiency is sufficient. At lower RPMs, not so and we get into difficulty.

A solution was found and if memory serves, (from 5 years back) that was to increase the compression ratio a bit along with using a hotter plug by way of a turbo plug adapter. I'd need to dig up my notes to be sure.

Another example might be the Norvel .074. It has five bypass ports and it's one of those very rare engines that WILL idle and transition perfectly despite not having an adjustable airbleed. BUT, you CAN adjust the idle mixture and that's done by way of it's unique, very broad, high speed needle range. You can turn the main needle a full 3/4ths of a turn with not much difference in the mixture setting at the top. That's where you get to adjust the mixture for your idle. Quite unique and the only other example of this that comes to mind is the OS Max .10. They have a spot for an adjustable air bleed cast into the body of the carb but it's not utilized because it was found to be unnecessary.

AndyW

My Feedback: (1)

AND, with the featured VA, with the Norvel piston, (with gobs of SPI) the same throttle was used for the glow and the diesel run. But linearity wasn't noted and it could be that under flying conditions, a smaller bore in the barrel of the diesel version might be beneficial. Attached is a shot into the exhaust port showing lots of the dreaded SPI.

earlwb

Wow!  I am surprised it runs with that much SPI.  I haven't seen a  engine with that much before.