Diesel Question's

[PROACE]

Thinking of playing with a Davis Diesel conversion on my TD .049 but I understand that the con rod really isn't designed for the pressure but something different from my PAW. Anyway does anyone know the actual compression ratio for model diesels? Now I got my interest up and maybe someone elses, are there any engineers lurking around who might know the difference in compression ratios for ignition for model diesel fuel and #2 grade auto/truck diesel?

[steve lewin]

The compression ratio is variable, that's what the screw on the top of the cylinder head is for ;). Sorry, couldn't resist.

Unfortunately there's no real comparison between model "diesels" and auto "diesels". Although they both use sort-of compression ignition, the other differences far outweigh any similarities. The fuels and their ignition requirements are completely different.

A typical normally aspirated auto/industrial engine runs at around 20:1 but I've never worked out what a model engine runs. Interesting question.

Steve

[PROACE]

I searched and found that an auto diesel runs 14:1 up to 22:1 but I am sure a model diesel wouldn't see ratio that high.
Anyway I concluded that the compression screw is for initial combustion pressure. As soon as ignition occurs then thermal pressure increases requiring the compression screw to be backed off and of course there is piston speed which is also a varible in combustion.
Anyway I still didn't find out the model engine compression ratio.
Ace :cool:

[easytiger]

It's not the conrod that fails on the conversions, it's the crankshaft. Davis sells a stronger replacement.
You can knock around just for fun all you want, but in the end, I think you will be happier buying another PAW!

[steve lewin]

Model 2-stroke glow engines already run high compression ratios, 12:1 to 15:1 is not unheard of. I'd be very surprised if a model 2-stroke diesel wasn't higher than that.

It's easy enough to measure the static compression. Volume at top of stroke / swept volume. I'l have a go some time if no-one knows the answer.

Steve

[1705493]

Lately, I've been running some Cox and Norvel conversions and I agree that it's the c/shaft that takes the beating. Broke two of these on only 5 tanks of fuel. The Norvels, so far, are holding up well. Both were run on an APC 7 X 3 prop. I had a Norvel crank break on me last year when I tried an 8 X 4 prop. Yes, I know that that is extreme for an .06 but I was just experimenting to see what happens. Now I know. I suspect that if I ran an 8 X 4 on glow, the crank wouldn't have broken. Higher compression as a diesel and the punch of diesel combustion seems to be the issue. That's why diesels, cube for cube, are built so much more robustly than glow.

However, with a ball bearing installed in the Norvel at the big end, I've run one on diesel with the 8 X 4 and with over six tanks of fuel, it's staying together, so far.

The real test will be in the flying though, and that'll have to wait till next year,,, when the snow melts. :^)

On compression ratio, I believe the typical 1/2A diesel runs at 18/1 but yes, diesels need much more compression than glow. Having only one diesel head, I made up a solid aluminum plug to replace the glow plug. This was installed with 10 copper gaskets and deleting four took me to a good setting to where the engine fired up everytime.

This was inspired by the recent piece in RCM, Engine Clinic I think, where Clarence Lee describes a thing called the gadget. This is like a stock glow plug with no element but a copper nipple sticking into the combustion chamber. This is supposed to retain enough heat to fire off the diesel fuel charge without resorting to compression adjustment. Also described is a copper disk for ,049s that works the same way. The copper disk is installed under your glow plug and again, the copper is supposed to retain enough heat to make the thing work. Hope to be able to try that eventually.

[BMatthews]

The auto diesel is probably not compatible with the model fuel. First off the model stuff has ether and some additive to aid ignition that is not in the auto stuff. Now if you wanted to try the auto stuff to replace the kerosene but still keep the ether and igniter additive then that would be a different story. I don't know it it would be any advantage but you can always try it. I wouldn't use that on the Davis though. Might be too rough.

Thanks to the ether and igniter additive I know that model diesel fuels flash off at much lower compression ratios than the auto stuff does. Just don't ask me the numbers on the model sizes. I know that auto stuff is in the 30:1 to 40:1 range though. I suspect model stuff is closer to 15:1 to 20:1. And then of course you have to vary it over a range to compensate for colder weather delivering denser air to the engine and a smaller amount for the temperature what with it being air cooled and all. Probably an atmosphere or so from summer to winter.

So the bottom line is that the flash point of the #2 auto stuff compared to the kerosene doesn't really matter. The ether is there to ignite first and set off the kero. The amyl nitrate or other additive aids that ignition of the kero. This is why the model size engines can get away with so much less compression ratio. So what you want to know is if the ether+igniter would fire off the #2 auto diesel. That's a good question and I'm not sure. Research the flash points of the two and you may see the answer.

[PROACE]

Here is a pic of the Davis Diesel head for those who never seen one.

[PROACE]

and attached to the Cox TD .049

[1705493]

Hey Proace,

How did you make out with your TD/Davis diesel conversion?

[PROACE]

It runs great. I haven't tached it yet but after the glow break in I started the break in process over with the diesel. I am using a 7x4 prop on it and it has THRUST! I am amazed at how steady it is, it seems the RPM is rock solid. If I get a chance this week I will go out and check it with a tach. I am uncertain if I want to use an 8x3 or larger on it. It also runs a lot cooler than glow.
Starting isn't bad either, I used an electric starter at first but now I have it "set" and I can usually get it going in two or three flips with a chicken stick.
Thanks,
Ace

[T. Lyttle]

There's another thread on this somewhere, lots of info there...

WATCH the improver, it is wild stuff, and yes, it is used as an additive to road diesels, auto and truck, in small quantities. On a model, it improves start/run quite nicely. As far as auto diesel is concerned, I have never felt the need to sacrifice a diesel to an experiment like that, mostly because kero is so readily available, and fuel for diesels is inexpensive.

Right on about the PAW recommendation: I have quite a few now, and many more before (sold off), and have NEVER had a failure. Even my older Darts &c will only wear out, not break, with one exception: I had a Webra 09 that broke its crank (in midair!) simply because it was being used on a trainer, and my students had busted ~30 props, nylon and wood, so I suppose one of the whacks cracked it.

[1705493]

Ace,

That's great news. After a lifetime of modelling, wondering what the heck all the fuss was about regarding diesels, now I know. Diesels appear to be somewhat underrrated and unappreciated. As you've discovered, there's something very unique about diesel combustion. It really seems to be able to haul the freight, 'specially with a big prop with a steadiness that's hard to define but quite distinct.

What fuel formula are you using?

I wonder if anyone out there has applied a diesel head to a Cox engine equipped with a sleeve type exhaust throttle?

[PROACE]

I am using the Davis "Diesel Power" fuel, airplane mix. The fuel can label doesn't specify the mix ratio of the kero, ether and oil.
I have a throttle sleeve for the TD but never thought to try it since I never used one in the past with Cox engines. Might have to experment some more..

[John_R_C]

Regarding the vehicle diesel fuel vs. kerosene, No. 2 diesel, the normal stuff, is a higher viscosity oil than kerosene, which is about equilavent to No. 1 diesel. Both are considered oils BTW. No. 2 is also household heating fuel oil - taxed differently and dyed accordingly!

Model diesel has the addition of either, amyl nitrate, and lubricant while vehicle diesel is only the fuel oil.

Hope this helps :o

[Projecthog]

Thinking of playing with a Davis Diesel conversion on my TD .049 but I understand that the con rod really isn't designed for the pressure but something different from my PAW. Anyway does anyone know the actual compression ratio for model diesels? Now I got my interest up and maybe someone elses, are there any engineers lurking around who might know the difference in compression ratios for ignition for model diesel fuel and #2 grade auto/truck diesel?


Hello there,

The ratios for diesel usually run from 11:1 in automotive to as much as 15:1 or higher in rc, I just did a conversion to my little .049 Baby Bee, and that was a Davis diesel conversion, I had a handfull of bad experience with them as they were very rude to me and will not come good for the short crank shaft for the .049 (I had to machine the nut down to get it not to bind), and the missing Oring for the .46 thunder tiger that was supposed to come with the .46 comversion. (Never mind the rest of the things they pulled.) Just a warning to be on your guard with them. Diesel fuel is another thing they wouldn't clearly answer for me. I use 45% ether, kerosene and straight castor, or klotz castor, all of that works but you can experiment and get optimum power if you care to do the experimenting.

Good luck and happy dieseling! PH.

[Projecthog]

Lately, I've been running some Cox and Norvel conversions and I agree that it's the c/shaft that takes the beating. Broke two of these on only 5 tanks of fuel. The Norvels, so far, are holding up well. Both were run on an APC 7 X 3 prop. I had a Norvel crank break on me last year when I tried an 8 X 4 prop. Yes, I know that that is extreme for an .06 but I was just experimenting to see what happens. Now I know. I suspect that if I ran an 8 X 4 on glow, the crank wouldn't have broken. Higher compression as a diesel and the punch of diesel combustion seems to be the issue. That's why diesels, cube for cube, are built so much more robustly than glow.

However, with a ball bearing installed in the Norvel at the big end, I've run one on diesel with the 8 X 4 and with over six tanks of fuel, it's staying together, so far.

The real test will be in the flying though, and that'll have to wait till next year,,, when the snow melts. :^)

On compression ratio, I believe the typical 1/2A diesel runs at 18/1 but yes, diesels need much more compression than glow. Having only one diesel head, I made up a solid aluminum plug to replace the glow plug. This was installed with 10 copper gaskets and deleting four took me to a good setting to where the engine fired up everytime.

This was inspired by the recent piece in RCM, Engine Clinic I think, where Clarence Lee describes a thing called the gadget. This is like a stock glow plug with no element but a copper nipple sticking into the combustion chamber. This is supposed to retain enough heat to fire off the diesel fuel charge without resorting to compression adjustment. Also described is a copper disk for ,049s that works the same way. The copper disk is installed under your glow plug and again, the copper is supposed to retain enough heat to make the thing work. Hope to be able to try that eventually.

Hey, that's a cool way to try to circumvent the adjustible screw and teflon disk idea, I'll have to try that and see, tinkering and trying, is a good way to get some knowledge for yourself, the copper disk is also a good news item. I am just getting back into diesel since trying factory items in the late sixties and seventies, in those days things didn't work very well, but I think that may have been because the internet wasn't there to speed things up a little, nice going thanks, PH.

[Projecthog]

I had said that automotive ratios started at around 11:1, which is wrong, They start at 12:1, but I think that rc ratios are not as high as I thought, it is a good subject to figure out, I'll try and come up with some more solid info for rc. PH.