Cox 0.049 Surestart Diesel with throttle fuel ecconomy

Martin AU Thu Oct 16, 2014 4:40 am

Hi Everyone,

I have a cox surestart with throttle and RJL Diesel head.

http://coxengines.ca/cox-.049-rc-diesel-engine.html

It has the stainless reed and I've fitted the Tee Dee cylinder and piston combination.

My question relates to what kind of fuel economy to expect vs throttle? The reasons I ask are as follows:

I set up the engine with a 6x3 propeller and a glass syringe fuel tank. Once tuned, I noticed that there were white mist like vapor puffs emerging from the exhaust port when I reduce the throttle from maximum. I though that it was running rich.

I next did a comparison of fuel flow between full open throttle and idle position. I noticed that near the end of the run when the fuel meniscus flows through the fuel tube, it actually flows faster on idle than on full open throttle.

I next set up the engine on the bench and started it and tuned it to max RPM (12,000) with a fully open throttle. I then decrease the throttle. The RPM decreases. I next tighten the needle valve on the suspicion it runs richer on throttle closure. After tightening the needle valve, the RPM increases. I close the throttle to a small slit and tighten the needle valve yet again. To my surprise, the max RPM has returned.

The measured run time for a 4mL fuel burn at 12,000 RPM was 1:26. With a throttle position providing 6,200 RPM, the fuel burn lasted 1:28. The needle valve was not adjusted between burns.

Has anyone come across this? Should the engine be burning less fuel when the throttle is closed? The similar burn times and the enriching of the mixture on throttle closure seems to suggest a near constant fuel consumption.

Here is the video of the needle adjustments:

https://www.youtube.com/watch?v=1AqHKrmlHmQ&feature=youtu.be

Thanks in advance,

Martin

Surfer_kris Thu Oct 16, 2014 4:57 am

There is nothing strange really, all is very logical once you understand how the "throttle" works.

The "throttle" is mounted on the "choke tube" and it is in fact a "choke" and not a carburetor with adjustable throttle. Its placement and function is identical to that of a cold-start choke that you would find on many larger engines. The drop in rpm comes from the choking effect that makes the fuel-ratio richer, so by re-tuning the needle you can get the correct fuel-ratio again and recover most of the lost rpm, as you have discovered...

Here is a schematics that illustrates the difference between the throttle valve and the choke valve;

Cox 0.049 Surestart Diesel with throttle fuel ecconomy Carburetor-01

Cox 0.049 Surestart Diesel with throttle fuel ecconomy Carburetor-01

Oldenginerod Thu Oct 16, 2014 5:42 am

The carb fitted to the SureStart choke tube is a different concept to most other R/C engine carbs. There are no actual fuel metering needles of air bleeds on the Cox carb, but as Surfer_kris has pointed out, it is simply a choke used to restrict airflow above the metering needle. It's a bit like some of the old 2 speed carbs which just had fast & slow. On slow, it just enrichened the misture so much the engine could do nothing more than run slow. Because the RPM drops, the airflow also drops and it should still use less fuel at slow speed than fast, but the mixture will be far richer.
The Cox carbs are a fairly crude concept, but apparently they're effective enough in most cases.

Rod.

Martin AU Thu Oct 16, 2014 5:55 am

Thanks Guys,

Thats all conformed my suspicions.

I'm used to working with the 2 stroke carburetors of nitro engines where there is an idle mix screw adjustment which performs the function of the second (lowest) butterfly valve in Surfer_kris's carburetor model.

Closing the throttle serves exactly the same function as blocking it with my finger to prime before starting.

My best analogy to this is that the reed valve and piston form a constant vacuum system on the upstroke. With the air path restricted by the "throttle", the only path for vacuum equalization is through extra fuel. Ideally, the throttle needs to restrict the fuel flow as well.

I guess the designers envisaged a fuel mixing/throttling mechanism analogous to blowing across a straw in a glass of water. The less airflow, the less water spray which in theory makes perfect sense if it is not a constant vacuum system.

The most effective "throttle" I've used for the cox engines is the exhaust constrictor type. I've found it to be extremely effective with the nitro engines but not so effective with Diesels as it starves the cylinder of the critical Ether. Use of the exhaust constrictor also seems to validate my constant vacuum theory as pressure on both sides of the piston may seem to equalize before all of the exhaust gasses have had a chance to escape, allowing less mixture into the crankcase.

Once again, thanks for the advice

Martin

Kim Thu Oct 16, 2014 8:13 am

Welcome to the Forum Mr. Martin !!!!!!!!

getback Thu Oct 16, 2014 11:28 am

Welcome to the Forum !! Sound's like you got her figured out there // Glad you found us. Getback Very Happy

Surfer_kris Fri Oct 17, 2014 6:56 am

Martin AU wrote:
The most effective "throttle" I've used for the cox engines is the exhaust constrictor type. I've found it to be extremely effective with the nitro engines but not so effective with Diesels as it starves the cylinder of the critical Ether.

A regular "proper" carb works very well with diesel fuel. In practice even a little better than on glow I think, as the fuel ratio is not as critical. While a glow engine can suddenly stop, the diesel engine keeps on running, it might just miss a little, that's all. The reed engine seems to work particularly well as diesels, in particular the often criticised Queen Bee becomes a very smooth running engine on diesel. The RJL .09 diesel head is a direct drop-in and the engine now gives a power of some other .09 engines. Here is a little video where I'm fiddling with the throttle stop on radio...;

Cox International Sat Oct 18, 2014 4:41 pm

Martin,

Just stumbled on this thread and, as per our correspondence, the throttle we sell actually does reduce the fuel flow as well due to the venturi effect induced by the choke tube. Not 100% proportional but sufficient to ensure the throttle works as intended.

The issues you are experiencing are due to the fact that you switched to a Tee Tee cylinder which, by default, is SPI (Sub Port Induction). Our throttle does not work well with an SPI setup as SPI permits air intake into the crankcase on upstroke, thus negating the throttles efforts to reduce airflow and, by association, fuel flow.

Surfer_kris Sun Oct 19, 2014 4:06 am

Cox International wrote:
Just stumbled on this thread and, as per our correspondence, the throttle we sell actually does reduce the fuel flow as well due to the venturi effect induced by the choke tube. Not 100% proportional but sufficient to ensure the throttle works as intended.

Have you actually measured the amount of under-pressure produced at different throttle settings?
Or has there been any modelling of the fuel-draw at different throttle settings?

Had the valve been positioned after the spray-bar, there would indeed had been a reduced venturi effect to lower the fuel draw. But with the valve before the spray-bar you have a choking effect that creates a vacuum which will dominate over the venturi effects. One has to very careful when talking about Bernoulli's equation and venturi effects in practical applications. The figure below illustrates the problem, there is always a reduced pressure downstream of a restriction.

The data given provided by running the engine (from the OP), clearly indicates a fuel draw that is constant with respect to time (although the rpm is reduced by a factor of two). This actually means that the fuel consumption per stroke has been doubled (!), as also indicated by the very rich running.

Cox 0.049 Surestart Diesel with throttle fuel ecconomy Bernoulliprinciple_clip_image001_0001

Cox 0.049 Surestart Diesel with throttle fuel ecconomy Bernoulliprinciple_clip_image001_0001

Cox International Sun Oct 19, 2014 11:56 am

No tests have been done except to run engines at various throttle settings. At full throttle, 25% Nitro and 5x3 prop and type "B" head conversion, about 10-12 min per ounce of fuel seems to be the norm; whereas that time increases when run on full idle. Test were not done on a diesel engine but am assuming a similar result.

Martin AU Wed Feb 25, 2015 4:48 am

Hi everyone and thanks for all the advice...

I've been on a little RC hiatus but have gotten back into the swing of things with Summer ending here... (Australian Summer is no good for Diesel fuel...)

Have to agree 100% with Surfer_kris. I have a TeeDee with the RJL head and proper carburetor based throttle. It is *the* ideal combination for diesel operation (for 0.049 displacement).

I've decided to take inspiration from this post and so modified the surestart "throttle" with a brass needle valve from a Thunder tiger engine...

I drilled and tapped the barrel for a M4 x 0.5mm tap and the needle valve assembly screwed right in.

Cox 0.049 Surestart Diesel with throttle fuel ecconomy Img_3112

Cox 0.049 Surestart Diesel with throttle fuel ecconomy Img_3112

Fortunately, the throttle barrel allows for choking of the flow of the mixture, not just the air. No air bleed adjustment but I'm not looking for 100% mixture control. Just better gross fuel economy on idle as pure solvent grade Ether is by no means cheap. So the idea is to block off the external fuel nipple and use the thunder tiger augmentation to take over broad mixture control. This modification does result in the fuel nipple rotating with throttle adjustment, but nothing a little extra fuel line won't fix.

Will post again once I have a chance to test it Smile

Thanks again!

roddie Wed Feb 25, 2015 7:51 am

Martin AU wrote:Hi everyone and thanks for all the advice...

I've been on a little RC hiatus but have gotten back into the swing of things with Summer ending here... (Australian Summer is no good for Diesel fuel...)

Have to agree 100% with Surfer_kris. I have a TeeDee with the RJL head and proper carburetor based throttle. It is *the* ideal combination for diesel operation (for 0.049 displacement).

I've decided to take inspiration from this post and so modified the surestart "throttle" with a brass needle valve from a Thunder tiger engine...

I drilled and tapped the barrel for a M4 x 0.5mm tap and the needle valve assembly screwed right in.

Fortunately, the throttle barrel allows for choking of the flow of the mixture, not just the air. No air bleed adjustment but I'm not looking for 100% mixture control. Just better gross fuel economy on idle as pure solvent grade Ether is by no means cheap. So the idea is to block off the external fuel nipple and use the thunder tiger augmentation to take over broad mixture control. This modification does result in the fuel nipple rotating with throttle adjustment, but nothing a little extra fuel line won't fix.

Will post again once I have a chance to test it

Thanks again!

I had to think about this and re-read.. but I understand it now. You will completely bypass the "Cox" NVA by plugging the (Cox) fuel nipple and sealing-off where the (Cox) needle enters? You could do this with two short pieces of plugged fuel line.. so as not to modify the backplate. This would leave the (Cox) spraybar intact within the venturi, which might not be an issue. One potential issue I see; is whether the reed-valve induction is capable of pulling fuel through that snorkel. Any air-leaks whatsoever will prevent a reliable fuel draw. I hope it works.. because it should provide more of a throttle.. than a choke.

Have you looked into the "Ace" plunger-style venturi-throttle, designed for the integral-tank Bee engines? The design works very well.. and I honestly think that it was not more popular, because of the overly-heavy radio gear available back during that time.. and the Babe Bee's limited power even at WOT. A full-house 1/2A radio today; can be assembled at a fraction of the weight.

The Ace design is so simple.. you can duplicate it with a short piece of .062" dia. music-wire with a short taper ground on one end. The standard .049 Bee backplate has a venturi-tube with a .0625" inner diameter. The small mesh-screen needs to be removed from the air-intake to allow the plunger (needle) to enter, but this is the only mod that is required to the engine. The "integral" needle/spray-hole is positioned just inside the air-intake.. so when the plunger enters through the back; it is actually metering the "mixture" to more of an extent.. than just the air. As crude as it is.. it works, and uses less fuel at lower rpm's. The "sure-start" (horseshoe) backplate cannot be used, because the spray-"bar" runs through the center of the venturi; unlike the unique design of the integrally-tanked engines where the venturi is wide-open with a spray-"hole" in it's housing.

Building a firewall for a Cox "Bee" using an Ace throttle involves drilling a small "5th" hole on center with the 4 engine mount hole-pattern; as a "guide" for the plunger. The space behind the firewall needs to accommodate a linkage to a horn/servo. Ace provided a small piece of fuel tubing in their kit; as an adjustable link for connecting to a pushrod.

Cox 0.049 Surestart Diesel with throttle fuel ecconomy Ace-ra10

Cox 0.049 Surestart Diesel with throttle fuel ecconomy Ace-ra10

Cox 0.049 Surestart Diesel with throttle fuel ecconomy Ace-ra11

Cox 0.049 Surestart Diesel with throttle fuel ecconomy Dsc02210

Cox 0.049 Surestart Diesel with throttle fuel ecconomy Dsc02211

These are various plunger-needles that I made from .062" music-wire. They have differing tapers/profiles.. but it's not critical. Inserting the plunger into the air-intake "past the taper-point".. will effectively stop the engine. This can naturally be set with the trim on the RC throttle channel.

Cox 0.049 Surestart Diesel with throttle fuel ecconomy Homema10

Cox 0.049 Surestart Diesel with throttle fuel ecconomy Homema10

A Babe Bee doesn't have much fuel capacity.. but you "can" bypass the integral tank for a larger one.

Cox 0.049 Surestart Diesel with throttle fuel ecconomy Qzr-t-12

Cox 0.049 Surestart Diesel with throttle fuel ecconomy Qzr-t-12

Here's my set-up bench-running in such a way..

https://www.youtube.com/watch?v=Sy9o2WDzLqc

Martin AU Wed Feb 25, 2015 1:47 pm

Thanks Roddie,

I have never see that throttle mechanism before, but makes perfect sense in the way it works. Completely blown away by how insanely simple it is Smile

Love it!

I might just machine up a back-plate and try out that technique... Will have to build another model though that will accommodate a linear travel throttle. I built a 75% scale version of the Boddington Mini Tyro so and a 0.049 looks surprisingly large by comparison... The fuel tank sits immediately behind the engine separated by a 5mm balsa firewall. Unfortunately there is no room for a linear movement from the venturi.

As for the modified throttle, well I happened to have a spare laying around so I figured "why not"? No big loss and I can always plug the hole with a nylon screw if it doesn't pan out...

Sorry for the brief description of the modification... The idea is to utilise the barrel of the caricature to block control the flow of mixture. The Barrel is a small aluminium dowel with a hold drilled transverse right through it. I drilled and tapped is to that the needle valve enters right down the middle of the barrel effectively forming a linked butterfly valve arrangement with reference to Surfer_Kris's carburetor diagram. In this set up, the "choke" and "throttle" butterflies are linked as they are both formed by the barrel.

Out of interest, have you operated the venturi throttle in a diesel configuration?

Thanks!

Martin

roddie Wed Feb 25, 2015 3:29 pm

Martin AU wrote:Thanks Roddie,

I have never see that throttle mechanism before, but makes perfect sense in the way it works. Completely blown away by how insanely simple it is Love it!

I might just machine up a back-plate and try out that technique... Will have to build another model though that will accommodate a linear travel throttle. I built a 75% scale version of the Boddington Mini Tyro so and a 0.049 looks surprisingly large by comparison... The fuel tank sits immediately behind the engine separated by a 5mm balsa firewall. Unfortunately there is no room for a linear movement from the venturi.

Out of interest, have you operated the venturi throttle in a diesel configuration?

Thanks!

Martin

Hi Martin, No.. I honestly haven't tried the Ace Ventura Laughing

throttle with diesel. If you ever decide to try it with glow-ignition.. a few more points about it's operation; (one of which is mentioned in the instructions photo) a "muffler" is recommended to keep the upper cylinder hot and the glow-head lit, at lower engine rpm's. (an aircraft application would be particularly sensitive to this..) therefore; a "Non" Sub-port induction cylinder is in order. You could possibly run a cowled-in engine without a muffler, and see good results though. Regarding the video.. I was using old.. Sig Champion 25% nitro.. the prop was a "Cox-grey" 6 x 3. The "glow-driver" was on during the video.. but probably because I forgot to turn it off.. as re-starts are common when tuning for the throttle. The engine "will" idle-down nicely without the glow-driver.

Lastly.. this throttle has only been "bench-tested" on glow-ignition by me thus far.

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Cox 0.049 Surestart Diesel with throttle fuel ecconomy

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Re: Cox 0.049 Surestart Diesel with throttle fuel ecconomy

Re: Cox 0.049 Surestart Diesel with throttle fuel ecconomy

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