COX exhaust throttle barrel - impact on engine life expectancy

balogh Wed Sep 25, 2013 1:46 pm

Sorry for the theoretical topic that may be a bit boring fro some of you. First of alll I confirm the exhaust throttle barrel lioke this one:
http://coxengines.ca/throttle-ring-for-cox-049-engine.html
is the most efficient and sensitive performance control means I have ever come accross with COX engines (carb throttles for the reedies and TD 050 seem to be less reliable as the engine tends to cut off when throttled due to the glow head cooling down with the carbie throttled).

My point is that the exhaust throttle seems to entail engine overheating due to the hot exhaust gases being hindered from fully discharging from the cylinder. The overheat may reach a point where - the otherwise useful - castor oil polimerization and varnishing at the combusting temperature will not provide the well-known lubrivcation and sealing as the deposit is charred and too stiff (one may look at the piston top that becomes soot-black). The piston skirt often gets discolored, a blueish shade at the crown indicates excessively high temperatures. The result is excessive engine wear, impaired compression and shortened engine life.

Anyone of you having similar experience?

roddie Wed Sep 25, 2013 8:38 pm

I believe that the referenced exhaust throttle that Cox International sells, is different than the "original" Cox exhaust throttle. "Original" Cox engines equipped with this throttle, and also the "original" replacement cylinder for these engines, featured a "ground" o.d. at the base of the cylinder... resulting in a thinner cylinder wall.

The referenced C.I. reproduction part is stated to fit most .049 cylinders. The i.d. of their throttle sleeve would be larger than the Cox "original" for this to work.

I do not know whether this is good or bad. Does the cylinder wall thickness at the base make a difference when using a sleeve? What mat'l. is the sleeve machined from? Is it aluminum? Would aluminum carry away too much heat compared to steel?

I personally don't like the idea of an exhaust throttle... even if it works well, because of the "coking" effect, which eventually cooks the castor, as you stated. The Cox engines MUST have castor based fuels to live a long life. "Exhaust-choking" the engine isn't the best way to throttle it... it's just "one way" to throttle it. The Cox engines were designed to run at W.O.T... "Throttling" them is truly a "luxury"... and at the expense of lost top-end performance and depending on the throttle type; prematurely worn-out cylinders.

I personally think that it's better to meter the fuel/air mixture to throttle a model engine. Using a muffler helps to keep the "head" hot and the engine from "quitting" at low idle and helps with throttle transition from idle to faster engine speeds. Great "throttle-response" is something we're "all" searching for with our small I/C engines... it's just a matter of trial and error... engine condition, prop. weight/dia./pitch, fuel, needle setting(s), plug type, atmospheric conditions... etc.

Have you given the "venturi" throttle a try? You can easily make one... Here's some info.

https://www.coxengineforum.com/t5486-049-reed-valve-venturi-throttle

tru168 Thu Sep 26, 2013 9:09 pm

Hi,

I have some throttled cox engines using throttle rings. some are ground cylinder. I found that the main purpose of ground cylinder is to smoothen cylinder surface for tighter seal between cylinder and throttle sleeve to get lower idle RPM. compare with ground cylinder and normal cylinder, only a tin layer ground off from cylinder surface.

venturi needle throttle can be quite good, but only limited to tanked engines. For horseshoe backplate type, most people will just use Bernie's choke tube throttle, a little hesitate on rpm change when throttle it though.

balogh Fri Sep 27, 2013 4:26 am

Although I am a COX-maniac, in my - very rare - objective moments I sometimes admit there are weak points in the venturi-throttle solutions in COX engines....my experience with ventury throttled reedies and the beautiful TEE DEE050 as well is that the venturi-throttled idle speed is far from being stable. (And while I have serious reservations against the very poorly fitted Norvel engines, i must also admit their venturi throttle is a very - albeit sole - reliable part).

Unfortunately the regular COX glow head (even the high compresion TeeDee head) tends to cool down at low rpm and this will shuit the engine off.

I tried COX conversion head with hot glow plugs, but while the conversion heads never worked well (no matter how many shims under them) and resulted in an engine overheat, despite the high temperature the idle was not stable either and the venturi-throttled engines tended to stop mid-air.

To the contrary though, the exhaust-throttle, while not adjustable with such accuracy as a venturi throttle for engine speed reduction, has always worked very reliably and would never shut the engine off. But. as nothing is perfect in ife, the exhaust throttle tends to bake the cylinder and piston due to the capture of high temp exhaust gases, and, i am afraid, will thus shorten engine life

coxaddicted Fri Sep 27, 2013 5:14 am

I have a throttled Cox Car engine.

It has a huge Kyosho muffler/ground cylinder and works with a valve. It has a very low noise at low rpm's but is screaming on high rpm's.

The idle is very stable. Don't know if it makes sense on a plane.

Here is a picture of the dissasembled engine:

COX exhaust throttle barrel - impact on engine life expectancy Dscf3310

A good solution for a ,,whispering'' Cox engine lol!

balogh Fri Sep 27, 2013 5:21 am

I guess the throttle principle is the same as that of the exhaust throttle barrel i.e. gradually closing the exhaust port and thus reducing engine noise too. I see your piston is rather dark showing the signs of castor coking on it at high temperatures. If excessively hot the castor varnish may lose its benevolent feature of sealing and separating the metal surfaces.

balogh Fri Sep 27, 2013 5:22 am

I guess the throttle principle is the same as that of the exhaust throttle barrel i.e. gradually closing the exhaust port and thus reducing engine noise too. I see your piston is rather dark showing the signs of castor coking on it at high temperatures. If excessively hot the castor varnish may lose its benevolent feature of sealing and separating the metal surfaces.

coxaddicted Fri Sep 27, 2013 5:49 am

Hi Balough,

oh yes. The engine is heavily used. It has runned through about 2 gallons of wrong fuel (only 12% Lube with 3% Castor).
But it is still working good.

tru168 Fri Sep 27, 2013 9:35 am

Some of cox car engines not only throttle by close the exhaust port, but also provide a feedback tube to venturi. when the exhaust port close slowly, it switch to the feedback pipe to feed the air intake of needle valve, because of lack of oxygen in exhaust gas which is feedback into air intake, the engine's RPM can go even lower.

coxaddicted Tue Oct 01, 2013 2:16 am

tru168 wrote:Some of cox car engines not only throttle by close the exhaust port, but also provide a feedback tube to venturi. when the exhaust port close slowly, it switch to the feedback pipe to feed the air intake of needle valve, because of lack of oxygen in exhaust gas which is feedback into air intake, the engine's RPM can go even lower.

-The Maples Throttle system isn't it??

tru168 Tue Oct 01, 2013 2:36 am

Yes, Mapple Throttle System, but only limited to car engines only.

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COX exhaust throttle barrel - impact on engine life expectancy

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Re: COX exhaust throttle barrel - impact on engine life expectancy

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Re: COX exhaust throttle barrel - impact on engine life expectancy

Re: COX exhaust throttle barrel - impact on engine life expectancy

Re: COX exhaust throttle barrel - impact on engine life expectancy

Re: COX exhaust throttle barrel - impact on engine life expectancy

Re: COX exhaust throttle barrel - impact on engine life expectancy