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Help! Dseielized Cox question

Post  fit90 Mon May 23, 2016 10:34 pm

I have been running some Cox reedies with the RJL head and am pretty happy with them.  I typically get a little over 10,000 RPM with a MAS 7X4.  With an engine with one of Bernie's "choke" type throttles I have been able to get a dependable idle as low as 4,000 RPM.  I am not running these at maximum compression and I am also leaving them a little rich. However, I am finding that the Cox reedies and other Cox type engines run very hot, not allowing me to comfortably adjust the compression with my fingers.  Has anyone else experienced this? Is this going to pose an issue for either dependability or longevity?  I am pleasantly surprised at how well the dieselized Coxes run.  Thank you for any and all help.  It is always greatly appreciated.

Bob
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Help! Re: Dseielized Cox question

Post  ian1954 Tue May 24, 2016 6:53 am

The Cox conversions do run hot, mainly because of inadequate cooling. The cylinder and head are not very substantial and don't soak the heat away.

Here is a comparison of a Cox head with an 049 diesel.

Dseielized Cox question Paw80v10

The running temperature can be reduced by changing the fuel mix and upping the ether to 40%. Less power but a cooler running engine.

If you compare your dieselized Cox to the OTM 0.8 you have - you will see the difference for cooling.

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Help! Re: Dseielized Cox question

Post  fit90 Tue May 24, 2016 8:39 am

Thanks Ian,

I am hoping that the air flow from flying will keep them cool enough. But, since it is nearly summer and our temps are usually in the mid to upper 90's I will definitely keep the modified fuel blend in mind. I am surprised at how many little differences there are between glow and diesel.

Thanks,

Bob
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Help! Re: Dseielized Cox question

Post  Surfer_kris Tue May 24, 2016 10:14 am

I've never had any heating problems with the PeeWee or the Queen Bee on Diesel. You do need to stay on the safe low compression side though. If the heat does start to build up the compression will be even more too high, and so on, in an escalating fashion (or unstable run-away, not sure what the proper wording would be here).

This is general general thing though, any diesel engine will behave like that. If you have throttled engine you need to be extra careful as at reduced throttle the engines tend to like a little higher compression setting and it is easy to get fooled into a too high setting, that will then over-heat at full throttle in the air. So during flying the engines should always hit and miss at reduce throttle settings, otherwise the compression ratio is too high for full throttle.

Bernie's "Throttle" is merely a cold start choke, so all the settings needs to be done att wide open throttle, and then you will just have to live with what ever throttling it can produce for that setting. Don't try to tune it at reduced settings, that will only trick you into something that doesn't work at full throttle...
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Help! Re: Dseielized Cox question

Post  KariFS Tue May 24, 2016 4:39 pm

I suppose the overheating problem or lack thereof is a matter of ambient conditions. I don't know where in Sweden you are Kris but here in mid-southern Finland we are mostly in the 70s or 80s F tops, whereas in FL 100s is a more usual figure in the summertime.

I've lived in Atlanta for a couple of years, felt weird to have the electricity bill skyrocket in the summertime, as opposed to how it is here in Finland where I spend fortunes to keep the house at 72F in January and February despite the 8" of thermal insulation in the walls and a foot and a half on top of the ceiling Smile

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Help! Re: Dseielized Cox question

Post  Surfer_kris Tue May 24, 2016 5:26 pm

Well, I guess 70-80F is usual here too on a good day in the summer. I didn't run any engines in australia when I was there two years ago, but their winter felt like a pretty good summer to me. Smile

To me it has less to do with ambient temperature, and more about the actual settings. You do need to avoid the escalating run-away situation. The fake throttle just adds to the problem, when trying to get a good idle you will tune away from the proper settings for full throttle, that is the real danger.

A diesel engine has something that one could call "positive feedback" of the temperature, i.e. when the temperature goes up, then the effective compression setting goes up to, thus producing more heat and so on. This will not stop until something breaks in the worst case. You can get this with any diesel engine in any ambient condition, e.g. if you tune a cold engine and take off flying straight away, in winter or summer.
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Help! Re: Dseielized Cox question

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