Like I said, you are using a sledge hammer to kill flies. The use of an EGT probe is so much vast overkill in this case. Besides, how are you going to calibrate it for 20k altitude? How do you know what the EGT should be at that altitude?
If you want to be real accurate, you need an oxygen sensor. That will tell you how rich or lean you are. The temp can change with load, air temp, etc.
You make it sound like an EGT probe is hard. The probes are inexpensive and all you need to install them is some high temp adhesive and a very small drill bit. As far as setting it. You don't set it to a specific temperature, you lean the engine till the temperature peaks and starts to drop off again (that is how real planes lean). As far as knowing what it should be at altitude I will have to use a transmitter to transmit the reading from the probe so I can make adjustments every few thousand feet. It shouldn't be that hard to transmit back since I will already be transmitting back GPS data so I know where it is and how high it is (there is no way you are going to see anything smaller than a 747 at that altitude (ok so that's a slight exaggeration but only slight). I think the reason cars use O2 sensors is because they are easier for the computer to use and I think they also aid in the emissions control systems that are irrelevant in this case. I will have to have some method of determining how much to lean while I climb because if the setting is for 2,000 feet, long before you get to
20,000 your engine will be running so rich it will probably die. So why not use the most direct method of determining how much to lean which is EGT?-- Chris W
"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."
-- Benjamin Franklin, 1759 Historical Review of Pennsylvania
Have you considered using a turbine, or maybe routing the exhaust out to pressuize the tank AND feed warm air into the carb in a feedback loop?
Mark
The basic advantage of an O2 sensor is that it can be used with a simple PIC device to automatically adjust the carb for optimum mixture. Adjusting by reacting to an EGT has the disadvantage that if you overshoot your adjustment, you can easily kill the engine at those extreme altitudes. Adjustments become much more sensitive when the engine is barely running! Remember, at 10k feet, half of all the air on the planet is below you!
The Omega catalog gives the information you will need to use the probes. There are look up tables and the conversion factors for all of the temp sensors that they sell. If you get the whole set of catalogs you will need a fair amount of shelf space to store them.
As far as the glue goes, you will have the same sorts of problems we all have trying to glue to a castor oil soaked substrate. If you clean it well enough, use the right glue, and get lucky, it will stick pretty well. If it pops off you get to try again. With the temp sensors you also need a glue that's got good heat transfer. The ceramic adhesive will probably work ok.
happy trails - Rob
Because the EGT max may not be the best mixture. Simple EGT measurements do not take into account advanced or retarded timing or other combustion factors. O2 sensors directly measure the unused O2 in the exhaust stream and can be calibrated for best combustion for any given situation. It will also automatically compensate for elevation or air pressure changes.
EGT has been the way you set the mixture in full scale airplanes for years. I would peak the EGT when at altitude and then richen the mixture for a drop of 50 degrees. This would produce the most efficient fuel burn without damaging the engine. Some people would richen 100 degrees depending on how much power they were running or if they were turbo charged.
Also carburetor ice is caused by humidity in the air passing over a cold venturi. The lowering of air pressure when air goes through a venturi and the evaporation of gas causes the carb to become very cold in that area. If flying in warm humid air the water vapor freezes in the venturi thus creating the dreaded "carb ice" and the need for carb heat. The problem is exacerbated when power is reduced for landing and the engine is not producing it's normal heat. That's why carb heat is used when landing.
That may be the case for full scale where millions of hours on each engine has determined where to set them. In a tiny, glow powered engine operating WELL outside its design envelope, how could you POSSIBLY determine where to set the mixture using EGT? An O2 sensor doesn't care how or where the engine is operated and can be used as a feedback device for automated systems. That's why they are used in automotive EFI systems. The sensor puts out a voltage (or resistance) depending on the O2 content of the exhaust gas. This content is used to determine the optimum mixture for best burn. The % content is very constant throughout elevation and humidity changes.
I am not saying that an EGT will not work, just that you will be operating it in an environment that there is no data for.
OK, do it the hard way. A PIC pocessor is a lot lighter than a telemetry transmitter with the power to transmit over 5 miles! You gotta carry all that stuff up to the altitude where you will be generating only about 10% of the lift and power that you do at sea level. I was only trying to make the system lighter.
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