Do any of you have experience installing exhaust gas temperature probes in our 2C gassers? I've been doing some browsing on the ultralight sites, and seen some info on the karting sites as well. There is some interesting information about how to tune using the EGT, which is familiar to me from full-scale aviation, but little information about specific mounting locations, except for the advice to center it in the exhast stream and keep it several inches from the piston. Maybe that's all I need to know, but I thought I would see what the community here might know.

Also on some of the karting sites I saw some relatively inexpensive adaptations of mixture measurement devices based on automotive oxygen sensors .. any of you ever try one of those? Wonder if our gas/oil mix would "foul" them?

Dave

www.aerosport.org
These guys have been doing this for years.

We have the Aero Sport data loggers on all our race planes..The 289cc shows 850 degrees F measured at the exhaust pipe flange...anything much over 900 is too hot and makes the piston part of the cylinder
The Aero Sport unit measures 2 separate temps, rpm, and airspeed...It saves the data so it can be downloaded in the pits after the flight and a graph made...Very handy for tuning...

Only 850 degrees fahrenheit? That seems awfully low. I used to race karts (yamaha 100cc piston port) years ago, and we shot for 1250 degrees F. I had a 400HP Subaru WRX. When tuning I shot for EGTs in the 1450 - 1500 degree range. Some guys liked to run as high a 1600. I also used to race in the SCCA Formula 500 class. On a 500CC rotax we were running about 1300 degrees.

I've stuck a few 2-stroke pistons in my day, but never under 1400 degrees. In all my experience with EGTs, wide open throttle at 850 degrees would be pig rich.

I trust that you know that you stick pistons at 900 degrees. But now you have me curious as to whether your particular thermocouple reads low, or RC engines really run that cold.

The engine manufacturer says they have run as high as 1000 on their dyno, but we stuck a piston hard enough to break the bottom of the cylinder off, running on a test stand..The thermocouple is right in the flange, about 1 inch from the piston....Piston clearance is only about .0025, maybe we should open it up a little ?

I know a little about temperature measurements. The temperature you measure depends greatly on the location of the thermocouple. If the TC is closer to the wall of the flange, it will measure colder than if it is in the center of the port. The colder wall will suck the heat from the TC.

I don't know if it's a piston clearance issue. I always let my engine builders deal with that. I never even bothered to ask. I awlays wanted to get out of there fast so I'd avoid the lectures on treating their engines better.

When I ran karts we went pretty heavy on the oil When trying to run close to the edge, I wouldn't have considered running 100:1 or even 50:1 for that matter. I don't remember the exact ratio I was running, but it was probably a lot closer to 20:1.

Timing could be another reason the manufacturer is getting 1000 degrees and you can only get 850 degrees. Timing that is overly advanced will cause detonation. If the detonation is not too bad it might not be very audible. Extended running at high RPM with mild detonation can cause migration of aluminum from the top of the piston to the cylinder walls. This can end up looking a lot like a traditional stuck piston due to a lean condition, as opposed to the more usual symptoms of bad detonation like holes in piston. You could try retarding your timing and see if that helps.

It could also be an issue of their choice of aluminum alloys for the piston and cylinder. Each alloy has a characteristic coefficient of thermal expansion. The piston clearance has to be matched to the type of alloy and operating temperature range. High expansion alloy pistons need more piston clearance to prevent seizure. Does the Hebrandson use an iron liner? Iron liners require more piston clearance.

Nope, aluminum with chrome liner..The cylinder alloy is different, theoretically expands a little more than the piston..The engines were used in 400 lb drones, cruised around for 5 or 6 hours running at 7000 rpm on gas, ported and timed for economy..We run 9400 rpm using methanol and glow plugs...We're not pushing the engines hard enough, the glow plugs lasted for 2 years and were still in the engine when the spinner hit the prop and the plane crashed at about 200 mph..Nothing left of the engine, but the glow plugs still work McCoy #9s.....Fuel was FAI with about 8 oz oil to a gallon.....
The thermocouple twisted wires stick out about 1/2 inch from the flange, right in the exhaust...

Ah, you run methanol. I bet the EGT for methanol is a lot cooler than for gasoline.

Hi,

Actually, knowing the absolute value of the EGT is not that important. It is quite sufficient to be able to determine your operating EGT with respect to peak EGT, without actually knowing the exact EGT value. So, within reason, it really doesn't matter too much where your probe is located.

I'm curious where you intend to run your engine based on the EGT values you measure?? Are you planning to run at peak EGT, ROP (rich of peak) or LOP (lean of peak), and by how many degrees if not at peak??

This is a very timely topic for me because I just took an engine management course (full scale) with Advanced Pilot Seminars in Ada, Oklahoma [link]http://www.advancedpilot.com[/link] that deals with all these issues. Even though we are talking about model engines, the operating principles are the same!

Thanks,

John

Yep, methanol and glow plugs. That's a world away from spark ignition gas motors. I'm sure that's why the EGTs are so much lower than what I'm used to.

Are you saying a piston will sieze at a lower temperature when you use methanol than when you use gasoline?

Don't know for sure, the 1000 degrees was on a dyno using gasoline....We never got to test one of the race engines on the dyno, but his estimate was about 32 hp...Stock on gasoline the dyno showed 26 hp at 7000 rpm.....The ThrustHp chart shows 44 hp with our 27 pitch prop at 9400 rpm..We KNOW that's wrong....The speed shows 240, got 230+ more than once on the radar gun..

Thanks all for the good input on this one. I will try mounting the sensor relatively close to the exhaust port where it is mechanically convenient for the mounting adapter that RCATS supplied.

John ... on your question, I had not assumed that absolute values were dependable/meaningful ... figured that if I used it to tune, I would set it on the rich side of peak EGT a la fullscale. For the most part, I have been pretty successful tuning by ear and tach, and backing it up with sparkplug inspections .. so this is additional data. Should be interesting to see it vary across the RPM range in real time via telemetry though.

Dave