[Katee]

I'm contemplating monitoring engine temperature via telemetry for a 30cc gas engine to ensure it is getting adequate cooling. However I'm not sure what temperature range I should be looking for. What would be max temperature for the engine without causing damage?

I assume that the best place to monitor the temperature would be near the top of the cylinder head.

Any thoughts would be appreciated.

[JoeT]

The hottest part of an engine is at the top of the cylinder head. On all my gas engines, I use a 10mm brass or copper ring terminal and attach a thermister on it to monitor engine temps. This ring terminal is placed under the spark plug.

The most popular thermister used on Telemetry is NTC 3950 100K Ohm @ 25 degrees C.

If you just put the thermister on the 1st or 2nd cylinder fin, and use a silicone hose to hold it in place, you'll see temps of 80 - 90 degrees C. Same engine same tune with the thermister under the plug, you'll get a consistent 90 - 120 degrees C average.

Mounting the Thermister lower (aka closer to the crank case) will get you cooler temps in the 70 - 60 degree C range.

Same engine, same tune, different placements.

Hope this helps.

[cravenjw]

Gassers with good baffling and/or cowl air flow, will run about 135-150C for normal flying. Pulling into a hover, sustained knife edge, and other lower speed moves, will you into the >160C. I don't spend any time above 165C. If my temps are hotter than 145C for normal flying, i address cowl airflow.

All these numbers for a wire loop sensor in groove between outer most 2 fins.

You will get 30-40C less with sensor on front of the head vs the back of the head.

You can see 15-30C difference between the two sides in a baffled setup.
As the prop rotates, one side has more air rammed into it because the prop wash is not straight but semi helical. With Jeti sensor both myself and two other guys have measured this. On twins, we put one sensor on the top side and the other on bottom side to get equal readings.

[Katee]

Joe and John, thanks very much for the useful info.

Just for clarification John, is your sensor at the back of the head for the ranges you are getting?

[cravenjw]

On my singles (typically no baffles) and the prop wash effect we seen on baffled twins isn't as distinct. So I usually put them at the back (hottest) location.

On my twins, I usually put them opposed top/bottom on the hotter side. This would be top of pilot right head and bottom of left head. When this is done you will get equal reading if the engine is in good condition and tune. Putting them opposed but opposite to above, you will get equal temps but cooler.

[Katee]

Thanks John. It all makes sense.

[kip51035]

I bought a laser temp gauge at Canadian Tire for $29.95 on sale, it works good. I usually point it at the cylinder head near the plug.

Normal Operating Temps
1. two stroke nitro around 300 deg. F
2. Four stroke nitro around 200 deg. F
3 two stroke gas around 325 deg. F
4. Four stroke gas around 275 deg. F

On my ultra-light aircraft engine (a Rotax) 475 deg F was max. Anything over that did result in internal damage. Like the guys said baffling is important and the size of the exit hole compared to the intake hole is also important. The exit hole should be 30% larger.

[RAMJET]

I check my engine with a infra red gun and locate the sensor so it's reads as close as possible to the same temps . I am getting between 190-220F on the ground and as much as 250 when pushing hard in the air . I tried the spark plug system but it read way hotter and did not respond nearly as quickly to temp changes . While I could compensate for the difference in temps (just take a % away from what I was reading ) temp changes were so slow that I was concerned if I did over heat I would not know until I was way past my set threshold .You want to take the reading from a area that most rapidly reflects the temp changes of the engine . EGT works best but a couple if fins just above the exhaust port is the next best thing. Exhaust Gas temperatures run around 1100-1350F and fluctuate wildly with throttle position and load , when these things melt down it will be because of EGT .

[Janeau]

How can you know the temp in flight with an IR gun ? Temp on the ground mean nothing be taken before or after flight.
The only real temp is the one taken with telemetry - in flight - with sensor (s) properly positionned - two fins down on the back of the engine cylinder(s). You get the incoming airflow , the engine working under flight conditions and this give you a real indication of whats going on.

Properly baffled - engine temp in the range of 75 to 100 c is what you want to see in flight. Gas engine will work with highter temp but at the cost of shorter lifespan and,will be prone to eary failure of components.

[cravenjw]

Your temp are too cold!

With sensor where you described, with a well baffled 3D, and a wheel tuned engine, normal (moving) temps would be 135-155c. Most guys will bail out of extended slow (low cooling) moves if the temp rise to 160-165c.

These temps are based on the loop sensor in between the out most fins and the loop being tight with good contact. True CHT sensors that are a copper ring clamped by the spark plug will read 100-125c more!

IR guns after the flight are useless. You only have to see telemtry log from a plane flight and see that 30s after a landing has nothing to do with what is happening during flight!