greenboot

My Feedback: (1)

I hear a lot about preventing overheating. But how do I know what is too hot? After running my ZDZ 80, it seems hotter than my glow engines. I'm not ready to invest in a temperature gun so I would like to use some form of "touch" method?

Tom

JWN

My Feedback: (42)

Gas engines run hotter than glow. Gas burns hotter than methanol.

John

OnTheEdge

These usually go on sale for 30 - 35 bucks.


http://www.radioshack.com/product.as...%5Fid=22%2D325

ramcfarland

My Feedback: (44)

I second that -temperature is the killer of a gas engine so ensure you have sufficient ducting with a larger area for air to exit the cowl as opposed to entering. You should get a 'reading' of 225 or less immediatly after landing.Try to direct the temperature gun between the crank case and first cooling fins on the cylinder. Would appreciate feed back to my recommendations-

greenboot

My Feedback: (1)

Richard,
That's the first actual number I've heard. Where did you hear it? Just wondering. I suppose you could just take a bunch of readings and use the highest one.

Tom

Diablo-RCU

My Feedback: (1)

You can use the sizzling spit test.....measures around 212F

greenboot

My Feedback: (1)

That did occur to me, but I'm not sure I could distinguish between 213 and 250.
Certainly a fun test anyway.


Tom

Diablo-RCU

My Feedback: (1)

If the spit doesn't move, it's less than 212F.

lucien

For most gas 2-strokes, nominal temp is about 300F (typically what you should have at the spark plug seat). In general, over 350F is the point where you should start looking for a problem. Less than about 275 or so is probably getting on the cold side.

LS

BME

Lucein,

You have done your homework. The only exception is where you shoot the cylinder with the laser gun. The only real way to tune an engine by temperature is by the exhaust gas temperature. I have tested the laser method a lot and found that I can get as much as 75 degrees difference between readings just depending on where the cylinder is shot. The hottest location always seem to be between the second or third fin from the top of the cylinder. The other thing is that the more effiecient the cylinder is at removing the heat the higher the reading will be.

Keith
BME

lucien

Agreed....
The big gassers that are too strong to restrain on the ground or use fixed jetting carbs are usually tuned with EGT. I'd say a good rule of thumb for most gassers is about 1000F at full throttle, but that'll probably vary from motor to motor. You want it to go down at full throttle, indicating a nice rich top end mix (our motors are primarily fuel cooled, the air cooling is pretty much only a small part of the cooling system).

Otherwise, I get the best results by setting the high speed mixture on the "knee" by manual tuning on the ground, like usual, and properly propping the motor for a good load at both full throttle and cruise.

Biggest mistake I see is running too lean or under/over propping. Underpropping is probably the worst of the two - a too unloaded of a prop in the air leads to a high rpm/low throttle opening situation which leads to unsufficient lubrication/cooling which leads to you-know-what.....

Overpropping is bad too - lugs the motor which puts a lot of stress on the moving parts, especially the bearings.

I like to run rich mixtures. Gives more carbon, but better lube and cooling. And, it doesn't hurt that motor to put out slightly less than full rated power on a continuous basis.....

LS

Diablo-RCU

My Feedback: (1)

Lucien:
I keep seeing the claim of gasoline 2-strokes being "primarily fuel cooled". That's not true, and here is the why. The Heat of Vaporization of gasoline is around 900 BTU/gallon of gas. The Lower Heating Value of gasoline is around 115,000 BTU/gallon of gas. Gasoline engines are about 30% efficient. This means that 70% of the energy of the fuel is not turned into mechanical energy, but is wasted as heat. If you burn a gallon of gas, approximately 80,000 BTU is wasted as heat and has to be removed. Only 900 BTU is available for cooling from the heat of vaporization. 79,100 BTU has to be removed by other means. This heat gets removed in 2 ways - through the cylinder fins to the air, or out the exhaust as waste heat.

Having a high heat of vaporization is valuable because it can increase the charge density of the incoming fuel air mixture which can raise the power of the engine. That's the same reason they use intercoolers on turbocharged engines. But, you wouldn't want to drain the radiator and count on the intercooler to keep the engine from overheating.

lucien

As a practical matter, the claim turns out to be true.

Leaving aside from the heat simply going out of the exhaust, you're left with two basic cooling systems: fuel (heat dissipation via vaporization of the fuel) and airflow over the cooling fins (in the case of an aircooled motor).

Of these two, heat dissipation through the cooling fins is by far the least effective cooling system. In particular, for the internal parts of the motor besides just the head/cylinder and outside of piston, such as the center of the piston crown, pin and bearings, and bottom end, air cooling plays a pretty miniscule role. As you know, these parts get pretty hot and the primary means of cooling them is the incoming fuel/oil charge, not air cooling.

Again, this is all apart from the matter of heat simply going out of the exhaust pipe. I'll grant that that's actually the primary means of cooling, to be really technical about it.

But for sure, fuel/oil cooling plays the most significant role in cooling a 2-stroke, particularly an aircooled one. A motor with enough cooling fin area to obviate fuel/oil cooling would be too large and heavy to be practical in an aircraft. So, a compromise is made - cooling fins are left smaller to keep size/weight down, and more emphasis is placed on cooling via the fuel. That's why we run them rich, particularly on the top end.....

LS

Kris^

My Feedback: (1)

Lucien is right. . . otherwise my BME Xtreme would have slagged itself by now