EGT vs CHT

[CDVdriver]

I still have trouble figuring out the correlation between EGT and CHT.
I do know that they peak at different mixture settings and different temps.
Also know about propagation of the flamefront in the cilinder head.
But does anybody have a link to a good article that deals with this?
What will in the long run be more damaging to an engine, high EGT or high CHT?
High EGT will burn the exhaust valve and high CHT will weaken the metal alloy of the cylinder head. Lycoming recommends running 50 F rich of peak EGT but this will result in high CHT.
Gamin(?) from the after market fuel injectors claim you can run 50F lean of peak with their equipment. So now there's a spitting match going on between the two.
There's a well documented case of a Cessna 402 with a double engine failure blamed on excessive leaning.
Flew a multi recently with about 30 hrs on new engines and CHT was peaking at a fuel flow 3gallons/hr higher then book figure for that altitude.
Now I now bookfigures but these are new engines.
Time has come to get completely confused now.
Can anybody recommend a book or something....

 

[bigD]

For more information regarding Gami injectors and operating LOP, see John Deakin's various articles on avweb.com. He also discusses at length the 402 accident. It's all pretty interesting.

 

[vetteracer]

CHT is the temp of the cylinder head as a whole, and for argument sake most of the top end of the engine.

EGT is just that, the exhaust gas temp. The critical value here is set to protect the exhaust valve, and that is it. As ignition takes place the heat pressure is building EGT more so then the flame. The intake valve is cooled by cool intake air and does not have high pressure exhaust gas forced over its surface.

So, what do you lean for? In larger equipment you lean for EGT and use the cowl flaps to control CHT. If you cannot get enough heat rejection using open cowl flaps to lower CHT, then you have to richen the mixture. Remember every molecule of air and fuel after Stoichemetric combustion has taken place is wasted. So when you richen the mixture you are subjecting the engine to more fuel then there are air molecules to combust with. Is this bad, no, in fact no engine runs at stoich.

This fuel in-excess is used for cooling; it is turned into steam or vapor, absorbs heat and goes out the exhaust. You can richen the engine too much, and this will be audible in smaller aircraft.

In the 402/310 I always walked the mixture up slowly over a ten-minute period. Most of the shock cooling comes in the form of slamming the mixtures to full rich at altitude, like it says to do on the check list for decent.

FYI 1/3 of an OTTO cycle engines power (heat) is wasted out the exhaust, 1/3 is absorbed and carried away by the heat sinks or radiators, and 1/3 is turned into mechanical power.

Mark

 

[CDVdriver]

Thanks so far..
Now I know EGT and CHT also depend on OAT, but is there a rule of thumb to determine healthy EGT? 1400F?1500F?
I understand CHT should not go above 450F...
Now Vetteracer if you need the cowl flaps full open to keep the CHT in limits aren't you running the engine too lean? EGT has peaked and is on the lower side in this situation..correct?

 

[vetteracer]

I think you are confusing the to events that are happening in the engine.

CHT is a function of heat rejection from the heat sinks on the head and cylinder. CHT is also a function of work performed by the motor. As I said earlier that a piston engine only use 1/3 of it’s heat energy to perform a mechanical function, and in this case turn a prop. The rest is wasted, and must be dealt with or it will destroy the motor. In a car, a radiator is used to reject the heat, in an aircraft engine heat sinks are used. These are all the little fins on the cylinder and head. Air must past over them to have heat transfer happen.

EGT is a small portion of the equation. With this instrument you are monitoring the exhaust temp. Why would you even care? Well, this is a good indication of efficiency and power. Stoichemetric is the result of every fuel molecule combusting with every air molecule. In layman’s terms, every atom finds a partner to bond with to make the exhaust hydrocarbons. This is a stoichemetric phenomenon. In the real world, this is just not practical. If your engine was running at stoich the flame front would be much like a cutting torch and toast the piston. The extra fuel is used as a shield. Luckily none of the fuel air metering devices are precise enough to have this occur.

Worry about CHT as this is more destructive then high EGT. If you are concerned about high EGT, just run the mixture up a tad. High CHT means the whole motor is hot. Heat means expansion, friction and wear. Aluminum is a very fussy metal; its coefficient of expansion is about 8 times that of steel or cast iron. Hot aluminum warps, cracks and deforms. The exhaust valve is most likely a hardened steel alloy. Dropping a valve is not that uncommon but I attribute that to the warping of the aluminum head in which the valve guide is. Warp this and the valve stem binds, then wears, and becomes sloppy. Most exhaust valves break at the steam.

When on climb out, worry about CHT, and in cruise, close the cowl flaps and lean the motor to 75% of stoich. To find stoich, lean the engine until it starts to run rough, and note the EGT reading, at its highest reading, moves the needle of the EGT. Run it at 75% of EGT. Remember that EGT reading change with altitude. So set your EGT at a normal cruising altitude.

Control CHT with the cowl flaps, and lean when appropriate. At 75% EGT, you should have no CHT issues that cannot be addressed with the cowl flaps.

This is how I fly a 402

1 Cowl flaps open for take off and climb.

2 Cowl flaps closed on level off

3 Set cruise power, and lean per 75%EGT

4 Decent, cowl flaps CLOSED

5 Increase in mixture 2 clicks per 1000 feet. Reduce Manifold pressure 1" per minute.

6 Land, mixture back about 2 inches to keep from fouling on ground.

7 Cowl flaps open.

Mark

 

[Sunnfun]

Hey man what's up?

Flew a multi recently with about 30 hrs on new engines and CHT was peaking at a fuel flow 3gallons/hr higher then book figure for that altitude.
Now I now bookfigures but these are new engines..

Just a thought, but there might be a problem with the baffles. Had the same problem with a 402 and rearranging the baffles got me the CHT 50 Degrees cooler with the same fuel flow in Cruise. But we fly the 402 with the fuel flow set as per the POH and EGT only as cross check.

Cheers,
Sun'n Fun