Engine Handling Thread

[ahramin]

Thought maybe we could have a productive thread on engine handling. Our very own Cat has stated that heat is one of an engine's biggest enemies, which also figures prominently in Lycoming's publications. So how do we control engine heat? For starters, for a given engine installation, what affects CHT?

Anyone? Beef, I see you're still awake?

[bigwalleye]

...

[hamstandard]

[quote="bigwalleye"]It's not nearly as difficult as many make it out to be. As I see it you have different phases of flight. The key to controlling engine temperatures is to think ahead. For example. On warm days ensure your ground operations are planned accordingly. Preferably into the wind, try if possibly to avoid prolonged ground running operations. [/quote]

Good points. One should not assume that running up into the wind only applies in warm temperatures. I have seen way back when..... temperatures climb high on a near zero degree day from a longer engine run due to not facing into wind.

[Masters Off]

Lets remember a few things from the mechanical side.

-I was flying an aircraft not too long ago that was mal-treated. It had a bad maintenance record, yet I was unaware. The cowling design didn't allow me to see much in the engine compartment besides the oil dipstick and fill areas.

The result: An engine running over 500 degrees (on consutation from a second AMO), with barely any baffling, bad exhaust piping and a partially blocked manifold. Therefore, note that it may not be your flying that controls everything. Once we had the problems fixed, the airplane hummed in the 350-390 range.

Furthermore, each aircraft is different. I flew one model of a SuperCub, while flying a homebuilt as well. The original model worked fine, never much above 400 in the climb whereas the homebuilt always required faster speeds, lower power climbs. The result was a big difference in performance for the same engine results.

[iflyforpie]

One thing that gets overlooked is the maintaining of the proper CHTs for the engine. This is usually found in the engine operating handbooks rather than the POH/AFM. For most engines, this is about 380F.

The engine is designed to spend most of its time at a specific operating temperature, because various parts are made from different metals and have different coefficients of expansion.

Pistons are made out of aluminum and the cylinder barrels are made from steel while the heads are made from aluminum as well. There is also a large thermal gradient between the top and bottom of the cylinder because of the combustion occurring at the top and the bottom being bathed in oil thrown from the crank bearings at the bottom.

So most cylinders are of a choked bore design... narrower at the top than the bottom. The common thought is that if the engine is too cold, the piston will get stuck in the choked bore, but this isn't the case. Putting a piston into a cylinder by hand (obviously a cold cylinder) reveals a very sloppy fit in comparison to other things like rocker shafts, lifters, and piston pins. When at operating temperature, the aluminum piston expands to a greater degree and fills the steel cylinder.

This isn't required for sealing--the piston rings do that. But if it is sloppy, it will cause piston slap and score the sides of the cylinder and in extreme cases tear pieces off the piston skirt.

So rather than focusing on keeping the engine cool (some pilots cruise with the cowl flaps open for a few minutes to 'cool the engine down'), I try to keep temperatures as close to optimal as I can.

[Instructor_Mike]

What about when you don't have an EGT?

I try to be slow with the mixture before decent (knowing ahead i'm going to give my student a forced for example) or I'll dial it in slow before my student does a hazel check, but usually we are only a few thousand up and a few miles from the airport. Probably no better, but some air work flights the mixture never has a chance to be lean until we are on the ground for the taxing back.

[Beefitarian]

Anyone? Beef, I see you're still awake?

Oh, sorry. I missed this last night. I'm actually trying to observe and learn in these ones. Though I was curious why everyone was getting so agitated.

For sure extremes in temperature can be very bad. Especially if things start to melt or if the metal is losing it's temper or starting to change in hardness.

I think as was mentioned when you do the more typical things that cause a relatively large temperature change, slow and steady wins the race.

[Beefitarian]

For starters, for a given engine installation, what affects CHT?

Mixture changes the temperature of the combustion.

Air flow, more relatively cool air flows through the fins removes more heat.

And I think how hard the engine is working.

[sidestick stirrer]

Iflyforpie has done an excellent job of describing cylinder "choke" and perhaps I could expand( no pun intended) on the subject a little more.
The unfortunate confluence of several factors makes this feature quite an issue, especially when starting in cold weather.The need for air cooling due to the weight and complexity of liquid cooling means that the engine experiences a wide range of temperatures when operating, especially acute between the head and base of the cylinder.
The need for a high and continuous power output at a low enough rpm to keep the propellor efficient while avoiding the weight and complexity of a gear-reduction between the engine and the propellor, mandates an unusually-large cylinder diameter, or bore.
These two factors mean that we have an engine that has large clearances internally until all the metal parts, stationary and moving, have reached their respective operating temperatures, causing considerable leakage or blowby of combustion gases from the combustion chamber into the crankcase, which we can see in the dark colour of the oil compared to the liquid-cooled gasoline engines in our vehicles.
The whole idea behind choke is to have the cylinder be just that: a perfect cylinder with the walls exactly parallel and therefore the diameter exactly the same from top to bottom, when the cylinder has stabilized at its operating temperatures.
These temperatures can vary widely between the top, where the majority of the heat is concentrated, and the base, which is much cooler as the burning fuel-air mixture has lost a lot of its heat through expansion by the time the descending piston uncovers the bottom of the cylinder.
Further increasing the temperature difference is the short timeframe that the base of the cylinder is exposed to this hot gas, for only a split second of each power stroke, while the top of the cylinder and the cylinder head are exposed to it continuously throughout each power stroke.
So, if we take this cylinder which is designed to be perfect with a temperature at the top of 370-385F and temperature at the bottom of perhaps 200F, what happens to its dimensions when it is cooled to, say, 20F?
its diameter is smaller at the top than at the bottom and this characteristic is actually designed into it and it is called choke.
The previous poster is correct that, even at quite cold temperatures, it is rare to have the piston jam in the cylinder at the top of its stroke. That it won't is more an indication of the tremendous forces at work, both while cranking and when running while cold. The aluminum piston is quite malleable even at cold temperatures and can be forced further up the cylinder even when the clearances are small to nil.
And somewhere in between there, trying to do their job, are the compression rings, thrust by gas pressure out against the cylinder walls, rapidly adjusting their diameter as they are forcibly carried up and down the cylinder by the piston.
When the cylinder is really cold and the piston has so little clearance at the top of its stroke that it can overcome the minimal oil film present on the walls during start and be scuffed by metal-to-metal contact, the rings have no where to go cannot contract either rapidly enough nor sufficiently and they break or chip.
All very technical and presumably of no interest or concern to us pilots. However, I hope this will indicate the severe stress and wear that we force upon an aircraft engine when starting without preheat in cold weather.
If it still can be turned by hand or spins sufficiently with the starter, does that mean the cylinders don't need preheating?
If the oil pressure comes up quickly does that mean preheating wasn't necessary?
What do you think, now that you know about the magic of choke?
Yes, one of the reasons for preheating the cylinders is to encourage fuel atomization and thus increase the chances of a successful start instead of a few, feeble combustion events followed by frost on the spark plugs.
But I personally feel the big reason is to avoid the absolute certainty of increased engine wear and shortened top-end life, not to mention reduce the chances of actual engine damage.

[Beefitarian]

Another thing that is probably a factor that the Colonel seemed to be suggesting. Different engines (mostly due to carberation and intake manifold design I am guessing but there could be other reasons as well.) might be damaged if I operate them in the same ways I do to avoid damage to another engine.

I have flown pretty much the same model C-172 for most of my hours. I'm used to doing things I believe are ok for that engine. Including the dreaded full rich prior to decent especially when flying with many instructors and checklists. Granted I don't pay for extra fuel because rentals are usually wet. So I operate them rich of peak not lean of peak. Therefore provided I'm flying level, not entering a low pressure area or paying attention to temperature gauges, the engine should be running on the cool side of warm.

We're creatures of habit, "This is how I always fly." could hurt a new type I'm flying some day.

I think his point in the other thread is, "Learn everything you can about the exact engine/airplane you're flying that day and look after it well." Even if he kind of wrote more words saying, "Fine, wreak your engines if you don't want to learn about them."

[Rowdy]

sidestick.. fantastic posting!

[iflyforpie]

I have flown pretty much the same model C-172 for most of my hours. I'm used to doing things I believe are ok for that engine. Including the dreaded full rich prior to decent especially when flying with many instructors and checklists. Granted I don't pay for extra fuel because rentals are usually wet. So I operate them rich of peak not lean of peak. Therefore provided I'm flying level, not entering a low pressure area or paying attention to temperature gauges, the engine should be running on the cool side of warm.

The Lycomings with carburetors are pretty much immune to poor and sloppy handling--as is the 172 itself. Going full rich on descent isn't really going to do much because the fuel vapourizes in the carburetor and then gets bathed in heat from the intake manifold which passes directly through the oil sump on the O-320. All you have to worry about is carb ice.

Unfortunately, that is the problem with the 172: it nurtures complacency. Go to a fuel injected plane and now that extra vapourizing fuel is stealing heat right from the aluminum cylinder head that was nearly 400F. Continental engines seem to be more fragile this way than Lycomings.

[bronson]

Good place to start is with the idea that the engine is subject to 2 stresses-thermal, which is measured at the cyl. head temp and oil temp, and mechanical which is measured at the tach. In general low rpm and high man. press. is the way to go. The exception to that is on takeoff, where reduced power (rpm) may actually be detrimental ( thinking of the 985 especially here but read the manual for your eng.). If the situation allows, reduce the rpm for the descent and leave the throttle alone. In cruise run the highest mp and lowest rpm allowed appropriate to your load. There's my 2 cents,cheers!