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Operating normally aspirated engines at high density altitudes - Setting a Target EGT?

There are a few practical things you can do when you want to depart from a short (dirt) strip in Africa. Depart early in the morning with lower outside temperatures. Use the 50/70 rule: at 50% of the runway length (also depending on obstacles at the end of the runway) you should be at 70-75% of your rotation speed. Sometimes when I doubt, I make a test run (takeoff) with 1 or 2 passengers less so I can get a feel of the distance left on the departure runway and the climb performance. When happy, I return and the extra passenger can come on board. When not happy, I postpone the departure until the temperature drops.

EHRD, Netherlands

How to live with your EGT

EGT is used in engine management because it is quick acting (seconds instead of minutes for CHT)
EGT is not a useful temperature nor a limit, it is only a navigation tool to know where you are on the rich-lean curve.
EGT is not the temperature of the exhaust gas, it is an average over 4 cycles, it is way cooler than the combustion temperature inside the cylinder.
CHT is the real temperature to watch, anything over 400°F is cause for action, regardless of the redline published by the manufacturer.
At peak EGT, you are already past the hottest CHT which occurs at about 40° ROP, leaning to peak EGT and then enriching to 25-50°F puts you right at the hottest CHT.
CHT is a proxy for Internal Cylinder Pressure (ICP), this is how hard you run the engine.
At 65% or below, there is no way you can damage the engine with mixture.
Do not set your mixture based on fuel-flow alone, in cold air, the engine will develop more power and needs more fuel than in hot air.
A good rule of thumb is to watch the CHTs during climb, if they are steady, all is fine, if they are rising, you need to be richer. If already full rich, you will want to adjust the fuel setup in the shop, this is one of the first things to watch is a new/overhauled engine has been installed.

EBKT

Thanks, AeroPlus.

And dirkdg as well, but are you sure about:

“EGT……. is way cooler than the combustion temperature inside the cylinder” ?

Swanborough Farm (UK), Shoreham EGKA, Soysambu (Kenya), Kenya

When I flew in SA (late october around Johannesburg, DA was 7500-9000ft), the PA28-140 I rented had no EGT probe. T/O leaning was determined during the run-up: Set 2000rpm, lean until the rpm increases (peak), enrich a quarter inch.
The PA28s I fly in Europe do have an EGT probe, but the scale has no absolute numbers, only that 1 tick is 25°F. My home base is 76ft MSL, so then I run the constant EGT method relative to the day’s max power EGT during the T/O run.

Last Edited by Arne at 28 May 16:33
ESMK, Sweden

EGT……. is way cooler than the combustion temperature inside the cylinder” ?

The combustion temperatures are about 3000°F, far above the melting temperature of the metal. The reason the cylinder doesn’t melt is that the temperature peak is only very short, due to the piston being only one time out of four in the power mode, and as soon as the piston goes down in the power cycle, compression and temperatures go down as well.

Both RPM (more RPM= later peak pressure) and mixture setting (leaner = slower burning) have a profound effect on the CHT.

EBKT

dirkdj wrote:

EGT is used in engine management because it is quick acting (seconds instead of minutes for CHT)

Yes true but I found myself fiddling with mixture left/right and EGT up/down (unless you know the exact position of the mixture)
My feeling even for EGT you may need 30 seconds to settle down and 5 iterations by the time your CHT start to move and you will be able to judge performance from your average TAS/ROC by then (an autopilot or long cruise does help for these, engine/fuel management when things are busy or inaccurate engine instrumentation is rather a symbolic act )

Last Edited by Ibra at 29 May 12:05
ESSEX, United Kingdom

Yes it takes a little while for the EGT to stabilise, following any mixture change.

I suspect this is due to mechanical sticktion/backlash within the mixture cable and the fuel servo, rather than anything inside the engine where the time constants of the relevant physics are likely to be a lot shorter, but who knows?

So, at top of climb I initially lean using the fuel flow (11.7GPH at low level, after setting 23" / 2400rpm) and then adjust for peak EGT and then slightly lean of there.

Actually at low level I just leave it at 11.7 since that is lean of peak, and fly everywhere like that.

Administrator
Shoreham EGKA, United Kingdom

I usually lean at top of climb, after acceleration, by doing the Big Mixture Pull; in about 5 seconds I go past peak until I feel a slight deceleration.
Depending on a fixed fuel flow is not a good idea, unless you always fly in ISA conditions. In colder air you need more fuel since you are producing more power.

EBKT
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