Current automotive technology enables race engines to be tuned in a very sophisticated manner. Mapping an engine on a rolling road is a fascinating process. It is startlingly apparent how a map from one engine does not suite an apparently identical second engine. Determining the correct air fuel mixture at a range of pot settings has a stunning impact on the power output of the engine, in how smoothly it runs and in the amount of “pollution” it produces.
Why is this relevant?
I was encouraged at some point in my flying to lean the engine using the EGT as the main tool for achieving the “correct” lean. In theory this seemed entirely reasonable before having a better understanding of engine mapping. Whilst I appreciate automotive engines are in many ways different animals they do bring home the extreme danger of operating an engine at peak fuel mixture efficiency unless you can be absolutely certain you can monitor and control all the factors that might just mean the engine has become over lean, even for a few minutes. For example, in a “modern” automotive engine the air pressure is constantly monitored, as are the emission gases, the air temperature and the head temperature never mind that during the mapping process the EMU will be programmed dynamically for that specific engine at 50 rpm intervals and the EMU will in many respects manage each cylinder individually! I can get 200 bhp from a 1,600 cc engine at 8,500 rpm with sound reliability.
In comparison whilst many of the tolerances on the engines we use are far greater equally the engine monitoring is primitive.
The most obvious result of over leaning an engine is a rapid rise in head temperature.
How often have you "leaned the engine correctly" for a given power setting and altitude and "forgotten" to adjust the mixture when you have changed power setting or altitude until some moments after or maybe even reaslsied you had left it just as is until you realised on short final. Poor airmanship, I guess, but you might only have to do it once to cause significant damage.
How often do engines require a top end overhaul before TBO? - nearly always. It is the top end that so often starts “playing up” before anything else. Of course one of the greatest causes of damage to the top end is over heating - I am not suggesting that is the only cause or the only reason for top end overhauls.
So in short I have come to the conclusion that most of the tools at our disposal are pretty useless at reliably telling us in all circumstances whether we are “cooking” the top end. An EGT, the usual next best thing (short of nothing but the mark 1 ear) is a pretty course tool, as it only gives an indication of what is happening to one cylinder and then some little while after it has happened. (I appreciate some monitor more than one cylinder but it is certainly not the norm). Of course a multi gauge CGT goes a long way to improve things but it stills relies on the human computer rather than an EMU to react to changes.
So in short running an engine rich of peak is less efficient, may cause in some engines a little more plug fouling, and is not environmentally friendly but it IS economically sound. Ah, I hear you cry, what about the extra fuel you are wasting. Well you are not because the extra fuel is providing a safety net, a cooling buffer if you will so it may cost a bit more every hour but its petty cash compared with the cost of an early top end overhaul!
In my view, lean a bit, and keep the bank manager happy, lean aggressively and keep your engineer happy and if the POH says differently stuff the POH.
