I think I spent over 12.000,00 Euros on fuel last year.... It doesn't bear thinking about.
As I wrote in my earlier post, I would like to confirm again that there is no leaning limitation put on Lycoming users. Lycoming keeps updating and changing their latest operational guidelines, but that is not a legal operational limitation.
By the way, if I follow the recomendation in my Cessna POH, by leaning to onset of engine roughness and then enriching sligtly, my EGT's are 60-70 LOP.
It just happens to be that most engines are operating with a fuel/air distribution that poorly one doesn't get past peak EGT when leaning without one cylinder beginning to miss.
"The reason ROP is cooler is because the combustion is making less heat. The same reason that LOP is cooler."
In reality ROP is cooler because the combustion process is slower and the end of combustion is arriving later. At that moment the combustion chamber is bigger due to the piston being already lower. Bigger volume means lower temperatures (adiabatic process).
The excess of fuel is used here to replace the variable ignition advance angle. The excess of air could be used the same way.
Miroc
LOP tends to be slower and comes later in the cycle so the cylinder is further past TDC - this is why pressures are lower and it tends to be a little more efficient (pushing when the crank is further past 12 o'clock).
As JasonC puts it, is how I understand, too.
Another point to consider is that at say 30 degress LOP one is flying "green".
The exhaust gases are clean.
You can't experience carbon monoxide poisoning when cruising LOP.
That doesn't mean to only fly LOP, but it is the cleanest and greenest method of operating our aircraft, when the engine is conforming.
If the engine is not conforming the power loss LOP will be too great from a pilot's perspective.
In reality ROP is cooler because the combustion process is slower and the end of combustion is arriving later. At that moment the combustion chamber is bigger due to the piston being already lower. Bigger volume means lower temperatures (adiabatic process).
That's what I was getting at. It isn't the vapourisation of avgas that makes ROP cooler.
In reality ROP is cooler because the combustion process is slower and the end of combustion is arriving later. At that moment the combustion chamber is bigger due to the piston being already lower. Bigger volume means lower temperatures (adiabatic process).
I am pretty sure the combustion process slows down about equally for any given % rich or lean of best power (which is about 50 ROP). I suspect a small amount of the reduction in peak pressure (and associated temperatures) is due to the marginally slower flame propagation (and hence marginally greater combustion chamber volume). However, the vast majority of the reduction in pressure/temperature is due to the lower total energy release (either due to fewer molecules -LOP, or incomplete combustion ROP)
" However, the vast majority of the reduction in pressure/temperature is due to the lower total energy release (either due to fewer molecules -LOP, or incomplete combustion ROP)"
Let's say there are 150 units of air and 10 units of fuel in the chamber, this is close to stechiometric mixture. All the fuel molecules will burn and release 10 units of energy. At the end there will be no unburned fuel particles nor free oxygen left.
Making the mixture richer under the same conditions by entering 11 units of gas there will be roughly the same energy released, because the 10 units will find oxygen, eleventh unit will remain unburned. 10 units of energy released.
Making the mixture leaner by allowing only 9 units of fuel to enter the chamber, all of fuel will be burned releasing 9 units of energy.
So by enriching the mixture slightly there will be the same energy released, by leaning there will be less energy released. That means by enriching the mixture there should be no observable change in ICP or EGT. That is not the case. I believe the relatively big change in EGT can be explained by the significant reduction in the speed of the combustion process.
Miroc
Mirco,
Unfortunately the fuel is not a 'unit' it is a complex set of hydrogen-carbon bonds that reform to be CO2 and H2O if stochiametric. However, in an excess fuel situation you don't have some fuel units fully burned and others unburned - you have most fuel units only partially burned and a material amount of the carbon only partially oxidised into CO (hence the risk of poisoning in ROP vs LOP). This partial oxidation releases less energy. The data sheets I have seen show flame propagation slowing equally for LOP and ROP settings - so the pressure reduction should be consistent for ROP and LOP, therefore there must be mechanism that reduces the energy release (the mechanism described above).
-- I think, but it is a long time since I did this, that fuel burned to CO and CO (rich mixture) has a energy release of 220 KJ/mol, where as fuel burned to CO2 (peak or LOP) has an energy release of 393 KJ/mol (ignoring the H part of the reaction in both cases). Hence why the flame of an oxy-acetelyne torch is pathetically cool with the O2 off and cuts steel with the O2 properly flowing
According to the APS handbook from GAMI, 40F ROP has the fastest burn time, Peak slower and 50F LOP slowest.
The reason LOP has lower ICPs than ROP is that the slower burn time means that the piston has moved further down before peak pressure from the combustion event has taken place.
Having thought about it, I think you are right, given how long the flame front takes to burn (which I had forgotten about) the variation in burn rate and hence increase in combustion chamber volume will be the primary reason for both ROP and LOP reducing CHT and internal pressures.
The chemistry then explains the two different reasons for reduced energy density and hence reduced flame propagation rate.
