View Full Version : Fuel injection and mixture control

Hairy Mary
29th Sep 2005, 16:50
Why do 21st century fuel injected aero piston engines still have manual mixture adjustment?
Cars have lambda sensors in the exhaust stream and a processor to manage fuel delivery for optimum stochiometric ratio.
Why not aero engines?

Standby Scum
29th Sep 2005, 17:12
Interesting post. I had to look up lambda and found :cool: :-


I can't answer your question though. My instructor's rating ran out a third of a century ago. :\

White Bear
29th Sep 2005, 23:57
This answer should be longer, but the mixture still needs to be adjusted in fuel injected engines for the same reasons you need to adjust the mixture in a carbureted engine, to lean out the fuel supply as you climb. Just because it is fuel injected does not make the system any smarter, you simply have better atomization of the fuel, and therefore better or more complete combustion.

You are quite right about car engines, and until FADEC (Fully Automatic Digital Engine Control) was introduced, piston engine aero technology was stuck in the 1930’s, primarily because of short sighted regulations, and the controlling authorities unwillingness to change.

New piston engines that are FADEC equiped use one lever power controls, computers control fuel mixture, ignition advance, manifold pressure, even prop pitch. It also monitors and controls cylinder head temperatures, by adjusting manifold pressure and mixture.
IMHO, it was a long time coming.
White Bear.

30th Sep 2005, 07:51
Simple answer : lambda probes do not work with high leaded fuel, the lead deposit on the probe leds to a ´drift´ in indicated O2 content.
And of course all ´modern´ aircraft injection systems are 100% mechanical, so no interference with electromagnetic fields, higher reliability (?), lightning strike safe etc.

On the other hand, a good hand mixed engine is better than an electronically controlled one. The signal of the lambda probe is rapidly changing at lambda=1 and more or less just ´high level´ or ´low level´ for lambda >1 or > 1, so it is perfect to adjust air to fuel ratio at exactly lambda=1. (which you need for the catalytic converter in motorcars) It is poor in adjusting any other lambda.
In Aircraft engines, we use lambda < 1 for takeoff power (best performance, economy is not relevant for this short period, internal cooling due to fuel, so higher peak performance allowed, HC and CO emissions are irrelevant) and lambda > 1 for cruise (best economy, the bad throttle response is irrelevant as we don´t change the throttle setting in cruise, NOx emissions are irrelevant). If we would fly with lambda = 1 all the time, takeoff power would be lower and cruise would be less economical.
(Both effects might be compensated by the better quality of the fuel injectors and hence better fuel vaporisation if you change to electronically controled injectors and high pressure fuel supply)

Hairy Mary
30th Sep 2005, 13:45
Thanks for the informed responses.
I am inclined to run on the rich side because I am terrified of making holes in pistons. I still think that the right technology could manage fuel far better than I ever could.

30th Sep 2005, 15:38
A few years ago I used to fly a plane that had no mixture lever. It was built in 1948 and had a german engine (Argus, 465 HP).
Design probably from WW2. Mixture and ignition timing fully automatic. No lambda or FADEC stuff and absolutely reliable.
So it would be feasable and reliable also these days even without electronics.
The problem is: most modern (american) piston engines are still 1940's technology with maybe an injection syatem and turbocharger added. And these companies do have some kind of monopoly and absolutely no will to start anything really new.

Cheers, jojodel