Starting F.I. Lycoming engines with mixture in cut off....

[Hobart Escin]

Hi,

This is more of a general knowledge question (also applicable to A2A's new 172 too though), but why does Lycoming recommend starting this model engine off of prime only with the mixture in cut off? Can fuel-injected engines backfire (assuming this is the reason)? Thank you.

[CodyValkyrie]

That's a great question that I wish I could answer with more authority. In older planes it was a safety concern when starting to not feed a fire (should one start) with adding in more fuel. I assume this falls somewhat along those lines but I am no expert. I'm curious to know the answer myself since when I flew 172s back in 03 we started with mixture forward, and not at cut-off.

[AKar]

I'd say the main reason is the Bendix RSA fuel injection system's fuel/air control unit.

It basically measures the air intake of the engine by comparing the differential between the intake total pressure (measured by four pitot-tube-like things within the unit) and the static pressure at the venturi throat. Those are applied to the different sides of a diaphragm. In the running engine, as the the throttle is increased, the "impact pressure", more accurately, the total pressure of the intake air on those pitots is more or less unchanged, but the static pressure on the venturi throat drops. That causes a force on the diaphragm. That force, assisted by a spring at low RPMs, opens the fuel servo valve and increases the fuel flow.

The movement of the servo valve is balanced by a similar diaphragm, but this one has the unmetered fuel pressure from the engine-driven fuel pump on the side that forces the servo valve to close, and the 'metered fuel pressure', more accurately perhaps a restricted fuel pressure on the other side. That pressure is 'restricted' by the mixture control, and that pressure regulates that closing force on the fuel diaphragm. The 'metered fuel' is what is discharged into the airstream for combustion via the servo valve opening. In a running engine, when you increase the throttle, the decreasing pressure in the venturi causes the valve to open until it is balanced by a more-or-less constant unmetered fuel pressure trying to close the servo valve against the dropping 'metered fuel pressure' (as the fuel flow increases due to opening servo valve, thereby lowering the "metered" fuel pressure on that side of the diaphragm).

Now, if you try to start an engine with the mixture full-rich, basically what happens is that there is hardly any pressure differential between the total and static air pressures, as the airflow into the engine is very small when the starter turns it. That causes the servo valve being open due to that spring assisting the air pressure differential at low RPMs ("a constant head spring"). Before the fuel pressure would build up, that will cause the engine to flood, as the fuel leaks through the servo valve.


Short answer: the Bendix servo does not work well without airflow into the engine, causing mixture to be too rich (noncombustible) if the 'metered fuel' fed into the engine is not restricted by the mixture lever. The engine won't start if you have the mixture forward.


(Probably some mistakes here, need to dig the old manuals under the dust layers some time! )

Edit: Obviously, I need to find a diagram from somewhere...

[rotrhed]

I've started 172's both ways but prefer to do it in cut off.

[CAPFlyer]

One thing to note is that aircraft fuel injection is not the same as what you have in your car and there is no computer and there is no real metering beyond what was mentioned above.

More information on the system can be found here -

http://www.avweb.com/news/maint/181624-1.html

This is on the Continental IO-360, but they use similar systems and have some of the same issues addressed.

[Hook]

AKar, I love your technical explanations of things, and you're obviously very knowledgeable. However, the whole time I was reading that I was hoping for someone to do a translation into layman's English, preferrably 25 words or less.

So, is this accurate?

"If the mixture is not at cutoff during start, the engine will flood because of the way the fuel injection system works."

Hook