Fuel servo question - zero time IO360

[Matt Ward]

Hey guys,

Hopefully a quick question for you. I just finished breaking in a zero time IO-360 in my M20E.  One thing I've noticed is that as I'm climbing (I'm based at nearly 6K feet), my fuel flow meter shows a steady, slow decline without making any power or mixture changes.  The engine runs great, temps are great, fuel pressure is great, performance is great, etc.  No changes in RPM but obviously MP declines as I get higher.  I suppose my old engine did this too and I just never noticed it but with a new engine I'm paying attention to everything.  It's nothing too drastic but enough change in the numbers to notice.  For reference, I normally try to take off in the 15 gph range and climb using the target EGT method, typically 12-13 gph I'd estimate.  FWIW I have confidence the fuel flow meter is accurate by looking at what it reports as burn vs. what it takes to fill up the tanks at the end of the flight.

Anyways, I'm guessing the answer lies in how the fuel servo works but that's a mystery box to me.

Does this seem normal to you guys?

[KSMooniac]

Normal behavior... The servo meters fuel based on mixture control and incoming air flow. Climbing leads to lower density, lower air flow, lower fuel needed.

Target EGT on takeoff and climb is the best way to operate! Good on ya. Just make sure your target EGT for installation corresponds to a sea level standard day takeoff with a rich mixture.

Sent from my LM-V405 using Tapatalk

[EricJ]

If you look down the inlet throat of the servo there are four ram air tubes sticking out into the air flow path.  Those feed pressure to one side of a diaphragm that helps control fuel flow, so that it helps regulate fuel flow to match air flow.   As you climb and the air density decreases the decreased pressure on the diaphragm will decrease the fuel flow.   It's not quite an automatic leaning system, but it helps.   It's analogous to the MAF sensor on a car that the engine computer uses to adjust fuel flow via injector pulse width.

I had noticed that in my usual trips I'd be doing incremental enrichment adjustments while descending but then when I got close to the bravo and put the nose down to get under it I'd have to pull it a little lean again.   The added pressure from the ram air tubes at increase speed will automatically increase the fuel flow.

This is a big difference between how Lycoming (Bendix) and Continental (TCM) fuel injection systems work.   The TCM fuel injection system on Continental engines don't have such a thing.

 

[N201MKTurbo]

The servo will keep the mixture constant with throttle changes, but not altitude changes. 
 

Unless you have a turbo like mine. I can set the mixture, climb to 20000 feet and back to sea level without ever touching it.

[Matt Ward]

Thanks for the responses guys, what an amazing resource this is!