Interpreting EGT, MAP and FF Settings: Help Requested

[Jeff_S]

Hello all.  I had a short flight to knock some rust off my brain today (boy did I need it...but that's too embarrassing to talk about right now) and pick up cheap fuel. On the return leg (back in the groove) I was cruising along at sightseeing altitude over the coast in my normal LOP configuration. After the flight I was studying my readouts on Savvy Analysis and noticed that at one point, when I had dialed back MAP from 24" to 20" (since I wasn't in a hurry) there was a corresponding rise in EGT.  I'm sure this is a normal reaction, but I've been thinking it through in my brain and it hasn't clicked "why" just yet.  

At LOP, reducing MAP means less air getting to the cylinder which would make the mixture richer...so that makes sense. But the FF declines as well, so there's less fuel getting into the cylinders.  But perhaps the new fuel/air ratio still left the mixture richer than before...so maybe I've answered my question but I know we have some better chemical engineers on this forum than me, so I'd be happy to get a more thorough explanation.  Here's a graph of the section in question:

And while I'm on the topic of things I know work but have never quite understood how, what is the mechanical link between FF and MAP?  When you increase or decrease throttle (FF) how does that automatically change the MAP?  Yeah, perhaps a very basic and dumb question that I should remember from PPL training, but I believe there's some hole in my knowledge of how an engine operates that needs to be filled.  I appreciate all kind and compassionate responses!

[gsxrpilot]

Just a quick note on this... you can post the link for the chart and that allows us to see more than just the snapshot in time that you post. It doesn't allow the public "into your account" or anything like that, but does make the one flight available.

[jaylw314]

 

1 hour ago, Jeff_S said:

And while I'm on the topic of things I know work but have never quite understood how, what is the mechanical link between FF and MAP?  When you increase or decrease throttle (FF) how does that automatically change the MAP?  Yeah, perhaps a very basic and dumb question that I should remember from PPL training, but I believe there's some hole in my knowledge of how an engine operates that needs to be filled.  I appreciate all kind and compassionate responses!

This webinar from Savvy Aviation below is a good start.  It starts with carburetors, and moves on to pressure carburetors which are the cousins to fuel injection, but the idea of "fuel metering" is analogous--you use some kind of pressure differential to adjust the flow of fuel from a fixed setting.  What pressure differential is used and how it controls the flow of fuel are the things that vary from one setup to the next.

Also, in many fuel injected engines, both MAP and FF are by-products of intake airflow, they're not directly the result of the other.

 

 

[carusoam]

Jeff,

As Jay points out above there is an interconnection between MP and FF...

So... When changing MP, it is normal to go through the leaning process again...

I never went through the effort to find out if there is a better short cut to get to the next leaning step. Just went back to peak, and leaned from there...

Let me know if I am missing something.  There could be a better way.

Best regards,

-a-

[Bob - S50]

I'm definitely no expert.  Hopefully someone who has taken the class in Oklahoma will chime in.  My guess as to why the EGT went up when you reduced manifold pressure is that a reduced pressure mixture will burn more slowly.  This will put the peak pressure further after TDC and more of the combustion event will still be continuing when the exhaust valve opens.