Reely340
26th Jul 2013, 12:13
Guys, I'm trying to get behind the concepts and technical workings of the S-300C (Hu269) fuel system,
more precisely: the relationship between throttle grip and mixture lever.
The S-300C features a 4 Cyl Boxer with mechanical(?) manifold fuel injection,
I'm familiar with the AFM starting procedure:
1) 1/2" throttle open
2) fuel pump on
3) mixture full rich for 2.5/3 secs,
4) mixture full lean (idle cutoff)
5) fuel pump off
6) 1/8" throttle open
7) crank
8) wait till engine fires
9) mixture to full rich
Why? (Mind you I can read the AFM, I'm talking technics here only!!)
So, what exactly do the controls do?
Throttle: changes orifice for air intake and somehow "adjusts" injected fuel quantity,
thereby maintaining a combustible mixture (hence neither rich nor lean, just more or less of it)
Mixture: from "idle cutoff" to "flight mode" seems to reduce the mixture's fuel component only.
Technical guesswork for AFM starting procedure:
At 5) there must be a puddle of liqud fuel (or cloud of fuel mist?) sitting in the manifold cavern.
Phase 7) cranking is where the mystery happens:
a) initial phase engine draws in
air for 1/8" open throttle, NO fuel (mixture at cutoff) PLUS vapor of said puddle of fuel.
That is a very undefined mixture most probably way too rich
b) waiting for ignition phase engine draws in
air for 1/8" open throttle, NO fuel (mixture at cutoff) PLUS increasingly less vapor of said puddle of fuel.
ingested mixture becomes leaner
c) at firing point mixture is correct but rather volatile
as the fuel still comes from the diminishing puddle of fuel and NOT from throttle!
d) after firing
we are meant to set mixture to "rich" fast,
due to imminent fuel starvation as puddle of fuel is being used up
e) after mixture "full rich"
we did it, now the 1/8" throttle settings air is complemented from mixture fuel only, puddle is gone.
Needless to say you ever get to c), d) is the tricky spot, be slow and she'll die on you.
Besides being very awkward to set starting mixture by half flooding the engine and dynamically
passing mixture control from "puddle of fuel" to a measuring mixture lever, somehow this feels the
wrong way around to me, especially with warn engine:
I'd rather set minimal throttle (1/8" or even 0), start to crank and progressively shove in the mixture.
That way the mixture would start way lean, become increasingly richer until idle conditions are met.
This would have the advantage of avoiding any accidental flooding.
In my view that fuel system misses a cold start mixture enrichment choke postition/switch/thingy.
Example: Yamaha WR450F motorcycle carburetor choke opens a huge additional fuel passage
AND a small additional air passage, hence cold start works with zero throttle.
Anybody got a detailed exploded view of the S-300C fuel metering system, graphs, tables
picturing mixture vs. level position, anything that explains HOW the thing is working?
TIA
Peter
PS: how is ambient pressure induced mixture enrichment done?
Why doesn't it drown from overly rich mixture at FL100?
more precisely: the relationship between throttle grip and mixture lever.
The S-300C features a 4 Cyl Boxer with mechanical(?) manifold fuel injection,
I'm familiar with the AFM starting procedure:
1) 1/2" throttle open
2) fuel pump on
3) mixture full rich for 2.5/3 secs,
4) mixture full lean (idle cutoff)
5) fuel pump off
6) 1/8" throttle open
7) crank
8) wait till engine fires
9) mixture to full rich
Why? (Mind you I can read the AFM, I'm talking technics here only!!)
So, what exactly do the controls do?
Throttle: changes orifice for air intake and somehow "adjusts" injected fuel quantity,
thereby maintaining a combustible mixture (hence neither rich nor lean, just more or less of it)
Mixture: from "idle cutoff" to "flight mode" seems to reduce the mixture's fuel component only.
Technical guesswork for AFM starting procedure:
At 5) there must be a puddle of liqud fuel (or cloud of fuel mist?) sitting in the manifold cavern.
Phase 7) cranking is where the mystery happens:
a) initial phase engine draws in
air for 1/8" open throttle, NO fuel (mixture at cutoff) PLUS vapor of said puddle of fuel.
That is a very undefined mixture most probably way too rich
b) waiting for ignition phase engine draws in
air for 1/8" open throttle, NO fuel (mixture at cutoff) PLUS increasingly less vapor of said puddle of fuel.
ingested mixture becomes leaner
c) at firing point mixture is correct but rather volatile
as the fuel still comes from the diminishing puddle of fuel and NOT from throttle!
d) after firing
we are meant to set mixture to "rich" fast,
due to imminent fuel starvation as puddle of fuel is being used up
e) after mixture "full rich"
we did it, now the 1/8" throttle settings air is complemented from mixture fuel only, puddle is gone.
Needless to say you ever get to c), d) is the tricky spot, be slow and she'll die on you.
Besides being very awkward to set starting mixture by half flooding the engine and dynamically
passing mixture control from "puddle of fuel" to a measuring mixture lever, somehow this feels the
wrong way around to me, especially with warn engine:
I'd rather set minimal throttle (1/8" or even 0), start to crank and progressively shove in the mixture.
That way the mixture would start way lean, become increasingly richer until idle conditions are met.
This would have the advantage of avoiding any accidental flooding.
In my view that fuel system misses a cold start mixture enrichment choke postition/switch/thingy.
Example: Yamaha WR450F motorcycle carburetor choke opens a huge additional fuel passage
AND a small additional air passage, hence cold start works with zero throttle.
Anybody got a detailed exploded view of the S-300C fuel metering system, graphs, tables
picturing mixture vs. level position, anything that explains HOW the thing is working?
TIA
Peter
PS: how is ambient pressure induced mixture enrichment done?
Why doesn't it drown from overly rich mixture at FL100?