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The Setup:
After recently installing the LASAR system (electronic ignition, magneto
backup) on my I/O 320, I became more sensitive to the temperature related
details of leaning, especially at max cruise power. The LASAR works as
advertised, reducing my peak EGTs by 80 degrees and raising my CHTs by
approximately 20 degrees as measured by Vision Microsystems EPI 800 System
with display increments of 10 degrees. This is a consequence of the
electronically managed hot-spark advance more completely burning the fuel in
the cylinder rather than the exhaust pipe, thus resulting in more power.
Utilizing Lycoming's lean recommendations, #2 cylinder (left front) was
always first to reach peak lean (EGT 1450-1460 degrees) and would remain the
leanest cylinder when enriched 100 degrees (less than 75 percent power) or
150 degrees (above 75 percent power). The other three cylinders were running
200 degrees "richer" and their CHTs were 20 degrees cooler. In full power
climbs, cylinder #2 had the hottest CHT by 20 degrees. Consulting with Tom
Giddings (IO 360, sealed plenum), I found he had similar characteristics,
i.e. cylinder #2 was always first to reach peak. He suggested that since the
injector was on the top forward part of the cylinder #2, it was subject to
high-speed, high pressure air and this may be causing a leaner mixture in
that cylinder. After all, Lycoming never expected 170-190 HP engines to be
operating in 190-200 Kt TAS environments.
The Experiment/Results:
Using a 3-inch high x 2-inch wide aluminum "shield" screwed to the lower
baffle and pop-riveted to the cylinder head baffle, the direct airflow to
injector #2 was blocked. On the first test flight, cyl #3 was first to peak
and on the second flight, #4 EGT peaked first at 1450 at 4500 ft, 25x25, 6
deg C, 192 Kts TAS, 120 degrees lean of peak, CHTs within 20 degrees (Avg
370) and EGTs within 100 degrees. Tom installed a similar block and a test
flight showed that cyl #2 was no longer the first to reach peak. The
conclusion is that the fuel/air mixture in normally aspirated engines may be
affected by differing air flows/pressures seen by the injectors.
The Survey:
If you are running an injected LNC2 with per cylinder instrumentation,
1. Do you find cylinder 2 the first to reach peak?
2. At cruise, is cylinder 2 running the leanest?
3. Are you using GAMI injectors?
4. Does Cylinder 2 CHT run the hottest in an extended climb at 135-140 Kts
IAS (we also think Cyl 2 needs a front baffle modification to increase its
cooling as we have done to Cyl 1)?
5. Do you think the use of injector shrouds connected to a common plenum
would help even out the combustion events in the normally aspirated engine
(turbo-charged engines use shrouded injectors plumbed to the deck pressure
plenum)?
Fly Safe, Fly fast,
Scott Krueger
N92EX
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