[Mark33]

Thanks Dan for the graph. To monitor my AFR I have an O2 sensor mounted in my exhaust system and a digital gauge for the read-out. The AFR O2 sensor is not directly connected in with the rest of EFII system and is there only for me to be able to monitor the AFR and make changes if I decide to. The system is setup like this so that if the O2 sensor goes bad, it won't directly effect the SDS-ECU or any part of the EFII system.

 

[DanH]

.... At the higher power levels, an air/fuel ratio around 13 is good....
It does seem to correlate with the graph from DanH above

Ummmm....no.

That said, remember...

(1)..an FAA dyno run is done at worst case temperature conditions.

(2)..data is absolutely valid for only one engine, reasonably valid for engines with the same particular characteristics, and only generally valid for generally similar engines.

Put another way, the above charts may look a bit different given a parallel valve cylinder, 25 BTDC timing, and "normal" temperatures.

Regardless, there is much to learn by studying them.

 

[rv6ejguy]

Most engines make best power around 12.5-12.8 to 1 AFR but if we encounter detonation, we may have to enrich and/or retard timing.

Dan's graphs show how critical these two parameters can be and this is on 100LL. On Mogas, this is FAR more critical so beware of too much timing, too lean AFRs at high MAP. Also note that the first chart is only at 24 inches MAP, the situation is much worse at 29 inches and much worse at 24 degrees timing than the 20 here.

In other words, be very careful running anywhere near stoich (14.7 AFR and peak EGT) with advanced timing on Mogas, even at relatively low MAP (23-24 inches). You can still get detonation here.

As an example on my turbocharged Subaru, I enrich the mixture to 11.5- 11.8 AFR above 30 inches. Takeoff power is limited to 35 inches on 91 octane Mogas and I retard timing from 32 BTDC (best power) to only 23 degrees here to avoid detonation. This is with 9.25 to 1 CR. The Subaru has vastly better detonation resistance than the Lycoming but you still must be careful as many Sube guys have found out the hard way.

The safe timing curves on 100LL vs. 91 Mogas will be quite different on any engine. 100LL has much greater detonation resistance than 91 Mogas.

 

[Mark33]

AFR

Thanks Ross, that's great information!!

 

[David-aviator]

The D-30 chart indicates detonation begins some 50 degrees EGT before peak. This would confirm we should not be running at LOP at this power setting. I like to use 23" MAP max as a simple marker for LOP ops.

 

[DanH]

The D-30 chart indicates detonation begins some 50 degrees EGT before peak. This would confirm we should not be running at LOP at this power setting. I like to use 23" MAP max as a simple marker for LOP ops.

Well, yes and no.

Detonation intensity peaks just before peak EGT and falls off on the lean side of peak EGT. This particular chart doesn't extend very far into the lean side of peak, but I've posted detonation study charts done at Lycoming in other threads, plus you've all seen the APS teaching charts. There is no debate on this point.

Bottom line is exactly what APS teaches: either run moderately rich or well on the lean side, but don't spend time in the area centered around 25~50 ROP. That's the whole idea behind the Big Pull; at higher power settings you want to go from plenty rich to a point well lean of peak as rapidly as possible, so as to not fiddle around in the detonation zone. Note the words "rapidly as possible". At high power, pilots who pull slowly through the detonation zone, or worse, fiddle around finding peak from the rich side are asking for trouble. At high power, you don't even want to be hunting for peak from the lean side.

BTW, in this context, let's call "high power" anything above 65%.

Bottom line, yes, you can run LOP at this power setting (2450/24), but not slightly LOP. And again, remember that the posted data is for an angle valve cylinder at 20 BTDC timing, run hot.

 

[DanH]

Quick follow up regarding A/F ratios and the hot angle valve cylinder...

For those who think one measurement is worth a thousand opinions, the entire Swift fuel study (containing lots of good 100LL data) can be downloaded here:

http://www.tc.faa.gov/its/worldpac/techrpt/ar0853.pdf

From the study, here is the entire range of A/F ratios for best power and detonation onset, for two different 100LL fuel samples, running in the IO-540-K. Take a really good look at detonation onset vs manifold pressure. You want to map your fuel delivery for higher A/F when undersquare, and lower A/F when oversquare:

Next, A/F ratio vs power and detonation onset, takeoff power, 2700 RPM and WOT. The Swift fuel data has been removed for clarity; this is again the two samples of 100LL. Note the power difference between 12:1 A/F and detonation onset at 13:1 A/F...not much. In fact, note the overall flatness of the curve in the best power mixture region. The 2 HP spread extends from 12 to about 13.6 A/F. The flatness is typical. In this case the power loss at 12 A/F is seven-tenths of one percent in return for a guarantee of no detonation.

Again, this is a hot cylinder, hot oil, 100F intake air, all pro-detonation. I'm sure you can push all the A/F settings upward given a cooler cylinder head. The problem is knowing how much.

 

[rv6ejguy]

Best power AFRs from this data agree pretty closely with combustion studies done back in the '40s on big radials I've seen in some of my old texts.

What surprises me here is that they are encountering the onset of detonation at such low timing values and around best power AFRs on 100LL. It appears to me that given this data, running 24 degrees timing at WOT, sea level and with 91 octane fuel, you'd ALWAYS be in sustained detonation on this engine and the same induction temps. Just seems that running on 91 octane Mogas would be suicide yet we don't hear of this or see it from people running Mogas. Would appear that only by running sick rich or way LOP would they be safe at lower MAP and still at full SL power, they might still have detonation.

I wasn't going to download and read 181 pages of data but was there some correlation between detonation onset and induction temperature? My old rule of thumb on turbo engines was to keep induction temps below 160F on Avgas.

 

[DanH]

What surprises me here is that they are encountering the onset of detonation at such low timing values and around best power AFRs on 100LL.

The data is taken under severe conditions per the engine certification FAR; 475F CHT on the hottest cylinder, all other cylinders within 50F of 475, inlet air 103F, oil inlet temperature 245F. The driver here is probably CHT, with intake air temp being the least notable. In fact, the SI that rolled back timing on angle valve motors was at least partially based on very low intake air temperatures, i.e higher charge density.

Re auto fuel, I kinda doubt the typical application (like a carbed 320 or 360 fixed pitch) is anywhere near best power mixture when WOT. Anyone have a quantified A/F ratio or relationship to peak EGT for that?

Sure would be nice if someone turned up hard data for a parallel valve. Everybody keep looking, please.

 

[Dorfie]

Detonation detection

Dan,
Do you know the method used for detonation detection during these engine tests you showed? Is it something that we can use with the EIS that is installed nowadays or is it more sophisticated equipment? I know CHT goes up rapidly when detonation occurs, but I have always assumed this to be rather late indication of detonation that is well established. Is there a fairly simple/cheap way for us to detect "early" detection during everyday flights?
Cheers.
Johan

 

[DanH]

Dan, Do you know the method used for detonation detection during these engine tests you showed?

Nothing more than what is written in the report...

As part of the previous tests, the cylinder assemblies had been removed, drilled, and tapped in the fin area to install high-temperature, water-cooled piezoelectric pressure transducers. One transducer was installed in the cylinder head of each cylinder with the transducer face as flush as possible with the cylinder cavity. The transducers were connected to charge-to-voltage amplifiers, and the amplifiers were connected to a data acquisition system. Analog cylinder pressure signals were digitized at the rate of 50 kHz per channel. The pressure data was fed to a numerical knock quantification analyses routine, as detailed in ASTM D 6424.

...but it doesn't appear to be something you'll easily add to your airplane.

 

[crabandy]

I'm assuming you must run your engine in the detonation zone in order to determine the peak EGT's for your specific engine at various power settings, then you can use those EGT temps to lean correctly. What is the best method to establish this baseline.
Thanks,
Andy

 

[rv6ejguy]

The data is taken under severe conditions per the engine certification FAR; 475F CHT on the hottest cylinder, all other cylinders within 50F of 475, inlet air 103F, oil inlet temperature 245F. The driver here is probably CHT, with intake air temp being the least notable. In fact, the SI that rolled back timing on angle valve motors was at least partially based on very low intake air temperatures, i.e higher charge density.

Re auto fuel, I kinda doubt the typical application (like a carbed 320 or 360 fixed pitch) is anywhere near best power mixture when WOT. Anyone have a quantified A/F ratio or relationship to peak EGT for that?

Sure would be nice if someone turned up hard data for a parallel valve. Everybody keep looking, please.

103F IAT is certainly not high if you fly in the hot states. Would be interesting to see to detonation margins at various CHTs. I'm thinking not many operate at 475F. The charge heating in the port and chamber would have to be substantial I'd think.

 

[crabandy]

I'm not sure how to save the graphs to post a readable display on the forums so here is a link to my data:

http://www.cirrusreports.com/flights/N117TR/

If you look at my flights from Jan 26 I was trying to establish some #'s with wheelpants but without gear leg fairings. OWI to K34 I was piddling with ROP with level offs at 5500, 7500 and 9500. I was searching for Peak EGT, some ROP and LOP numbers.
From what I can tell at full throttle #1,4 EGT peaks first followed by #3 with #2 lagging quite a bit. #3,4 are close to 100 degrees LOP when #2 is 20-30 degrees LOP. The engine isn't quite happy at this and is surging slightly. It runs smoother with #3,4 40-50 degrees LOP but this puts #2 close to peak EGT.
If you look at the Jan 26 flight from K34 to OWI, I think I found a happy place for my engine combo where it runs very smoothly. With the throttle partially closed in the 55-60% power range the EGT's are much more aligned and the CHT's all drop to similar values. Fuel flow jumps around the lower 6 gph and TAS around 133-140.
I've got more playing around to do but appreciate any input anyone would like to offer.
Thanks,
Andy

 

[crabandy]

More piddling around with LOP, starting etc.

My cold starts haven't been the best (O-360 without primer), but I believe it's my procedure that's to blame. 12 years ago I caught the airbox of a C-150 on fire and haven't pumped the throttle without the prop turning since. My cold starts (preheated below 40 *F) have consisted of hit the starter and pump the throttle twice, usually takes 2-3 tries for a successful start. Recently I've started using the more conventional approach of pumping the throttle twice and then hitting the starter, starts easily in 2-3 blades. New start procedure adopted, I'm amazed how easy it starts now.

Runup has been the same 10-15 RPM and EGT change with one ignition shut down. Even with the Catto prop my static idle stop is set at 480 RPM, much lower and it starts to get rough.

My engine doesn't have an even enough fuel distribution to run LOP at full throttle, I thought I got some screen shots but apparently not. At full throttle, while leaning the mixture #4 cylinder will be close to 100 LOP when #2 cylinder is getting close to peak. No wonder #4 is my hottest cylinder and #2 is my coolest.

Something magic happens when I partially close the throttle, everything evens out. I could change my data fields around on the G3X to better represent the situation, but this was just after leveling out at about 7000 Density Altitude and setting the mixture LOP. All my EGT's peak about 1450 when slowly enriching the mixture from LOP. I was showing about 155knts TAS, disregard the GS noted in the screen shot as the winds were not consistent. I can go about 30-40 *F leaner but I can feel the engine surging, small changes in mixture make a big difference LOP. My fuel flow is also not steady from 6-8 GPH, perhaps the way the needle and float is opening and closing.

Here's a screenshot of about 100 *F ROP, I couldn't tell any difference in speed only fuel burn.

The above situations were about 80-85 *F on the ground, I was showing much better cylinder head temps several months ago. Thinking about redoing the cowl exit for better cooling.

 

[I-MKLK]

EFII installation pictures

For the same purpose of crabandy I post some picture of my installation.It's a dual ECU ignition and fuel injection system.
ECUs

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Connectors

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Fuel pumps

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Red Cubes

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Throttle body

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Map Sensors

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Dual SuperB batteries

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Fuel Rail

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Spark Harness

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