Toobuilder
Well Known Member
One of the design goal for my engine in the Rocket is to be able to go fast on auto fuel. Not airport or marina "mogas", but the garbage dispensed from the "regular" nozzle at any California gas station. One major consideration toward this goal is ignition timing and I validated earlier flight test with a timing sweep on the Ly-Con dyno in this thread:
https://vansairforce.com/community/showthread.php?t=182190&highlight=Timing,+dyno
Last few flights have been working in more and more auto gas and today I flew with a 100% load of gas station regular auto junk in one tank. Start, TO, hard climb to 11,500 and cruise for data collection were nominal. As was the climb to 13,500 cruise, descent, T&G, and landing. The initial climb was more aggessive than normal (2600 feet to 9500 in 2 minutes), so I was not babying the engine.
Data was interesting: (and keep in mind this is data from an engine monitor, not calibrated test equipment) level at 11.5 MSL and normal LOP setting, I took a look at the difference in engine behavior between the alchohol laced auto gas and the 100LL. As expected, with a constant FF of 10 GPH (open loop EFI), the chemically "more lean" auto gas drove the expected behavior:
Auto fuel = 1321 EGT
100LL = 1345 EGT (24 degrees "richer")
OK, flip the SDS LOP switch off and return to best power mixture/ignition advance and I get:
Auto fuel = 1309
100LL = 1293 (16 degrees "richer")
Ok, the auto gas is leaner at the same volume than 100LL, no surprise there, but what if we do a "constant EGT target" to see what it does to FF?
With the 10.0 GPH and 1345 LOP EGT as the baseline for 100LL, how much more auto gas do we need to throw at the engine to match the 100LL EGT? Its a simple thing to just tweak the manual mixture knob on the SDS EFI to add or subtract a percentage of FF. So with the engine running on car gas again, I richened the mixture until I hit my target EGT of 1345. Resulting FF: 10.5 GPH.
Now hold on a second, it's going to cost me a half gallon per hour of fuel economy to save a few cents at the pump? But thats not the full story, and this is the part that does not make much sense -
The 10.0 GPH figure for both 100LL and car gas produce the exact same speed (192 KTAS today), despite the significantly "leaner" (and presumably chemically less powerful) auto gas. Weird stuff, huh?
Anyway, thought the group would like the info.
https://vansairforce.com/community/showthread.php?t=182190&highlight=Timing,+dyno
Last few flights have been working in more and more auto gas and today I flew with a 100% load of gas station regular auto junk in one tank. Start, TO, hard climb to 11,500 and cruise for data collection were nominal. As was the climb to 13,500 cruise, descent, T&G, and landing. The initial climb was more aggessive than normal (2600 feet to 9500 in 2 minutes), so I was not babying the engine.
Data was interesting: (and keep in mind this is data from an engine monitor, not calibrated test equipment) level at 11.5 MSL and normal LOP setting, I took a look at the difference in engine behavior between the alchohol laced auto gas and the 100LL. As expected, with a constant FF of 10 GPH (open loop EFI), the chemically "more lean" auto gas drove the expected behavior:
Auto fuel = 1321 EGT
100LL = 1345 EGT (24 degrees "richer")
OK, flip the SDS LOP switch off and return to best power mixture/ignition advance and I get:
Auto fuel = 1309
100LL = 1293 (16 degrees "richer")
Ok, the auto gas is leaner at the same volume than 100LL, no surprise there, but what if we do a "constant EGT target" to see what it does to FF?
With the 10.0 GPH and 1345 LOP EGT as the baseline for 100LL, how much more auto gas do we need to throw at the engine to match the 100LL EGT? Its a simple thing to just tweak the manual mixture knob on the SDS EFI to add or subtract a percentage of FF. So with the engine running on car gas again, I richened the mixture until I hit my target EGT of 1345. Resulting FF: 10.5 GPH.
Now hold on a second, it's going to cost me a half gallon per hour of fuel economy to save a few cents at the pump? But thats not the full story, and this is the part that does not make much sense -
The 10.0 GPH figure for both 100LL and car gas produce the exact same speed (192 KTAS today), despite the significantly "leaner" (and presumably chemically less powerful) auto gas. Weird stuff, huh?
Anyway, thought the group would like the info.
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