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Experimenting With Car Gasoline -EFI 540

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.
 
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That's pretty interesting.

Out of curiosity did you try letting SDS takeover completely with closed loop control?
 
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?

A few cents ???? Must be talking MoGas with/without alcohol.

How much between MoGas and Avgas-----that is where the real savings would be.
 
Closed loop is not an option with the high probability of mixing fuels. Maybe when unleaded Avgas becomes widely available.
 
A few cents ???? Must be talking MoGas with/without alcohol.

How much between MoGas and Avgas-----that is where the real savings would be.

Avgas is 3.65; car gas is 3.10 around here. Did you already forget what it's like out here in California?
 
A few cents ????
How much between MoGas and Avgas-----that is where the real savings would be.

A viewpoint from another part of the world (Ontario, Canada)...
Regular 87 octane, 10% ethanol mogas is roughly equivalent to US$3.00 /US gallon
100LL Avgas is roughly equivalent to US$5.25/US gallon

That US$2.25/gallon difference adds up VERY quickly.

I burn a fair bit of 91 octane "premium" 0% ethanol mogas in my aircraft. It's price is currently running about US$3.70 /US gallon. Even with only a $1.55/gallon price differential that's US$14/hour or nearly 30% reduction in direct operating cost. That's serious money!
 
Michael,

You have a few variables going on there, and not all your comparisons are going to be apples for apples so to speak. I will see if I can explain this well.

Avgas 100LL SG = 0.717kg/L
E10 Ethanol 15% = 0.735kg/L
Shell V Power 98PULP = 0.754kg/L

Other fuels will vary slightly place to place but you can see the variation. The ULP's have higher density and more calorific value per volume, and thus the same power will come from slightly less volume of fuel.

What you can measure is based on what equipment you have. Having done a bit of this with George Braly and Tim Roehl at GAMI with the G100UL, I can assure the difference is real, and measurable, but it is in the kinds of proportions you see above. At best about 4%.

So given you tested at a constant volume, that confuses things a little. Peak EGT for one fuel will be different to the other so when switching from ULP to 100LL or from LOP to ROP, you need to run your tests based on the new peak EGT for each fuel. Not compare one with the others peak.

Auto fuel = 1321 EGT
100LL = 1345 EGT (24 degrees "richer")
This is a false positive. At the same flow rate the ULP will have been the richer mixture as the F/A ratio got more pounds/hr of fuel on the ULP. The lower EGT would have been a later Theta PP and lower peak pressure thus a higher EGT (less expansion of the gasses)

Auto fuel = 1309
100LL = 1293 (16 degrees "richer")
Now that is possibly a weird result but it would be assuming the test method was identical. I suspect you have a different peak and the order in which you did things was mirrored.

The denser and thus richer fuel you would expect to produce a lower EGT value, but the latency of the fuel effects the Theta PP and thus expansion ratio of the gases. So quite likely the faster burning (albeit richer) fuel has caused a higher EGT value than the leaner but slower burning avgas.

One of my aviation buddies is a guy called Tom Gresham, many of you who follow the shooting/outdoors sports will know Tom or his dad Grits. One of his favourite sayings is at any time two things can be true, but in broader terms one scenario is correct.

So that may have scrambled the eggs a bit, I hope I have helped somewhat.

Have a good day!
 
I used to run pure ethanol free 90 octane "rec fuel" in one tank on my RV-10. I also would TO/Land on the 100LL tank just to be safe, then switch over to the rec fuel tank. I didn't pay a lot of attention to my egt's, but I saw no difference in the fuel flow or TAS. I plan to do the same on the -9A but will take a more analytical approach in my testing. The price difference between 100LL and the rec fuel was right around $1.75 per gallon at that time.
 
Don't forget that other things are going on. As your exhaust temperature changes, so does the tuning of the exhaust system (speed of sound change) as well as the mass of residual gas in the clearance volume - not insignificant in a low-ish CR engine.

Flame speed/burn rate is also a function of combustion chamber surface temperature as well as oxygen concentration (i.e. internal EGR rates)

BTW, if you plot the data that Michael gave in the thread originally linked, it looks like the 25° point is at least a couple of degrees beyond MBT ignition. The ignition-torque curve usually closely approximates a quadratic function so if you plot the data and apply a least squares fit you can pick off your MBT ignition angle easily enough - much more accurately than eyeballing numbers.

Additionally, I think this means that Michael's original knock clearance he thought he had has shrunk at least several degrees.
 
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I'm curious if the standard fuel pump has any issue with ethanol fuel. Im more concerned about damaging fuel components. Or are you running electric pumps?
 
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I'm running SDS EFI - all automotive fuel parts. No issues with alcohol or any other junk in the fuel.

I'm running SDS as well on my IO360 - just a datapoint on the timing perspective so that you can test on your own engine. I run Wal-Mart grade 91 premium (with ethanol) in my engine and it's quite happy - but in early testing I played with inducing detonation at advanced timing, which the SDS is quite happy to do with the LOP function. I found that at low altitude and full power and a warm engine, I could induce mild detonation in the "Red Box" pretty much at will with the timing set at 31BTDC. I have set my maximum advance at 29BTDC now, and have a couple more degrees taken out at higher manifold pressures to give myself some margin. You'll obviously want to test your engine for your own parameters.
 
Thanks for that info Greg. I still have some testing to do when LOP, but based on my dyno testing, my timing is pulled all the way back to 17 degrees at typical TO power. It doesn't even advance to the dataplate value of 25 until around 6000 MSL.

Clearly, the auto fuel has less detonation margin than 100LL, but the timing changes afforded by the SDS system allow me to buy back some of that margin. The piston jets pull a lot of heat off the piston crown too (and that's why I installed them).
 
Thanks for that info Greg. I still have some testing to do when LOP, but based on my dyno testing, my timing is pulled all the way back to 17 degrees at typical TO power. It doesn't even advance to the dataplate value of 25 until around 6000 MSL.

Clearly, the auto fuel has less detonation margin than 100LL, but the timing changes afforded by the SDS system allow me to buy back some of that margin. The piston jets pull a lot of heat off the piston crown too (and that's why I installed them).

Yep - love the piston squirters, I'm using them too. My takeoff timing is currently 21 degrees at 2700' MSL.
 
Were you able to keep your original sized oil cooler? Industry info seems to indicate going "up one size" cooler with squirters - and my results seem to confirm that. My CHT's are very much in control, even in the incredible heat we've had this Summer, but my oil temps are marginal. Clearly, the oil jets move a lot of heat off the pistons, and my once adequate oil cooler (pre jet mod) is having a tough time today.
 
Were you able to keep your original sized oil cooler? Industry info seems to indicate going "up one size" cooler with squirters - and my results seem to confirm that. My CHT's are very much in control, even in the incredible heat we've had this Summer, but my oil temps are marginal. Clearly, the oil jets move a lot of heat off the pistons, and my once adequate oil cooler (pre jet mod) is having a tough time today.

No - I had to bump it up early in Phase I to a 10-row cooler with 4" scat hose feeding air to it. My cooler is mounted on the firewall behind cylinder #4. My experience is just as yours - I don't have trouble keeping CHT's under control, but the oil needs a lot more cooling.
 
I'm curious if the standard fuel pump has any issue with ethanol fuel. Im more concerned about damaging fuel components. Or are you running electric pumps?

I have been running mogas 91 E10 for the last 600 hours over a period of 8 years with an AFP fuel injection and mechanical fuel pump in my RV-10.
As of this writing I have not been able to detect any issues with any component of the fuel system including the mechanical fuel pump.
I did exhaustive testing early on to make sure my engine would run on mogas in all regimens of flight without any limitations.
There is one noticeable difference between 91 mogas and 100 LL that I don't quite understand. I am able to run .8 gph leaner on mogas than on 100LL before the engine starts to run "rough". I can see no noticeable difference in speed or temperature in my equipment when switching from avgas to mogas.
A slight increase in EGT (25 degrees) on mogas perhaps.
Oil temp is rock steady at 180F and my cylinders run well below 350F except for operating on the hottest days in climb but even then they rarely approach 400F by the end of a climb with a full load.
Glad some people still consider mogas, it is my alternate fuel and with that in mind, I don't care when or if the "gumbiment" ever gets around to ban 100LL.
 
A thing to keep in mind with different fuels is that in the end (assuming you've addressed all the chemical compatibilities upstream of the cylinder), this all comes down to a chemical reaction between a fuel MIX and oxygen in the cylinder, and the precise timing of when it starts to combust, and how fast that flame front moves. 100LL is a much "cleaner" mix of hydrocarbons than is automotive fuel - meaning fewer variables of the chemical chain. 100LL is much closer (still not close, but closER) to a pure single hydrocarbon being burned with oxygen in the cylinder, which is ultimately predictable. The further you vary from that, the fuzzier the results become.

At the refinery, there are a certain set of standards required to produce an acceptable mix of fuel for a particular product range - defined by the ASTM test procedure set for that fuel. Some are a big target, some are small - and all are defined by temperatures and pressures in the refining process, each of which is subject to the measurement error of the device used for measurement. There is no given single set of hydrocarbon component mixtures that will give 100LL or Mogas or 91/93E10 standards - and some states even vary that mixture by summer/winter and even by altitude of the filling station for mountainous states. You get a fuel that meets the standards - though one time it might be in the high left hand quadrant, and the next time it might be in the low right hand quadrant.

We like to think that "x" amount of fuel with "y" amount of air solves all our problems - but it just ain't so. 100LL comes close, but mogas or E10 mixes take all bets off the table, especially with seasonable variability and oxygenate additives, which are variable by state and calendar. Any time you are taking a load of "non-100LL" fuel out of the tanks you are getting a different mixture than you did the last time, and some supervisory oversight over the feeding and wellbeing of your power plant is required.

I don't know how to say this without hurting some feelings, but this is not an area to be trod by those who are fearful or afraid of RTFM. If you want to play here, knowledge is not just handy - it's required. You'll melt pistons if you aren't careful - and sometimes even if you are. What you are buying for your dollar by fueling with 100LL is sure knowledge of where you are on the field.

Yes it's fun - and there is a feeling about cruising at oxygen levels on sub-$2 fuel that can't quite be described... but.... "here there be dragons"

Caution is advised.
 
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