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Lycoming “O” versus “IO” Approved Automotive Fuels

Kuhtenia

Active Member
Came across Lycoming Service Instruction No. 1070AB (dated 4/8/2020), “Specified Fuels for Spark-Ignited Gasoline Aircraft Engine Models” (https://www.lycoming.com/sites/default/files/SI1070AB Specified Fuels.pdf). [Document is technical and irrespective of separate FAA certification considerations.]

Question is this – For automotive fuels that meet Lycoming specs, Table 3 of the document ("Fuels and Fuel Grades Approved for Use in Lycoming Engine Models") includes many of the “O” variety engines, but relatively fewer of the “IO” engines. Is there a particular technical reason for this, is it more about what has actually been tested over the years, something else?
 
Compression Ratio

Came across Lycoming Service Instruction No. 1070AB (dated 4/8/2020), “Specified Fuels for Spark-Ignited Gasoline Aircraft Engine Models” (https://www.lycoming.com/sites/default/files/SI1070AB Specified Fuels.pdf). [Document is technical and irrespective of separate FAA certification considerations.]

Question is this – For automotive fuels that meet Lycoming specs, Table 3 of the document ("Fuels and Fuel Grades Approved for Use in Lycoming Engine Models") includes many of the “O” variety engines, but relatively fewer of the “IO” engines. Is there a particular technical reason for this, is it more about what has actually been tested over the years, something else?

Most if the IO-360 series excluded from the auto fuel columns have higher compression ratios (8.7:1) than the O-360 (8.5:1). Th IO-390 which is also excluded has an 8.9:1 CR.

I’m not too familiar with the -540 models but I assume the IO models that are excluded is also due to higher compression ratio.

Skylor
 
Automotive fuel

It is all about the copression ratio and the different combustion cambers.
If you are looking at the IO-360 there are two completely different engines.
1. the angle valve IO-360 compression ratio 8,7, 200 hp or more.
2. the parallell valve IO-360 compression ratio 8,5, 180 hp.
I have the parallell valve IO-360 compresson ratio 8,5, 180 hp and
using autofuel for more than 10 years with no problems.
This engine is also sold as the O-360 with carburator 180 hp.

Good luck
 
By limiting timing at high MAP, you can safely run Mogas in any naturally aspirated Lycoming with standard compression ratios. You may have a loss in maximum power at higher manifold pressures but cruise performance should be unaffected.

This is best addressed with an EI having user programmable timing curves rather than fixed timing mags.
 
The Peterson STCs (one airframe, one engine) I applied to my Cherokee 140 (O-320-E2D) stated the primary concern with MoGas is vapor lock, which could be dealt with by turning on the boost pump.
 
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Depends.

Haven’t read that particular STC; however, usually one each is required for airframe and power plant. Making a low wing airframe acceptable for Mogas is typically not a complex undertaking. A high wing often takes zero modification to hardware or operation. It is important to consider/differentiate between the two; AF and PP.
 
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Haven’t read that particular STC; however, usually one each is required for airframe and power plant. Making a low wing airframe acceptable for Mogas is typically not a complex undertaking. A high wing often takes zero modification to hardware or operation. It is important to consider/differentiate between the two; AF and PP.

For mogas, that's generally true - but for car gas with ethanol, caution is advised. The ethanol blends will not play nicely with natural rubber compounds in the fuel system like O-rings, hoses, bladders or diaphragms. You'll need to replace those with Viton or Teflon.

It's not actually the ethanol that attacks the rubber - but ethanol is an octane booster, which allows the refinery to start with a lower-quality base stock and still get to the same target octane after they add the ethanol. That lower-quality base stock has more aromatic hydrocarbon rings like benzene and related compounds, and those are what will attack the rubber.
 
Mogas with ethanol

Lycoming introduced fluorosilicone rubber in their fuelinjection 1987.
This is compatible with ethanol blended fuel.
I have a IO-360 parallel valve compression ratio 8,5, 180 hp.
Previously I had the Lazar electronic magnetoes with large spark advanse running with no problem. Now I have the Surefly (2) with 25 deg + spark advance up to 38 deg working fine.
I have not seen any reduction in power or increased fuel consumption.
There is about +5 F higher CHT compared to 100 LL.
CHT is around 320-350 F and never goes higher than 400 F.
Cylinders and pistons are very clean.
Mogas in EU may not be produced to the same standard as Mogas in the US.

Good luck
 
For mogas, that's generally true - but for car gas with ethanol, caution is advised. The ethanol blends will not play nicely with natural rubber compounds in the fuel system like O-rings, hoses, bladders or diaphragms. You'll need to replace those with Viton or Teflon.

It's not actually the ethanol that attacks the rubber - but ethanol is an octane booster, which allows the refinery to start with a lower-quality base stock and still get to the same target octane after they add the ethanol. That lower-quality base stock has more aromatic hydrocarbon rings like benzene and related compounds, and those are what will attack the rubber.

Yes Sir. My brief and poorly worded response was specific to the RVP property. The typical engine in the subject Piper has no problem with the lower Octane. As I’m sure you know, that’s only the half of it. Most people who’ve been around long enough have at least heard of improper use of Mogas/car gas/pump gas/. sometimes with very bad consequences. Here in the states (and probably everywhere) fuel is still the cheapest thing you put in an aircraft.
 
Gasoline boils at a relatively low temperature. If the gas boils problems result. Pumps move liquid, not vapors for example.

Car gas standards vary from state to state, but 100LL manufactured to standards that control things like its boiling point. More research is needed to know what that point is, but 150F for autos was one result the trusted internet spit out. Winter blend fuels will have a different (lower) boiling point than summer blends.

So keep things that might be a heat sink like the electric fuel pump, and gascolator cool and out of the engine compartment to help make an installation tolerate auto-fuel. Also keep your fuel line lengths as short as possible and away from exhaust. A 3/8" DIA line will move fuel faster over its length than a 1/2" line. You might also insulate your fuel lines and your fuel totalizer hardware, and place a cooling shroud on your engine driven fuel pump.

Maybe this is the reason why one person's aircraft burns auto fuel with no problem yet another one has issues. So, its more to it than compression ratio to determine rubber compounds.
 
Maybe this is the reason why one person's aircraft burns auto fuel with no problem yet another one has issues.

Absolutely true - and I'm happy to share the path that I took to enlightenment and running 91E10 - but that really should be another thread.
 
Just say no

To 100LL..... recently running pump gas 80% of the time, 93 octane alcohol free in my 1991 O320 D1A 160 hp. I convert my fuel cost savings to flying faster burning more fuel. AMERICA :)
 
100LL

Please bring on the 100 unleaded av gas. Cant wait for the lower maintenance (no plug fouling, less slug in oil, etc).
 
Browsing the internet yesterday, I stumbled across this document from Lycoming. SL270 titled "Extended Maintenance Intervals for Spark-Ignited Engines Operated on Unleaded Fuels".

Seems Lycoming is happy to recommend doubling the oil change hour internal when running entirely on unleaded fuel. Still need to change the filter at 50hrs, but the oil can keep going.

Yes G100ul costs more. Does that offset the reduced cost of oil changes? I suppose it's very specific to each engine and how much fuel and oil it consumes. An oil guzzling O-235 will be a different story than a clean TIO-540 chugging back 20gph.
 

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My foray into running 91 non-ox in my IO-360 ended after about 15 minutes. I had 100 LL in the left tank and mogas in the right. I took off with 100 LL and tried cruising on mogas. The first thing I noticed was a drop in fuel pressure from 24 psi to 7. I turned on the electric fuel pump. This happened 4 times. During the time in between loss of fuel pressure I tried leaning. The engine did not seem to lean as well. EGT was around 1300 and CHT 290-320.

I returned home and drained the right tank. Then I flew to a local airport to fill up with 100 LL. I did verify the right tank filled with 18 gallons.

My mechanical fuel pump is about 10 years old. It pumps 100 LL at 24 psi. Most of the time the pressure is steady but occasionally there are brief fluctuations to as low as 18 psi. I may replace the pump but I am hesitant to try the mogas experiment again.
 
Browsing the internet yesterday, I stumbled across this document from Lycoming. SL270 titled "Extended Maintenance Intervals for Spark-Ignited Engines Operated on Unleaded Fuels".

Seems Lycoming is happy to recommend doubling the oil change hour internal when running entirely on unleaded fuel. Still need to change the filter at 50hrs, but the oil can keep going.

Yes G100ul costs more. Does that offset the reduced cost of oil changes? I suppose it's very specific to each engine and how much fuel and oil it consumes. An oil guzzling O-235 will be a different story than a clean TIO-540 chugging back 20gph.

Lycoming still recommended oil change at four mos.
 
... I may replace the pump but I am hesitant to try the mogas experiment again.

Sounds like you experienced some vapor lock. I'd have a look at the tubing between the tank and your fuel pump. There may be a place where the flow is constrained, and it's causing the fuel to change state from liquid to gas.
 
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Sounds like you experienced some vapor lock. I'd have a look at the tubing between the tank and your fuel pump. There may be a place where the flow is constrained, and it's causing the fuel to change state from liquid to gas.

I was surprised to have vapor lock while in cruise. I plan to measure fuel flow from the left and right tanks using the boost pump to see if they differ.
 
I was surprised to have vapor lock while in cruise. I plan to measure fuel flow from the left and right tanks using the boost pump to see if they differ.

Reach out to Airguy, posted on Feb 5th. I'd be interested in hearing what he has to say.

As you look over your installation think "Hot fuel. Look where the fuel passes and it distance between warmth or things that touch fuel might get warm. As the fuel heats up it will vaporize at some point (or the fuel pressure will drop which lowers the boiling point) So a 90 degree AN fitting in a fuel line might be a weakish spot.
 
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