G100UL™ Avgas vs 100LL Operating Characteristics

Two short video clips showing the small differences in operating characteristics as between 100LL and G100UL™ unleaded Avgas. (August 4, 2011)

Part One

This is a very short (41 second) video clip of the AVIDYNE digital engine screen display in the TN SR 22 Cirrus (Experimental N223TN).

At the beginning, the engine has been running on G100UL™ Avgas and the EGT display has been "normalized" to a zero baseline on all EGT and the TIT values.

The engine is operating at 30" MAP x ~2520 RPM and lean of peak at about 16.9 to 17.1 gph. The aircraft is at 5,500 feet. The density altitude is about 7600 feet and the OAT was around 79°F. (Hot day).

The first thing that is shown is the fuel selector sitting on the right hand position pointing to the G100UL™ unleaded Avgas wing tank.

At about 5 seconds into the short video clip, the fuel is switched and then the camera is re-directed to show the "normalized" screen EGT display.

It takes about 15 to 20 seconds for the fuel in the lines to "clean up". At that point, you will see the EGTs begin to drop as the 100LL fuel reaches the cylinders.

The EGTs drop because the Air / Fuel ratio (which is already "lean") becomes slightly leaner due to the slightly lower (~ 3%) volumetric energy density of the 100LL fuel as compared to the G100UL™ unleaded Avgas.

The changes are on the order of 15°F to 20°F in EGT and TIT value.

Part Two

In this video clip the process is simply reversed and the fuel selector is again changed, this time from 100LL back to G100UL™ unleaded avgas. At that point, after fuel clean up, the EGTs return to their original normalized baseline.

Also, of note, the fuel in the right tank is "mongrel". That is, it is not "pure G100UL™ unleaded Avgas". It has a smaller fraction of 100LL that was added during a recent cross country. It is around 2/3rds to 3/4 G100UL™ Avgas. Thus, when a comparison is made between 100LL and "pure" G100UL™ Avgas, the delta in the EGTs should be somewhat larger than is observed in these video clips. In that case, the delta EGT values would increase to around 20-30°F. This testing was done, in part, because there was some questions raised about the "fungible" nature of the two fuels. As shown in these two clips, the mixture of the two fuels is transparent to the operation of the engine.


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