The devil is in the details with unleaded avgas
MAY 4, 2015 BY BEN VISSER 1 COMMENT
In the 1970s, the automotive world switched from leaded to unleaded fuels and the oil companies did a lot of research on knocking and how to prevent it. One of the big projects involved octane requirement increase (ORI).
In this program, cars were rated for octane requirement when new and then every 2,000 miles. The octane requirement increased until it leveled off at about 20,000 miles.
Usually the ORI was about 6 to 10 numbers. We then removed the heads and cleaned the deposits off all of the pistons and re-checked the octane requirement. The requirement usually dropped a number or two.
We then removed the heads and cleaned the heads and valve faces. When we re-checked the octane requirement, it had dropped another number or two, but still was not back to new levels.
The really interesting part came when we removed the head a third time, took the intake valves out, cleaned the deposits off the under head part of the valve, but not the seat. When the engines were reassembled, the octane requirement of almost every one dropped a number or two.
This means that the change in intake flow pattern caused by the intake valve tulip deposits were enough to increase the octane requirement of the engine by 1 or 2 numbers.
The ORI for passenger car engines is significantly higher than for aircraft engines because of ash-containing oils and non-bottomed fuels. But the ORI for aircraft engines can still be significant.
A second interesting point was that when we would purchase a pair of identical cars, their octane requirement was usually a number or two different. So here we have two identical engines produced on a mass production line, but when they got into the field, their octane requirement was different.
In aviation, we have a lot of different people assembling engines, and we add an additional variable with PMA parts. The FAA is testing a particular model of aircraft engine with the new unleaded fuels and expecting that this test will cover all of the combinations of original and PMA parts that will be seen in the field. Will a cylinder head with a different casting method and port design produce the same octane requirement as the original manufacturer’s parts?
To compound these problems, pilots need to adjust the mixture strength during flight. In an operating aircraft engine, the carburetor (or fuel injection) does not supply a completely homogenous mixture to all cylinders.
Most pilots know the proper way to lean out their mixture strength, but many others do not even know what a carburetor is, let alone how to lean it properly.
At the stoichiometric air/fuel ratio, the temperature and pressure is at the maximum and continued operation here will lead to knocking and possible engine damage.
In the real world, pilots will lean until the engine runs rough and then richen up until it smooths out.
With the problems described above, which can change the A/F ratio distribution in a given engine, plus improper leaning procedures and a very small octane safety margin with unleaded fuels, this can lead to one or more cylinders operating at a dangerously high temperature.
Another problem is that a lot of planes are equipped with single point exhaust gas temperature gauges. This assumes that one cylinder is always the hottest and as long as the pilot leans to keep this particular cylinder cool enough to live, the rest of the cylinders will be OK.
But if deposits or different parts change the cylinder to cylinder distribution, will the “chosen” cylinder still be the hottest? Will it be necessary for all PMA parts to be tested for their effect on octane requirement of the host engine?
In the future, if an engine has a knocking-related failure on an unleaded avgas, who will the lawyers sue? Will it be the fuel supplier, the FBO, the engine builder, the PMA parts supplier, or the FAA?
It really is a trick question because we know they will sue all of them. The devil is always in the details.
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