In my last column, I did an update on the status of the 100 motor octane unleaded fuel programs by the Piston Aviation Fuels Initiative (PAFI) steering group members, as presented at Oshkosh. In this column I would like to talk about my visit with George Braly from General Aviation Modifications Inc. (GAMI) about the company’s unleaded avgas development, which is not part of the PAFI program.
Talking to the folks at GAMI is always a pleasure because they seem to understand the “show” and are doing some basic research into the actual working of an aircraft engine. By comparison, most of the other parties are just aiming at a 100 motor octane aviation fuel.
As an example, George explained that they have a 97 motor octane unleaded candidate that operates with no detectable detonation in their aircraft test engines. But they have also tested 100 and 102 motor octane candidate fuels that result in significant detonation in the same aircraft engine tests.
We discussed the many reasons for this lack of correlation between the motor octane of a fuel and its performance in actual aircraft engines.
We also agreed that this is one of the major problems with developing a new unleaded aviation fuel because pilots will automatically buy the fuel with the highest motor octane in the belief that it will perform better in their aircraft engines.
We also discussed the correlation between motor octane and the original lean rating octane number, which was correlated to actual aircraft engine detonation levels.
Many years ago, avgas sales were declining due to commercial aircraft switching over to jet engines. In an effort to reduce costs, the industry decided to stop supporting the lean rating test method and used a chart to correlate a motor octane rating test result to the lean rating.
The biggest problem here is that the industry ran a series of motor octane and lean rating tests on fuels to develop the chart that is included in the ASTM D-910 specification. The concern is that they only tested fuels made primarily from alkylate. They have never run tests to examine the correlation of the motor octane rating to the lean rating on unconventional fuels like the 100 motor octane candidate fuels.
George also asked me if knock and detonation were the same thing. I feel that they are not. Detonation is any auto ignition of end gases near or at the peak pressure during the power stroke. But if the detonation is significant enough that it caused an audible response, then it is knock.
So when is detonation or knock harmful to an engine and how do we determine where the knock limited power should be set?
If one monitors the pressure versus crank angle in an engine, you will see that the pressure goes up during the compression stroke, then goes way up with the ignition occurrence. Now if there is detonation, you will notice some small static or mini spikes at or near the peak. As the detonation increases, the size of the spikes increase until you get into knock.
So where do you rate the engine: At the first appearance of static or when the spikes get to be a certain percentage of the normal peak pressure?
The reason this is critical is that many engines — like large radials — will need to have a limited boost to prevent knocking or the engines will need to be modified.
But how do you set the limit or determine what modifications are necessary? Because of the high noise level in aircraft, you will usually not be able to hear knock until after some engine damage has occurred. And you cannot put pressure sensors in every cylinder of every engine type and operate under almost every possible condition.
It was interesting to talk to the GAMI people. They do not have all of the answers, but at least they understand that they do not have all of the answers.