100LL vs 91 octane

[blandess]

Hello All,
i just read an article in AOPA Pilot about the ongoing debate of the future of 100LL. I have a question that came to me on the way to work this morning and would like all your input. My former supervisor built a 550hp Factory Five GTM, normally aspirated. Chevy's ZR1 'Vette puts out 638 supercharged horsepower, Ferrari has 612hp V12's...all on pump gas...

Imagine driving one of these cars to Flagstaff, AZ (7000' MSL), or somewhere like it, and smashing the pedal. I can reasonably assume it will pin you to your seat. I understand that these cars are not operated on the brink of redline all the time as in aircraft engines but HOW do engineers say that the high compression twin-turbo Columbia's, Cirrus', Mooney's etc. are prone to detonation when those engines don't generate half as much power as the above mentioned engines?

My apologies for the lack of RV subjects but if 100LL goes away I would like to know there will be some kind of fuel to propel the beast. Oh, one more thing while I'm ranting...what about turbines in an RV? I remember a company that had one in a -4 but went under. There will probably always be jet fuel so why not have a small one in an RV?

Phew...I feel better...

[Kyle Boatright]

Originally Posted by blandess

Hello All,
i just read an article in AOPA Pilot about the ongoing debate of the future of 100LL. I have a question that came to me on the way to work this morning and would like all your input. My former supervisor built a 550hp Factory Five GTM, normally aspirated. Chevy's ZR1 'Vette puts out 638 supercharged horsepower, Ferrari has 612hp V12's...all on pump gas...

Imagine driving one of these cars to Flagstaff, AZ (7000' MSL), or somewhere like it, and smashing the pedal. I can reasonably assume it will pin you to your seat. I understand that these cars are not operated on the brink of redline all the time as in aircraft engines but HOW do engineers say that the high compression twin-turbo Columbia's, Cirrus', Mooney's etc. are prone to detonation when those engines don't generate half as much power as the above mentioned engines?

My apologies for the lack of RV subjects but if 100LL goes away I would like to know there will be some kind of fuel to propel the beast. Oh, one more thing while I'm ranting...what about turbines in an RV? I remember a company that had one in a -4 but went under. There will probably always be jet fuel so why not have a small one in an RV?

Phew...I feel better...

I'll give it a stab...

Our engines have very large cylinder volumes and operate at low RPM. In a large cylinder the flame front takes more time to travel the length and breadth of the cylinder than on a small cylinder like in most auto engines. Which means the compressed air/fuel mixture in the most distant corners of the combustion chamber has to wait for a relatively long time before the flame front gets there. Hold a heated, compressed, fuel/air mixture at that temperature and pressure for too long and it detonates. This is also related to the fact that the cylinder pressure is ever-increasing until the pretty late in the combustion process, so your 8:5:1 compression cylinder may be (effectively) 12:1 before the flame front reaches the fuel/air mix in the far corner of the cylinder

To avoid the problem, you either spin the engine faster while making the same amount of power, reduce the cylinder volume but add cylinders, increase the cylinder volume while decreasing compression, or you increase the octane rating.

And since we want to maximize the low RPM torque from our engines, we want slow turning engines with large cylinder volumes. Which, unfortunately, are more sensitive to octane or lack thereof.

As to the turbine issue, nobody has developed a small, inexpensive, fuel efficient turbine. And due to the engineering and production realities, I doubt you'll ever see an inexpensive, fuel efficient (at our altitudes) turbine. Small, we can do...

[Ted Johns]

...HOW do engineers say that the high compression twin-turbo Columbia's, Cirrus', Mooney's etc. are prone to detonation when those engines don't generate half as much power as the above mentioned engines?

Simple. They flap their lips up and down, moving their tongue as needed.

This is a complicated subject, but in a nutshell, a twin-turbo Continental or Lycoming is probably putting out a higher BMEP (brake mean effective pressure) during the firing event than a Chevy ZR1. Couple that with firing event twice as long (less than half the RPM) and a combustion surface temperature twice as high (air cooled), and you need 100LL.

If you wound the twin-turbo Continental to 6000RPM, you would have well over 600hp. Until it broke.

[Geico266]

I've been running 92 octane fuel in my planes for years and hundreds of hours. No problems.

[the_other_dougreeves]

Let's think about what automotive engines have:

* Good fuel distribution between cylinders using multi-point or direct injection
* Good fuel/air mixture control using mass air flow (MAF) sensors. Continuously controled via the ECU.
* Multiple fuel mappings, controlled by the ECU
* Variable spark timing, continuously controlled by the ECU
* Variable valve timing, either step-type of continuously variable, controlled by the ECU
* Knock sensors, giving feedback to the ECU
* Electronic ignition

Most airplane engines don't have any of these (if your does, great).

TODR

[David-aviator]

Originally Posted by Geico266

I've been running 92 octane fuel in my planes for years and hundreds of hours. No problems.

With or without ethanol, Larry?

I'd like to consider mogas in the IO360 but all pump gas around here has ethanol. The one component in the current fuel system I am not sure about with regard to ethanol is the engine drive fuel pump. With the Subby it did not matter. I ran fuel with ethanol for 4 years and it was ok.

[Geico266]

I have access to 92 octane no lead around here.

If you have a 100 / 500 gallon tank you might want to call the distributor and see if you can buy bulk without ethanol. The distributors cannot send ethanol through the pipeline system so if you may be able to buy gas before ethanol is added. It's worth a phone call. Call a bulk supplier and tell him what you are doing. Just an idea.

There are a couple of VAF guys that run ethanol gas with no problems, but I personally cannot recommend it because I have no experience with it.

[pvans]

Our typical RV engines ( without turbos) only need 100LL during the first few minutes while we climb up to 5000 feet or so, after that unleaded would work great.

How about a small tank used for take off only, and a larger tank or two of common gas for cruise and descent?

Ok, more pilot workload, more chance of problems, and it does not solve the problem for the users of the bulk of 100LL, which are the big bore Continental and Lycomings used on aircraft like the Navajos, Cessna Turbo 210s, etc.

Sooner or later we will be faced with fuel changes, there is no doubt about that.

[frankh]

No need to drag it all back up here as this has been discussed at length.

But the bottom line is your Lyc with reasonable CR (8.5:1 is the accepted upper limit) will be perfectly fine on premium grade unleaded with or without ethanol.

It will NEVER need 100LL.

Now here is the health warning..with the standard fuel pump setup it will be more prone to vapour lock issues, and there is some debate about phase seperation but I have never expereinced to 15,500ft at least.

I have developed and electric only wingroot mounted pump setup and dispensed with the mecahnical fuel pump.

Lots of info in the archives on how to make one if your interested.

other than that the engine will run lean of peak and you can use electronic ignitions to advance the spark at altitude and get some really good economy.

My IO360 has not seen 1ooLL for a couple of hundred hours

Frank

[frankh]

I honestly don't know if the (ghastly) mechanical fuel pump will tolerate ethanol or not..Purely personally i think they should go to the same place my vacuum pump went..I.e the trash can...

Frank