Is your plane faster with 100ll?

[CTSW Bob]

Kicking around a thought…

 

I normally fly using 93 octane non-ethanol fuel.  The other week I flew into an airport and had to buy some av gas in order to waive the facility fee.  On the way back, besides a bit of a tail wind, I was booking it at roughly 130+ knots.  So I was wondering if our planes fly faster with the higher octane av gas versus auto gas.  Do the Rotax engines compensate for higher octane by advancing the timing?  
 

Curious in Georgia

[Skunkworks85]

12 hours ago, CTSW Bob said:

Kicking around a thought…

 

I normally fly using 93 octane non-ethanol fuel.  The other week I flew into an airport and had to buy some av gas in order to waive the facility fee.  On the way back, besides a bit of a tail wind, I was booking it at roughly 130+ knots.  So I was wondering if our planes fly faster with the higher octane av gas versus auto gas.  Do the Rotax engines compensate for higher octane by advancing the timing?  
 

Curious in Georgia

No

[FlyingMonkey]

12 hours ago, CTSW Bob said:

Kicking around a thought…

 

I normally fly using 93 octane non-ethanol fuel.  The other week I flew into an airport and had to buy some av gas in order to waive the facility fee.  On the way back, besides a bit of a tail wind, I was booking it at roughly 130+ knots.  So I was wondering if our planes fly faster with the higher octane av gas versus auto gas.  Do the Rotax engines compensate for higher octane by advancing the timing?  
 

Curious in Georgia

Higher octane means more resistance to detonation…higher octane fuel actually has less fuel energy.  The best power for any engine comes from the *minimum* octane fuel that the engine will run on without detonation.
 

Higher octane = higher power is a myth, unless the engine is detonating at lower octane.

[FlyingMonkey]

2 minutes ago, Skunkworks85 said:

No

Correct and succinct, lol.
 

I recently saw a 19 year old kid with a brand new fully loaded F-150 putting 100LL from the airport pumps in his truck.  I asked him what he was doing and he explained he wanted more power and always come to the airport for gas.  I gently explained why he wasn’t going to get more power, and that he likely has destroyed the emissions system on his vehicle with leaded gas.

[Eddie Cesnalis]

4 hours ago, FlyingMonkey said:

Higher octane means more resistance to detonation…higher octane fuel actually has less fuel energy.  The best power for any engine comes from the *minimum* octane fuel that the engine will run on without detonation.
 

Higher octane = higher power is a myth, unless the engine is detonating at lower octane.

But, but... here's an article saying if I upgrade to premium my horsepower goes up in my Ford Ranger from 270 to 300 https://tfltruck.com/2018/12/can-the-2019-ford-ranger-make-300-hp-on-premium-fuel/

 

[Anticept]

Stuff like that is true if you have an ECU that changes timing in response to knocking.

[GlennM]

It would need knock sensors, also.

[Anticept]

Edited my comment because the iS does change timing, but not for knocking. It is a possible future possibility that rotax put in the plugs and support for if the regs change to allow it, but right now it's not a thing and isn't enabled.

[CTSW Bob]

Thank you for the replies.  I know from my motorcycle days you could advance the timing to take advantage of premium gas (carbed engine) and supposedly make more power.  I’ve heard one of my cars could not hit the speed limiter on 87 but I (I mean they) could hit it no problem with premium 93.  
 

laughed at the “no” response too!  

[Skunkworks85]

22 hours ago, Anticept said:

Stuff like that is true if you have an ECU that changes timing in response to knocking.

I didn't read the article, But yes, Timing advance based on knock sensor feedback, or more boost, again, based on knock sensor feedback.

[Skunkworks85]

On 12/8/2022 at 6:23 AM, FlyingMonkey said:

Higher octane means more resistance to detonation…higher octane fuel actually has less fuel energy.  The best power for any engine comes from the *minimum* octane fuel that the engine will run on without detonation.
 

Higher octane = higher power is a myth, unless the engine is detonating at lower octane.

One point to elaborate on here is the comment "higher octane fuel actually has less fuel energy" . This is *sort of* true. The proper term is "less energy density", which means that is has less BTU per unit volume. (Or Mj/kg) HOWEVER, that does not mean less power. Let me explain. 

Most premium fuels are oxygenated fuels, Most are oxygenated by blending alcohols. Alcohols have oxygen molecules, This means that the stoichiometric ratio of the fuel is less. In doing so, the Fuel brings its own oxygen to the party, Therefore you rely less on the engine to supply oxygen and can burn more fuel per cycle.  Hence more power. 

This is the primary reason that top fuel dragsters use methanol, Methanol is something like 40% less energy dense than gasoline, But if the engine can burn 80% more of it per combustion event, your net result is more power. (additionally, Alcohols also have a much lower vapor pressure which allow more dynamic compression ratios due to cooling effects)

All of above does not apply to the OP original question since all of the above statements need more advanced fuel management systems than exist in 99.9% of aircraft.

[FlyingMonkey]

2 hours ago, Skunkworks85 said:

One point to elaborate on here is the comment "higher octane fuel actually has less fuel energy" . This is *sort of* true. The proper term is "less energy density", which means that is has less BTU per unit volume. (Or Mj/kg) HOWEVER, that does not mean less power. Let me explain. 

Most premium fuels are oxygenated fuels, Most are oxygenated by blending alcohols. Alcohols have oxygen molecules, This means that the stoichiometric ratio of the fuel is less. In doing so, the Fuel brings its own oxygen to the party, Therefore you rely less on the engine to supply oxygen and can burn more fuel per cycle.  Hence more power. 

This is the primary reason that top fuel dragsters use methanol, Methanol is something like 40% less energy dense than gasoline, But if the engine can burn 80% more of it per combustion event, your net result is more power. (additionally, Alcohols also have a much lower vapor pressure which allow more dynamic compression ratios due to cooling effects)

All of above does not apply to the OP original question since all of the above statements need more advanced fuel management systems than exist in 99.9% of aircraft.

All that is true...but since I was talking about the fuel itself and not the capabilities of how much fuel a given engine can burn to make power, I stand by my statement. 

Fuel energy density is what I was referencing.