Using higher than recommended octane fuel can result in incomplete combustion?

[meep]

Technically, there might be some basis for truth in that article but I really don’t know enough to state one way or another. Higher octane fuel does have a slower flame speed, which is really easy to understand if you look at it this way. Higher octane fuel requires a higher ignition temp, which is why it is more resistant to detonation. Since the gas/air charge has to heat more prior to lighting off, it takes just a scoche longer for the ignition wave to propagate. Does it have any meaningful difference in reality? I don’t know.

it’s plausible that the flame propagates faster when under more compression, since the molecules are more tightly packed, suggesting it’s less of a problem in higher compression motors.

I’ll also note that if you take the headers off an engine and watch it run, fuel is still being burned for another 6” outside of the heads, as each exhaust port shoots out blue flame with every exhaust stroke.

 

[MRC01]

"

... BMEP is essentially torque per unit volume of displacement. ...

Close, but not exactly. BMEP is directly proportional to torque per unit displacement. ...

If your point is that the only difference is units conversion, then yes I agree. That's why I said "essentially".
Note: In the same way that power is torque * rotational speed, or that force is mass * acceleration. It captures the qualitative relationship which is important conceptually, but to solve practical problems you have to do units conversion which adds constants to the equations.

 

[Skippy722]

I’ve always been an advocate for using the octane called for by the OEM. My understanding higher octane = slower to ignite which can be beneficial in many(FI/Euro engine/small engine) cases. Basically, for higher compression. As the old tuner’s adage, lean is mean but low octane increases the change of blowing holes or cracking ring lands in your pistons once you add boost or compression.

The Japanese have always said higher octane for better performance but “regular” octane in the US is adequate. In the US, our super/premium is Europe’s regular.

Europe’s 95 is about 89-90 AKI, or mid grade in the States. 98 RON comes out to our 93 premium.

 

[ZeeOSix]

The main difference is torque and, as a result, power.

It all starts with the "big bang".

 

[RDY4WAR]

The octane rating is really just a representation of resistance to auto-ignition with nothing beyond that. It has no real measurable effect on burn rate. Once the mixture is lit, the octane rating is no longer relevant.

Look at gasoline vs kerosene. Kerosene has a very low octane rating of ~15 which is reflective of its low auto-ignition temperature (~420°F) compared to pump gas (~540°F). Yet, kerosene has a slower burn rate which is reflective of its higher flash point (~100°F vs -45°F), higher specific gravity (0.820 vs 0.740), higher boiling point (~400°F vs ~120°F @ 10% distillation) and higher viscosity (2.39 cSt vs 0.600 cSt @ 68°F).

Methane has a MON of >120 but burns much faster than gasoline reflective of its much lower specific gravity (0.55 vs 0.74), flash point (-306°F vs -45°F), and boiling point (-258°F vs 120°F @ 10%).

 

[CBR.worm]

That F150 in the charts had an incredible gain in real-world performance. I'd be interested in seeing the dyno curves of that engine with the two fuels. The performance difference indicates that there must be much more area under the curve w/ the higher octane gas - more than just the additional HP/TQ numbers would indicate. That would surely be noticeable in everyday driving. Seeing results like that makes me wish I had a well-managed turbo or two. Most of the time, I appreciate the simplicity of a port injected 5 liter 32 valve V8 w/ variable valve timing and lift. In the past, I never would have thought my old lump would be simple compared to today's engines.

 

[slacktide_bitog]

In the US, our super/premium is Europe’s regular.

Not correct. The scales are different.

US scale	European RON scale
87	91
91	95
93	98

 

[nthach]

Not correct. The scales are different.

US scale	European RON scale
87	91
91	95
93	98

I always thought US 91/92 was Euro regular, seeing for years many BMWs/Mercedes call for premium and VW/Audi for the exception of the older 1.8T/VR6 and Audi 2.8/3.2/4.2 engines regular in the US/Mexico.

 

[ZeeOSix]

That F150 in the charts had an incredible gain in real-world performance. I'd be interested in seeing the dyno curves of that engine with the two fuels. The performance difference indicates that there must be much more area under the curve w/ the higher octane gas - more than just the additional HP/TQ numbers would indicate. That would surely be noticeable in everyday driving.

Only if every day driving was a lot of high load, near WOT driving conditions. Just driving down the road at normal speeds and light throttle (low HP levels) isn't going to matter regardless what the difference under the curve is at WOT high HP output. The area under curve difference between fuels when the engine is only making 30-40 HP probably couldn't even be measurable on a normal chassis dyno.

 

[RDY4WAR]

Let's confuse things even more... (because someone mentioned flame propagation)

Burn speed and flame speed are not the same thing. One refers to the rate of flame propagation from the point of ignition while the other refers to the rate of chemical reaction a molecule undergoes once the flame reaches it. A fuel can have a fast flame speed but slow burn rate and vice versa.

Ethanol (and methanol) has a faster flame speed than gasoline but a slower burn rate. This is the other way that alcohol diminishes the risk of detonation as the flame front travels too quickly for a hot spot to ignite ahead of it. (the other way being cooling) It's also why obtaining an accurate octane rating of alcohol in a CFR engine is darn near impossible.

 

[Graham Piccinini]

 

[Thermo1223]

The real basics for me...

You run higher octane fuel because the engine requires it to make the rated power. If not needed to make rated power why would you waste your money and use it?

If the engine will run fine on lower octane fuel and that is all it requires to make rated power then that settles it for me. One reason I enjoy our new Tiguan, it doesn't need 91 to run properly.

 

[DGXR]

Ignition is not the same thing as combustion. I agree with Thermo1223 post above, if your engine compression does not require high octane fuel, and your engine has no buildup or other issues requiring high octane, there is no need to use high octane. (Yes some engines can extract more power from higher octane fuels but that is a different topic.)
Gasolines of equal ethanol blend have the same energy density, regardless of octane rating. Too many people thinking high octane fuels have more energy density... they do not. High octane is simply more resistant to pre-ignition, this makes it possible for high compression engines to run without detonation, resulting in higher specific output (power per unit displacement) and higher energy efficiency (power per fuel energy input).

 

[nicholas]

What the octane rating of gasoline means and what it does not mean is widely misunderstood and part of the problem comes in because there is more than one way to alter the rating. All else being equal the energy content is the same regardless of the rating but an engine that can accommodate a higher rating through advancing the timing can achieve a higher efficiency. But running higher octane fuel in an engine designed for a lower octane fuel has no benefit nor does it cause harm. It burns just as completely. The instance where fuel does not burn completely is when there is pre-ignition because the rating is too low.

This..........
Octane is the fuels ability to fight pre-ignition.
It will all burn the same in an engine spec'ed for 87.
Mazda engines are over 13:1 comp....14:1 in Europe yet they get away with 87, very lean burn.


However I noticed a definitive response while using 91 in the summer in our skyactive mazda 6.

 

[ZeeOSix]

What's the difference between premium grade and regular gasoline?

MIT School of Engineering | » What’s the difference between premium-grade and regular gasoline?

engineering.mit.edu

 

[edyvw]

People seriously think about this stuff way too much

Like half the vehicles on the road (hyperbole) are turbocharged. So if you have a turbocharged car that calls for premium but you just putt around all day and go pick up milk and what not and never see even close to max cylinder pressure... Do people really think this is going to cause a long term problem?

Engine doesn’t know whether you are picking milk or not. Max turbo pressure in BMW N55 engine is already at 1,200rpms. New Toyota turbo engine has mx pressure already at 1,500-1,600rpms. When you picking up milk, do you go above 1,500rpms or you idle to the store?

 

[Jimmy_Russells]

Engine doesn’t know whether you are picking milk or not. Max turbo pressure in BMW N55 engine is already at 1,200rpms. New Toyota turbo engine has mx pressure already at 1,500-1,600rpms. When you picking up milk, do you go above 1,500rpms or you idle to the store?

Ok sure, but max boost pressure is only achieved if your foot is on the floor. If you’re asking if I occasionally drive to the store or work and never see any boost, yes that probably happens somewhat frequently.

 

[twouvakind]

No clue about combustion but this behavior will empty your wallet quicker.

 

[edyvw]

Idk what turbo you are driving, but my VW sees boost all the time. If it didn’t, it would be major problem.

 

[Jimmy_Russells]

Idk what turbo you are driving, but my VW sees boost all the time. If it didn’t, it would be major problem.

I have an RS3. Driving to work when I am half asleep almost never sees any boost. Not really unusual. My commute is hardly long enough for it to get warm anyway.