Thread: What Octane Is and Does.

Sticky Material???

Lets start with chemistry!
Methane, Ethane, Propane, Butane etc. These are all carbon strings of fuels. The longer the string, the more energy that can be released, and the characteristics of the burn are changed. Methane has one carbon chain, ethane has two and so on. Longer chains have more byproducts generally, as they have more molecular structure to keep the fuel stable. The simpler chains burn very clean. Octane is one of those strings, and has 8 carbon chains linked together. As strings get longer, they are more easily made into liquid form. Gasoline is made of 2 different strings mixed together, Octane and Heptane. Heptane combusts very easily under compression.

So what is the Octane rating? The octane rating is really a mixture of the Motor Octane Number/MON, and the Research Octane Number/RON and you get the rating for gasoline by adding both numbers and taking the average. Ron+Mon/2. A 100 octane fuel is 100% octane with no heptane mixed in. You pay extra for that higher concentration of octane. 91 octane fuel is approx 91% octane and 9% heptane, but not completely! There are some other additives in gasoline that are used to raise the "rating".

RON number is related to the actual amount of Octane in the fuel, where the MON number is the resistance to spontaneity of ignition. The higher MON fuels have additives like MTBE and Tetra-Ethyl-Lead to help raise the knock resistance of fuel. This is how they did things in the old days to raise the octane of fuels. They would just add Lead to the mix! This same practice is still used in race fuels today. That's how they get octane ratings over 100!


Now on to Physics!
The ideal gas (PV=NrT) law says that compressing a gas (physical form, not gasoline) will increase the temperature of gas proportionally the further it is compressed. The more compression, the more heat. All fuels have a certain flash point in which the temperature will cause spontaneous ignition. This is how a diesel engine works. It compresses the fuel to its flash point until it ignites.

Now try this experiment. Get a small amount of gasoline of 85 octane. Now get a small amount of high octane, this experiment works well for many of them but not all. Make a stream of each fuel and light them at the same time and you can SEE the difference between octanes.

The result you will get is that the higher octane fuel will burn slower.

On to the next point, higher octane doesn’t necessarily have to burn slower or faster to get the rating it has. Because of different additives in gasoline you can have the properties of a very quick burning fuel with a high detonation threshold. You do give up some burn time for having a higher octane fuel generally, but you also get a higher threshold. This higher threshold allows more room to make power before you experience detonation.

Higher octane fuel/air mixture can be compressed more and see more heat before it reaches its flash point. This is good because cylinder pressures in high performance cars can get very high and will need the higher octane to keep from causing cylinder pressures from spiking quickly and breaking stuff (detonation). Detonation and preignition cause these rapid spikes in cylinder pressures. Detonation is the spontaneous ignition of the air/fuel mixture after the ignition sequence has started. After the spark plug fires, there is a pressure increase in the cylinder. If the cylinder pressure and heat go beyond the flash point of the fuel, spontaneous ignition occurs. Go here http://www.streetrodstuff.com/Articles/Engine/Detonation/ to direct you to a better explanation of what detonation is. Thanks BenFenner.

If you need to push a car bumper to bumper for a certain distance, you want to do it with finess rather than slamming into it at 30mph. Pressure spikes can cause serious engine damage as there is a finite amount of pressure your pistons and block can see before things break. Just like pushing another car with your bumper. You want to have pressures increase, otherwise torque would not be generated, but you don't want the pressure to spike and cause engine damage.

It isn't always the pressure spike that causes engine damage, but temperature as well. After the spark plug fires there is a pressure spike as the gas starts to expand. As the pressure spike occurs, cylinder temps rise as well (Ideal gas law) If temps get too high, spontaneous ignition of the remaining unburned fuel occurs.

Higher octane is good, but not in all circumstances. IF you can keep the octane low, then you can take advantage of the quicker burn that these fuels have, thus yielding faster gas expansion and more power. This is a common honda tuner trick from what I understand.

Higher octane fuels can also use more aggressive timing maps. Since the higher octane generally burns slower you can advance timing to take advantage of this and start building cylinder pressure a few degrees sooner. By the time the piston has reached TDC there is a lot of pressure already there. So you can take advantage of this extra pressure and can make more power by optimising it. Advancing timing increases cylinder pressure and as cylinder pressures rise, temperatures increase also. So it is important to keep the timing reasonable.

In Naturally aspirated engines the higher octane will generally lower power unless the A/F ratio and timing are changed to take advantage of it. Here’s the neat thing I have seen in many cars. The higher octane fuel burns slower, so when timing is not advanced you have more fuel going through the engine unburned or incompletely burned. This extra fuel is seen by the O2 sensor and the computer makes adjustments to the short term fuel trim to LEAN out the mixture and then you can see gains from the higher octane.

Leaner mixture means more power, and more heat as well. Advancing timing too much can still cause cylinder pressures to rise too fast when on the compression stroke If the cylinder pressure gets too high it will put a lot of wear on the engine, even if detonation never happens. Detonation will just make quicker work of it. This is seen on engines with advanced timing that require use of an ignition retard setup to get the car started as the expansion of gas in the combustion chamber before TDC puts a lot of stress on the starter.

You still with me?

In turbo cars high octane is the way to go. The higher the octane, the more cylinder pressure you can handle and the more power you can make before that spontaneous combustion happens causing damage to the engine.

More fuel also has a cooling effect therefore extending the detonation threshold. This is how E85 can make so much power even though it has a low octane (RON) rating. You inject more fuel and as the fuel atomizes it absorbs a large amount of heat to change from a liquid state to a gaseous state and that cools the charge. This is why the MON rating for E70 and E85 are so high. This again goes into the ideal gas law and extends the detonation threshold by lowering the temperature in the combustion chamber. Interesting indeed.

The Toyota Prius utilizes an extremely high 13:1 compression ratio (Not 100% sure on this, but it is somewhere in this range.) and is rated to use 85 octane. The engine’s timing and fuel maps were designed for efficiency instead of power and the engine itself is governed for power output, which is why it only makes 65hp or so even though the compression ratio is very high. The efficiency of this engine on the lower end of the operating range I would imagine is extremely high, reflecting that you don’t need high revs to improve VE if the engine is designed right. I am just putting this in there for perspective, its relevance to the subject is minimal, but you can get an idea of how the tune is a major factor on what octane fuel needs to be run before you burn the house down.

Higher octane = more timing, higher pressures, less fuel, and more power.

Feel free to add or correct info as needed.

^ i agree with the above, i have been talking with my tuner about getting a race fuel map made up, he said that he averages around 40-60whp gain with race fuel tune over pump gas tune. given the race gas gives you the ability to get in more timing and lean out the air fuel a little more, is where you see those 40-60whp gain over pump gas.

Uh, I couldn't get past paragraph 3 without running into problems. For a technical article, this is horrible. Aside from my personal gripes with phrases like "My understanding of fuels is that", the mention of compressing (liquid) fuel is preposterous.

C-

cylinder pressure is directly related to torque. detonation usually starts at the torque peak. just more info for you to play with coheed. torqueless high CR hondas on 87 octane.....


RON+MON is that like Rob+Fab

I will edit it to make it easier to read, I just wrote this up before I went to work. You can in fact compress liquids, but they don't like it and don't compress easily. I am not going to go into that, but what I was getting at is when the injectors spray the fuel it is still in liquid form and under a lot of pressure. When the pressure is released it wants to evaporate and as it does so it cools the air as it absorbs energy in order to change states.

I will fix that phrase as it was taken out of context. Of course the liquid fuel is not compressed in the diesel engine. it is in a semigaseous state, IE like a fog.

edited for easier reading. hope this helps.

This is paragraph 7:



This really doesn't make sense even in context. What "causes cylinder pressures to rise too high too fast and break stuff"?
Yes, there is a finite amount of pressure an engine can take before damage occurs, but the damage isn't usually directly caused by the high pressure. I haven't read further, but before talking about this, you should probably bring up detonation, and pre-ignition. Things will make much more sense if you've laid out what damages the engine first, and then go on to explain why/how higher octane prevents this damage. That's how I'd do it anyway.

I would recommend reading this article on detonation and pre-ignition, as it is extremely informative, and shows some problems with your explanation:

http://www.streetrodstuff.com/Articles/Engine/Detonation/

It should be a prerequisite to any discussion of octane.

fixed. I have thrown a little bit in there on detonation, but I will leave the rest in the link you provided. I didn't want to write something that would get out of control and be too long to read. Keep the input coming though so I can iron out all the kinks. Like I said, I kinda rushed this before going to work so I could use the proofreading.

I guess I should add that the high cylinder pressures usually don't cause the most damage (at least to pistons) it is usually the high heat associated from detonation that melts the pistons. I never really could figure out why detonation seems to focus on the quench areas, usually burning through the valve reliefs...

Detonation is a vicious cycle. High cylinder pressure and heat (both connected by the ideal gas law) cause detonation, which in turn increases cylinder pressure even more, therefore causing even more heat.

It should also be said that the combustion event itself is a controlled burn, and not really a combustion. The spontaneous combustion of the end gases (fuel that has yet to be burned after ignition has already started) is detonation. This description is in the link BenFenner has provided. I added the link my post. Thanks Ben. I think I have ironed out all the kinks. Thanks for the proofreading, I hope it is easier to read and understand now.

While you do have the concept correct, fuel does have a cooling effect. Hence you never run 14.7 at WOT.

But Ethanol has a higher octane rating ranging from 100-105. Thus it's as good as race gas but much cheaper, this is why it's used by so many people.

Now E85 burns colder. Thus you get lower gas mileage. You basically get less bang out of 1 gallon of Ethanol then you would out of 1 gallon of gasoline.

so the prius is race ready, all motor...just gotta tune it! LOL!!!

but no really...VERY informative! this actually just came up in discussion the other day as questions to me. i didnt really have a technical answer other than, "detonation is bad." i kind of had some info to answer further, but didnt know what i know now. thank you!