Octane Rating Explained By An Engineer

Hey All,

I was browsing through the home page today and an old thread popped up (see below):

http://www.tacomaworld.com/forum/2nd-gen-tacomas/36847-gas-octane.html

As I read through it, I realized that there was a lot of misinformation in it, and instead of trying to quote, comment, and correct a thread from 2009, I thought I would start a new one. I am a degreed Mechanical Engineer from the Georgia Institute of Technology. While I was there, I had the pleasure of taking an internal combustion engines class, and had the octane ratings of fuels explained to me by an industry expert. This post will serve more as a lecture for anyone that gets on TW looking for information about the octane ratings of fuels.

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ABSTRACT:
This post defines what the octane rating of a fuel is, and explains why different engines require different octane ratings. Knock will also be defined and discussed as it relates to the octane rating of a fuel. Finally, this information will be used to better evaluate the quality of gasolines at the pump. Only spark-ignition engines will be discussed (this excludes diesels).

PART 1: The Octane Rating
First, the self-ignition temperature of a fuel is defined as the temperature above which the fuel will ignite without an external igniter (spark). Ignition delay is defined as the time difference between when the fuel reaches the self-ignition temperature and when the fuel actually ignites. Ignition delay is usually on the order of milliseconds. Figure 1 illustrates the two above definitions as they relate to the temperature inside the combustion chamber.


Figure 1. Self ignition characteristics of fuels

[Excerpts from Pulkrabek (1) with editing]
The fuel property that describes how well a fuel will or will not self-ignite is called the octane number or octane rating. This is a numerical scale generated by comparing the self-ignition characteristics of the fuel to that of standard fuels in a specific test engine at specific operating conditions. The two standard reference fuels used are isooctane (2,2,4 trimethylpentane; C8H18), which is given the octane number (ON) of 100, and n-heptane, which is given the ON of 0. Note: this means that the gasoline that you buy at the pump is compared to isooctane, it does not contain any octane itself.

The two most common methods of rating gasoline (at the publication of Pulkrabek) and other automobile SI fuels are the Motor Method and the Research Method. These give the motor octane number (MON) and research octane number (RON). The engine used to measure MON and RON was developed in the 1930s. It is a single-cylinder, overhead valve engine that operates on the four-stroke Otto cycle. It has a variable compression ratio which can be adjusted from 3 to 30.To find the ON of a fuel, the following test procedure is used. The test engine is run at specified conditions using the fuel being tested. Compression ratio is adjusted until a standard level of knock is experienced. The test fuel is then replaced with a mixture of the two standard fuels. The intake system of the engine is designed such that the blend of the two standard fuels can be varied to any percent from all isooctane to all n-heptane. The blend of fuels is varied until the same knock characteristics are observed as with the test fuel. The percent of isooctane in the fuel blend is the ON given to the test fuel. For instance, a fuel that has the same knock characteristics as a blend of 87% isooctane and 13% n-heptane would have an ON of 87. Table 1 (4-3 in image) shows the test conditions for octane number measurement.
[end excerpts from Pulkrabek (1)]


Table 1. Test conditions for octane number measurement

At the fuel pumps, the fuel rating will be reported as the Anti-Knock Index (AKI), which is the average of the RON and MON: (RON+MON)/2. It may also sometimes be called the Pump Octane Number (PON). So when purchasing "regular" gasoline, the number "87" is the AKI.

The higher the octane number of a fuel, the higher the self-ignition temperature, and therefore the less likely it will self-ignite under normal operating conditions.

As the piston moves up the cylinder towards top-dead-center (TDC), the pressure in the cylinder increases. Assuming the air-fuel mixture is an ideal gas (for sake of example), the temperature and pressure inside the combustion chamber can be related by Equation 1:

PV = nRT ___________________________(1)

Where P is the pressure, V is the volume of the combustion chamber (this is the volume above the piston including the clearance volume), n is the number of moles of the air-fuel mixture, and T is the temperature of the mixture. From equation 1, it can be seen that if the volume of the combustion chamber decreases (piston moving up) causing the pressure of the fuel to rise, and the amount of air/fuel is the same, the temperature of the fuel must also rise.

Engines with low compression ratios - defined as the volume of the cylinder with the piston at bottom-dead center (BDC) divided by the volume of the cylinder at TDC - can use fuels with lower octane numbers because the pressure (and therefore temperature in the cylinder) is not expected to climb very high (relatively) and therefore a high self-ignition temperature is not required to prevent self-ignition. On the other hand, high-compression engines must use high-octane fuels because they run the very real risk of causing the fuel to reach or exceed the self-ignition temperature BEFORE the scheduled ignition.

PART 2: Engine Knock
This brings us to engine knock. Engine knock (or ping) is defined as the occurrence of self-ignition before or during regularly scheduled combustion that results in unwanted pressure pulses in the engine that are often in the audible frequency range. Figure 2 illustrates this concept graphically from the side


Figure 2. Graphic depiction of types of engine knock

and Figure 3 illustrates self-ignition beyond the flame-front in a top view of a single cylinder.


Figure 3. Self-ignition beyond the flame front

The flame-front is the edge or boundary between unburnt and burnt fuel - the edge of the flame. Figure 4 illustrates how engine knock results in varying cylinder pressure, which can cause damage if the pressure exceeds the engineered factor of safety for the engine.


Figure 4. Cylinder pressure over time with varying degrees of engine knock

Obviously, damage to the engine is to be avoided, so engine knock is a bad thing. This means that the fuel used in an engine needs to be matched to the compression ratio of the engine - the fuel needs to have a self-ignition temperature above that which the cylinder will ever experience. Look in the owner's manual (or online, because we all know Google is easier than digging around in the glove box) and find what [minimum] octane rating the engine was designed for.

PART 3: Picking A Fuel
So what about using fuels with a higher octane number? All that means (all else being equal) is the fuel will self-ignite at a higher temperature. If one puts high-octane fuel into an engine that will never get close to the self-ignition temperature, then the "extra temperature buffer" (and money) is wasted.

This is not to say that using ALL high octane fuels is a waste, or as some have commented in the other thread, will not result in better performance. There is a lot more than octane rating that differentiates the varying grades of fuel (regular, medium, and premium grade - with what ever octane ratings those come in for your area). Gas companies put additives in the higher grade fuels to help differentiate their fuel from the competition. Without being a petroleum engineer, I can only assume that their claims of "cleaner burning" and "less carbon build-up" have at least some truth.

The other big difference could be ethanol content. Ethanol blended petroleum fuels have less chemical energy that non-blended petroleum fuels, simply because ethanol contains less chemical energy than gasoline. Pure ethanol contains ~26.8 MJ/Kg, whereas gasoline contains ~44.4 MJ/Kg. So ethanol contains about 40% less energy than gasoline; meaning the higher percentage of ethanol blended into the gasoline, the lower total chemical energy per unit of fuel. All this to say that one will experience better performance and gas mileage from non ethanol-blended fuels.

So when deciding to put regular or premium into your Tacoma, stating that the higher octane number automatically makes the premium better is incorrect. It still may be better, but it is not because of the octane rating - unless of course you are running a supercharger or higher compression ratio than stock and need the higher octane rating to avoid knock. If buying premium fuel means that it won't be blended with ethanol, it is very possible to make the money back in better fuel economy. However, if all the grades of fuel are blended (or unblended), the octane rating alone is not going to increase the performance of the engine.


REFERENCES:

1. Pulkrabek, Willard W. Engineering Fundamentals of the Internal Combustion Engine. Upper Saddle River, NJ: Pearson Prentice Hall, 2004. Print.

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I hope this was helpful and informative for anyone looking into this. If I have made any errors, I'd appreciate a comment so that I can amend the original article. If there are any further questions, I can try and adress them by augmenting the article or by replying to the post.

Thanks,

Hunter

 

Well this was interesting. Is it bad that I learned more in this post than at school this week? (Super easy classes cause seniors rule)

Thanks for the breakdown here.

 

This is what causes a lot of confusion. People say you don't get better mileage out of premium fuel, but a lot of us run 91 because we live in states like MO that require 10% blend in all fuel except premium. I get enough extra mileage out of 91 (due to the lack of ethanol) that it's usually worth it to me in cost per mile to do so, unless the premium is oddly (like, 40 cents/gal) higher than regular, which does happen sometimes. I only use 91 non-eth gas unless I can help it.

 

Ahh, what you did not reference was the aspect of engine timing. If the engine is equipped with knock sensors and an ECU that can adjust timing based on knock, then the choice between regular and premium as it relates to performance/milage gets a bit more complicated.

 

The reason why YOU will get better mileage on high octane fuel, is because your ignition timing will be more advanced.

 

Right, but you have a high compression engine that brings the fuel up to a higher temperature -- closer to self-ignition. Because of the physical properties of the engine, you actually *require* higher octane fuel in order to run optimaly. The difference is that it *can* run on low octane fuel by delaying the ignition slightly, but your fuel economy and performance will be compromised.

Because the difference is so minor and the engine runs on the "edge" of detonation, you can get by with simple ignition delay.

If you look at OP's figure 2, you can see that what is happening in the detonation column, is that the spark ignition is causing a pressure wave through the compressed fuel AHEAD of the actual combustion front. This increases the pressure through the combustion chamber and sets off a separate ignition and a second wave that impacts against the first.

What the engine does, is it delays the spark until a point where the piston is already on its downstroke, pressure and temperature are sufficiently lower that the primary pressure wave won't set off a second combustion, hence no detonation.

 

Yes sir, in theory you are correct (except the part with spark occuring after TDC), but in the real world YMMV. Personally I've run premium at a couple of points in my Tacoma's life. I normally run 4-5 tanks at a time. While in theory, greater perf/milage is possible, my data showed no significant difference in either for my driving style. So reg it is for me.

 

Well written and great explanation.
Thanks!

 

In the interests of providing a simplified explanation. In reality, its all about timing things "just right". Spark happens BTDC because it takes some amount of time for it to build just perfectly... etc.

As for your observations... being such a fine line, all kinds of different factors exist that can affect whether the fuel you use will require consistent delayed ignition or not. I suspect, given your location, that the fuel is adjusted to a slightly higher octane rating than it would be in cooler climates.

Also, of course, driving habits play a big part in this. If you're the type to floor it frequently and drive near the vehicle's limits, you'll be pinging more than someone who never exceeds half throttle. Why? Well obviously if there is less air in the cylinder, the pressure it reaches will be lower, and so also the temperature.

 

Mmmm....well, I sure don't get the mileage with 91 eth (which I use out of town when I can't get to my normal gas station) that I get with 91 non-eth. Down around 19 mpg vs. the 20.6 or so I get on 91 no-eth. But thanks for the remote diagnosis.

 

Hunter,

Well written, though...
The published Octane Number at the pump is typically the average of the 2 test values. These MON and RON values can be as much as 10 points apart.
Also, the Octane Number scale allows for values greater than 100. Which by your example would require >100% Iso-Octane. Remember the 100 value for Iso-Octane fuel is just a reference number.
I took a similiar class a "few" years ago.
I remember seeing a good paper a couple of years ago on a new flame type testing method. Quick Google; http://ir.uiowa.edu/cgi/viewcontent...K37oA#search="octane rating test method pump".

 

Nice write up.

Its nice to find a butter bar that didnt get their degree in underwater basket weaving.

 

Hey Chris,

I reviewed the thesis you linked to. Interesting.

You are absolutely correct about the Anti-Knock Index. Thank you for pointing that out to me. I just forgot to add it in as I was compiling everything I wanted to say and typing it out. I included it in the original post to clarify to any new readers.

In response to your comment about the model's limitation at an ON of 100, I pulled the following passage from page 3 of the thesis:

This essentially reiterates what is described in Pulkrabek. However, I do completely agree with you that there are fuels with an octane number greater than 100. As you can see from the bold sentence, other fuels are used to extend the scale; this does not invalidate the original model (which I obviously didn't create), as it is only for reference anyway.

Thanks again for taking the time to help provide quality information to TW.

Hunter

 

HAHAHA, not this guy - and I'm infantry no less I have a BA from Emory University in Economics and Mathematics and a BS from Georgia Tech in Mechanical Engineering.

 

I need some more personal time after reading all this . . . .

 

bhahah i understood most of this but damn my brain is frazzled.

May I ask what you are an engineer in op? im currently going to school for accounting but I would like to learn more about what school is like when going for engineering.

 

I am a Mechanical Engineer. The best way to put engineering school is - "welcome to the suck". It is really hard and it takes a lot of work; Tech was way harder than Emory, mainly because there is no room for bullshit. In a paper about ethics, one can argue just about anything, and if the logic is sound, it will probably receive a passing grade. In engineering school, it is either right or wrong. If you don't know it, then that's it. My biggest advantage was that I was a math major at Emory before coming to Tech, so if I didn't know a formula or relationship for something on a test, I could sometimes derive it using my knowledge of math and physics. But there is also a lot of science in engineering that I couldn't reason my way into, it just has to be studied. I had several tests where the class average was a 45%, and getting a 55% was a "B". I graduated with a 3.34, which gave me honors. At Tech, we don't say we've graduated, we say we made it out.