We’ve covered on what petrol octane means and what it helps with in yesterday’s article. (Please click on the link below to read more.)
To reiterate, the higher a fuel’s octane rating, the more it resists self-ignition when it’s being compressed in the ignition chamber. Self-ignition or more correctly known as pre-igntion caused engine knock which could destroy the engine in a matter of minutes.
But how is the octane number derived and how is the octane increased? It surely isn’t an arbitrary number picked out of the sky, right?
The name “octane” is actually one of a family of hydrocarbons (HC) resulting from the refinery process of crude oil. When the crude is “cracked” (broken down into different types of substances), different lengths of hydrocarbon chains are were produced. These are then separated and blended to form different fuels, such as methane, propane, butane, among others.
Methane consists of a single carbon atom. Propane has three carbon atoms chained together. Butane has four, pentane with five, hexane with six, heptane has seven and octane has eight carbon atoms.
Octane is a family of colourless liquids that boil at approximately 125 oC. A member of the octane family, 2,4,4-trimethyl pentane which we now call “iso-octane” is used a reference standard to determine the tendency of gasoline or LPG to resist self-ignition.
A test engine is used to measure the octane rating, by comparing to a mixture of iso-octane and heptane. The mixture of the two types of HCs by volume is the octane number of the fuel, i.e. 95% iso-octane and 5% heptane means 95 octane.
The test engine, known as a Cooperative Fuel Research engine is a specialist single-cylinder with a bore and stroke of 82.5 mm and 114.3 mm respectively which equates to 612 cc, with a variable compression ratio from 4:1 to 18:1. The piston has four compression rings and one oil control ring. Both the head and cylinder are one piece and can be moved up and down to obtain the desired compression. It has a four-bowl carburettor, allowing for quick switching between the reference fuel and samples.
Knock is detected by using a magnetorestrictive sensor in the combustion chamber and measured on a “knockmeter.” A complete system costs in the regions of USD 200,000 and is made by only one specialist Waukesha Engine Division of the Dresser Industries in Wisconsin, USA.
This is how it works. The CFR engine is turned at 600 RPM and technician will test the sample fuel corresponding to the iso-octane/heptane mixture’s knock resistance properties. The octane rating called RON (Research Octane Number) is produced, it’s tested in a controlled environment.
However, if we go on further, a certain rating doesn’t mean the gasoline has only the corresponding mixture of iso-octane and heptane, as fuels commonly contain other HCs and additives. Because of this, as some fuels are more knock-resistant than pure iso-octane, the RON could go above 100.
Racing fuels, avgas (aviation gasoline), LPG and alcohol based fuels such as methanol may have octane ratings higher than 100. Octane boosters such as additives include MTBE, ETBE, iso-octane and toulene. Tetraethyllead or more commonly known as just “lead” was once used widely as an additive, but has since been banned as lead is poisonous to the environment and humans.
There are however, other fuel octane ratings, one of those called Motor Octane Number. Testing is similar to that for the RON rating. However, the engine is run at 900 RPM, the fuel is pre-heated, engine is run at higher speeds and ignition timing varied to determine the fuel’s knock resistance. Depending on the fuel’s composition it’s MON rating may be between 8 to 12 octane lower than RON, although there isn’t a direct link between the two.
Because of RON and MON ratings, certain countries require petroleum companies to specify the Anti-Knock Index (AKI) or more commonly, (R+M)/2. The United States and Canada are among some countries who specify this rating on the pumps. It’s also called Posted Octane Number (PON) (which is sometimes mistakenly called “Pump Octane Number”).
There is also Observed Road Octane Number (RdON) which is produced from testing petrols in real-world multi-cylinder engines at full throttle. First developed in the 1920s, it’s still reliable until today. As you may have envisioned, early testing was performed on cars on the road. As digital and other technologies advanced further, testing has moved onto dynanometers in environmental controlled quarters for consistency.
We’re so used to listening to the term “high octane” especially when TV networks or event managers using that term to imply an exciting event: “Get ready for high octane racing at the Malaysia MotoGP!” the announcer screams.
It’s probably from this kind of “nurturing” that we mistakenly associate higher octane fuels with big performance.
But, does higher octane fuel really guarantee enhanced performance? How is a fuel’s octane, called Octane Number, rating determined, anyway?
We first need to understand about “engine knock.”
Let’s assume a cylinder fills with a volume of 650.5cc of fuel/air mixture from the intake stroke. The piston now rises back up to TDC for its compression stroke, squeezing that mixture into a smaller and smaller space of just 50cc (the piston crown is typically less than 1mm below the cylinder head). This is how we derive at an engine’s “compression ratio” as seen in the spec sheet. We divide the volume at BDC with the volume at TDC: 650cc/50cc = 13/1, denoted as a ratio of 13:1.
A higher compression ratio (higher cylinder pressure) is directly related to power and thermodynamic efficiency of an internal combustion engine, allowing it to extract more energy from a given amount of fuel.
However, the temperature of fuel/air mixture rises as it’s compressed, making it easier to combust efficiently. (In fact, diesel engines work by compressing air to extreme compression ratios, heating up the air enough to ignite when diesel is injected into the combustion chamber.)
With the pressure and heat building up, the fuel/air mixture may spontaneously combust before the spark plug emits its spark at the correct moment. This uncontrolled combustion, called pre-ignition, produces compression pressure waves that bounce back and forth in the combustion chamber, leading to an audible knocking or pinging. Left alone, engine knock is devastating to the engine, as the rising piston attempts to resist the force of the still expanding pressure of combustion, besides the abnormal heat produced. (Please be aware that “pre-ignition” and “detonation” are two different phenomenas.)
A dirty engine with a thick layer of carbon deposits on the piston tops and valve surfaces actually increases the compression ratio. Besides that, the heated carbon will contribute to local hotspots, further raising the chances of pre-ignition.
Modern engines are typically equipped with knock sensors. They send the information to the engine ECU which then alters the ignition timing, fuel injection timing or fuel/air mixture ratio, to combat the knock. Consequently, engine performance and efficiency suffers.
So how do we avoid engine knock without having the ECU knocking back on our enjoyment?
That is why the manufacturer of high performance motorcycles recommend higher octane fuels, in order to avoid pre-ignition and knocking, and the ECU from intervening.
Does that mean higher octane fuels mean higher quality? Not necessarily so, honestly. Most, if not all, petrol grades of the same friendly neighbourhood petrol brand may already contain virtually the same fuel-system cleaning, combustion enhancing, etc., additives.
A fuel’s octane determines its resistance to pre-ignition, nothing more.
So why do motorists swear to being rewarded with a better performance when they used higher octane fuels? Psychology? Maybe.
However, there are test data that reported increase in performance as a result of using higher octane fuels, such as those reported by Petron regarding their Blaze RON 100 petrol, recently. (Please click on the link below to know more.)
Tested: Petron’s Blaze 100 fuel – Does it make a difference on motorcycles?
Again, a higher RON rating doesn’t make the fuel contain more energy. For the sake of enlightenment, MotoGP rules state the minimum of RON 95 to a maximum of RON 102.
It is due to the absence of knock, which implores the ECU to run the optimum fuel mixture injection timing and amount, and ignition timing among other factors, allowing the engine to produce a “better performance.”
Stay tuned as we explain the process of how the octane or the RON number we see on the pumps is determined, in the next edition.
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