Explained: Octane & cetane numbers, fuel myths & mixing different fuels

Premium is the better gas in terms of octane levels, but that doesn’t mean it’s better for your car. Your owner’s manual will tell you if it’s required or not.

BHPian NomadSK recently shared this with other enthusiasts.

After huffing and puffing through all the pages, thought of giving technical insight and clearing some air, regarding:

Octane Number, Cetane Number, Fuel Myths, Mixing Different Fuels

Octane number, also called Antiknock Rating, measure of the ability of a fuel to resist knocking when ignited in a mixture with air in the cylinder of an ICE. The octane number is determined by comparing the knock intensity of the fuel with that of blends of two reference fuels: iso-octane, which resists knocking, and heptane, which knocks readily. The octane number is the percentage by volume of iso-octane in the iso-octane–heptane mixture.

Iso-octane has an octane number of 100 and is high in its resistance to knocking; n-heptane is quite low (with an octane number of 0) in its resistance to knocking. Blends of n-heptane and isooctane thus serve as a reference system for gasoline and provide a wide range of quality used as an antiknock scale. The exact blend, which matches identically the antiknock resistance of the fuel under test, is found, and the percentage of isooctane in that blend is termed the octane number of the gasoline. For example, gasoline with a knocking ability which matches that of a blend of 90 percent iso-octane and 10 percent n-heptane has an octane number of 90.

In general, it was found that higher cracking temperatures and lower pressures produced higher octane gasoline, but unfortunately more gas, cracked residual, and coke were formed at the expense of the volume of cracked gasoline. I will leave it here, else will deep dive into the refining process.

If this is a bit clear, let’s go into a bit of chemistry, for those who are interested in the know-how:

Of the 18 isomers of normal octane (C8H18), octane gets its name from the 2,2,4-Trimethylpentane compound, which is highly resistant to auto-ignition. This iso-octane has been assigned the reference value of 100 for testing purposes. The extremely unstable normal heptane (C7H16) molecule is the 0-octane reference fuel.

Octane ratings are measures of “how stable is fuel” at different compressions/pressure. These ratings are based on the pressure at which a fuel will spontaneously combust (auto-ignite) in a testing engine. The octane number is actually the simple average of two different octane rating methods—motor octane number (MON) and research octane rating (RON). The higher the octane number, the more stable the fuel. It is a standard measure of a fuel’s ability to withstand compression in an internal combustion engine without detonating. The difference between the research and motor octane is an indicator of the sensitivity of the performance of the fuel to the two types of driving conditions and is known as the ‘‘sensitivity’’ of the fuel.

RON- The research octane number indicates the combustibility of engine fuel at low speeds and temperatures. It’s designed to reflect the behavior of fuel under idling conditions and during normal acceleration. The higher the RON rating, the more compression it can withstand in a spark-ignition engine before igniting.

MON- The motor octane number denotes how a fuel behaves at full-throttle range (hard acceleration, pedal to metal types). The rating is calculated at high speeds and temperatures designed to simulate fuel combustion on expressways and highways. When the MON rating is too low, the air/fuel mixture doesn’t combust properly in the engine and creates a pinging sound known as “knocking”.

Altitude affects several properties of gasoline, the most important of which are losses by evaporation and octane requirement.

Effects of Variables on Octane Requirements as per ASTM

  • Altitude -3 RON per 1000 ft (305 m) increase in altitude
  • Humidity -0.5 RON per 10% increase in rel. humidity at 70°F (21.1°C)
  • Engine speed -1 RON per 300 rpm increase
  • Air temperature +1 RON per 20°F (11.1°C) rise
  • Spark advance +1.5 RON per 1° advance
  • Coolant temperature +1 RON per 10°F (5.6°C) increase
  • Combustion chamber deposits +1 to 2 RON per 1609 km up to 9650kms

A Bit of Cetane Number

Cetane number is a measure of the tendency of a diesel fuel to knock in a diesel engine. The scale is based upon the ignition characteristics of two hydrocarbons n-hexadecane (cetane) and 2,3,4,5,6,7,8-heptamethylnonane. Cetane has a short delay period during ignition and is assigned a cetane number of 100; heptamethylnonane has a long delay period and has been assigned a cetane number of 15. Just as the octane number is meaningful for automobile fuels, the cetane number is a means of determining the ignition quality of diesel fuels and is equivalent to the percentage by volume of cetane in the blend with heptamethylnonane, which matches the ignition quality.

Octane number beyond 100 is possible?

Yes. It is possible to have a fuel having an octane number greater than 100. This means the fuel has better knocking characteristics (like higher self-ignition temperature) than the reference fuel iso-octane whose octane number is 100. Iso-octane is just a point on the scale.

Science Behind Octane Number

In a normal or low-compression engine, the fuel gets compressed inside the chamber and at a precise moment, the spark ignites the compressed fuel/air mixture, thereby thrusting the piston downward in the power stroke. The exhausted gas gets vented and then the fresh fuel/air mixture is brought into the combustion chamber and the process of compression begins all over again. There is no need to delay the ignition with special high-octane fuel.

So why do high-performance cars with higher compression engines need the higher-octane fuel? In short, the trouble with gas is that if it is compressed too much it starts igniting on its own (auto ignition property of the fuel), without a spark, rather than when you want it to. In fact, diesel engines work on this exact principle with no need for spark plugs and that’s why they are high compression engines and diesel is auto ignited under extreme pressure. However (for gasoline), in a high-compression gas engine, the air is packed more tightly within the combustion chamber and ordinarily (if it’s having low octane fuel) that would ignite the fuel/air combination a bit sooner than it should be. What would be the purpose of the ignition given by the spark plugs when the fuel in the combustion chamber is already ignited under high compression? Makes sense? Therefore, it needs something that delays the ignition a fraction of a moment. That is what higher octane does. Higher-octane fuels allow engine manufacturers to design more powerful and fuel-efficient engines.

Means – The higher the octane rating, the higher the compression a fuel can withstand before igniting. In crux, the higher octane gives more control over the combustible fuel allowing the engine more compression before firing.

So, in short, low/normal compression engines require low-octane fuel and higher compression engines require high-octane fuel. Also, high-octane fuel is more stable than low-octane fuel.

So, how do I know which gas to get for my car?

To know which gas your engine is designed for you should read your owner’s manual. Also, if your engine is a high-performance engine it most likely will have the higher-octane rating posted on the gas filler door as a reminder.

What happens if I mix with 91 octane and 98 octane?

You get an octane number that is higher than 91, but lower than 98. You haven’t discovered anything magical. Normal compression engine cars work the same. Higher octane number fuel in low compression engines isn’t harmful as compared to low octane number fuel in high compression engines.

If you fill low-octane fuel where the engine demands high-octane fuel (high compression engine), compressing the air-fuel mixture may cause fuel to detonate (auto-ignite) before the flame front from the spark plug reaches it. The detonation is extreme and produces much higher pressure than the engine may be able to withstand. In modern-day engines, this is mostly taken care of.

Let me explain it with real life scenario

At the gas station nearest to me, I can choose 91, 93, 95 or 98 octane (whatever). The highest-octane gas always costs the most. The question is this: If all 4/5 of these grades of gas cost the same amount of money, which one would you choose? Maximum people would say the 98 is the best quality gas. That is the myth and you would be wrong to believe it.

That expensive gas does not make your car go faster, get better gas mileage or make your engine last longer. Yet it is not surprising that most people would think that 98 is a better gas than 91. The Oil refineries have done their part to condition you into believing the higher-grade gas is better. Of course, they do this because there is a better profit margin for them on “premium” gas. The fact that they even call it “premium” implies that the gas is better. Aren’t Premium things usually are.

In an effort to market to customers who want the “higher quality” but can’t quite afford the price of premium, the fuel manufacturers created the additional mid-octane fuel solely to give people more choices hoping that at the very least, most people would choose the middle option given a choice over the cheap (read low-octane) stuff. Indeed, that happens to be the case. At just about any gas station you can tell most people choose the mid-grade fuel based on the button wear and tear. Again, most people are choosing too high of a grade and spending too much money. There is absolutely no benefit to running premium fuel in a regular engine. It just and just means that the lower the octane rating, the more easily the gasoline will detonate/explode/ignite, nothing more.

Some Myths that come to my mind and need to be busted

Premium gas is better for your car

It’s true that premium is the better gas in terms of octane levels, but that doesn’t mean it’s better for your car. Higher-octane fuel is designed for high performance vehicles with high compression ratio. Your owner’s manual will tell you if it’s required for your car.

The average car is designed to run at peak performance on unleaded fuel. Using premium in your normal won’t hurt your car, but it certainly won’t make it perform better either.

Off-brand petrol/diesel will hurt your car.

Don’t stress yourself!

All fuel has to meet the same API/ASTM standards. Using off-brand petrol/diesel won’t hurt your car. Go ahead. Find the gas, and fill up your tank. (Here, I’m not mentioning, if adulteration is being done by the bunk owners or anyone)

Premium gas gives you better gas mileage and runs cleaner

Just like premium gas isn’t better for your car, it also won’t give you better gas mileage or run cleaner. If your manual doesn’t require it, don’t reach for it.

Bottom line: if your engine is designed for 91-octane fuel, don’t waste your money on premium gas that won’t do anything extra for your car.

You get more bang for your money when you gas up in the morning

This may be our favorite myth, but it’s still false. For years, a myth has persisted that if you buy gas in the cooler part of the day — say in the morning during summer — you get more for your money, since a cooler liquid is denser. This theory may sound plausible when you’re at the pump during the heat of the day. At filling stations, however, gasoline is almost always pumped from storage tanks underground that are naturally insulated from large temperature swings. Because of this, any slight change in the temperature of the gas is so small you wouldn’t notice any appreciable savings.

Stop tailoring your gas-buying habits to this old myth. Just Buy gas when you need it.

Aftermarket additives and devices can dramatically improve your fuel economy

Excluding full conversions that meet all Environmental Protection certification standards, tests have shown that such devices and additives do not improve fuel economy and may damage your engine and/or increase your tailpipe emissions. Customers in general and car owners in particular always want to find some magic fix in a bottle. That’s why a typical auto-parts store usually has an entire shelf dedicated to various potions promising better mileage. There’s no denying that the addition of certain chemical compounds can help to clean out the fuel system. Petrochemical/Refineries would have already added said compounds into their petrol – any extra expression is superfluous. Also, fuels have already been optimized by their suppliers – extra additives will not give you any of the benefits they claim to have.

Also, An octane booster is a product which is designed to raise the octane rating of the fuel. Many octane boosters come in “lavish and shining” packaging which makes extravagant claims about their effectiveness. Testing of octane boosters in controlled settings has suggested that these products are actually NOT EFFECTIVE, with a few exceptions, and that if people want high octane fuel, they should simply pay for it at the gas station. Since many octane boosters are quite expensive, it’s often also more cost-effective to buy high octane fuel than it is to use an octane booster, even if that product works.

Even the claims on such products are not verified, and statements like “adds up to eight octane points” can be very misleading. Customers might think that this claim turns 90 octane gas into 98 octane, but in fact the “points” are usually expressed in 10ths, so the octane booster is really capable of increasing the octane up to 89.8 octane, and often much less in practical conditions, which will not generate much of an improvement.

A dirty air filter leads to lower mileage

The engines in older vehicles pulled air straight through the air filter into the carburetor, so a clogged filter could affect gas mileage to some degree. But today’s advanced engines have a computerized engine control module (ECM) to precisely regulate the air-to-fuel ratio. In a modern vehicle, air goes through the filter and then through a mass airflow (MAF) sensor that lets the ECM gauge the airflow and adjust the fuel accordingly; less airflow means less fuel is sent to the engine. While gas mileage may not be affected, dirty air filters can lead to sluggish acceleration.

Sugar in the Gas Tank Can Ruin an Engine

This is an old myth that’s remarkably persistent. It seems like most people know a friend whose cousin’s neighbor’s car got ruined after a rival poured sugar into the gas tank. As the myth goes, the sugar will be drawn into the engine with the gas, where it will caramelize and wreck the engine. However, this isn’t true. Sugar doesn’t dissolve in gasoline, so it won’t change the nature of the gas itself. At the worst, the sugar could clog up the fuel filter. This would be inconvenient, and you might have to pay to have the filter replaced, but it’s a far cry from destroying the engine.

You’ll Suck Up Sludge/Debris If Your Tank Runs Low

Thing is, fuel pumps are designed to be submerged in gas/petrol. They draw fuel from the bottom of the tank regardless of the amount of petrol. This means that sludge will be dredged through while there is plenty of fuel in the system. Thankfully, there are fuel filters to ensure these particles do not make it into your engine. That being said, constantly running your car on close-to-empty can cause pump damage. These components utilize the gas/petrol itself to cool and lubricate their mechanical rotating components – take the petrol away, and the parts may overheat, leading to reduced lifespans or failure in extreme scenarios.

Mixing altogether different fuels

Putting petrol in a diesel car

The probability of filling petrol in a diesel car is high as the cross-section of petrol dispenser nozzle is smaller, and it can easily fit into a diesel fuel tank. It is also an unfortunate scenario as petrol in a diesel engine can cause extensive damage to the internal components of the engine.

Diesel cars use fuel as a lubrication oil, ensuring engine parts run smoothly to prevent wear and tear. When petrol is added to diesel the mixture acts like a solvent, dissolving the lubricant and killing its lubricity. Switching on your ignition circulates the mixture and increases friction between components, damaging parts including your fuel lines and pump. Eventually Killing your engine. The worst part is, it’s not easy to know the presence of petrol in a diesel car. And by the time you realize it may cause significant damage to the engine.

Putting diesel in a petrol car

In reality, it’s difficult to end up with diesel in a petrol car. That’s because the diesel fuel dispenser nozzle generally does not fit into the petrol car’s fuel filler nozzle. Putting diesel in a petrol car is less serious than putting petrol in a diesel car.

After starting your engine, the diesel will coat the spark plugs and the fuel system, which often leads to misfiring. Your engine may give off smoke, cut out, or fail to start at all. Petrol is a thin fuel and diesel is thick. Hence, the petrol engine is not designed to cope with the properties of diesel. If you crank a petrol engine with diesel in the fuel tank, the fuel filter may clog up because it’s not designed to handle the greasy property of diesel.

Next, if the fuel somehow moves any further, it can clog up the fuel injectors. The spark plugs may also fail due to the soot build-up due to the mixture of petrol and diesel.

You should drain the fuel system as soon as possible.

Now, What if you mistakenly put the wrong fuel.

Diesel in a petrol car

While filling up the fuel

  • If you realize the mistake while filling diesel in a petrol car, there’s not much to worry about.
  • Because if you fill diesel for less than 3-5% of the total tank capacity, all you need to do is fill the rest of the tank with petrol.
  • Since diesel easily mixes with petrol, you can continue driving, and there should not be any problem, technically.
  • However, to avoid fuel contamination, it’s better to drain the diesel out of the tank and flush it with petrol.

After filling up the fuel

  • In case you realize the mistake after filling diesel for more than 5% of the tank capacity, the situation can be controlled.
  • All you have to do is not start the engine and call for a towing service.
  • Tow the vehicle to the nearest service station and drain the fuel tank. Once done, flush the tank and fill it up with petrol, and you are good to go.

After cranking the engine

  • If you have started the engine after filling diesel in a petrol car, turn off the engine immediately.
  • Tow the vehicle to a service center, drain the fuel and flush the tank.
  • Since you also cranked the engine, the fuel lines need a thorough flush with petrol. You may also need to clean the spark plugs.
  • After that, you can refuel with petrol and drive without any problems.

After driving the car for some time

  • It is the worst out of all the mentioned scenarios. The first thing you should do is immediately stop the vehicle and turn off the engine.
  • Call roadside assistance/tow service and transport the car to the nearest service station.
  • Get the fuel tank and fuel lines cleaned and flushed with petrol to avoid fuel contamination.
  • Additionally, you may also change the fuel filter to prevent further complications. After that, you can refuel with petrol and drive without any worries.
  • Anyway, this will not kill your engine, might injure it.

Petrol in a diesel car

The general remedial measures remain the same as in the above scenario. If you realize that your diesel car has petrol in the tank, immediately tow the vehicle to the service station. Get the fuel tank and fuel lines cleaned and flushed before refueling diesel. However, there are two scenarios you should be extremely careful about. Below are more details on the same.

If you start the engine

  • Even if you turn the ignition key on, the fuel pump may send fuel through the lines.
  • That means the tank and fuel lines need a thorough clean and flush.
  • If you start the engine, the procedure gets a bit complicated as the wrong fuel may have damaged the engine’s internal components.
  • Hence, the engine needs a thorough and complete inspection.

If you drive the car for some time

  • It’s the worst-case scenario. If you have driven the diesel car with petrol fuel, the petrol may have entered the nook and corner of the engine.
  • Immediately switch off the engine and call for roadside assistance/tow service.
  • Tow the vehicle to the nearest service center and get help from an expert mechanic because it requires more than cleaning and flushing the fuel tank and fuel lines.
  • The amount of work and repair bill depends on the extent of damage to the engine components.
  • This can do extensive damage to your engine and can kill it.

So, the bottom line is the octane number is just a number (on a scale), which shows the measures of fuel stability and is not to be confused with the quality of the fuel. Octane numbers are based on a scale on which iso-octane is 100 (minimal knock) and heptane is 0 (max knock).

Happy, if the post has debunked some myths and elucidated some members and Sorry in advance, if it has confused some.

PS – Various Manuals/References/Standards are referred to collate some of the above data.


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