[NomadSK]

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 would deep dive in refining process.

If this is bit clear, let’s go in 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 an 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 octane number greater than 100. This means the fuel has better knocking characteristic (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 engine require low-octane fuel and higher compression engines require high-octane fuel. Also, high-octane fuel is more stable than the 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 engine isn’t harmful as compared to low octane number fuel in high compression engine.

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 that 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, The lower the octane rating, the more easily the gasoline will detonate/explode/ignite, nothing more.

Some Myths which comes to my mind and needs to be busted;

1: 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.

2: 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)

3: 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.

4: 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.

5: 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. It’s not 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 do so.

Also, An octane booster is a product which is designed to raise the octane rating of 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.

6: 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.

7: 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.

8: 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 scenario.

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 it's 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 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;

1. Diesel in a petrol car

-While filling up 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 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.

2. Petrol in a diesel car

The general remedial measures remain the same as 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, bottom line is, Octane number is just a number (on scale), which shows the measures of fuel stability and 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.

Cheers

[Jeroen]

Originally Posted by NomadSK

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

Thanks very much for this excellent write-up. At long last some facts and figures into this thread

Originally Posted by NomadSK

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

Thanks for providing these. Folks need to understand these effects. By and large, they outdo any effects you might see between an octane 95 and 98 fuel.

You mentioned how the Octane number is derived. Essentially a comparison between the performance of 100% IsoOctane and a given fuel.

A few years ago I stumbled upon this Octane number testing machine. It has a variable compression to find the tested fuel's respective octane number.



It is known as a Waukesha motor. Do you know how they test for the octane number nowadays??

A few additional thoughts on this topic.

On this thread several times references have been to to the USA. The USA does not use the RON system. The USA is one of the few, if not the only country that uses the PON System (Pump Octane Number).

As a rule of thumb:
PON 91 = RON 95
PON 93 = RON 98

So the idea that US fuel has a much lower octane rating than the rest of the world is just a myth. Or more precisely, they use a different method.

With this US low octane number comes the myth that cars for the US markets have a special tuning, to accommodate the lower Octane rating. Again, this is simply not true. Because by and large, when using the same method to classify fuels there is no difference between them.

The so-called US tune has more to do with the very strict emission regulations. Compared to many other countries, including the EU the US regulation tends to be more strict. So say a BMW destined for the US will have a slightly different tune than the same BMW staying in the European market. Nothing to do with the fuel, all to do with emission. In practice outside US engines could run slightly more rich. That tends to give a little power increase and improve driveability a bit.

Of our six cars, three of them (Alfa Romeo Spider, Jaguar XJR and the Jeep Cherokee) come from the USA and were produced specifically for the US market. They run fine in Europe and there is no non-US tune. It is all identical! I never changed anything on them.

Here in Europe we also have different contents of ethanol in our fuels. Standard these days is Euro 95 (Octane 95) E10, (Maximum 10% ethanol). However, most Euro 98 (Octane 98) are E5 (Maximum 5% ethanol). Fuel companies such as BP, Shell, Esso and a few others guarantee no ethanol at all in their Euro98 E5! Which is great news for classic car owners such as me. So all my cars run fine on 95 but wont do so well with high ethanol contents. So I put Euro98E5 in all my cars. Not because I need the high octane, but because I don't want ethanol in my fuel. Doesnt make the slightest bit of difference on performance. If there is a fuel efficiency gain it is probably more likely to be due to the lack of ethanol than the higher Octane rating.

People have been reporting improved performance when using a higher octane fuel. In most cases, to your point, I don't think it is true. It is probably a case of wishful thinking You spend a lot of extra money on your beloved car, sure you are going to see an improvement.

The most relevant engine parameter, when it comes to determining the optimal octane number is compression ratio. That is fixed. Next through the clever piston, cylinder head design and so on you can influence it some more. Also, modern injection techniques can have an influence.

On some cars/engines going from say Octane 95 to Octane 98 fuel might give some improvements. Modern engines have pretty fancy knock sensors and might be able to adjust the pre-ignition (and some very fancy engine, fuel injection).

Engines designed with Octane 95 in mind, but with a relatively high compression ratio are the most likely candidates to see positive effects of a higher octane fuel.

But in most cases, the effects are pretty small.

If you believe you are getting more power from Octane 98 compared to 95, and you have a somewhat older car, there might be another effect coming into play.

As cars/engines age, most over time will suffer from some coking effects on the piston, top of the cylinder, valves etc. What this does is effectively increase the compression ratio!! This means your ECU is likely to retard the ignition timing. Changing to 98 will allow the ECU to advance the ignition again.

Contrary to popular Internet belief, a so-called Italian tuning will not get rid of it!

So if you believe you are getting better performance you need to ask yourself some honest questions:

- Do I have a set of relevant long-term measurements before and after that factually prove it?
- Might I have a somewhat coked-up engine?

Jeroen

[NomadSK]

Thanks for your reply. Thats the whole reason for putting up the post to clear some doubts about the fuel.

Originally Posted by Jeroen

It is known as a Waukesha motor. Do you know how they test for the octane number nowadays??

Yes, we still use the same engine for testing octane value. However, I didn’t know this was called “Waukesha motor”. We know it by the name, CFR engines. It's great to hear what actually knocking sound is.

The only changes I can see from the pic your shared, is that we have now digital gauges and no more analogue gauges, no other change, even the colour is same. I think no one wants to re-invent the wheel. The fuel is generally taken from various sample point (we call it bombs), from heaters/reactors/pipelines/tanks and send to laboratory for testing at a predefined frequency.

A few additional thoughts on this topic.

On this thread several times references have been to to the USA. The USA does not use the RON system. The USA is one of the few, if not the only country that uses the PON System (Pump Octane Number).

As a rule of thumb:
PON 91 = RON 95
PON 93 = RON 98

So the idea that US fuel has a much lower octane rating than the rest of the world is just a myth. Or more precisely, they use a different method.

In practice, however, the spark is advanced at higher elevations to improve engine performance and the net effect is to reduce the PON of the gasoline marketed by about two numbers for a 1524-m increase in elevation. Octane requirements for the same model of engine will vary by 7 to 12 RON because of differences in tune-up, engine deposits, and clearances (as mentioned in my previous post for variables). Since the posting of octane numbers on the service station pump has been required in the US, the posted octane number (PON) is the one most well-known by the typical driver. In the United States, this is the octane number that by federal law must be posted on gasoline dispensing pumps.

PON = (RON+MON)/2

Hence you get the lower value of the PON, since it’s the arithmetic average of the two. Octane numbers quoted are usually, unless stated otherwise, research octane numbers.

Another Example

The requirements are to produce a 50/50 split of premium and regular gasoline lines having 91 and 97 posted octane numbers, respectively, and Reid vapor pressures of 10.2 psi. For this split between regular and premium, PON needed is 94.0 (Average of 2). The available basic stocks are then selected for blending. Hence depending on the blending of the stock one can get the product ranging in between octane 91 to 97.

This is a trial-and-error process at this stage. After selecting the blending stocks, the quantity of n-butane required to give the desired vapor pressure is calculated first because the n-butane contributes significantly to the octane of the finished product. Apologies, if it's getting too technical.

There’s another PON – Pool octane number – It’s the average octane of the total gasoline production of the refinery, if the regular, mid-premium, and super-premium gasolines are blended together. The resultant is called Pool octane number. This is generally required for R&D purposes, As new designs are introduced into the market to implement various technologies for improving fuel efficiency.

[Cresterk]

Originally Posted by NomadSK

1: 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.

3: 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.

Thank you for this excellent write up. I have pretty much given up on telling people that putting higher octane fuel in their car that was designed to run on lower octane fuel is just a waste of money. It's not a "higher quality" of fuel as is often thought, it just means that it can be compressed more before the spark plug ignites the mixture. If you don't have an engine capable of compressing it more, then there is no point to using it. Whatever fuel mileage or noise improvements they feel is just placebo OR the fuel they were using previously was adulterated.

Also here is another fact that most people don't know. Ethanol blended fuels have lower energy density but also have higher octane by default which is why some cars like the Koenigsegg Jesko make considerably more power while running E85 rather than normal petrol. Mileage will be lower, but power produced will be more.

[Omkar]

Explained : Octane Number, Cetane Number, Fuel Myths & Mixing Different Fuels - Posts moved to a new thread.

[careind]

Originally Posted by NomadSK

In an effort to market to customers that 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, The lower the octane rating, the more easily the gasoline will detonate/explode/ignite, nothing more.

Cheers

Thanks for the write up @NomadSK.

I do have a few doubts on going through the material you had provided

Disclaimer : I am just an IT consultant who has collated knowledge from the internet and not an expert in any way.

Now that THAT is out of the way,
Is it just really just ONLY a marketing ploy by the oil companies to sell high octane/cetane fuels and the average normal customer does not benefit in anyways?

I , like so many of us, was attracted to the below by IOC that says about its XtraGreen
https://iocl.com/pages/XtraGreen

My thought process while adopting XtraGreen was that apart from the additional cetane %, it also (says it) brings down the Carbon emissions and NOx emissions and it would be beneficial in other angles, like degree of pollution - keeping performance aside.
Hopefully it would slightly lessen the soot generation as well as it is a more "cleaner" Diesel and hence (maybe?) could slightly reduce the REGEN frequency

Your valuable thoughts?

[Silver Knight]

Originally Posted by NomadSK

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.


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.


Cheers

Not correct, the nozzles are similar in most cases. You can even interchange the nozzles. At some very high-volume outlets where many trucks fill the nozzle size of diesel dispensers is slightly big.

[Sam800]

Originally Posted by NomadSK

6: A dirty air filter leads to lower mileage


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

I have couple of noob questions.

1. If dirty air filter doesn't affects the mileage in newer cars, wont the mileage drop if one presses pedal to metal to overcome the sluggish acceleration due to clogged filter?
Pressing the accelerator harder will obviously draw more fuel to the engine. I understand the engine won't pump more fuel by itself but the driver will push the pedal harder to overcome the sluggishness. And i think older carb car will be affected the same. Correct me if i am wrong, sir.

2. I understand carb engines had mechanical fuel pump which was operated by crankshaft and new ECU engines have electrical fuel pumps. The carb fuel filter used to be small netted cylinder and modern fuel filters are much better. But often in restoration posts we have read the fuel lines and pump got clogged due to rust and debris in fuel tank. Now these vehicles are mainly carb ones.

So in such cases, does the fuel filter can't perform its job or it is simply too small for debris or the lines get clogged before reaching the filter??

[Kkumar]

A wonderful article that can also be titled “MYTH BUSTER”. It has cleared a lot of misconceptions when it comes to fuel. Nice to see such well researched and lucid article. It is also one of the reasons why I joined this forum.
Thank you once again.

[norhog]

Thank you for the well written article.

What is the OCTANE numbers of gaseous fuels.
LPG which is Buthane and Propane at various mixing levels.
CNG which is Methane with some impurities.

[dj96]

Thank you for such an extensive write up. I just wanted to ask a few questions.

1. How does methanol injection come into play in all of this? I saw Jay leno's 600 grosser video where he mentions that he has added methanol injection as california gasoline comparatively low octane.

2. Please recommend some suitable octane boosters which one can carry as in India, high octane fuel is not readily available.

[Jeroen]

Originally Posted by Silver Knight

Not correct, the nozzles are similar in most cases. You can even interchange the nozzles. At some very high-volume outlets where many trucks fill the nozzle size of diesel dispensers is slightly big.

I really can’t remember how things are in India in this respect. But certainly in Europe diesel en petrol nozzles are different to prevent using the wrong fuel. But you can still mess up, ask me how I know��

Originally Posted by Sam800

I have couple of noob questions.

1. If dirty air filter doesn't affects the mileage in newer cars, wont the mileage drop if one presses pedal to metal to overcome the sluggish acceleration due to clogged filter?
Pressing the accelerator harder will obviously draw more fuel to the engine. I understand the engine won't pump more fuel by itself but the driver will push the pedal harder to overcome the sluggishness. And i think older carb car will be affected the same. Correct me if i am wrong, sir.

2. I understand carb engines had mechanical fuel pump which was operated by crankshaft and new ECU engines have electrical fuel pumps. The carb fuel filter used to be small netted cylinder and modern fuel filters are much better. But often in restoration posts we have read the fuel lines and pump got clogged due to rust and debris in fuel tank. Now these vehicles are mainly carb ones.

So in such cases, does the fuel filter can't perform its job or it is simply too small for debris or the lines get clogged before reaching the filter??

There is a fundamental difference between a carb engine and one with (electronic) fuel injection and control.

On a carb engine a clogged filter creates additional vacuum so more fuel gets pulled in through the carburettors. But the amount of air does not change or rather reduces. Essentially your engine will start running rich.
Which means less fuel efficient. The higher the load of the engine and or RPM the worse it gets.

On an electronically controlled fuel injected engine the air fuel ratio is always maintained to be the most optimum. So a clogged filter doesn’t really matter. Your ECU will adjust accordingly. So little or no influence on fuel efficiency.

Both cases as mentioned are likely to feel more sluggish.

There are plenty of carb engines that don’t have a mechanical but electrical fuel (petrol) pump. My W123 springs to mind.

Clogging of fuel line and or filters happens due to a number of reasons. In fact not all car fuel systems have filters build in. Modern engines with injection always do.

One of the problems with classic cars is they stand idle for months sometimes years. All fuel will go off over time. Normal petrol will turn into an almost jelly type of stuff, very sticky. It has happened to me once when my Alfa Spider was in storage for more than a year. We had to take the complete fuel system apart.

The current fuels with high ethanol contents can be particular devastating on old classic fuel systems, ruining and clogging up all parts. Particularly when not being used much

Jereon

[Sam800]

Originally Posted by Jeroen

On an electronically controlled fuel injected engine the air fuel ratio is always maintained to be the most optimum. So a clogged filter doesn’t really matter. Your ECU will adjust accordingly. So little or no influence on fuel efficiency.

Both cases as mentioned are likely to feel more sluggish.

Okay, understood. Does the fuel pump in fuel injected engine pumps the fuel at constant pressure all the times or the pressure is regulated as per demands from the engine? I understand the fuel is supplied to the cylinders through the nozzles but what about the pressure from fuel pump?

Originally Posted by Jeroen

There are plenty of carb engines that don’t have a mechanical but electrical fuel (petrol) pump. My W123 springs to mind.

Wow, Thats a news to me. I don't have much exposure to different vehicles than Maruti 800, Maruti Van or Tata indigo. There's always something to learn for good

Originally Posted by Jeroen

One of the problems with classic cars is they stand idle for months sometimes years. All fuel will go off over time. Normal petrol will turn into an almost jelly type of stuff, very sticky. It has happened to me once when my Alfa Spider was in storage for more than a year. We had to take the complete fuel system apart.

Oh dear, thats terrible. What can be done so that neither petrol turns sticky, nor the tank gets rusty? Are there any additives for the petrol which can be added to it to prevent jelly-fying?

Originally Posted by Jeroen

The current fuels with high ethanol contents can be particular devastating on old classic fuel systems, ruining and clogging up all parts. Particularly when not being used much

This is my 2nd most greatest fear for my vehicles after rusting.


I am a newbie member in team bhp and im honoured to share the platform with brightest minds on the forum. I'm looking forward to learn as much possible. Thanks OP for such informative post and sir Jeroen for clearing my doubts and to pass your knowledge to learners like me.

[Sai Teja]

Originally Posted by NomadSK

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

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

I can somewhat comprehend the variability of RON due to altitude, humidity and air temperature, however the other factors like spark advance, coolant temperature and combustion chamber deposits make me think whether it's really the fuels ability to resist detonation or there is some part of the engine architecture that is making the fuel to hold up longer from detonation. Am I missing any part of the puzzle here? may be someone could help me clarify this.

Anyways, this was something really insightful I've ever come across online, thanks for posting.

Mod Note: Please quote ONLY the relevant bits of a post. Quoting a full, long post inconveniences our mobile readers. Thank you.

[Zippy_wheels]

Originally Posted by NomadSK

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.

So I have a fairly simple question. The manual of my car (1.0 TSI) says use 95 octane and in emergency it is okay to use 91 (slight power loss and lower fuel efficiency) Attached is the part of the manual that describes this. Can you please break it down for us whether its okay to use 91 regularly in normal city and occasion highway driving conditions?

Is the only difference power and mileage or can it cause more engine wear and tear/damage?

Thanks!