What Is Octane Number: Difference Between Octane Number And Cetane Number [PDF]

Do you know that the higher the octane number, the better the fuel is for spark ignition engines? Do you know why high-performance cars use high-octane rating fuels?

Lets us find answers to all those questions and explore what is octane number and what are the differences between an octane number and a cetane number

Octane Number

Octane number is the measurement of resistance of fuel or gasoline against denotation or knocking in a spark-ignition internal combustion engine. The higher the octane number, the better the fuel is against denotation. The octane number is often called an octane rating.

High-performance cars use high-octane number fuel because engines are designed to have a higher compression ratio.

Due to the high compression ratio, those engines can produce more power. However, the high compression ratio can lead to early burn of fuel before the actual spark occurs. To avoid early burns of fuel, the high-performance engine uses high-octane number fuel.

Octanes are a family of hydrocarbons. Iso-octane is one of the hydrocarbons used in gasoline. Octane numbers are based on a scale in which iso-octane is 100 and n-heptane is 0 (Best-case scenario). The higher the proportion of iso-octane, the better the octane number.

The proportion of the presence of iso-octane is the benchmark for determining the octane number. If I say the octane number of fuel is 90, you may think that it has 90% iso-octane and 10% heptane. But that is not true. Because fuel has other additives too.

What the number 90 means is that the fuel will have the same performance characteristics as if the fuel has 90 % iso-octane. Many additives are mixed in the fuel to boost the octane number.

Detonation or Knocking

The reason we use high-octane rating fuel is to avoid detonation or knocking. Right? So let us understand what is that detonation or knocking.

We all know that the petrol engine is a spark-ignition engine. The engine compresses the air and fuel mixture and at the end of the compression stroke, a spark plug starts igniting the fuel.

Now if the unburnt fuel is heated and compressed too much ( As in the case of high-grade engines), the air-fuel mixture starts pre-igniting ( or self-igniting) before the actual spark occurs.

Pre-ignition creates shock waves and those shock waves create too much pressure inside the engine. No engine parts are designed to withstand that extra pressure.

Due to this, the engine starts detonating and produces a knock-knock sound. If the knocking is too severe then it may even damage the engine.

Why do diesel engines use low-octane-rated fuel?

Diesel engines are compression ignition engines. Only air is compressed and fuel is injected into the compressed air which is already heated. There is no spark plug in diesel engines to start ignition. The required heat to ignite fuel is generated by the compression of air.

Since there is no way that air and fuel mixture gets compressed in a diesel engine, so there is no chance of denotation or knocking.

As I said earlier the whole concept of using high-octane fuel is just to avoid knocking. Since there is no chance of knocking so why use high-octane fuel? That is the only reason, diesel engine uses low octane rating fuel.

Cetane Number

Cetane number or cetane rating is the performance indicator of the combustion speed of diesel. It is an inverse function of an octane number. High octane number fuel delays the ignition, but high cetane number fuel speeds up the ignition.

Unlike the octane number where the proportion of iso-octane sets the benchmark, in the case of diesel, it is the cetane (Hexadecane) that sets the benchmark. Often diesel uses additives like Alkyl nitrates and di-tert-butyl peroxide to raise the cetane rating.

Difference between octane number and cetane number

How to measure octane numbers?

There are two different methods of measurement of octane number as shown below.

• Research Octane Number ( RON)
• Motor Octane Number (MON)

Research octane number (RON)

This method uses a test engine with a variable compression ratio and measures the anti-knocking capability of fuel with different proportions of iso-octane and heptane. The compression ratio is varied to check the anti-knocking capability.

Motor octane number (MON)

This method uses the same type of test engine but with a high RPM ( 900 instead of 600 in RON). It uses variable ignition timing and preheated air-fuel mixture to check any knocking capability of fuel with different proportions of iso-octane and n-heptane. Usually, the value obtained in MON is lower by about 8-12 compared to the research octane number.

Anti Knock index

Most countries use the mean of the Research Octane number and Motor Octane number to determine the final value of the octane number which is called the anti-knock index.

Anti Knock Index = (RON+MON)/2

Fuel Sensitivity

The difference between research octane number and motor octane number is called fuel sensitivity

Fuel Sensitivity = RON-MON

How to measure the cetane number?

The cetane number of diesel can be measured by following test methods.

• Cooperative fuel research method ( CFR)
• Ignition quality tester (IQT)
• Fuel ignition tester

Cooperative fuel research method

In this method, the technician increases the pressure inside the engine chamber until the ignition delay is about 2.407 MS. Based on that ignition delay time, the technician checks the proportion of cetane and accordingly gets the Derived Cetane Number ( DCN)

Ignition quality tester

In this method, fuel is injected at 575°C and 310 PSI. The time between fuel injection and the time when the chamber reaches 310 PSI pressure is measured which is called the ignition delay. This ignition delay is then compared with the proportion of cetane present in the fuel and thus the cetane number is obtained.

Fuel Ignition Tester

In this method, fuel is injected at 575°C. The time is measured between the fuel injection and fuel combustion. That time is then compared with the percentage of cetane present in the fuel and accordingly, the cetane number is derived.

The octane number of petrol

The typical octane number of petrol is about 85-95. In the United States, unleaded gas has an octane number of about 87, and Mid-range gas has a value of about 88-90. The premium-grade petrol has an octane number of about 95. Most of the cars are suitable for 85-90 octane rating. Aircraft engines use fuel with an octane rating of 100

Cetane Number of Diesel

The typical cetane number of diesel varies from 40-50 and premium diesel has a cetane number of about 48-50. The higher the cetane number, the better the diesel fuel is.

Conclusion

Both the octane number and cetane numbers are performance indicators for fuels. Different countries use different test methods and different indexes. But ultimately they all check the same thing and follow the same logic of testing fuel. High octane-rated petrol is good for petrol engine and high cetane-rated diesel is good for diesel engines.

I hope you got a fair idea about the octane number and cetane number. Still, if you do have any queries or questions, you are always welcome to post a comment with your questions and I will be happy to answer you.

Frequently Asked Questions ( FAQ)