Question

Which of the following statements about a petrol engine's octane and cetane rating is TRUE?
1. Diesel should have higher octane as well as cetane number
2. Diesel should have higher octane but lower cetane number
3. Diesel should have lower octane but higher cetane number
4. Diesel should have lower octane as well as cetane number

Answer 1

Correct Answer - Option 3 : Diesel should have lower octane but higher cetane number

Concept:

• Octane Number (ON) is the measure of the resistance of gasoline to detonation or preignition of the fuel in the SI engine.
• It is measured relative to the mixture of iso-octane (2, 2, 4 trimethyl pentane) (ON = 100) and n-heptane with (ON = 0).
• Higher octane fuel has a greater resistance to autoignition under higher combustion pressure and heat.
• Cetane Number (CN) is the measure of the combustion quality of diesel oil or it is the measure of the ignition delay.
• The cetane number is calculated by determining rich mixture of cetane (hexadecane)(CN = 100) and iso-cetane (CN = 0).
• The main difference between the cetane and octane number of a fuel is that the octane rating represents how well a fuel can resist pre-ignition due to compression which ensures that the fuel ignites only from a spark from the spark plug.
• However, the cetane number measures the delay in the ignition time of the fuel.
• Hence, both numbers are inversely related to each other and a fuel having high octane number will have a low cetane number and vice-versa.

Here, Diesel should have lower octane but higher cetane number