Consider the following statements in the light of octane number and cetane number of fuels: 1. Cetane number is used for gasoline and octane number is used for diesel. 2. Octane number of n-hexadecane is 100 and cetane number of isooctane is 100. Which of the statements given above is/are correct?
- A.1 only
- B.2 only
- C.Both 1 and 2
- D.Neither 1 nor 2
Correct answer
Explanation
The correct answer is D, Neither 1 nor 2. Both statements swap the two scales: octane number rates petrol, cetane number rates diesel, and their reference fuels are the other way round.
Octane number measures how well petrol (gasoline) resists knocking in a spark-ignition engine. Its scale sets isooctane (2,2,4-trimethylpentane) at 100 and n-heptane at 0. Cetane number measures how readily diesel ignites under compression in a diesel engine, where quick ignition is wanted. Its scale sets n-hexadecane, called cetane, at 100 and alpha-methylnaphthalene at 0. So a good petrol has a high octane number and a good diesel a high cetane number.
A is wrong because statement 1 reverses the fuels: octane is for gasoline and cetane is for diesel. B is wrong because statement 2 reverses the reference compounds: isooctane has octane number 100, and n-hexadecane has cetane number 100. C is wrong because neither statement is correct.
Exam tip: Octane = petrol, isooctane 100, n-heptane 0; Cetane = diesel, n-hexadecane 100.
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