250+ TOP MCQs on Octane and Cetane Number and Answers

Engineering Chemistry Multiple Choice Questions on “Octane and Cetane Number”.

1. Which of the following compound is considered for calculating the octane number?
a) n-heptane
b) n-hexane
c) iso-octane
d) iso-butane
Answer: c
Clarification: The octane number of a fuel is numerically taken as the percentage of iso-octane in a mixture of iso-octane and n-heptane. The octane number of iso-octane is 100 and of n-heptane is 0.

2. Which of the following compound readily goes under the process of knocking?
a) 2-ethly butane
b) n-heptane
c) benzene
d) toluene
Answer: b
Clarification: n-heptane is a straight chain hydrocarbon which knocks readily and hence its octane number is zero. Presence of straight chain hydrocarbon in gasoline reduces its rate of combustion.

3. Which of the following compound is considered for calculating the cetane number?
a) α-methyl naphthalene
b) n-hexane
c) iso-octane
d) cetane molecule
Answer: d
Clarification: Cetane, C₁₆H₃₄, is a saturated hydrocarbon that has a very short ignition lag as compared to any commercial diesel fuel. Its cetane number is 100. Cetane number is the percentile of cetane in a mixture of cetane and α-methyl naphthalene.

4. What should be the cetane number of middle speed diesel engine?
a) At least 35
b) At least 25
c) At least 65
d) At least 45
Answer: a
Clarification: The cetane number of high speed diesel engine should be at least 45, that of middle speed diesel engine should be at least 35 and that of low speed diesel engine should be at least 25.

5. The knocking characteristic of gasoline fuel are expressed in terms of cetane number.
a) True
b) False
Answer: b
Clarification: The knocking characteristics of diesel fuel are expressed in terms of cetane number and that of gasoline fuel is expressed in terms of octane number. The molecule with the highest octane number has a least cetane number.

6. Which of the following substance is used to decrease knocking in diesel fuel?
a) Tetra ethyl lead
b) Benzene
c) Sodium hydroxide
d) Acetone peroxide
Answer: d
Clarification: The knocking in diesel fuel can be reduced by adding substances like ethyl nitrite, ethyl nitrate, isoamyl nitrate etc. This process also increases the cetane number of diesel fuel.

7. Which of the following is used to reduce the surface tension in diesel fuel?
a) Chemical additives
b) Inhibitors
c) Poly hydrocarbon
d) Dopes
Answer: a
Clarification: Chemical additives are used to reduce surface tension thus promoting the formation of finer spray. Inhibitors are used to delay or prevent gum formation. Poly hydrocarbons are used to preserve the fluid properties.

8. By which process does the knocking starts in diesel engine?
a) Due to sudden spontaneous combustion of last portion of fuel
b) Due to delay in spontaneous combustion of last portion of fuel
c) Due to the rise in temperature of diesel engine
d) Due to the parts of diesel engine
Answer: b
Clarification: Due to the presence of impurities in diesel, its spontaneous combustion of the last portion of fuel takes some time. Due to this their produces an uneven sound in the engine which is called knocking.

9. Which of the following exhaust gas produces least harmless components after its combustion?
a) Carbon monoxide
b) Ethyne
c) NO_x
d) N₂
Answer: d
Clarification: Gases produced by the compression ignition engines are said to be exhaust gases. This engines operate with air-fuel ratios of 14:1 on load and 70:1 on idling.

10. Which of the following has the highest cetane number?
a) n-heptane
b) n-hexane
c) n-pentane
d) n-butane
Answer: a
Clarification: n-heptane has maximum branched atom due to which it has the highest cetane number of all. This fuel is good for diesel engines.