Engineering Chemistry Multiple Choice Questions on “Knocking”.
1. Which compound of fuel is oxidised in the process of knocking?
a) Coke
b) Gasoline
c) Hydrocarbon
d) Compounds containing sulphur
Answer: c
Clarification: In an internal combustion engine, a mixture of air and petrol vapour is compressed and ignited by an electric spark and the essential chemical reaction is the oxidation of the hydrocarbon molecules.
2. On what factor does the rate of oxidation of a hydrocarbon molecule depend?
a) On the calorific value of the compound
b) On the number of carbon atoms
c) On the rate of combustion
d) On its moisture content
Answer: b
Clarification: The rate of oxidation of a hydrocarbon molecule depends on the number of carbon atoms in the molecule, on the structure and on the temperature. The rate of oxidation should be moderate.
3. How will be the knocking sound produce in an engine?
a) Due to an increase in the rate of oxidation of hydrocarbon
b) Due to a decrease in the rate of oxidation of hydrocarbon
c) Due to the impurities present in fuel
d) Due to an increase in the percentile of ethanol molecule
Answer: a
Clarification: In the mixture of air and petrol vapour, there occurs a condition when the rate of oxidation increases to such a value that it starts producing sound in the engine which are called a knock. This decreases the octane rating of gasoline.
4. On what factor does the temperature of the engine depend?
a) Rate of oxidation
b) Size of the engine
c) Compression ratio of the engine
d) Fuel used in the engine
Answer: c
Clarification: Compression ratio is the ratio of the cylinder volume at the end of the suction stroke to the volume at the end of the compression stroke of the piston. Rate of oxidation is depended on the temperature.
5. The power output of engine increases continuously with an increase in the compression ratio.
a) True
b) False
Answer: b
Clarification: With the help of a variable compression engine, in actual practice, the power increases to a maximum value and then falls rapidly with a further increase in the compression ratio.
6. At what value of compression ratio, a knock can be heard?
a) At the minimum value
b) At the maximum value for power output
c) At the maximum value above the power output
d) In between of lowest and highest value
Answer: c
Clarification: The compression rate, corresponding to the maximum power output is known as a useful compression ratio at which a slight metallic knock can be heard. This knock becomes louder when the compression ratio goes on increasing.
7. Which of the following compound produces the least knocking?
a) Benzyl
b) Paraffin
c) Olefin
d) Diesel
Answer: a
Clarification: Aromatic compounds produces least knocking due to their cyclic structures. Straight chain hydrocarbons produces more knocking due to this effect.
8. How is the knocking tendency increased?
a) Due to the compression ratio of the engine
b) Due to size of the engine
c) Due to the decrease in the engine speed
d) Due to a defect in an engine
Answer: c
Clarification: The knocking tendency is increased by reducing the engine speed and by advancing the ignition of the engine. The engine should work in its original way.
9. Which of the following compound has the least knocking properties?
a) n-heptane
b) n-hexane
c) n-pentane
d) n-butane
Answer: d
Clarification: The anti-knock properties decreases with an increase in the length of the hydrocarbon chain. The octane number of n-butane, n-pentane, n-hexane and n-heptane are 90, 60, 29 and 0 respectively.
10. Olefins have higher anti-knocking properties than the corresponding ______________
a) Paraffin
b) Benzene
c) Toluene
d) Nitrobenzene
Answer: a
Clarification: The anti-knocking tendency increases as the position of a double bond approaches the centre of the chain. Paraffin consists of a single bond and are straight chained due to which they are less anti-knocking then olefins.