Assertion (A): A very high compression ratio is favoured for a CI engine, in order toattain high mechanical efficiency without knocking.
Reason (R): The delay period in CI combustion affects rate of pressure rise and hence knocking.
a)
Both A and R are individually true and R is the correct explanation of A
b)
Both A and R are individually true but R is not the correct explanation of A
c)
A is true but R is false
d)
A is false but R is true
Correct answer is option 'D'. Can you explain this answer?

Question

Assertion (A): A very high compression ratio is favoured for a CI engine, in order toattain high mechanical efficiency without knocking.
Reason (R): The delay period in CI combustion affects rate of pressure rise and hence knocking.

a)
Both A and R are individually true and R is the correct explanation of A
b)
Both A and R are individually true but R is not the correct explanation of A
c)
A is true but R is false
d)
A is false but R is true

Correct answer is option 'D'. Can you explain this answer?

Answer

Assertion: A very high compression ratio is favored for a CI engine in order to attain high mechanical efficiency without knocking.

Reason: The delay period in CI combustion affects the rate of pressure rise and hence knocking.

Explanation:
Compression ratio is defined as the ratio of the total cylinder volume at bottom dead center (BDC) to the total cylinder volume at top dead center (TDC) in an engine. In a compression ignition (CI) engine, the air-fuel mixture is compressed to a high pressure and temperature before ignition occurs due to the heat of compression. This compression causes the air-fuel mixture to self-ignite, leading to combustion without the need for a spark plug.

High compression ratio for high mechanical efficiency:
1. A high compression ratio leads to increased air density in the combustion chamber, resulting in better combustion efficiency.
2. A higher compression ratio allows for more work to be extracted from the combustion process, leading to increased mechanical efficiency.
3. The higher the compression ratio, the greater the expansion ratio during the power stroke, which results in better conversion of heat energy into mechanical work.
4. Thus, a high compression ratio is favored in a CI engine to achieve higher mechanical efficiency.

Delay period and knocking:
1. Delay period refers to the time between the start of fuel injection and the start of combustion in a CI engine.
2. During the delay period, the fuel is atomized and mixed with air, and the fuel droplets vaporize and start to react with oxygen.
3. The duration of the delay period affects the rate of pressure rise during combustion.
4. If the delay period is too short, rapid pressure rise can occur, leading to knocking.
5. Knocking is an undesirable phenomenon that occurs when the air-fuel mixture detonates instead of burning smoothly, resulting in a knocking sound and potential engine damage.
6. The delay period is influenced by factors such as fuel injection timing, fuel properties, and engine operating conditions.
7. Hence, the delay period in CI combustion affects the rate of pressure rise and can contribute to knocking.

Conclusion:
The assertion is false because a very high compression ratio is not favored for a CI engine to attain high mechanical efficiency without knocking. The delay period in CI combustion does affect the rate of pressure rise and hence knocking. Therefore, option D is the correct answer.

Top Courses for Mechanical EngineeringView all

General Aptitude for GATE

Engineering Mathematics

General Awareness & Knowledge

GATE Mechanical (ME) Mock Test Series 2025

Strength of Materials (SOM)

Top Courses for Mechanical Engineering

Top Courses for Mechanical Engineering

Suggested Free Tests

Related Content

Mock Test Series

Subject-wise Courses

Subject-wise Courses

Other Engg Branches

Company