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Combustion in SI engines MCQs

1. What is the primary factor influencing flame development and propagation in an internal combustion engine?

a) Fuel octane rating
b) Air-fuel mixture ratio
c) Engine compression ratio
d) Ignition timing

Answer: b) Air-fuel mixture ratio

Explanation: The air-fuel mixture ratio plays a crucial role in flame development and propagation. An optimal mixture ensures efficient combustion, while deviations can lead to incomplete combustion or knock.

2. In a Pressure-Crank Angle diagram, what does the area under the curve represent?

a) Engine displacement
b) Indicated mean effective pressure
c) Combustion chamber volume
d) Exhaust gas temperature

Answer: b) Indicated mean effective pressure

Explanation: The area under the Pressure-Crank Angle curve represents the indicated work done by the engine per cycle, commonly known as indicated mean effective pressure (IMEP).

3. Which stage of combustion is characterized by the rapid release of energy due to the burning of the air-fuel mixture?

a) Ignition
b) Flame propagation
c) Flame stabilization
d) Afterburning

Answer: b) Flame propagation

Explanation: Flame propagation involves the rapid expansion of the flame front through the combustion chamber, releasing energy as the air-fuel mixture burns.

4. What is the term used to describe the delay between ignition initiation and noticeable combustion in the engine?

a) Combustion delay
b) Ignition lag
c) Detonation lag
d) Flame quenching

Answer: b) Ignition lag

Explanation: Ignition lag refers to the time delay between the initiation of ignition and the start of noticeable combustion in the engine cylinder.

5. How does increasing air density affect combustion in an engine?

a) Decreases combustion efficiency
b) Increases combustion stability
c) Reduces flame propagation speed
d) Enhances fuel vaporization

Answer: b) Increases combustion stability

Explanation: Higher air density improves the mixing of air and fuel, promoting better combustion stability and efficiency.

6. What effect does higher engine speed generally have on ignition timing requirements?

a) Requires advanced ignition timing
b) Requires retarded ignition timing
c) No effect on ignition timing
d) Depends on the fuel type

Answer: a) Requires advanced ignition timing

Explanation: Higher engine speeds necessitate advancing the ignition timing to ensure that the air-fuel mixture ignites at the optimal point in the engine cycle.

7. Which factor contributes to increased turbulence within the combustion chamber?

a) Larger piston displacement
b) Lower compression ratio
c) Narrower intake manifold
d) Swirl-inducing intake ports

Answer: d) Swirl-inducing intake ports

Explanation: Swirl-inducing intake ports promote turbulence within the combustion chamber, aiding in better mixing of air and fuel for improved combustion efficiency.

8. What is the primary cause of abnormal combustion in internal combustion engines?

a) Incorrect spark plug gap
b) Insufficient fuel octane rating
c) Excessive engine coolant temperature
d) Early ignition of the air-fuel mixture

Answer: d) Early ignition of the air-fuel mixture

Explanation: Abnormal combustion, such as pre-ignition and detonation, is primarily caused by the premature ignition of the air-fuel mixture within the combustion chamber.

9. How does pre-ignition differ from detonation in internal combustion engines?

a) Pre-ignition occurs after spark ignition, while detonation occurs before.
b) Pre-ignition involves multiple flame fronts, while detonation involves a single flame front.
c) Pre-ignition is caused by excessive air-fuel mixture richness, while detonation is caused by lean mixtures.
d) Pre-ignition is initiated by a hot surface, while detonation is initiated by pressure and temperature spikes.

Answer: d) Pre-ignition is initiated by a hot surface, while detonation is initiated by pressure and temperature spikes.

Explanation: Pre-ignition occurs when the air-fuel mixture ignites prematurely due to a hot surface within the combustion chamber, whereas detonation is caused by pressure and temperature spikes leading to uncontrolled combustion.

10. Which type of combustion chamber design is typically associated with diesel engines?

a) Hemispherical
b) Pent-roof
c) Swirl
d) Direct injection

Answer: d) Direct injection

Explanation: Direct injection combustion chambers are commonly found in diesel engines, where fuel is injected directly into the combustion chamber at high pressure for efficient combustion.

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