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In an IC engine, combustion was found to proceed during the expansion stroke also. The reason could be:
  • a)
    rich mixture with ignition advance
  • b)
    the high calorific value of the fuel
  • c)
    a weak mixture with ignition advance
  • d)
    weak mixture without ignition advance
Correct answer is option 'D'. Can you explain this answer?
Verified Answer
In an IC engine, combustion was found to proceed during the expansion...
Lean mixture: Mixture which contains more air than the stoichiometric requirement. There is more oxygen than required to burn the amount of fuel completely; after combustion, there is excess oxygen in the exhaust gases.
Rich Mixture: Mixture which contains less air than the stoichiometric requirement. There is not enough air to completely burn the amount of fuel; after combustion, there is unburnt fuel in the exhaust gases.
Spark advance/Ignition Advance:
The purpose of the spark advance mechanism is to assure that under every condition of engine operation, ignition takes place at the most favorable instant in time, i.e., most favorable from the standpoint of engine power, fuel economy, and minimum exhaust dilution. By means of these mechanisms, the advance angle is accurately set so that ignition occurs before the TDC point of the piston.
Ignition timing is the correct instant for the introduction of a spark near the end of the compression stroke in the cycle. Several factors affect the ignition timing of an engine, like compression ratio, Engine speed, Engine load, Quality of fuel, Mixture strength, etc.
In the case of a rich mixture, the combustion rate is faster and less ignition advanced is required. And in the case of a lean mixture, the combustion rate will be slower, thus requiring more ignition advances.
When ignition occurs early in the compression stroke, the engine is said to be advanced. A retardation ignition takes place when the piston is just near the compression stroke.
If the ignition is advanced too much, it will be completed before the end of the compression stroke. Under these conditions, the crankshaft and connecting rod will have to push the piston upward, compressing the gases. In such a case, the force might not be sufficient to overcome the pressure and the engine would stop or stall.
If the ignition is retarded too much, the combustion of fuel will continue during the power stroke of the piston (Expansion stroke), and the maximum pressure will not be developed. Less work will be obtained from heat energy. Under these conditions, comparatively more exhaust gases will be going out of the engine cylinder overheating the exhaust valve.
Conclusion:
Rich Mixture with ignition advance: It can complete before the compression stroke
Weak Mixture without ignition advance: Combustion can be found to proceed during the expansion stroke
The rich mixture should have retarded ignition advance, and the lean mixture should have more ignition advance.
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Most Upvoted Answer
In an IC engine, combustion was found to proceed during the expansion...
Combustion during the expansion stroke in an internal combustion engine (IC engine) can occur due to a weak mixture without ignition advance. This phenomenon is known as knock or detonation.

1. Combustion in an IC engine:
In an IC engine, combustion occurs in a series of four strokes: intake, compression, power, and exhaust. During the power stroke, the combustion of the air-fuel mixture generates high pressure and temperature, converting chemical energy into mechanical work.

2. Knock or detonation:
Knock or detonation refers to the uncontrolled combustion of the fuel-air mixture, which leads to a sudden increase in pressure and temperature. This can cause severe damage to the engine components, reduce efficiency, and result in a loss of power.

3. Weak mixture:
A weak mixture refers to a fuel-air mixture that has a lower ratio of fuel to air. In such a mixture, there is a limited amount of fuel available for combustion. This can lead to incomplete combustion, reduced power output, and increased emissions.

4. Ignition advance:
Ignition advance refers to the timing of the spark plug firing in relation to the position of the piston. Advancing the ignition timing means firing the spark plug earlier, allowing more time for complete combustion before the expansion stroke.

5. Weak mixture without ignition advance:
When a weak mixture is present in the combustion chamber without ignition advance, the combustion process may not be able to complete during the power stroke. This incomplete combustion can result in unburned fuel entering the expansion stroke.

6. Combustion during the expansion stroke:
If the unburned fuel-air mixture ignites during the expansion stroke, it causes an additional pressure rise in the cylinder. This can lead to knock or detonation, which is undesirable and can cause engine damage.

7. Reason for combustion during the expansion stroke:
The reason for combustion during the expansion stroke in this scenario is a weak mixture without ignition advance. The limited amount of fuel and the lack of proper timing for ignition result in incomplete combustion during the power stroke. The unburned fuel-air mixture then ignites during the expansion stroke, leading to knock or detonation.

In conclusion, combustion during the expansion stroke in an IC engine can occur when a weak mixture without ignition advance is present. This phenomenon, known as knock or detonation, can cause engine damage and reduce efficiency. Therefore, it is important to ensure proper fuel-air mixture and ignition timing to avoid this issue.
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In an IC engine, combustion was found to proceed during the expansion stroke also. The reason could be:a)rich mixture with ignition advanceb)the high calorific value of the fuelc)a weak mixture with ignition advanced)weak mixture without ignition advanceCorrect answer is option 'D'. Can you explain this answer?
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