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In an SI engine very high compression ratio cannot be used because
- a)The engine efficiency would be unmanageably high
- b)The power required for compression would be high, so cylinders will require very thick walls
- c)Cylinder will require very thick walls
- d)Self-ignition may take place before the spark occurs
Correct answer is option 'D'. Can you explain this answer?
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In an SI engine very high compression ratio cannot be used becausea)Th...
If a very high compression ratio is used, than the petrol will reach its self to the ignition temperature before the spark and the knocking will also occur. So compression ratio of petrol engine is less.
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In an SI engine very high compression ratio cannot be used becausea)Th...
Explanation:
In a spark ignition (SI) engine, the combustion process is initiated by a spark plug. The fuel-air mixture is ignited by the spark, resulting in the expansion of gases and the generation of power. The compression ratio in an SI engine is the ratio of the volume of the cylinder at bottom dead center (BDC) to the volume at top dead center (TDC).
Reasons why very high compression ratios cannot be used:
1. Risk of self-ignition: In an SI engine, the fuel-air mixture should be ignited by the spark plug at the correct moment. If the compression ratio is too high, the temperature and pressure inside the combustion chamber increase significantly during the compression stroke. This can lead to self-ignition of the fuel-air mixture before the spark plug fires, causing knocking or detonation. Knocking is undesirable as it can lead to engine damage and reduced performance.
2. Thermal efficiency limitations: While it may seem counterintuitive, increasing the compression ratio does not always result in improved engine efficiency. The efficiency of an SI engine is influenced by various factors, including the expansion ratio, combustion efficiency, and heat losses. Very high compression ratios can lead to increased heat losses, especially through the cylinder walls. This can reduce the overall thermal efficiency of the engine.
3. Increased mechanical stresses: Higher compression ratios require the cylinder walls to withstand higher pressures during the compression stroke. If the compression ratio is excessively high, the cylinder walls may need to be thickened to prevent deformation or failure. However, thicker cylinder walls increase the weight of the engine and can lead to increased frictional losses and decreased performance.
4. Power requirements: The compression stroke in an SI engine requires work to be done on the air-fuel mixture to compress it. As the compression ratio increases, the power required for compression also increases. This means that a higher compression ratio would result in increased energy consumption for compression, reducing the overall power output of the engine.
Therefore, option D is the correct answer as the risk of self-ignition occurring before the spark plug fires is a significant limitation in using very high compression ratios in SI engines.
In a spark ignition (SI) engine, the combustion process is initiated by a spark plug. The fuel-air mixture is ignited by the spark, resulting in the expansion of gases and the generation of power. The compression ratio in an SI engine is the ratio of the volume of the cylinder at bottom dead center (BDC) to the volume at top dead center (TDC).
Reasons why very high compression ratios cannot be used:
1. Risk of self-ignition: In an SI engine, the fuel-air mixture should be ignited by the spark plug at the correct moment. If the compression ratio is too high, the temperature and pressure inside the combustion chamber increase significantly during the compression stroke. This can lead to self-ignition of the fuel-air mixture before the spark plug fires, causing knocking or detonation. Knocking is undesirable as it can lead to engine damage and reduced performance.
2. Thermal efficiency limitations: While it may seem counterintuitive, increasing the compression ratio does not always result in improved engine efficiency. The efficiency of an SI engine is influenced by various factors, including the expansion ratio, combustion efficiency, and heat losses. Very high compression ratios can lead to increased heat losses, especially through the cylinder walls. This can reduce the overall thermal efficiency of the engine.
3. Increased mechanical stresses: Higher compression ratios require the cylinder walls to withstand higher pressures during the compression stroke. If the compression ratio is excessively high, the cylinder walls may need to be thickened to prevent deformation or failure. However, thicker cylinder walls increase the weight of the engine and can lead to increased frictional losses and decreased performance.
4. Power requirements: The compression stroke in an SI engine requires work to be done on the air-fuel mixture to compress it. As the compression ratio increases, the power required for compression also increases. This means that a higher compression ratio would result in increased energy consumption for compression, reducing the overall power output of the engine.
Therefore, option D is the correct answer as the risk of self-ignition occurring before the spark plug fires is a significant limitation in using very high compression ratios in SI engines.
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In an SI engine very high compression ratio cannot be used becausea)The engine efficiency would be unmanageably highb)The power required for compression would be high, so cylinders will require very thick wallsc)Cylinder will require very thick wallsd)Self-ignition may take place before the spark occursCorrect answer is option 'D'. Can you explain this answer?
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