4.What is the effect of increase in temperature and pressure on knocking in a CI engine.

 In a Compression Ignition (CI) engine, commonly known as a diesel engine, knocking refers to the premature ignition of the air-fuel mixture in the combustion chamber, which causes a sudden and abnormal pressure rise. The effect of temperature and pressure on knocking in a CI engine is as follows:

1. Effect of Increase in Temperature:

• Increased Knocking Tendency: As the temperature inside the combustion chamber rises, it accelerates the rate of chemical reactions, leading to a quicker ignition of the fuel-air mixture. This can cause the fuel to ignite too early before the piston reaches its optimal position.
• Auto-Ignition: Higher temperatures reduce the ignition delay, making it more likely for the fuel to auto-ignite, which can cause knocking. This is because fuel reaches its auto-ignition temperature faster.
• Heat of Compression: Increased temperatures can also result from higher compression, contributing to higher pressures and promoting knocking.

2. Effect of Increase in Pressure:

• Increased Knocking Tendency: As pressure increases, it also increases the temperature of the air-fuel mixture in the combustion chamber (since pressure and temperature are directly related in thermodynamics). This can lead to an earlier auto-ignition of the fuel, promoting knocking.
• Increased Compression: In CI engines, high compression ratios are used for better fuel efficiency, but higher compression also raises the temperature and pressure, which can result in knocking.
• More Premature Combustion: High pressure compresses the air-fuel mixture more, which may cause it to ignite before the desired time, triggering a knocking event.

Summary:

• Increased temperature and increased pressure both tend to make knocking more likely in a CI engine because they reduce the ignition delay, causing the fuel to ignite prematurely.
• To minimize knocking, engine parameters like fuel quality, compression ratio, and engine cooling systems need to be optimized to control both pressure and temperature in the combustion chamber.

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