The tendency of knocking in a Compression Ignition (CI) engine can be reduced by injecting fuel just before the Top Dead Center (TDC). This is the correct answer because it helps to control the combustion process and minimize the occurrence of knocking. Let's understand why this is the case in detail.

What is knocking in a CI engine?
Knocking refers to the undesirable phenomenon in CI engines where the air-fuel mixture ignites spontaneously at multiple points within the combustion chamber instead of being ignited by the spark plug. This creates high-pressure waves that collide with each other, resulting in a knocking sound. Knocking causes engine damage, decreased efficiency, and increased emissions.

How does injecting fuel just before TDC reduce knocking?
Injecting the fuel just before the TDC helps to control the combustion process and prevent knocking. Here's how it works:

1. Delaying ignition: By injecting the fuel just before TDC, the ignition delay period is increased. This delay allows the air-fuel mixture to mix more thoroughly, resulting in a more homogeneous mixture. A homogeneous mixture burns more uniformly and reduces the likelihood of knocking.

2. Reducing peak pressure: Injecting fuel just before TDC also helps in reducing the peak pressure inside the combustion chamber. When the fuel is injected earlier, it has more time to mix with the air, resulting in a higher peak pressure. This high pressure can lead to knocking. By injecting fuel just before TDC, the peak pressure is controlled, reducing the chances of knocking.

3. Optimizing combustion: Injecting fuel just before TDC allows for better control over the combustion process. The fuel can be injected in a way that promotes efficient and complete combustion, minimizing the formation of unburned fuel molecules that can contribute to knocking.

Other options and their effects:
Let's briefly discuss the other options mentioned in the question and their effects on knocking:

a) High self-ignition temperature of fuel: While high self-ignition temperature can reduce the chances of knocking, it is not as effective as injecting fuel just before TDC. High self-ignition temperature alone may not be sufficient to prevent knocking in all operating conditions.

b) Decrease in jacket water temperature: Decreasing the jacket water temperature can have some impact on reducing knocking by lowering the overall temperature inside the combustion chamber. However, this approach is not as effective as injecting fuel just before TDC.

d) Decrease in injection pressure: Decreasing the injection pressure can affect the fuel atomization and mixing, but it may not be as effective as injecting fuel just before TDC in reducing knocking.

In conclusion, injecting fuel just before the Top Dead Center (TDC) is an effective way to reduce the tendency of knocking in a Compression Ignition (CI) engine. It helps to control the combustion process, delay ignition, reduce peak pressure, and optimize combustion, resulting in a more efficient and reliable engine operation.

The tendency of knocking in CI engine reduced by injection of fuel just before TDC.

Knocking is a phenomenon that occurs in internal combustion engines when the air-fuel mixture in the combustion chamber ignites spontaneously before the spark plug fires. This causes a rapid increase in pressure and temperature, resulting in a knocking or pinging sound. Knocking can lead to engine damage and reduced efficiency. In a Compression Ignition (CI) engine, knocking can occur due to various factors including the properties of the fuel, temperature, and pressure conditions in the combustion chamber.

Effects of the given options on knocking:

1. High self-ignition temperature of fuel:
- Self-ignition temperature refers to the temperature at which a fuel can ignite spontaneously without an external ignition source.
- Fuels with high self-ignition temperatures are less prone to knocking because they require higher temperatures to ignite.
- While high self-ignition temperature of fuel can reduce knocking to some extent, it is not the most effective option among the given choices.

2. Decrease in jacket water temperature:
- The jacket water temperature refers to the temperature of the cooling water that circulates around the engine to maintain its operating temperature.
- Decreasing the jacket water temperature can lead to higher combustion chamber temperatures, which can increase the tendency of knocking.
- Therefore, decreasing the jacket water temperature is not an effective way to reduce knocking in a CI engine.

3. Injection of fuel just before TDC:
- TDC stands for Top Dead Center, which is the position of the piston at the end of the compression stroke.
- Injecting the fuel just before TDC allows more time for the fuel to mix with the air in the combustion chamber before ignition.
- This improves the uniformity of the air-fuel mixture and reduces the chances of localized areas with high temperature and pressure, which can cause knocking.
- Therefore, injecting the fuel just before TDC is an effective way to reduce knocking in a CI engine.

4. Decrease in injection pressure:
- Injection pressure refers to the pressure at which the fuel is injected into the combustion chamber.
- Higher injection pressure helps in better atomization of the fuel, leading to improved mixing with air and reduced chances of knocking.
- Therefore, decreasing the injection pressure can increase the tendency of knocking in a CI engine.

Conclusion:
Among the given options, the most effective way to reduce knocking in a CI engine is to inject the fuel just before TDC. This allows for better mixing of the air-fuel mixture and reduces the chances of localized areas with high temperature and pressure, which are the main causes of knocking.