Answer & Solution
Answer: Option
B
Solution:
Let's understand this concept step by step.
Compression Ignition (C.I.) Engine, also known as a diesel engine, works differently from a petrol engine. In a C.I. engine, only air is compressed inside the cylinder. Due to the high compression ratio, the air temperature rises significantly. Diesel fuel is then injected into this hot compressed air, where it ignites automatically without the need for a spark plug.
What is knocking in a C.I. engine?
Knocking in a compression ignition engine is a sharp metallic noise caused by rapid and uncontrolled combustion of fuel. It mainly occurs when there is a long ignition delay, which is the time interval between fuel injection and the start of combustion.
During a long ignition delay, more fuel continues to be injected into the combustion chamber before the first portion of fuel ignites. As a result, a large quantity of fuel accumulates inside the cylinder. When ignition finally starts, this accumulated fuel burns almost simultaneously, producing a very high rate of pressure rise. This sudden pressure rise creates vibration and the characteristic diesel knocking sound.
Effect of Higher Combustion Chamber Wall Temperature
When the combustion chamber walls are at a higher temperature, they transfer additional heat to the compressed air and the injected fuel droplets. This increases the temperature inside the combustion chamber even further.
Because the fuel is surrounded by hotter air, it reaches its self-ignition temperature much faster. Consequently, the ignition delay period becomes shorter, and combustion starts soon after fuel injection.
Since the ignition delay is reduced, only a small amount of fuel accumulates before combustion begins. The fuel then burns gradually and smoothly instead of burning all at once. This prevents a sudden pressure rise and significantly reduces the knocking tendency.
Key Points to Remember
• Higher combustion chamber wall temperature → Higher in-cylinder temperature.
• Higher temperature → Shorter ignition delay.
• Shorter ignition delay → Less fuel accumulation before combustion.
• Less accumulated fuel → Smoother combustion and lower pressure rise.
• Therefore, higher combustion chamber wall temperature reduces knocking in C.I. (diesel) engines.
Exam Tip: In diesel (C.I.) engines, knocking is mainly controlled by reducing the ignition delay period. Any factor that shortens ignition delay, such as higher compression ratio, higher combustion chamber temperature, or higher cetane number fuel, helps reduce knocking. In contrast, petrol (S.I.) engines require a longer ignition delay to avoid knock.
