JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III IC ENGINES AND GAS TURBINES Previous Question Paper (November/December - 2020 - R16)

In a two-stroke engine, the P-V diagram consists of a compression stroke (upward curve), combustion at constant volume, expansion stroke (downward curve), and exhaust/scavenging occurring near the bottom dead center (BDC). The area enclosed represents the net work done per cycle.

The valve timing diagram for a two-stroke engine is characterized by the absence of intake and exhaust valves, relying instead on ports. The sequence is as follows:

• Exhaust port opens before BDC (typically 50° to 60° before BDC).
• Transfer port opens shortly after the exhaust port opens.
• Scavenging occurs while both ports are open.
• Transfer port closes after BDC (typically 50° to 60° after BDC).
• Exhaust port closes shortly after the transfer port closes, marking the start of compression.
• Ignition occurs before Top Dead Center (TDC).

Ignition quality in CI engines is measured by the cetane number, which represents the ignition delay period. Its influence is critical in the following areas:

• Cold Starting: High cetane fuels have a shorter ignition delay, which is essential for cold starting. During cold starts, the cylinder walls are cool, reducing the air temperature at the end of compression. A fuel with high ignition quality ignites readily under these lower temperature conditions, preventing misfiring.
• Engine Roughness: Engine roughness is caused by diesel knock, which occurs when a large amount of fuel accumulates in the combustion chamber during a long ignition delay, leading to a rapid, uncontrolled pressure rise upon ignition. High cetane fuels reduce the ignition delay, ensuring a smoother, more controlled combustion process and reducing noise and vibration.
• Compression Ratio: The compression ratio is primarily selected to ensure the air temperature at the end of compression is high enough to auto-ignite the fuel. If a fuel has a high ignition quality, the engine can theoretically operate with a lower compression ratio while still achieving reliable ignition. Conversely, low-quality fuels necessitate higher compression ratios to compensate for the longer ignition delay and ensure combustion stability.