Jawaharlal Nehru Technological University Hyderabad B.Tech II THERMAL ENGINEERING-I Previous Question Paper (November/December - 2020 - R15)

In a two-stroke SI engine, the port timing diagram illustrates the opening and closing of the intake, transfer, and exhaust ports relative to the crank angle. Unlike the ideal diagram, which assumes instantaneous port opening and closing at Top Dead Center (TDC) and Bottom Dead Center (BDC), the actual diagram accounts for the physical constraints of gas dynamics and mechanical operation.

The actual port timing diagram incorporates the following features:

• Exhaust Port Opening (EPO): Occurs before BDC (typically 50° to 60° before BDC) to allow high-pressure combustion gases to escape, reducing pressure before the transfer port opens.
• Transfer Port Opening (TPO): Occurs after EPO (typically 40° to 50° before BDC). The delay ensures that the cylinder pressure drops below the crankcase pressure, preventing exhaust gases from entering the crankcase.
• Transfer Port Closing (TPC): Occurs after BDC (typically 40° to 50° after BDC).
• Exhaust Port Closing (EPC): Occurs after TPC (typically 50° to 60° after BDC). This delay allows for scavenging, where the fresh charge helps push out remaining exhaust gases, but it risks some fresh charge escaping through the exhaust port.
• Inlet Port Opening/Closing: Controlled by the piston skirt, typically opening before TDC and closing after TDC to utilize the pressure drop in the crankcase during the upward stroke.

The comparison between the two is summarized as follows:

The actual diagram is essential for engine design to balance the trade-off between efficient scavenging and the loss of fresh charge through the exhaust port, a phenomenon known as short-circuiting.

A high-performance ignition system is essential for the efficient operation of an internal combustion engine. The following are the critical qualities required for a good ignition system:

• Reliable Spark Generation: The system must consistently produce a spark of sufficient intensity to ignite the air-fuel mixture under all operating conditions, including varying speeds, loads, and compression ratios.
• Precise Timing: It must deliver the spark at the exact required moment in the cycle. Accurate spark advance is necessary to account for engine speed and load to ensure maximum pressure development occurs at the optimal crank angle.
• High Voltage Capability: The system must be capable of generating a voltage high enough to overcome the electrical resistance of the spark plug gap, which increases significantly under high-pressure conditions within the combustion chamber.
• Durability and Reliability: Components such as the ignition coil, distributor, and spark plugs must withstand extreme thermal, mechanical, and electrical stresses without failure over long periods of operation.
• Low Maintenance: A good system should require minimal adjustment or replacement of parts, ensuring long service intervals and ease of maintenance for the user.
• Efficiency and Energy Consumption: The system should consume minimal electrical power from the battery or charging system while maintaining high spark energy.
• Compact and Lightweight Design: To facilitate easier integration into the engine assembly, the components should be compact and add minimal weight to the overall engine package.
• Resistance to Environmental Factors: The system must be well-insulated and protected against moisture, dust, heat, and vibration to prevent short circuits or timing drift.
• Rapid Recovery: At high engine speeds, the system must have a fast recovery time to recharge the ignition coil between successive firing events to prevent misfiring.