Jawaharlal Nehru Technological University Hyderabad B.Tech II THERMAL ENGINEERING - I Previous Question Paper (August/September - 2022 - R15)

Internal combustion engines generate high temperatures during the combustion process, with gas temperatures reaching 2000°C to 2500°C. If this heat is not dissipated, it leads to several critical issues: the lubricating oil film breaks down, causing metal-to-metal contact and seizure; structural components like pistons, cylinders, and valves lose mechanical strength; and thermal stresses cause distortion or cracking. Furthermore, excessive heat leads to pre-ignition and knocking, reducing engine efficiency and life. Thus, a cooling system is essential to maintain engine components at a safe operating temperature.

An air cooling system removes heat directly from the cylinder walls and head to the atmosphere. It is primarily used in small engines like motorcycles, lawnmowers, and some aircraft engines. The system relies on the following design features:

• Fins: The outer surface of the cylinder block and head is provided with extended surfaces called fins. These increase the surface area available for heat transfer, significantly enhancing the rate of heat dissipation to the surrounding air.
• Airflow: Heat is carried away by the flow of air over these fins. This may be achieved by the natural motion of the vehicle (ram air effect) or by a forced draft provided by a fan or blower driven by the engine crankshaft.
• Baffles: Metal shrouds or baffles are often used to direct the airflow specifically over the hottest parts of the cylinder and head to ensure uniform and efficient cooling.

The sketch of an air-cooled engine consists of a cylinder block with a series of parallel fins cast onto the exterior. A fan is typically mounted on the flywheel to force air through the fin passages. The heat flows by conduction from the inner cylinder wall to the fins, and then by convection from the fin surfaces to the moving air stream.

Advantages include simplicity, lower weight, and no risk of leakage or freezing. Disadvantages include higher noise levels, limited cooling capacity for large engines, and non-uniform temperature distribution.

A solid injection system, also known as airless injection, is a fuel injection method where liquid fuel is injected directly into the engine cylinder at a very high pressure without the use of compressed air. In this system, the fuel is pressurized by a high-pressure pump and atomized through a nozzle into the combustion chamber. This system is standard in modern compression ignition (diesel) engines because it eliminates the need for a bulky air compressor, resulting in higher thermal efficiency and a more compact engine design.

The jerk pump is a common type of solid injection system that delivers a metered quantity of fuel at high pressure to the injector at the precise moment required for combustion. Its working principle is based on a reciprocating plunger driven by a cam mechanism.

The working process of a jerk pump involves the following stages:

1. Suction Stroke: As the cam rotates, the plunger moves downward due to a spring force, creating a vacuum in the pump barrel. Fuel flows from the supply tank into the barrel through the inlet port.
2. Compression Stroke: As the cam continues to rotate, it pushes the plunger upward. Initially, the fuel is pushed back into the supply line until the plunger covers the inlet port.
3. Injection Stroke: Once the inlet port is covered, the fuel is trapped and pressurized as the plunger continues to rise. When the pressure exceeds the spring force of the injector, the delivery valve opens, and high-pressure fuel is forced through the nozzle into the combustion chamber.
4. Pressure Relief: The plunger typically features a helical groove. As the plunger rises further, the helical edge uncovers the spill port, which allows the high-pressure fuel to escape back to the suction side, causing the pressure to drop instantly and terminating the injection process.

The quantity of fuel injected is controlled by rotating the plunger, which changes the effective stroke length by altering the point at which the helical groove uncovers the spill port relative to the cam position.