JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II THERMAL ENGINEERING-I Previous Question Paper (July/August - 2021 - R15)

In an ideal four-stroke SI engine, the valves open and close exactly at Top Dead Center (TDC) and Bottom Dead Center (BDC). However, in an actual engine, valve timing is modified to account for gas inertia, pressure differences, and the time required for valve opening and closing, resulting in the actual valve timing diagram.

The key events in the actual valve timing diagram are as follows:

• Inlet Valve Opening (IVO): Occurs 10° to 20° before TDC (Valve Lead) to ensure the valve is fully open when the suction stroke begins.
• Inlet Valve Closing (IVC): Occurs 30° to 40° after BDC (Valve Lag). This utilizes the inertia of the incoming air-fuel mixture to increase volumetric efficiency.
• Spark Timing: Occurs 20° to 40° before TDC to allow for combustion delay and ensure peak pressure is reached shortly after TDC.
• Exhaust Valve Opening (EVO): Occurs 30° to 50° before BDC. This allows high-pressure exhaust gases to escape early, reducing back pressure during the exhaust stroke.
• Exhaust Valve Closing (EVC): Occurs 10° to 15° after TDC. This helps scavenge residual gases from the combustion chamber.

The period where both inlet and exhaust valves are open simultaneously is known as Valve Overlap. This is absent in the ideal diagram.

Comparison Summary:

The pressure feed lubrication system is a forced-feed method used in modern internal combustion engines to ensure reliable lubrication of critical moving parts, such as crankshaft bearings, connecting rod bearings, and camshaft bearings, regardless of engine speed or load.

The system operates through the following components and process:

• An oil pump, typically of the gear or rotor type, is driven by the engine camshaft or crankshaft. It draws lubricating oil from the oil sump through a strainer.
• The oil is pumped through a filter to remove impurities and contaminants before entering the main oil gallery.
• From the main oil gallery, the oil is distributed under pressure through drilled passages in the engine block and crankshaft to the main bearings and connecting rod big-end bearings.
• A pressure relief valve is incorporated into the circuit to maintain a constant oil pressure by bypassing excess oil back to the sump when the pressure exceeds a set limit.
• After lubricating the bearings, the oil drains back into the sump by gravity, where it is cooled and recirculated.

A sketch of the system would show the oil sump at the bottom, connected to the oil pump, which feeds into an oil filter. The filter connects to a main oil gallery running along the engine block. From this gallery, lines branch off to the main crankshaft bearings, with internal oil drillings in the crankshaft allowing oil to reach the connecting rod bearings. A pressure gauge is connected to the gallery to monitor system performance, and a pressure relief valve is positioned between the pump outlet and the gallery to regulate flow.

This system is superior to splash lubrication because it provides a constant, high-pressure film of oil, effectively cooling the bearings and removing heat from the friction surfaces, which is essential for high-speed, high-output engines.