JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II THERMAL ENGINEERING I Previous Question Paper (July/August - 2021 - R13)

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, fluid friction, and the finite time required for valve opening and closing.

The actual valve timing diagram incorporates the following features:

• Intake Valve Opening (IVO): Occurs before TDC (typically 10-20° BTDC) to ensure the valve is fully open when the piston begins the suction stroke, maximizing charge intake.
• Intake Valve Closing (IVC): Occurs after BDC (typically 30-40° ABDC). Due to the high velocity of the incoming air-fuel mixture, inertia keeps the charge flowing into the cylinder even as the piston starts moving upward.
• Ignition Timing: Spark occurs before TDC (typically 20-30° BTDC) to allow for flame propagation delay, ensuring peak pressure occurs slightly after TDC for maximum power.
• Exhaust Valve Opening (EVO): Occurs before BDC (typically 30-50° BBDC). This releases high-pressure exhaust gases early, reducing the work required by the piston to push gases out during the exhaust stroke.
• Exhaust Valve Closing (EVC): Occurs after TDC (typically 10-15° ATDC). This utilizes the inertia of exhaust gases to scavenge the remaining burnt gases from the clearance volume.

The valve overlap period is the interval where both intake and exhaust valves are open simultaneously, occurring between the end of the exhaust stroke and the beginning of the intake stroke. This is essential for effective scavenging.

Comparison Summary:

In a pressure feed lubrication system, oil is supplied to the engine components under positive pressure using a mechanical pump. This system ensures consistent lubrication regardless of engine speed or load, making it the standard for modern internal combustion engines.

The system operates through the following components and cycle:

1. An oil pump, typically driven by the camshaft or crankshaft, draws oil from the oil sump through a strainer.
2. The oil passes through an oil filter to remove impurities and contaminants.
3. The pressurized oil is then delivered to a main oil gallery, which runs the length of the engine block.
4. From the main gallery, oil is distributed through drilled passages to the main bearings of the crankshaft.
5. Oil flows through internal passages in the crankshaft to reach the connecting rod bearings (big-end bearings).
6. In some designs, oil is further directed to the camshaft bearings, valve train components, and timing gears.
7. After lubricating the moving parts, the oil drains back into the sump by gravity, where it is cooled and recirculated.

A pressure relief valve is integrated into the system to maintain a constant oil pressure; if the pressure exceeds a set limit, the valve opens to bypass excess oil back to the sump. A pressure gauge or warning light is provided on the dashboard to monitor the system's health.

Sketch description: The diagram should show the oil sump at the bottom, a gear-type oil pump connected via a pickup tube, an oil filter in the main line, a main gallery running horizontally, and branches leading to the crankshaft main bearings and big-end bearings, with a return path for the oil to the sump.