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

An efficient cooling system in an internal combustion engine must maintain the engine components at an optimal operating temperature to ensure structural integrity and lubrication efficiency. The key characteristics are:

• Uniform Cooling: The system must ensure uniform heat distribution across the cylinder head and block to prevent thermal stresses, warping, or distortion of components.
• Rapid Warm-up: It should allow the engine to reach its design operating temperature quickly to minimize fuel consumption and wear during the cold-start phase.
• Temperature Regulation: The system must maintain a constant temperature regardless of load or ambient conditions, typically achieved through thermostats that regulate coolant flow.
• Compactness and Reliability: The cooling system components, such as the radiator, pump, and fan, should be lightweight, compact, and highly reliable to minimize parasitic power losses.
• Corrosion and Scale Prevention: The coolant must be chemically stable to prevent corrosion of metal surfaces and the accumulation of scale deposits that impede heat transfer.
• Low Power Consumption: The energy required to drive the cooling pump and fan should be minimal to maximize the overall thermal efficiency of the engine.

A simple carburetor suffers from several limitations due to its basic design, which relies on a single venturi and fuel jet. The primary limitations include:

• Mixture Strength Variation: As engine speed increases, the air-fuel ratio tends to become richer because the density of air decreases while fuel flow increases disproportionately, leading to poor fuel economy.
• Starting Difficulty: During cold starts, the air velocity is too low to create sufficient pressure drop to draw fuel, resulting in a lean mixture.
• Idling Issues: At low speeds, the venturi effect is insufficient to lift fuel from the main jet, causing the engine to stall.
• Acceleration Lag: Rapid opening of the throttle causes a sudden drop in vacuum, leading to a momentary lean mixture and engine hesitation.

These limitations are rectified by incorporating auxiliary systems into the carburetor:

• Idling System: A separate fuel passage bypasses the main venturi to supply fuel directly to the intake manifold during idling.
• Economizer System: A valve reduces fuel flow at high loads to maintain the correct air-fuel ratio and improve economy.
• Acceleration Pump: A small pump injects an extra charge of fuel into the venturi when the throttle is opened suddenly to prevent stalling.
• Choke Valve: A butterfly valve restricts air intake during cold starts, increasing the vacuum and ensuring a rich mixture for ignition.
• Compensating Jet: An air bleed or compensating jet is used to lean out the mixture at high speeds, counteracting the richness caused by the main jet.