JNTU
JNTU B.Tech III IC ENGINES AND GAS TURBINES Previous Question Paper (February/March - 2022 - R16)
Answer any five questions. All Questions carry equal marks.
Main
What is Volumetric efficiency? Explain in detail how the volumetric efficiency is influenced by inlet and outlet valve settings.
Answer
Volumetric efficiency is defined as the ratio of the actual mass of air induced into the engine cylinder during the suction stroke to the theoretical mass of air that should occupy the swept volume of the cylinder at ambient atmospheric conditions. It serves as a measure of the engine's ability to breathe and is expressed as:
The volumetric efficiency is significantly influenced by valve timing settings:
- Inlet Valve Opening (IVO): Opening the inlet valve before Top Dead Centre (TDC), known as valve lead, allows the valve to be partially open when the piston begins its downward stroke, ensuring maximum flow area during the high-velocity intake period.
- Inlet Valve Closing (IVC): Closing the inlet valve after Bottom Dead Centre (IVC), known as valve lag, utilizes the inertia of the incoming air column. Even as the piston starts moving upward, the momentum of the air continues to force charge into the cylinder, increasing the total mass trapped.
- Exhaust Valve Opening (EVO): Opening the exhaust valve before BDC allows for the blowdown process, reducing cylinder pressure early and minimizing backpressure during the subsequent exhaust stroke.
- Exhaust Valve Closing (EVC): Closing the exhaust valve after TDC, known as valve overlap, helps scavenge residual gases from the clearance volume, creating a slight vacuum that assists in drawing in fresh charge, though excessive overlap can lead to charge short-circuiting.
Compare and contrast two stroke and four stroke engines.
Answer
| Feature | Four-Stroke Engine | Two-Stroke Engine |
|---|---|---|
| Cycle Completion | Requires two revolutions of the crankshaft. | Completed in one revolution of the crankshaft. |
| Power Strokes | One power stroke every two revolutions. | One power stroke every revolution. |
| Valve Mechanism | Uses valves and a camshaft/timing gear assembly. | Uses ports cut into the cylinder walls; simpler construction. |
| Thermal Efficiency | Generally higher due to better scavenging and separation of processes. | Lower due to charge short-circuiting and incomplete scavenging. |
| Lubrication | Separate lubrication system (sump). | Often uses oil mixed with fuel (petroil) or total loss system. |
| Weight/Size | Heavier and bulkier due to complex valve train. | Lighter and more compact with higher power-to-weight ratio. |
| Torque | More uniform torque delivery. | Less uniform due to frequent power impulses. |
Explain in detail how (i) Time loss factor (ii) Heat loss factor causes variation between Ideal and Actual cycles of IC engine.
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List the internal combustion engine components along with their materials.
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Discuss the various air fuel mixture requirements according to the vehicle operating conditions. (Idling, cruising and acceleration).
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Explain the working principle of fuel injector with neat diagram.
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Explain the factors influencing flame speed during combustion process in S. I engine.
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Explain different stages of combustion in CI engines along with a P-Q diagram.
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Design bore and stroke of 4s petrol engine for the following data: Brake Power = 73.5 kW, Speed = 400 rpm, Brake MEP = 8.5 bar, Mechanical efficiency = 80%, Brake SFC = 0.346 kg/kWh, Calorific value = 44100 kJ/kg, Compression ratio = 6. Also calculate indicated power, BP air standard and relative efficiency and indicated MEP. Assume that bore is equal to stroke.
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Discuss the various changes in fuel injection system in S.I. engines with neat sketches.
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Explain a brief review on design changes to achieve high efficiency in modern automotive engines.
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Draw neat schematic diagram of simple gas turbine cycle used in air-craft propulsion and explain its working.
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Explain the use of inter cooling, reheating and regeneration of a gas turbine.
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Derive the efficiency expression of turbo jet cycle with the help of P-v and T - s diagrams.
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Discuss about different types of gas turbine combustion chambers with relative merits and demerits.
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