JNTU
JNTU B.Tech III THERMAL ENGINEERING I Previous Question Paper (August - 2022 - R16)
Answer any five questions
All questions carry equal marks
Main
Why valves are not required for two stroke engine? Explain the significance of ports in two stroke engine.
Answer
In a two-stroke engine, valves are not required because the piston itself acts as a sliding valve. As the piston moves, it covers and uncovers openings in the cylinder walls, known as ports, to control the flow of gases. This design eliminates the need for complex valve actuation mechanisms like camshafts, pushrods, and rocker arms, which are necessary in four-stroke engines to time the intake and exhaust events.
The ports are critical for the engine's operation:
- Inlet Port: Allows the fresh air-fuel mixture or air to enter the crankcase during the upward stroke of the piston.
- Transfer Port: Connects the crankcase to the combustion chamber, allowing the pre-compressed charge to transfer into the cylinder for combustion.
- Exhaust Port: Provides a path for the burnt gases to escape from the cylinder once the piston uncovers it during the downward power stroke.
The precise geometry and timing of these ports determine the engine's scavenging efficiency and overall performance.
Differentiate between spark ignition engine and combustion ignition engine based on principle of operation.
Answer
| Feature | Spark Ignition (SI) Engine | Compression Ignition (CI) Engine |
|---|---|---|
| Fuel Type | High volatility fuel (Petrol) | Low volatility fuel (Diesel) |
| Working Cycle | Otto Cycle | Diesel Cycle |
| Charge | Air-fuel mixture | Air only |
| Ignition | Spark plug | Self-ignition due to high compression |
| Compression Ratio | Low (6:1 to 10:1) | High (14:1 to 22:1) |
| Governing | Quantity governing (Throttle) | Quality governing (Fuel injection) |
In an SI engine, the air-fuel mixture is compressed and ignited by an external spark provided by a spark plug. The combustion process is theoretically at constant volume. In contrast, a CI engine compresses only air to a very high pressure and temperature. Fuel is injected at the end of the compression stroke, and it self-ignites upon contact with the hot compressed air. The combustion process in a CI engine is theoretically at constant pressure.
Define Octane number and Cetane number of fuels and explain how to improve the quality of fuels.
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Explain the constructional and operational features of magneto ignition system with a suitable diagram.
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What is the significance of flame speed on performance of the engine? Discuss the influence of different parameters on flame speed.
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Explain different ill effects of knocking on engine and discuss how to control the knocking phenomena in SI engines.
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Explain different stages of combustion in CI engine and describe the effect of operating variables on different stages.
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Draw the pre-combustion indirect combustion chamber for CI engine and explain the advantages and limitations.
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How to measure the frictional power for multi cylinder engine? Explain in detail along with the required equations.
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A twin cylinder oil engine coupled to an electric DC generator, when tested for fuel consumption recorded the following data: Line current = 25 A at 220 V, Time for consumption of 10 cc of fuel = 21 s, speed of the engine = 1200 rpm, efficiency of generator = 80 %, Specific gravity of fuel = 0.835, Calorific value of fuel oil = 44500 kJ/kg. Calculate the specific fuel consumption in kg/kWh and the brake thermal efficiency.
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Derive the equation for optimum pressure ratio of two stage reciprocating air compressor with perfect inter cooling.
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The delivery pressure of two stage compressor is 25 bar and the clearance volume of the LP cylinder is equal to 5% of the swept volume. The atmospheric conditions are 1.01325 bar and 302 K. The index of the polytropic law of compression and expansion in both stages is 1.25. Determine the volumetric efficiency and work required per kg of air based on optimum conditions.
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Draw the schematic diagram of vane sealed compressor and explain its advantages and limitations.
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A rotary air compressor working between 1 bar and 3.2 bar has internal and external diameter of impeller as 340 mm and 680 mm respectively. The vane angle at inlet and outlet are 28° and 41° respectively. If the air enters the impeller at 25 m/s find (i) speed of the impeller in rpm (ii) work done required by the compressor.
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Explain the importance of vapour absorption refrigeration system along with the suitable diagram.
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Define and derive the equations in terms of partial pressure of water vapour for relative humidity and specific humidity.
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