JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III THERMAL ENGINEERING – I Previous Question Paper (July - 2021 - R18)
Answer any five questions. All questions carry equal marks.
Main
Compare SI and CI engines with respect to: (i) Basic cycle (ii) Fuel used (iii) Ignition (iv) Compression ratio (v) Speed (vi) Efficiency (vii) Weight.
Answer
| Feature | SI Engine | CI Engine |
|---|---|---|
| Basic Cycle | Otto Cycle | Diesel Cycle |
| Fuel Used | High volatility (Petrol) | Low volatility (Diesel) |
| Ignition | Spark plug | Compression ignition |
| Compression Ratio | 6:1 to 10:1 | 14:1 to 22:1 |
| Speed | High speed | Low to medium speed |
| Efficiency | Lower thermal efficiency | Higher thermal efficiency |
| Weight | Lighter construction | Heavier construction |
Discuss the relative advantages and disadvantages of internal combustion and external combustion engine.
Answer
Internal Combustion (IC) engines and External Combustion (EC) engines differ fundamentally in where the fuel is burnt and the power conversion process.
Advantages of IC Engines:
- Higher power-to-weight ratio, making them ideal for automotive applications.
- Compact design and smaller overall footprint.
- Quick starting capability and rapid response to load changes.
- Higher thermal efficiency compared to most EC engines.
Disadvantages of IC Engines:
- Limited fuel flexibility; they require high-quality, refined fuels.
- Higher maintenance costs due to complex moving parts and lubrication requirements.
- Lower starting torque compared to steam engines.
Advantages of EC Engines:
- Ability to burn a wide variety of fuels, including low-grade solid fuels.
- High starting torque, which is beneficial for heavy machinery and locomotives.
- Smoother operation with less vibration due to continuous combustion.
Disadvantages of EC Engines:
- Low power-to-weight ratio and bulky size due to boilers and condensers.
- Longer startup time required to build up pressure.
- Lower overall thermal efficiency due to heat losses in the external heat exchanger.
Explain various stages of combustion in S.I engines with the help of P-θ diagram.
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Describe with neat sketch the working principle of pressure lubrication system.
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Describe with a neat sketch the construction and working of a single-stage single acting reciprocating air compressor.
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A gas engine working on Otto cycle has piston diameter 20 cm and stroke length 60 cm. It works on the following conditions rpm = 225, Misfire per minute = 10, Mean effective pressure = 8 bar, Mechanical efficiency = 85%, Assuming the engine to be working on four stroke cycle principle. Determine: (i) IP (ii) BP (iii) FP.
Answer
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Enumerate the various engine efficiencies and discuss their importance.
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A single-acting two-stage air compressor deals with 4 m3/min of air under atmospheric conditions of 1.016 bar and 15°C with a speed of 250 rpm. The delivery pressure is 78.65 bar. Assume complete intercooling. Find the minimum power required, bore and stroke of the compressor. Assume a piston speed is 3 m/s, mechanical efficiency of 75% and volumetric efficiency of 80% per stage. Assume the polytrophic index (n) of compression in both stages is 1.25 and neglect clearance.
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Explain the working of Roots Blower for compressing the air and describe the efficiency reciprocating air compressor.
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Derive an expression for volumetric efficiency of air compressor.
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Explain the deviation of actual gas turbine cycle from ideal Brayton cycle.
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In a gas turbine power plant, operating on Joule’s cycle, air is compressed from 1 bar and 20°C through a pressure ratio of 8. It is then heated to 727°C in the combustion chamber and expanded back to 1 bar. Calculate the net work done, cycle efficiency and work ratio. Isentropic efficiency of turbine is 80% and of compressor is 75%.
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Explain the principle of operation of centrifugal compressor with neat sketch.
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An axial flow compressor, with compression ratio as 6, draws air at 23°C delivers it at 187°C. The main blade speed and flow velocity are constant throughout the compressor. Assume 50 % reaction blading and take the blade velocity as 180 m/s. Find the flow velocity and number of stages. Take work factor = 0.84, α = 12°, β = 42° and Cp = 1.005 kJ/kg K.
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What is diesel knock? How to minimize knocking in C.I. engine? Explain.
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Describe with a neat sketch the working principle of Mechanical fuel pump.
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