JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year THERMAL ENGINEERING I Previous Question Paper (March - 2024 - R16)
Part A is compulsory, which carries 25 marks. In Part A, Answer all questions. In Part B, Answer any one question from each unit. Each question carries 10 marks and may have a, b as sub questions.
Part A
Define scavenging in IC engines.
Answer
Scavenging is the process of flushing the burnt exhaust gases out of the engine cylinder and replacing them with a fresh charge of air or air-fuel mixture. This process is essential in two-stroke engines to ensure the cylinder is prepared for the next combustion cycle, as the exhaust and intake processes occur simultaneously near the end of the expansion stroke and the beginning of the compression stroke.
What are the different types of cooling in IC engines?
Answer
Internal combustion engines primarily utilize two methods of cooling to maintain optimal operating temperatures and prevent thermal failure of engine components:
- Air Cooling: This method relies on the direct transfer of heat from the cylinder walls and cylinder head to the surrounding atmosphere. Extended surfaces known as fins are cast onto the exterior of the cylinder block to increase the surface area for heat dissipation. This system is simple, lightweight, and typically used in small engines like motorcycles and lawnmowers.
- Liquid (Water) Cooling: This method uses a circulating coolant (usually water mixed with additives) to absorb heat from the engine block and cylinder head through internal passages called water jackets. The heated coolant is then circulated through a radiator where it releases heat to the atmosphere before returning to the engine. This system is more efficient, allows for uniform cooling, and is standard in most automotive and heavy-duty engines.
Discuss the factors which affect the combustion in SI engine.
Answer
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Differentiate auto ignition and pre-ignition.
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Define mean effective pressure and how is it related to indicated power of an IC engine?
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Discuss about free air delivery and its significance.
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What is the air standard efficiency of a 4-stroke gas engine having a compression ratio of 6.5?
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What are a slip factor and a pressure coefficient?
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Define subcooling and show the effect of subcooling of condensate with the help of T-s and P-h diagram in a vapor compression refrigeration cycle.
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Two kg of air at 40°C DBT and 50% RH is mixed with three kg of air at 20°C DBT and 100% RH. Calculate the resultant condition of the air after mixing.
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Part B
Differentiate the SI and CI engines.
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Construct ideal and actual valve timing diagram of a 4-stroke SI engine.
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Construct the schematic diagram of simple carburetor and explain its operational features.
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Compare air cooling and water cooling of IC engines.
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Explain the effects of engine variables on flame propagation and its significance.
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Explain compression and combustion induced turbulence in Diesel engine.
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Explain the four stages of combustion in CI engines.
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Discuss delay period and its importance.
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The following details were recorded in a 4 cylinder, 4 stroke engine bore=100 mm, stroke =120 mm, speed=1600 rpm, fuel consumption = 0.2 kg/min, calorific value of fuel=44 MJ/kg, difference in either side of the pulley is 40 kg, brake circumference is 300 cm. If the mechanical efficiency is 80%, calculate the brake thermal efficiency and indicated thermal efficiency.
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Explain the heat balance sheet with various parameters involved.
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Derive an expression for minimum work required for two stage reciprocating air compressors with perfect inter-cooling and neglect clearance volume.
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A single stage single acting reciprocating air compressor running at 900 rpm delivers air at 12 bar. The induction and free air conditioning can be taken as 1 bar and 300 K and the free air delivery as 0.5 m3/min. Calculate the bore and stroke, the volumetric efficiency, the indicated power and the isothermal efficiency. Assume the index of compression and expansion is 1.3.
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Explain the working of the vane blower with a neat sketch.
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Derive an expression for the polytropic efficiency of centrifugal compressor in terms of inlet pressure delivery pressure, inlet temperature and the ratio of specific heats.
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Differentiate the rotary and reciprocating air compressors.
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An axial flow compressor with an overall isentropic efficiency of 85% draws air at 20 C and compresses it in the pressure ratio of 4:1. The mean blade speed and flow velocity are constant throughout the compressor. Assuming 50% of reaction blading and taking blade velocity as 180 m/s and work factor as 0.82. Calculate the flow velocity. Take α1 =120, β1 =420.
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Explain the working of vapour absorption refrigeration system with a sketch.
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Discuss the different methods of improving the COP of simple vapour compression refrigeration cycle.
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Atmospheric air having DBT of 16 C and RH of 25% is passed through a furnace and then through a humidifier to maintain a final DBT of 30 C and 50% RH. Find the heat and moisture added to the air during the process.
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Explain the working of year-round air-conditioning system with a sketch.
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