JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III THERMAL ENGINEERING – I Previous Question Paper (September - 2021 - R18)
Answer any five questions
All questions carry equal marks
Main
Draw the actual and fuel air cycles for both S I Engine and C I engine and explain the salient features.
Answer
The Fuel-Air Cycle accounts for the variable specific heats of the working fluid and the chemical composition of the fuel-air mixture, whereas the Actual Cycle further incorporates real-world losses such as pumping losses, heat transfer, combustion inefficiency, and valve timing effects.
For SI engines, the actual cycle shows a rounded pressure peak due to finite combustion time and heat loss, with a gradual pressure drop during the expansion stroke. For CI engines, the cycle exhibits a more complex pressure-volume diagram due to the delay period and the controlled rate of fuel injection, resulting in a flatter combustion peak compared to the theoretical cycle.
Salient features include:
- Pumping Losses: Significant in actual cycles due to throttle position in SI engines.
- Specific Heat Variation: Actual and fuel-air cycles account for the increase in specific heat at high temperatures, reducing peak pressure and temperature.
- Dissociation: Occurs at high temperatures in actual cycles, limiting the maximum energy release.
- Time Loss: Combustion is not instantaneous, causing a deviation from the constant volume or constant pressure assumptions.
What is the role of carburetor in spark ignition engine? Explain how the load variations are taken care by the carburetor?
Answer
The carburetor is responsible for atomizing the fuel and mixing it with the incoming air in the correct proportions (air-fuel ratio) to ensure efficient combustion across various engine operating conditions. It operates based on the Bernoulli principle, where air flowing through a venturi creates a pressure drop, drawing fuel from the float chamber into the airstream.
To handle load variations, the carburetor utilizes specific circuits:
- Idling System: Provides a rich mixture during low-load, low-speed idling when the throttle is nearly closed.
- Main Metering System: Supplies the required air-fuel ratio during normal cruising loads.
- Economizer System: Delivers a leaner mixture at part-throttle to maximize fuel efficiency.
- Acceleration Pump: Provides an extra burst of fuel during sudden throttle openings to prevent engine stalling due to the inertia of liquid fuel.
- Choke System: Supplies a very rich mixture during cold starting by restricting air intake, compensating for poor fuel vaporization.
Why cooling is required for the better operation of internal combustion engines? Explain different types of cooling systems.
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What are different operating parameters influence the knocking phenomena in spark ignition engine? Discuss the methods to minimize them.
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Differentiate between uncontrolled combustion and controlled combustion in compression ignition engine.
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What is the need of air movement in compression ignition engine? How to achieve it? Explain different methods.
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How the fuels are being rated? What are different anti-knocking agents added to petrol and diesel for better combustion? Explain.
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Explain the principle of operation of retardation in internal combustion engines? Explain.
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A twin-cylinder two-stroke engine has a swept volume of 150 cm3. The maximum power output is 19 kW at 11000 rpm, bsfc is 0.11 kg/kWh and the air/fuel ratio is 12. If ambient test conditions are 25°C and 1.01325 bar and the fuel has a calorific value of 44 MJ/kg. Calculate the bmep, overall efficiency and the volumetric efficiency.
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What is the function of intercooler in two stage compression? Explain with p-v diagram.
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Draw the schematic layout of Lysholm rotary compressor and explain the constructional and operational features.
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A two stage air compressor receives 0.238 m3/s of air at 1 bar and 27°C and discharges it at 10 bar. The polytropic index of compression is 1.25. Determine: (i) the minimum power required for the compression, (ii) the power needed for single stage compression to the same pressure, (iii) the maximum temperature and (iv) the heat removed from the inter cooler.
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Draw the pressure variations and velocity variations in the impellor and blades of a centrifugal compressor and explain salient features.
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An axial flow compressor with compression ratio of 5, draws air at 20°C and delivers it at 50°C. Assuming 50% degree of reaction, find the velocity of flow, if the blade velocity is 100 m/s. Also find the number of stages if work factor = 0.85, α = 10°, β = 40° and Cp = 1.005 kJ/kg K.
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A closed cycle gas turbine using has a two stage compression with perfect inter cooling. The overall pressure ratio is 9 and pressure ratio in each stage is equal. Each stage has an isentropic efficiency of 85%. The turbine is also two stage with equal pressure ratio with inter change reheat to original temperature. Each turbine stage has an isentropic efficiency of 90%. The turbine inlet temperature is 1100 K and the compressor inlet is 303 K. Find (i) work done per kg of fluid flow (ii) work ratio (iii) The overall cycle efficiency.
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Explain the influence of regeneration on net work output, heat addition and efficiency of gas turbine power plants.
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