JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year THERMAL ENGINEERING Previous Question Paper (February - 2022 - R16)
Answer any five questions
All questions carry equal marks
Main
Draw the line diagrams and explain any two types of fuel injection systems with all salient points.
Answer
Fuel injection systems are critical for delivering precise amounts of fuel into the combustion chamber of internal combustion engines. Two primary types are the Air Injection System and the Solid (or Airless) Injection System.
1. Air Injection System: In this system, fuel is injected into the cylinder using a blast of high-pressure air provided by a multi-stage compressor. The fuel pump delivers fuel to the fuel valve, where high-pressure air atomizes it before entering the combustion chamber.
Salient points include:
- Requires a bulky, power-consuming multi-stage air compressor.
- Provides excellent atomization and mixing, allowing for the use of lower-grade, heavier fuels.
- The engine has a lower power-to-weight ratio due to the compressor weight.
- Commonly used in large, slow-speed stationary or marine engines.
2. Solid (Airless) Injection System: This system injects fuel directly into the combustion chamber at very high pressure using a high-pressure fuel pump and an injector nozzle, without the need for compressed air. It is categorized into Common Rail, Individual Pump, and Distributor systems.
Salient points include:
- Eliminates the need for a bulky air compressor, resulting in a more compact and lighter engine design.
- Higher thermal efficiency due to the absence of compressor power losses.
- Requires high-precision manufacturing for pumps and injectors to handle extreme pressures and prevent leakage.
- Standard in modern high-speed automotive and industrial diesel engines.
Explain the factors influencing the flame speed in SI engines.
Answer
Flame speed in spark-ignition engines is the rate at which the flame front travels through the air-fuel mixture. Several factors influence this speed:
- Air-Fuel Ratio: Flame speed is maximum at slightly rich mixtures (approximately 10-20% excess fuel) because they provide higher flame temperatures and faster chemical reaction rates.
- Turbulence: Increased turbulence intensity significantly enhances flame speed by increasing the surface area of the flame front and promoting heat and mass transfer between the burnt and unburnt gases.
- Compression Ratio: Higher compression ratios increase the pressure and temperature of the charge at the time of ignition, which reduces ignition delay and accelerates the flame propagation.
- Engine Speed: As engine speed increases, the turbulence level also increases, which generally leads to a higher flame speed, allowing the combustion process to keep pace with the piston movement.
- Inlet Temperature and Pressure: Higher initial temperatures and pressures decrease the ignition delay and increase the burning rate, thereby enhancing flame speed.
- Residual Gases: The presence of residual exhaust gases dilutes the fresh charge, lowering the flame temperature and slowing down the combustion process.
Explain the stages of combustion in SI engines elaborating the flame front propagation.
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Explain in detail the method of preparing the heat balance sheet with all necessary equations and draw one model sheet.
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What are the differences between in cylinder pressure and mean effective pressure?
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The air flow to a four-cylinder stroke oil engine is measured by means of a 5 cm diameter orifice, having a coefficient of discharge of 0.6. During a test on the engine the following data were recorded: Bore = 10.5 cm; Stroke 12.5cm; engine speed 1200 rpm; brake torque 147 Nm; fuel consumption 5.5 kg/hr; calorific value of fuel 43,100 kJ/hr; head across orifice, 5.7 cm of water, ambient temperature and pressure, 20oC and 1.013 bar respectively. Calculate:
The thermal efficiency on b.p. basis
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The break mean effective pressure
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The volumetric efficiency based on free air condition.
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Explain the effect of intercooling in a multistage reciprocating compressor.
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When a multi-stage compression used for air? What are its advantages?
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The following details represent the Gas turbine data using Air as working fluid at a rate of 1 kg/s; Ambient Temperature is 827oC; pressure at compressor inlet is 1 bar; pressure ratio is 4; compressor efficiency is 82%. Heating value of the fuel is 42,000 kJ/kg. Combustion chamber efficiency is 0.9. Determine:
a) Compressor work
b) Heat supplied
c) Work ratio and
d) Air/fuel ratio.
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Mention any three boiler mountings and three Accessories with neat sketches.
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Draw the line diagram and explain the working of high pressure boiler.
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The speed of single stage impulse turbine is 3000 rpm and means blade diameter is 1 meter. The nozzle of the turbine is inclined at 20oC to the plane of the wheel and the moving blade inlet and exit angle are 35o and 30o. Assuming friction factor (blade velocity, coefficient) of 0.8. Determine:
The power developed for a steam consumption rate of 9000 kg/hr.
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The blade diagram efficiency.
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The axial thrust assumes that the steam enters the blades without shock.
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