JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II Year THERMAL ENGINEERING - I Previous Question Paper (August/September - 2021 - R18)
Answer any five questions. All questions carry equal marks.
Main
Draw the cross section of a single cylinder spark ignition engine and discuss the important components.
Answer
- Cylinder Block: The main structural component that houses the cylinder, providing the space where combustion occurs and the piston reciprocates.
- Piston: A cylindrical component that moves inside the cylinder, transmitting the force of combustion to the connecting rod. It is equipped with piston rings for sealing and heat transfer.
- Cylinder Head: Mounted on top of the cylinder block, it contains the combustion chamber, spark plug, and valves (inlet and exhaust).
- Spark Plug: An electrical device that initiates combustion by producing a high-voltage electric spark to ignite the compressed air-fuel mixture.
- Connecting Rod: A mechanical link that connects the piston to the crankshaft, converting the reciprocating motion of the piston into the rotary motion of the crankshaft.
- Crankshaft: The rotating shaft that receives power from the piston via the connecting rod and delivers it to the transmission system.
- Valves (Inlet and Exhaust): Mechanisms that control the flow of the air-fuel mixture into the cylinder and the discharge of burnt exhaust gases, respectively.
- Crankcase: The lower part of the engine housing that supports the crankshaft and acts as a reservoir for lubricating oil.
Differentiate between two stroke engine and four stroke engine and explain.
Answer
| Feature | Two-Stroke Engine | Four-Stroke Engine |
|---|---|---|
| Power Strokes | One power stroke per revolution of the crankshaft. | One power stroke every two revolutions of the crankshaft. |
| Valve Mechanism | Uses ports in the cylinder wall instead of valves. | Uses poppet valves operated by a camshaft. |
| Thermal Efficiency | Generally lower due to scavenging losses. | Higher due to better combustion and scavenging. |
| Weight/Size | Lighter and more compact for the same power output. | Heavier and bulkier due to complex valve train. |
In a two-stroke engine, the entire cycle of suction, compression, power, and exhaust is completed in two strokes of the piston (one revolution). The intake and exhaust processes occur simultaneously through ports. Conversely, a four-stroke engine completes these four distinct processes over four strokes of the piston (two revolutions), allowing for more precise control over gas exchange and higher overall efficiency.
Explain the effect of various engine variables on SI engine knocking phenomena.
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Why cooling is required for IC engine? Explain different methods of cooling.
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Explain the phenomenon of knock in CI engines and compare it with SI engine knock.
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How does the fuel rating influence the knocking? Explain.
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Explain the method of motoring test for obtaining friction power of an engine.
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The following data is referenced to a four stroke petrol engine: diameter 100 mm, stroke 125 mm, speed 2000 rpm, indicator area 2.5 mm^2, length of spring 2.5 cm, spring constant 10 bar/cm. Calculate: i) indicated mean effective pressure ii) indicated power iii) indicated thermal efficiency if fuel rate is 0.02 kg/s and CV = 44000 kJ/kg.
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Determine the size of the cylinder for a double acting air compressor of 37 kW in which air is drawn in at 1.01325 bar and 15C and compressed according to the law PV^1.2 = C to 6 bar. The compressor runs at 100 rpm with the average piston speed of 152.5 m/min, neglect clearance.
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Explain the significance of positive displacement compressor used to compress the air.
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With the help of neat sketches explain a roots air blower. How its PV diagram is different from a reciprocating air compressor.
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What is the role of impeller in the centrifugal compressor? Explain.
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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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Discuss the methods of improving the work ratio, specific out put and cycle thermal efficiency by employing multi state system gas turbines with Inter cooling and reheating.
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