JNTU
JNTU B.Tech III Year THERMAL ENGINEERING I Previous Question Paper (February - 2022 - R16)
Answer any five questions. All questions carry equal marks.
Main
How the air mixture variation is being taken care against the engine load in the carburetor? Explain.
Answer
A simple carburetor provides a rich mixture at high loads and a lean mixture at low loads, which is contrary to engine requirements. To maintain the correct air-fuel ratio across varying loads, modern carburetors incorporate several auxiliary systems:
- Idling System: At closed throttle, the main jet is ineffective due to low air velocity. A separate idling circuit with a small jet bypasses the throttle valve to provide a rich mixture necessary for combustion at low speeds.
- Economizer System: During part-load operation, the engine requires a leaner mixture for fuel economy. The economizer valve restricts fuel flow to the main jet. At full load, the valve opens to provide additional fuel for maximum power.
- Acceleration Pump: When the throttle is opened suddenly, air velocity increases faster than fuel flow, causing a temporary lean mixture. A mechanical pump injects an extra charge of fuel into the venturi to prevent engine hesitation.
- Choke System: During cold starting, fuel vaporization is poor. A choke valve reduces the air intake area, creating a high vacuum that draws a very rich mixture into the engine cylinder.
How to estimate the Octane number for petrol and Cetane numbers for Diesel? Explain the importance of these numbers.
Answer
The Octane Number and Cetane Number are standard ratings used to define the ignition quality of fuels.
Octane Number (Petrol): It is defined as the percentage by volume of iso-octane in a mixture of iso-octane and normal heptane that matches the knocking characteristics of the fuel in a standard test engine. Iso-octane (high anti-knock) is assigned 100, while n-heptane (high knocking) is assigned 0. A higher octane number indicates better resistance to auto-ignition and knocking.
Cetane Number (Diesel): It is defined as the percentage by volume of cetane (n-hexadecane) in a mixture of cetane and alpha-methyl naphthalene that matches the ignition delay of the fuel. Cetane (short ignition delay) is assigned 100, and alpha-methyl naphthalene (long ignition delay) is assigned 0. A higher cetane number indicates a shorter ignition delay and smoother combustion.
Importance: These numbers ensure the fuel is compatible with the engine's compression ratio and operating conditions. Using fuel with an incorrect rating leads to engine knocking, reduced thermal efficiency, overheating, and potential mechanical damage to the piston and cylinder assembly.
Draw the port timing diagrams for ideal, actual for both S I Engines and C I Engines of two stroke engines and explain the salient points.
Answer
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Explain the influence of engine variables on the flame speed in S I engine combustion chamber.
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What are different antiknock agents are to be added in petrol and diesel fuels? Explain the ill effects of these antiknock agents.
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How does the knocking phenomena is different in petrol engine compared with diesel engine? Explain the differences clearly.
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What is the importance of divided type combustion chamber? How is it better than open type combustion chambers in C I Engine? Explain.
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How to measure the exhaust gas composition for the internal combustion engines? Discuss the effects of these components.
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The output of a single cylinder four stroke IC engine is measured by a rope brake dynamometer. The diameter of the brake pulley is 750 mm and rope diameter 70 mm. The dead load on the tight side of the rope is 600 N and the spring balance reading is 120 N. The bore is 200 mm and stroke is 275 mm. The engine consumes 5 kg/h of fuel at rated speed of 1200 rpm. The calorific value of the fuel is 43.5 MJ/kg. Calculate the brake specific fuel consumption, bmep and the brake thermal efficiency. If the mechanical efficiency is 80%, calculate the IP, imep, indicated specific fuel consumption and indicated thermal efficiency.
Answer
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What is the effect of clearance volume on volumetric efficiency of a reciprocating air compressor? Explain.
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A two stage single acting reciprocating air compressor draws 6 kg/min of air at 1.01325 bar and 20°C and the same is delivered at 14.75 bar. Assume perfect inter-cooling and optimum intermediate pressure with compression index of 1.25. The clearance factors for low pressure and high pressure cylinders are 0.04 and 0.07. Determine: i) Shaft power required with mechanical efficiency of 85%. ii) Isothermal efficiency iii) Net heat transferred in L.P. and H.P. cylinder during compression and also in the intercooler iv) Volumetric efficiency of each stage and free air delivered. v) The ratio of cylinder diameters for the same stroke for both the cylinders. Assume R = 0.287 kJ/kg K, Cv = 0.717 kJ/kg K.
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Differentiate between positive displacement type compressors and dynamic compressors.
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A centrifugal compressor operating at a pressure ratio of 4:1 has inlet temperature of 15°C. Calculate the overall diameter of impeller given that speed of operation 15000 rpm. Slip factor 0.9; Power input factor 1.03; Isentropic efficiency 0.85.
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What is the effect of sub cooling and super heating on the performance of vapour compression refrigeration system? Explain on p-h chart.
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Explain the principle of operation of winter air conditioning system along with the processes on psychrometric chart.
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