Mechanical Engineering 2021 Paper II 50 marks Compulsory Explain

Q5

(a) What do you understand by the term EGR? Explain how EGR reduces NOₓ emission in CI engines. (10 marks) (b) The flue gas composition measured by Orsat apparatus for a boiler burning a fuel of unknown hydrocarbon CₓHᵧ is given as follows: CO₂: 8·0%, CO: 0·9%, O₂: 8·8% and N₂: 82·3%. Determine (i) the composition of the fuel (ii) the air fuel ratio on mole and mass basis (iii) the percentage of excess air used. (10 marks) (c) Describe the following terms with reference to stream nozzle: (i) Efficiency (ηₙ) (ii) Velocity coefficient (Cᵥ). (10 marks) (d) Explain the concept of balance point between the compressor and the capillary tube in refrigeration systems. (10 marks) (e) Briefly explain the 'Equal Friction Method' of air-conditioning duct design procedure. (10 marks)

हिंदी में प्रश्न पढ़ें

(a) EGR पद से आप क्या समझते हैं? समझायें कि EGR, NOₓ उत्सर्जन को कैसे कम करता है। (10 अंक) (b) अज्ञात हाइड्रोकार्बन CₓHᵧ के ईंधन को जलाने वाले वाष्पित्र के लिए ऑर्सेट उपकरण द्वारा मापी गई फ्लू गैस संरचना निम्नानुसार दी गई है: CO₂: 8·0%, CO: 0·9%, O₂: 8·8% और N₂: 82·3%। ज्ञात करें (i) ईंधन की संरचना (ii) मोल और द्रव्यमान के आधार पर वायु ईंधन का अनुपात (iii) प्रयोग की गई अतिरिक्त हवा का प्रतिशत। (10 अंक) (c) धारा तुंड के संदर्भ में निम्नलिखित शब्दों का वर्णन करें: (i) दक्षता (ηₙ) (ii) वेग गुणांक (Cᵥ)। (10 अंक) (d) प्रशीतन प्रणालियों में संपीड़क और केशिकानली के बीच संतुलन बिंदु की अवधारणा की व्याख्या करें। (10 अंक) (e) वातानुकूलन वाहिनी अभिकल्प प्रक्रिया के लिए 'समान घर्षण विधि' को संक्षेप में समझाएं। (10 अंक)

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Directive word: Explain

This question asks you to explain. The directive word signals the depth of analysis expected, the structure of your answer, and the weight of evidence you must bring.

See our UPSC directive words guide for a full breakdown of how to respond to each command word.

How this answer will be evaluated

Approach

Begin with a concise definition of EGR for part (a), then explain the thermal and dilution mechanisms for NOₓ reduction. For part (b), set up the combustion equation using Orsat data and solve systematically for fuel composition, A/F ratios, and excess air—this numerical part demands ~35% of time. For (c), define nozzle efficiency and velocity coefficient with clear formulas. For (d), explain the balance point concept with a pressure-enthalpy diagram. For (e), outline the Equal Friction Method steps with a simple duct layout example. Conclude each descriptive part with practical implications for Indian conditions (e.g., BS-VI norms, tropical climate design).

Key points expected

  • Part (a): EGR definition as Exhaust Gas Recirculation; thermal effect (reduced flame temperature below 1800K) and dilution effect (lower O₂ concentration) suppressing NOₓ formation via Zeldovich mechanism
  • Part (b): Carbon balance to find x=7.3, hydrogen balance to find y=14.6 giving fuel formula C₇.₃H₁₄.₆ or approximated as C₇H₁₅; stoichiometric O₂ from (x+y/4); actual O₂ from N₂=82.3% using 79:21 ratio
  • Part (b): A/F mole ratio ~18.5, mass ratio ~12.8; excess air ~35% using (actual-theoretical)/theoretical × 100
  • Part (c): Nozzle efficiency ηₙ = (actual KE)/(isentropic KE) = (h₁-h₂)/(h₁-h₂s); Velocity coefficient Cᵥ = √(actual KE/isentropic KE) = √ηₙ; typical values 0.95-0.99
  • Part (d): Balance point as intersection of compressor pumping curve and capillary tube flow characteristic; subcooling control; stable operation zone; effect of condensing pressure variation
  • Part (e): Equal friction method maintains constant pressure drop per unit length (typically 0.5-1.0 Pa/m); sizing ducts using friction chart/Moody diagram; iterative velocity adjustment; suitable for Indian commercial buildings with uniform loading

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness22%11For (a) correctly identifies thermal and dilution mechanisms with temperature ceiling ~1800K; for (b) applies correct combustion stoichiometry with nitrogen balance; for (c) distinguishes ηₙ and Cᵥ with proper square-root relationship; for (d) explains balance point as stable intersection of curves; for (e) states 0.5-1.0 Pa/m design criterion.States EGR reduces NOₓ but confuses mechanisms; combustion equation set up with minor element balance errors; defines ηₙ and Cᵥ but misses Cᵥ=√ηₙ relation; describes balance point vaguely; mentions constant pressure drop without numerical range.Confuses EGR with SCR or catalytic reduction; fundamental errors in combustion equation (e.g., ignoring CO, misusing Orsat data); swaps ηₙ and Cᵥ definitions; no understanding of balance point; describes equal friction as equal velocity method.
Numerical accuracy18%9Part (b): x=7.3, y=14.6 (fuel C₇.₃H₁₄.₆), mole A/F=18.5, mass A/F=12.8, excess air 34-35%; all calculations shown with 3 significant figures; nitrogen balance verified (82.3% N₂ → 21.9% O₂ supplied).Correct methodology but arithmetic slip in x or y (e.g., x=7, y=14); A/F ratio within 10% of correct; excess air calculation uses wrong base but formula correct.Major errors: treats Orsat percentages as mole fractions without basis, ignores CO in carbon balance, or computes A/F as 1:1; no verification using nitrogen balance.
Diagram quality18%9For (a) schematic of EGR loop with cooler and valve; for (d) P-h diagram showing balance point intersection with compressor and capillary curves; for (e) simple duct layout with main and branches showing velocity/pressure annotations; all diagrams labelled with Indian equipment references (e.g., EGR in Tata BS-VI engines).At least two relevant diagrams drawn but missing key labels (e.g., no EGR cooler shown, balance point not marked on curves); duct layout without pressure drop annotations.No diagrams or irrelevant sketches (e.g., T-s diagram for nozzle instead of h-s); diagrams drawn but completely unlabelled.
Step-by-step derivation22%11Part (b): General combustion equation written, element balances (C, H, O, N) set up systematically, four equations solved stepwise; for (c) derivation of ηₙ from h-s diagram with stagnation states; for (d) derivation of capillary tube flow equation showing choked vs non-choked regimes.Combustion equation written but jumps to solution; element balances stated but simultaneous solution not shown; nozzle formulas stated without h-s diagram derivation.No derivation—only final answers stated; or incorrect equation setup (e.g., CₓHᵧ + O₂ → products without N₂ inclusion).
Practical interpretation20%10For (a) links EGR to BS-VI compliance in Indian diesel vehicles (e.g., Mahindra, Tata) with trade-off on PM; for (b) comments on boiler efficiency implications of 0.9% CO (incomplete combustion); for (d) discusses seasonal variation effects on balance point in tropical Indian climates; for (e) compares with velocity reduction method for Indian high-rise buildings.Mentions one practical application (e.g., EGR for emission norms) but no trade-offs; notes CO indicates incomplete combustion without efficiency quantification.No practical context; treats all parts as purely theoretical exercises with no mention of Indian standards, equipment, or operating conditions.

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