(a) With the help of P-θ (pressure-crank angle) diagram, compare the knock in SI and CI engines. Explain that "the factors which tend to prevent knock in SI engines in fact promote knock in CI engines". (10 marks)

(b) Describe the phenomenon of 'blo

Question

(a) With the help of P-θ (pressure-crank angle) diagram, compare the knock in SI and CI engines. Explain that "the factors which tend to prevent knock in SI engines in fact promote knock in CI engines". (10 marks)

(b) Describe the phenomenon of 'blowby losses'. What are the factors that increase the blowby losses? What are the effects of increased blowby losses on the engine performance? (10 marks)

(c) What are the advantages and disadvantages of supercritical pressure boilers as compared to that of subcritical boilers? Also, draw the Rankine cycle (T-s diagram) for a steam power plant employing supercritical boiler with single stage of reheating. (10 marks)

(d) Derive the expression as given below for draught h in mm of water column being created by a chimney of height H metre:

h = 353H[1/T_a - ((m_a + 1)/m_a)(1/T_g)]

where m_a is mass of air supplied per kg of fuel, and T_a and T_g are ambient air and hot gas temperatures in Kelvin, respectively. (10 marks)

(e) What is the chemical name of R134a? Is R134a an ecofriendly refrigerant? Clarify. (10 marks)

UPSC Answer Check · Accepted Answer

Begin with part (a) by drawing and comparing P-θ diagrams for SI and CI knock, then explain the inverse relationship of factors. For (b), describe blowby mechanism before listing factors and effects. Part (c) requires a balanced comparison of supercritical vs subcritical boilers followed by the T-s diagram with supercritical Rankine cycle and reheat. Part (d) demands rigorous step-by-step derivation of the chimney draught formula using ideal gas assumptions and density relationships. Conclude with (e) by stating R134a's chemical name and critically evaluating its environmental impact through GWP and ODP analysis. Allocate approximately 20% time each to parts (a), (b), (c), (d), and (e) respectively.
- Part (a): P-θ diagram showing SI knock as sharp pressure spike near TDC vs CI knock as gradual pressure rise; explanation of how high compression ratio, high inlet temperature, and early injection prevent SI knock but promote CI knock
- Part (b): Definition of blowby as gas leakage past piston rings into crankcase; factors including worn rings, high cylinder pressure, low viscosity oil, poor honing; effects on power loss, oil contamination, increased blowby gas handling requirement
- Part (c): Advantages of supercritical boilers (higher efficiency ~45%, no drum, reduced fuel consumption, lower emissions) vs disadvantages (high capital cost, materials stress, water treatment criticality); T-s diagram showing supercritical heating, expansion, reheat, condensation with correct slopes and critical point marked
- Part (d): Derivation starting from hydrostatic pressure balance, ideal gas law for air and flue gas, density substitution ρ = P/RT, pressure difference ΔP = H(ρ_a - ρ_g)g, conversion to mm of water using ρ_w, final algebraic manipulation to given formula
- Part (e): Chemical name 1,1,1,2-tetrafluoroethane (CF3CH2F); evaluation as eco-friendly regarding zero ODP but high GWP of 1430 over 100 years, comparison with R12 and R22, mention of Kigali Amendment implications for India
- Part (d) numerical: Correct handling of temperature in Kelvin, proper unit conversion from Pa to mm of water (1 mm H2O = 9.81 Pa), recognition that 353 = 353 K·mm/Pa or equivalent constant derivation

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	20%	10	For (a), correctly identifies SI knock as pre-ignition/auto-ignition of end gas vs CI knock as ignition delay causing rapid pressure rise; for (b), accurately describes blowby as unburnt mixture/combustion gas past piston rings; for (c), correctly distinguishes supercritical (P>22.1 MPa, no phase change) from subcritical with phase change; for (e), correctly identifies R134a as HFC with zero ODP but significant GWP	Basic understanding of knock differences but confuses cause-effect relationships; blowby described vaguely as 'leakage' without specifying path; supercritical advantages listed but without clear thermodynamic reasoning; R134a identified correctly but environmental assessment superficial	Confuses SI and CI knock mechanisms entirely; describes blowby as valve leakage or exhaust blowback; treats supercritical as simply 'high pressure' without critical point significance; wrong chemical name or claims R134a is CFC/HCFC
Numerical accuracy	15%	7.5	Part (d) derivation shows correct substitution of ρ_a = P/(RT_a) and ρ_g = P/(RT_g) with proper algebraic manipulation; constant 353 correctly derived as 353000/(g×ρ_w) or equivalent showing unit consistency; temperature strictly in Kelvin throughout	Final formula correct but intermediate steps skip unit conversions or assume 353 without derivation; minor algebraic errors that cancel out; temperature conversion errors that are partially corrected	Numerical errors in density expressions; uses Celsius in gas law without conversion; wrong constant derivation leading to incorrect final formula; order of magnitude errors in mm of water calculation
Diagram quality	20%	10	Part (a) P-θ diagram shows two distinct curves: SI with smooth rise then sharp spike (detonation) vs CI with delayed ignition then steep rise; both with correct TDC reference and crank angle scale; Part (c) T-s diagram clearly shows supercritical region above critical point (22.1 MPa, 374°C), continuous heating line, expansion, reheat at intermediate pressure, second expansion, condensation below critical pressure; critical point marked, constant pressure lines correctly sloped	Diagrams drawn but lack precision: P-θ diagrams show pressure rise but knock characteristics not clearly distinguished; T-s diagram shows reheat but supercritical region not clearly marked or treated as simply high-pressure subcritical	Diagrams missing or incorrect: P-θ shows pressure vs volume or wrong axes; T-s diagram shows standard Rankine cycle without supercritical features or omits reheat; no labels or values indicated
Step-by-step derivation	25%	12.5	Part (d) shows complete derivation: (1) pressure at chimney base due to air column Hρ_a g, (2) pressure due to gas column Hρ_g g, (3) draught pressure ΔP = Hg(ρ_a - ρ_g), (4) ideal gas substitution ρ = PM/RT with M_air ≈ 29, M_gas ≈ 29-30, (5) simplification using m_a relationship, (6) conversion to mm water; each step logically connected with assumptions stated	Key steps present but skips some algebraic manipulation or assumes relationships without proof; jumps from pressure difference to final formula with 'hence' or 'it can be shown'; partial derivation with correct final result	No derivation shown, only final formula stated; or derivation with fundamental errors (e.g., treating gases as incompressible, ignoring temperature effect, wrong molecular weight assumption); circular reasoning
Practical interpretation	20%	10	For (a), explains practical implications: SI engines need high octane fuel, spark timing retard, combustion chamber design; CI engines need cetane number, injection timing advance, pre-chamber design; for (b), links blowby to Indian context: BS-VI emission norms requiring positive crankcase ventilation (PCV) and oil consumption limits; for (c), cites Indian supercritical plants like NTPC Kudgi (2×800 MW) or Mundra UMPP; for (e), discusses India's phase-down schedule under Kigali Amendment, transition to R1234yf or natural refrigerants	Mentions practical applications but generic: 'knock is bad for engines', 'blowby causes pollution', 'supercritical saves fuel'; no specific Indian examples or current regulatory context for R134a	No practical interpretation; treats all parts as purely theoretical; or gives incorrect practical implications (e.g., claims SI and CI can use same anti-knock strategies, suggests blowby increases power)

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