(a) (i) How does the mixture combustion in the combustion chamber of a C.I. engine differ from that of an S.I. engine? (ii) What is meant by combustion induced swirl? Show with sketches two important designs of C.I. combustion chamber using this meth

Question

(a) (i) How does the mixture combustion in the combustion chamber of a C.I. engine differ from that of an S.I. engine? (ii) What is meant by combustion induced swirl? Show with sketches two important designs of C.I. combustion chamber using this method of swirl. (20 marks)

(b) In a single-heater regenerative cycle the steam enters the turbine at 30 bar, 400°C and the exhaust pressure is 0·1 bar. The feed water heater is a direct-contact type which operates at 0·3 MPa. Find the efficiency of the cycle neglecting pump work. (At 30 bar, 400°C: h = 3230·9 kJ/kg and s = 6·9212 kJ/kg K. Also use steam tables given towards the end of booklet for steam/water properties). (20 marks)

(c) A moist air sample has dry bulb temperature of 30°C and specific humidity of 11·5 gm of water vapour per kg dry air. If the saturation vapour pressure of water at 30°C is 4·24 kPa and the total pressure is 90 kPa then what is the relative humidity of the air sample? (10 marks)

UPSC Answer Check · Accepted Answer

Explain the fundamental combustion differences between CI and SI engines with clear physical reasoning for part (a), supported by well-labelled sketches of swirl chambers. For part (b), set up the regenerative cycle analysis methodically: identify states, apply energy balance at the feed heater, and compute cycle efficiency with proper steam table interpolation. For part (c), apply psychrometric relations precisely to find relative humidity. Allocate approximately 40% effort to part (a) given its 20 marks and diagrammatic demand, 40% to part (b) for its computational complexity, and 20% to part (c).
- Part (a)(i): CI engines use heterogeneous diffusion-controlled combustion with fuel injection late in compression; SI engines use homogeneous premixed flame propagation with spark ignition; contrast flame speed, ignition delay, and combustion duration characteristics
- Part (a)(ii): Combustion-induced swirl is organized air motion created by piston motion into specially shaped pre-chamber or main chamber; explain how it enhances air-fuel mixing and reduces ignition delay
- Part (a)(ii): Two designs—Swirl Chamber (Ricardo Comet V) with tangential throat creating vigorous swirl, and Pre-combustion Chamber with restricted passage creating pressure differential and swirl; both sketched with flow arrows
- Part (b): Identify turbine inlet (state 1: 30 bar, 400°C), extraction pressure (0.3 MPa ≈ 3 bar), and condenser pressure (0.1 bar); determine state 2s (isentropic expansion to 3 bar) using s1 = s2 and steam tables
- Part (b): Energy balance at direct-contact heater: y·h2 + (1-y)·h4 = h6 where y is extraction fraction, h4 = hf at 0.1 bar, h6 = hf at 3 bar; solve for y
- Part (b): Compute h3s (isentropic expansion to 0.1 bar), actual h3 if efficiency given (or assume ideal), then W_turbine and Q_boiler; efficiency η = (W_net)/Q_boiler with pump work neglected
- Part (c): Apply ω = 0.622·(φ·ps)/(p - φ·ps) or rearrange to find φ = ω·p/(0.622·ps + ω·ps); substitute ω = 0.0115 kg/kg, p = 90 kPa, ps = 4.24 kPa
- Part (c): Correct numerical substitution yielding φ ≈ 0.305 or 30.5%; show intermediate calculation of partial pressure of vapour pv = ω·p/(0.622 + ω)

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	20%	10	For (a), correctly distinguishes heterogeneous vs homogeneous combustion, diffusion vs premixed flame control, and explains swirl generation mechanism physically; for (b), correctly identifies regenerative cycle principle and direct-contact heater operation; for (c), applies correct psychrometric relation between specific humidity, relative humidity and partial pressures	Basic distinction between CI and SI combustion modes given but missing diffusion/premixed terminology; regenerative cycle understood but minor error in energy balance setup; psychrometric formula correct but substitution error	Confuses CI and SI combustion characteristics (e.g., claims CI uses spark ignition); treats regenerative cycle as simple Rankine cycle; uses wrong psychrometric formula (e.g., confuses specific with absolute humidity)
Numerical accuracy	20%	10	Part (b): Correct steam table interpolation for h2s at 3 bar with s = 6.9212 kJ/kg·K, accurate extraction fraction y ≈ 0.18-0.22, final efficiency ≈ 35-38%; part (c): φ ≈ 30-31% with correct pv calculation; all values carry appropriate significant figures	Correct method but minor interpolation error in steam tables or arithmetic slip in extraction fraction; part (c) within 5% of correct φ due to calculation error	Major errors: wrong state identification (e.g., uses 30 bar saturation properties), ignores extraction in efficiency calculation, or part (c) result outside ±10% due to formula misuse
Diagram quality	20%	10	Part (a): Two clear sketches—Swirl Chamber showing spherical pre-chamber with tangential throat, arrow indicating swirl direction, piston at TDC; Pre-combustion Chamber showing restricted passage, main chamber, flow pattern; part (b): T-s or h-s diagram showing states 1-6, extraction line, and feed heater represented; all diagrams labelled with pressures, temperatures, and flow directions	One of the two combustion chambers well-drawn with labels, other sketch incomplete or missing arrows; cycle diagram shows basic shape but states not clearly marked or heater location ambiguous	No sketches for combustion chambers despite explicit demand; cycle diagram missing or drawn without thermodynamic states; diagrams confuse open/closed heater types
Step-by-step derivation	20%	10	Part (a): Logical progression from combustion mode → swirl necessity → chamber design; part (b): Explicit statement of assumptions, clear state table with h, s, x values, isentropic relations stated, energy balance equation written and solved for y, efficiency formula derived; part (c): Formula rearrangement shown before substitution	Key steps present but some jumps (e.g., states h2s without showing interpolation method); energy balance correct but y solved without showing algebra; part (c) jumps to final formula	No derivation shown—final answers only; or major logical gaps (e.g., cannot explain why extraction improves efficiency, or confuses mass and energy balances)
Practical interpretation	20%	10	For (a), links swirl chambers to Indian automotive context (e.g., Hindustan Ambassador diesel, Mahindra DI engines), notes trade-off: swirl chambers improve cold starting and emissions but reduce thermal efficiency vs direct injection; for (b), comments on why 0.3 MPa heater pressure is chosen (optimum bleed pressure for maximum cycle efficiency), notes 5-8% improvement over simple Rankine; for (c), interprets 30% RH as uncomfortable conditions, relevant to HVAC design for Indian climates	Mentions that regenerative heating improves efficiency without quantifying; notes swirl helps mixing; states humidity affects comfort without specific context	No practical interpretation; treats all parts as purely academic exercises; or gives irrelevant applications (e.g., discusses rocket engines for automotive CI engines)

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