Question 1

Four-stroke petrol engine: expansion (power) stroke timing In a conventional four-stroke spark-ignition engine, the power or expansion stroke ideally begins near top dead centre after combustion and continues as the piston travels toward bottom dead…

Accepted Answer

Introduction / Context: The expansion (power) stroke in a four-stroke spark-ignition engine converts the high-pressure, high-temperature gases produced by combustion into useful mechanical work. Understanding when the expansion begins and ends on the crank angle diagram is fundamental to valve-timing, combustion phasing, and performance analysis. Given Data / Assumptions: Four-stroke petrol (spark-ignition) engine with conventional valve timing. Combustion is initiated before top dead centre (spark advance), but the useful expansion predominantly occurs after top dead centre. Textbook idealization treats valve events as instantaneous for clarity. Concept / Approach: In the ideal four-stroke cycle, combustion is assumed to occur rapidly around top dead centre (TDC). The power stroke therefore begins at TDC and continues as the piston moves toward bottom dead centre (BDC). Real engines introduce small departures: spark occurs before TDC for proper phasing, and the exhaust valve opens slightly before BDC to reduce pumping work during blowdown. Nonetheless, the conceptual description aligns with TDC-to-BDC for the expansion stroke. Step-by-Step Solution: Identify ideal event: expansion begins when piston leaves TDC after combustion.Track piston travel: expansion continues through the downward stroke.End of ideal expansion: at BDC (before exhaust stroke). Real engines may open exhaust early, but the stroke is still considered TDC→BDC in ideal diagrams. Verification / Alternative check: Valve-timing diagrams show ignition advance before TDC and exhaust valve opening before BDC. Despite these optimizations, the expansion stroke is conventionally defined as the piston’s motion from TDC to BDC when cylinder pressure is doing positive work on the piston. Why Other Options Are Wrong: Option (b) suggests expansion starts before TDC, which is incorrect; pressure may rise due to combustion, but expansion stroke formally begins at or just after TDC.Option (c) fixes specific angles that are not standard nor universally correct.Option (d) ignores definite kinematic limits set by TDC and BDC.Option (e) reverses the direction of the power stroke. Common Pitfalls: Confusing the start of combustion (spark advance before TDC) with the start of the expansion stroke; also assuming exhaust always opens exactly at BDC in reality. Ideal definitions are used for conceptual clarity. Final Answer: starts at top dead centre and ends at bottom dead centre

Question 2

Air–fuel ratio at idle (spark-ignition engines) During idling of a petrol (gasoline) engine, which mixture strength is generally required for stable combustion and smooth running?

Accepted Answer

Introduction / Context: Idle operation presents a challenging combustion environment: low air motion, low manifold pressure, and low in-cylinder temperatures. Carburetor idle circuits and modern ECU idle strategies enrich the mixture to maintain a stable flame and prevent misfire. Recognizing the appropriate mixture at idle is vital for troubleshooting rough idle and setting carburetors or ECUs. Given Data / Assumptions: Spark-ignition engine operating at low speed and very light load (closed or nearly closed throttle). Conventional homogeneous-charge operation. Goal is smooth, reliable combustion without misfire. Concept / Approach: At idle, air velocity through the intake is low, fuel atomization and vaporization are poorer, and flame propagation can be inconsistent. A slightly rich mixture (more fuel than stoichiometric) improves flame speed and stability and reduces the chance of lean misfire. Historically, carburetors include dedicated idle passages calibrated to deliver this richer mixture; modern engines may use closed-loop control but still target a mixture richer than the nominal stoichiometric in many conditions during cold or unstable idle. Step-by-Step Solution: Identify idle constraints: low turbulence, low temperature, weak mixture tolerance is reduced.Rich mixture → faster flame development and improved stability → smoother idle.Therefore, select “rich” as the correct characterization. Verification / Alternative check: Service literature for carbureted engines specifies idle mixture screws that typically enrich for highest stable idle speed. Modern ECU maps may use enrichment during cold idle and stabilization phases to prevent misfires and maintain stable rpm. Why Other Options Are Wrong: “Lean” or “ultra-lean” increases lean misfire risk at idle due to poor ignition quality.“Chemically correct” is a laboratory ideal and often too lean for stable idle under cold or low-turbulence conditions.“Oxygen-enriched” is not a practical mixture description for automotive idle control. Common Pitfalls: Confusing warm, well-calibrated closed-loop idling (near stoichiometric) with cold-start and idle stabilization phases that still use enrichment for stability. Final Answer: rich

Question 3

Indicated power vs. brake power Choose the correct statement regarding the relationship between indicated power (IP) and brake power (BP) of an internal combustion engine.

Accepted Answer

Introduction / Context: Engine performance uses three related measures: indicated power (IP), brake power (BP), and friction power (FP). IP is the power developed in the cylinder based on pressure–volume work, BP is the useful shaft output, and FP accounts for mechanical and pumping losses. Understanding their relationship is essential for interpreting dynamometer tests and efficiency calculations. Given Data / Assumptions: Well-defined IP from indicator diagrams or calculations. BP measured at the crankshaft via dynamometer. Mechanical efficiency eta_m = BP / IP is less than 1 due to losses. Concept / Approach: The fundamental relationship is IP = BP + FP. Friction power includes bearing friction, piston and ring friction, valve train losses, pumping work, and accessory loads. Therefore BP, being IP minus these losses, must be lower than IP. No practical condition yields BP > IP; at best, reductions in friction raise BP closer to IP, but never beyond it. Step-by-Step Solution: Write energy balance: IP = BP + FP.Since FP > 0 in real engines, IP > BP.Hence the statement “BP is always greater than IP” is false. Verification / Alternative check: Mechanical efficiency values (typically 0.75–0.90 for many engines) confirm BP/IP < 1. Morse tests and motoring tests independently demonstrate positive friction power, reinforcing IP > BP. Why Other Options Are Wrong: Equality (option c or d) would require zero losses, which is unrealistic. Option e adds an engine-type condition that does not alter the fundamental inequality. Common Pitfalls: Confusing brake mean effective pressure (BMEP) with indicated mean effective pressure (IMEP); BMEP is always lower than IMEP for the same engine and conditions because of mechanical and pumping losses. Final Answer: Disagree — indicated power is greater than brake power due to mechanical losses

Question 4

Reactor control sensing: what is measured? In the automatic control system of nuclear reactors, the sensing element is intended to measure the level (intensity) of neutron flux to infer reactor power. Which option correctly reflects this purpose?

Accepted Answer

Introduction / Context: Reactor power in fission systems is directly proportional to neutron population and thus to neutron flux. Control and protection systems rely on detectors that respond to neutron flux to regulate reactivity and maintain safe power levels. Recognizing what the sensors measure clarifies how the control loop is structured. Given Data / Assumptions: Standard power reactors using ion chambers, fission chambers, or boron-lined detectors. Flux is commonly expressed in neutrons per square centimetre per second (n/cm^2·s). Goal is real-time indication of reactor power and rate-of-change (period). Concept / Approach: Neutron detectors do not measure temperature, volume, or “density” of flux; they measure flux intensity (or its proportional current) and sometimes its time derivative. Reactor period meters, source range, intermediate range, and power range channels are all based on neutron count or ionization current linked to flux level. Therefore “none of these” is the best choice because the listed options do not state “neutron flux” or “flux intensity.” Step-by-Step Solution: Identify controlled variable: neutron flux (reactor power proxy).Map sensor principle: ionization or fission products create current proportional to flux.Conclude none of the provided distractors matches “flux level,” so choose “none of these.” Verification / Alternative check: Plant instrumentation schematics show neutron detectors feeding the reactor protection system and regulating rods via power/period signals, not via temperature or volume. Thermal sensors exist but for coolant monitoring, not primary reactivity control. Why Other Options Are Wrong: Temperature and pressure are thermohydraulic variables; volume is irrelevant; “density” does not apply to flux. Common Pitfalls: Confusing coolant temperature control with neutron power control; they are coupled but sensed by different instruments. Final Answer: none of these

Question 5

Measuring indicated power (multi-cylinder SI engine) Which test method is commonly used to determine the indicated power of a multi-cylinder spark-ignition engine?

Accepted Answer

Introduction / Context: Indicated power (IP) quantifies the net work developed in the cylinders. Direct measurement on multi-cylinder engines is difficult without cylinder pressure indicating equipment. The Morse test offers a practical alternative to estimate IP from accessible brake measurements by cutting out cylinders sequentially. Given Data / Assumptions: Multi-cylinder spark-ignition engine operating at constant speed. Ability to disable cylinders one at a time (e.g., shorting spark plugs). Brake power measured on a dynamometer. Concept / Approach: The Morse test estimates the indicated power of each cylinder by observing the drop in brake power when that cylinder is cut out. Summing the per-cylinder indicated contributions gives total IP. Since BP is measured directly, and losses are assumed nearly unchanged at constant speed, the differences yield a reasonable IP estimate without in-cylinder pressure transducers. Step-by-Step Solution: Run engine at constant speed and record BP (all cylinders firing).Disable one cylinder → measure new BP; the drop approximates that cylinder’s indicated contribution.Repeat for all cylinders; sum indicated contributions to obtain total IP. Verification / Alternative check: Comparisons with indicated diagrams show acceptable accuracy for tuning and diagnostics, though precision studies still prefer pressure-indicating equipment. Why Other Options Are Wrong: Prony brake test measures BP, not IP. Motoring test assesses friction power by driving the engine. Heat balance test partitions energy flows but does not directly yield IP. Willans line uses fuel flow versus power for efficiency mapping, not IP per se. Common Pitfalls: Assuming friction and pumping losses remain perfectly constant; slight variations can introduce error, so results are approximate. Final Answer: Morse test

Question 6

First nuclear power plant in India Identify the location of India's first commercial nuclear power station (commissioned as TAPS).

Accepted Answer

Introduction / Context: Historical knowledge of nuclear installations is common in engineering general studies. India’s first commercial nuclear power plant set the stage for subsequent developments in the nation’s nuclear program. Given Data / Assumptions: TAPS stands for Tarapur Atomic Power Station. Commissioned in the late 1960s as India’s first commercial nuclear station. Other sites listed host later nuclear facilities or are unrelated. Concept / Approach: Matching the acronym and historical commissioning confirms the correct location as Tarapur, Maharashtra. Noting this helps avoid confusion with other reactor sites like Kota (Rajasthan Atomic Power Station) or Kalpakkam (Madras Atomic Power Station). Step-by-Step Solution: Recall acronym: TAPS → Tarapur.Associate with India’s first commercial nuclear plant.Select Tarapur as the correct answer. Verification / Alternative check: Official records and energy-history sources consistently list Tarapur as India’s first commercial nuclear power station. Why Other Options Are Wrong: Kota refers to Rajasthan Atomic Power Station; Bareilly is not a nuclear site; “none of these” conflicts with known facts; Kalpakkam hosts MAPS built later. Common Pitfalls: Confusing research reactors with power stations; TAPS is a commercial power plant. Final Answer: Tarapur

Question 7

Normal cetane (n-cetane) used in diesel knock rating Which statement(s) about normal cetane are correct?

Accepted Answer

Introduction / Context: Diesel fuel ignition quality is characterized by the cetane number scale. The reference fuels for this scale include normal-cetane at the high-reactivity end and alpha-methylnaphthalene (historically) or heptamethylnonane (modern) at the low-reactivity end. Understanding what “cetane” is helps interpret diesel ignition behavior. Given Data / Assumptions: Normal cetane denotes n-hexadecane. It is used as a primary reference fuel for the cetane number determination. Straight-chain paraffins typically have high cetane numbers (short ignition delay). Concept / Approach: In the cetane rating method, a test diesel’s ignition delay is matched to that of a blend of reference fuels. n-Hexadecane, with excellent ignition quality, is assigned cetane number 100. Its long, straight-chain structure enhances reactivity, explaining its role as the “good” end of the scale. Therefore, all listed statements about normal cetane are correct. Step-by-Step Solution: Identify the reference fuel: normal cetane = n-hexadecane.Recall its property: very short ignition delay (high cetane number).Confirm structure: long straight-chain paraffin (C16H34). Verification / Alternative check: Engine test standards for cetane number routinely cite n-hexadecane as the 100-point reference, with low-reactivity reference adjusted in modern practice (e.g., HMN). Why Other Options Are Wrong: Options restricting correctness to subsets ignore that all three statements are simultaneously true. Common Pitfalls: Confusing octane (SI knock resistance) with cetane (CI ignition quality); they measure different phenomena. Final Answer: all of the above

Question 8

Factors increasing diesel knock tendency Which of the following fuel properties/conditions increase the probability of knocking in compression-ignition (diesel) engines?

Accepted Answer

Introduction / Context: Diesel knock relates to the rate of pressure rise when a significant amount of fuel ignites almost simultaneously after a long ignition delay. Fuel properties that lengthen ignition delay or degrade spray preparation exacerbate knock tendency. Identifying these properties is essential for fuel selection and injector design. Given Data / Assumptions: Compression-ignition engine with direct or indirect injection. Constant injection timing and in-cylinder conditions for comparison. Focus on fuel properties and their effect on ignition delay and mixture preparation. Concept / Approach: High self-ignition temperature fuels resist auto-ignition, increasing the ignition delay. Longer delay allows more fuel to accumulate before ignition, producing an abrupt heat release and stronger knock. Low volatility hinders fuel evaporation, slowing mixing with air and contributing to non-uniform combustion onset. High viscosity degrades atomization, leading to larger droplets, slow vaporization, and poor penetration–mixing patterns, all of which can prolong delay and promote knock intensity. Step-by-Step Solution: Link self-ignition temperature to delay: higher → longer delay → greater premixed burn → more knock.Link volatility to vaporization: lower → slower evaporation → delayed ignition and uneven burn.Link viscosity to atomization: higher → larger Sauter mean diameter → poorer mixing → increased delay. Verification / Alternative check: Empirical correlations of ignition delay include temperature, pressure, and fuel cetane quality. Laboratory tests demonstrate reduced knock with higher-cetane, properly volatile, lower-viscosity fuels within injector design limits. Why Other Options Are Wrong: “None of these” contradicts established combustion science; each listed factor is known to worsen diesel knock when considered in isolation. Common Pitfalls: Assuming only cetane number matters; spray atomization and volatility are equally important for ignition and mixture formation dynamics. Final Answer: all of these

Question 9

Why light-water reactors need enriched uranium When ordinary (light) water is used as both moderator and coolant, why is enriched uranium fuel required?

Accepted Answer

Introduction / Context: Reactor criticality depends on neutron economy: enough neutrons must survive moderation and non-productive losses to sustain fission. The properties of the moderator strongly influence whether natural uranium suffices or enrichment is required. Given Data / Assumptions: Light water (H2O) acts as moderator and coolant. Fuel is uranium with fissile U-235 present at 0.7% in natural uranium. Goal is to maintain a multiplication factor near unity under operating conditions. Concept / Approach: Hydrogen in light water possesses a relatively high absorption cross-section for thermal neutrons compared with heavy water or graphite. Consequently, many neutrons are captured by the moderator instead of inducing fission in U-235. To offset these parasitic losses and achieve criticality, the fuel must be enriched so that a higher fraction of neutrons find fissile nuclei. Heavy-water or graphite moderators, with much lower absorption, can often use natural uranium instead. Step-by-Step Solution: Identify loss mechanism: parasitic absorption in H2O.Effect on neutron economy: fewer neutrons remain to cause fission.Compensation strategy: increase U-235 fraction by enrichment to restore criticality. Verification / Alternative check: Comparative designs: CANDU (heavy water) operates on natural uranium; PWR/BWR (light water) require low-enriched uranium, illustrating the absorption effect of the moderator choice. Why Other Options Are Wrong: Option (b) is a consequence statement but the core reason is the higher absorption; option (c) scattering helps moderation rather than causing the need for enrichment; option (d) is the opposite of the truth; option (e) confuses neutronics with thermal hydraulics. Common Pitfalls: Conflating moderating power (a product of scattering effectiveness and low absorption) with scattering alone; low absorption is crucial for good neutron economy. Final Answer: Because light water has a high neutron absorption cross-section

Question 10

Definition in I.C. engines: scavenging In reciprocating internal combustion engines, what is the process called by which the burnt gases are removed from the cylinder at the end of the power stroke and during the exhaust stroke?

Accepted Answer

Introduction / Context: Exhaust gas displacement is essential for the next fresh charge to enter and for maintaining volumetric efficiency. The term used for actively clearing residual combustion products from the cylinder is standard in both two-stroke and four-stroke engine discussions. Given Data / Assumptions: Cylinder has just completed the power stroke. Exhaust valve opens (four-stroke) or ports uncover (two-stroke). Fresh charge may assist in displacing residuals (especially in two-stroke). Concept / Approach: Scavenging denotes the organized removal of spent gases from the cylinder so that the incoming mixture or air can occupy the cylinder for the next cycle. Effective scavenging reduces residual fraction, improves combustion stability, and enhances performance. In two-stroke engines, dedicated scavenging flows (cross-flow, loop, or uniflow) are designed. In four-stroke engines, exhaust blowdown plus piston-driven displacement and valve overlap accomplish the task. Step-by-Step Solution: Identify the phase: end of expansion and through exhaust events.Define action: displacement and removal of burnt gases.Name the process: scavenging. Verification / Alternative check: Textbook valve-timing diagrams include overlap specifically to aid scavenging by using pressure differentials to draw in fresh charge and push out residuals. Why Other Options Are Wrong: Detonation is abnormal combustion; supercharging increases intake pressure; polymerisation is a chemical process unrelated to cycle gas exchange; after-treatment refers to exhaust emission control devices outside the cylinder. Common Pitfalls: Equating scavenging only with two-stroke engines; while more critical there, four-stroke engines also benefit from effective scavenging via valve overlap and exhaust tuning. Final Answer: scavenging

Question 11

Supercharging in internal combustion engines A supercharger draws air from the surrounding atmosphere, compresses it to a higher intake pressure, and then delivers this pressurised air to the engine intake valve or intake manifold. Evaluate the corr…

Accepted Answer

Introduction / Context: Supercharging is a classic method to increase the mass of air inducted per cycle so that more fuel can be burned without exceeding safe air–fuel limits. This raises the indicated mean effective pressure and power density. The question asks whether the concise operational description of a supercharger is correct. Given Data / Assumptions: Air is drawn from ambient surroundings. A mechanical compressor raises intake pressure above atmospheric. Pressurised air is supplied to the intake valve or manifold before the intake stroke. No restriction to a specific engine type is implied; concept applies to both SI and CI engines. Concept / Approach: A supercharger is an air compressor mechanically driven by the engine crankshaft (gears, belt, or direct coupling). Unlike a turbocharger, it does not use exhaust gas energy for the turbine drive. Its essential function is to increase intake air pressure and density, boosting trapped air mass per cycle and enabling higher fuel flow at similar equivalence ratios. Step-by-Step Solution: Identify the medium handled: fresh atmospheric air, not exhaust gas.Identify the process: compression to a higher-than-ambient pressure.Identify delivery: compressed air fed to intake port/valve/manifold.Match with definition: this is precisely the purpose and pathway of a supercharger. Verification / Alternative check: Manufacturer schematics show the supercharger upstream of the intake manifold. Intercoolers may be added for charge temperature control but are not required to satisfy the core definition. Why Other Options Are Wrong: Incorrect: contradicts the accepted definition. Correct only for diesel: SI engines are supercharged as well. Correct only with intercooling: intercooling is beneficial but optional. Exhaust-only handling: describes turbocharger turbine flow, not a supercharger. Common Pitfalls: Confusing turbocharging (exhaust-driven) with supercharging (mechanically driven). Both compress intake air, but the energy source differs. Final Answer: Correct

Question 12

Typical brake specific fuel consumption (BSFC) for diesel engines For a compression-ignition (diesel) engine operating at or near rated load, the approximate specific fuel consumption per brake power hour is closest to which value?

Accepted Answer

Introduction / Context: Brake specific fuel consumption (BSFC) measures how much fuel mass is required to produce one unit of brake power for one hour. It is a key indicator of engine efficiency, commonly expressed as kg per brake power hour or g/kWh. The question seeks the characteristic value for diesel engines. Given Data / Assumptions: Engine type: compression-ignition diesel. Operating region: near rated load and speed where BSFC is typically minimized. Units: kg per brake power hour. Concept / Approach: Diesel engines generally demonstrate superior thermal efficiency compared with spark-ignition engines due to higher compression ratios and unthrottled operation. Typical BSFC values for well-tuned medium-speed diesels cluster around 0.2 kg per bp·h (approximately 200 g per bhp·h). Petrol engines usually show higher values, around 0.3 kg per bp·h at comparable conditions. Step-by-Step Solution: Identify typical diesel BSFC band: roughly 0.18–0.25 kg per bp·h depending on size and tuning.Select the nominal textbook value representing efficient diesel operation: 0.2 kg per bp·h.Compare with alternatives: 0.3–0.35 kg per bp·h is characteristic of petrol engines or poorly loaded diesels. Verification / Alternative check: Engine maps from manufacturers show minima near 190–220 g per kWh for many modern diesels, in agreement with the 0.2 kg per bp·h magnitude. Why Other Options Are Wrong: 0.25 kg and 0.3–0.35 kg: higher than the typical optimum diesel value; plausible for smaller or poorly tuned units but not the standard figure. 0.15 kg: unrealistically low for conventional diesels at rated conditions. Common Pitfalls: Confusing indicated specific fuel consumption (ISFC) with brake values; mechanical losses make BSFC higher than ISFC. Final Answer: 0.2 kg

Question 13

Octane number interpretation for SI fuels A petrol fuel with an octane rating of 75 has the same knocking tendency as which reference mixture by volume in the CFR test engine?

Accepted Answer

Introduction / Context: The octane scale for spark-ignition fuels is defined using two primary reference fuels: iso-octane, which resists knock well, and n-heptane, which knocks readily. The octane number quantifies a fuel’s knock resistance by equating it to a blend of these two references. Given Data / Assumptions: Octane number of the test fuel is 75. Reference fuels: iso-octane (100) and n-heptane (0). Mixtures are expressed in percent by volume for the CFR test engine comparison. Concept / Approach: An octane rating of 75 indicates the fuel behaves, in terms of knock tendency, like a blend containing 75% iso-octane and 25% n-heptane under standard test conditions. That is the operational definition of the research octane number scale. Step-by-Step Solution: Map rating to blend: Octane 75 → 75 parts iso-octane + 25 parts n-heptane (by volume).Exclude distractors that swap the percentages or replace the references with unrelated fuels.Select the mixture that matches the definition. Verification / Alternative check: Standard fuel testing protocols (RON/MON) rely on CFR engine comparisons to these defined blends. Why Other Options Are Wrong: Swapped percentages invert the rating (would imply Octane 25). Petrol–diesel mixtures are not reference pairs for octane testing. Aromatic or alcohol mixtures may alter knock resistance but are not the scale’s endpoints. Common Pitfalls: Confusing octane number (knock resistance) with calorific value or flash point; they are independent properties. Final Answer: 75% iso-octane and 25% n-heptane

Question 14

Ignition source in petrol (spark-ignition) engines In a conventional petrol engine, the premixed air–fuel charge in the cylinder is ignited by which device?

Accepted Answer

Introduction / Context: Petrol (spark-ignition) engines rely on externally initiated ignition of a premixed air–fuel charge. The reliability and timing of the ignition source are crucial for power, efficiency, and emissions control. Given Data / Assumptions: Engine type: spark-ignition gasoline engine. Mixture is air plus atomised fuel prepared by a carburettor or fuel injection. Goal: identify the component that actually initiates combustion. Concept / Approach: Spark plugs provide a high-voltage electrical discharge across electrodes, igniting the compressed mixture at a precise crank angle. Other listed components either meter fuel (pump/injector) or are relevant to diesel engines (glow plug) and do not create the timed ignition spark in SI engines. Step-by-Step Solution: Determine which device delivers a controlled spark → the spark plug.Eliminate components that deliver fuel but do not ignite it (pump, injector).Eliminate diesel-specific aids (glow plug) and implausible items (heated valve). Verification / Alternative check: Ignition systems comprise ignition coil, control electronics or distributor, leads, and spark plugs to ensure reliable kernel formation. Why Other Options Are Wrong: Fuel pump/injector: regulate fuel delivery, not ignition. Glow plug: assists cold starting in diesel, not used to trigger SI combustion. Heated valve: not an ignition device. Common Pitfalls: Assuming fuel injectors always imply compression ignition; many SI engines are fuel-injected but still require a spark plug. Final Answer: Spark plug

Question 15

Relative cylinder size for gas engines versus petrol engines Considering typical design practice and energy density of the inducted charge, the cylinder of a gas-fuelled engine (running on gaseous fuel) is generally made ________ than that of a comp…

Accepted Answer

Introduction / Context: Engine sizing is influenced by the energy per unit charge and volumetric efficiency. Gas engines (using gaseous fuels such as natural gas or producer gas) often deliver less energy per inducted cylinder charge than petrol engines under similar conditions. Given Data / Assumptions: Comparing engines of similar technology and speed class. Gas engine uses a gaseous fuel–air mixture; petrol engine uses gasoline–air mixture. Goal: achieve similar brake power. Concept / Approach: Gaseous fuels displace part of the intake air volume, lowering the mass of oxygen available per cycle relative to liquid-fuelled SI engines where only vaporised fuel occupies minimal volume. Additionally, some gaseous fuels have lower volumetric energy density. To maintain comparable power, designers increase cylinder displacement (larger cylinder) or boost intake density. Step-by-Step Solution: Recognise reduced oxygen/fuel energy per charge for gaseous mixtures.Compensate by increasing displacement (larger cylinder dimensions).Therefore select “larger.” Verification / Alternative check: Practical engine catalogues show natural gas variants often having lower specific power or larger displacement than gasoline counterparts at the same ratings. Why Other Options Are Wrong: Smaller or same size: would reduce power further unless compensated by boosting. “Variable” options do not address the general design trend taught in fundamentals. Common Pitfalls: Confusing gaseous-fuel turbocharged engines (which can recover power) with naturally aspirated designs; the general statement refers to unboosted or lightly boosted cases. Final Answer: larger

Question 16

Compression-ignition (diesel) knock and compression ratio For a given fuel in a CI engine, how does changing the compression ratio influence the knocking tendency?

Accepted Answer

Introduction / Context: Diesel knock arises from rapid pressure rise caused by accumulated fuel that auto-ignites after a long ignition delay. Compression ratio directly affects in-cylinder temperature at the end of compression and thereby influences ignition delay and knock severity. Given Data / Assumptions: Fuel quality is fixed. Engine speed and injection timing are within normal ranges. We examine the qualitative effect of compression ratio on ignition delay and knock. Concept / Approach: Higher compression ratio increases end-of-compression temperature and pressure, reducing chemical ignition delay. Shorter delay means less fuel accumulates before ignition, thereby mitigating the abrupt energy release that causes knock. Conversely, lowering compression ratio reduces temperature, lengthens ignition delay, allows more premixed fuel to accumulate, and heightens knock intensity. Step-by-Step Solution: Increase CR → higher T and p at TDC → shorter ignition delay.Shorter delay → less premixed charge → smoother pressure rise → less knock.Decrease CR → lower T at TDC → longer delay → more premixed fuel → stronger knock.Therefore, knocking tendency is enhanced by decreasing compression ratio. Verification / Alternative check: Engine calibration practices exploit higher CR in diesels to obtain quick self-ignition and smoother combustion, within mechanical stress limits. Why Other Options Are Wrong: Enhanced by increasing CR: opposite of the typical diesel behavior. Dependent only on injector size/Unaffected by CR: both ignore the dominant thermodynamic link between CR and end-gas conditions. Exhaust backpressure effect is secondary here and does not reverse the fundamental trend. Common Pitfalls: Transferring SI engine knock intuition to diesel; in SI, higher CR can increase knock, but CI relies on auto-ignition by design. Final Answer: Enhanced by decreasing compression ratio

Question 17

Nuclear versus conventional steam power stations Compare a nuclear power plant with a conventional coal-fired steam power station. The principal difference lies only in the steam-generating section that supplies energy to the working fluid. Assess t…

Accepted Answer

Introduction / Context: Both nuclear and conventional thermal power plants use a heat source to raise steam, expand it through a turbine, and convert thermal energy into electricity via a generator. This question asks whether the fundamental difference is limited to the heat source (steam generator/boiler section). Given Data / Assumptions: Downstream equipment: steam turbine, condenser, feedwater system, and generator are broadly similar. Nuclear plant uses a reactor as the heat source; a coal plant uses a boiler fired by fossil fuel. Focus is on thermodynamic cycle hardware rather than nuclear-specific safety systems. Concept / Approach: Thermodynamically, both plants implement the Rankine cycle. The major change is the primary heat generation method: chemical combustion in boilers versus nuclear fission in reactors with associated heat exchangers (steam generators) or direct steam production. Auxiliary systems such as control, shielding, and safety are unique to nuclear plants, but the turbine island is largely the same. Step-by-Step Solution: Identify cycle commonality: Rankine cycle with similar turbine–condenser–feedwater trains.Identify the difference: reactor core plus steam generator replaces the coal-fired boiler.Therefore, the principal thermodynamic difference is indeed the steam-generating part. Verification / Alternative check: Plant layouts show a “nuclear island” (reactor and safety systems) feeding a conventional “turbine island.” Why Other Options Are Wrong: Disagree: ignores the shared turbine-condenser-generator architecture. Conditions about specific reactor types or gas turbines are irrelevant to the basic comparison. Common Pitfalls: Conflating thermodynamic equipment with unique nuclear safety and control systems. The statement is thermodynamic-equipment centric. Final Answer: Agree

Question 18

Natural uranium isotopic composition Identify the approximate fractional composition of the three naturally occurring uranium isotopes in un-enriched natural uranium by percentage.

Accepted Answer

Introduction / Context: Natural uranium consists primarily of U-238 with small fractions of U-235 and a trace of U-234. Knowing these percentages is essential for reactor fuel discussions, enrichment calculations, and neutron economy considerations. Given Data / Assumptions: Material: natural (un-enriched) uranium. Three isotopes considered: U-238, U-235, U-234. We seek widely accepted average percentages. Concept / Approach: Standard references report natural uranium as roughly U-238 ≈ 99.28%, U-235 ≈ 0.71%, and U-234 ≈ 0.005–0.006%. Minor variations occur by deposit and measurement method, but these values are the canonical figures used in educational settings. Step-by-Step Solution: Select the option with U-238 near 99.28%.Ensure U-235 is about 0.71% and U-234 a trace near 0.006%.Option (a) matches the accepted composition. Verification / Alternative check: Reactor physics texts and nuclear data tables corroborate these percentages used in enrichment benchmarks. Why Other Options Are Wrong: Swapped or exaggerated values contradict known natural abundances. Any option making U-235 or U-234 the dominant isotope is physically incorrect for natural uranium. Common Pitfalls: Confusing the composition of enriched uranium (higher U-235) with natural uranium. Final Answer: U234 = 0.006%, U235 = 0.712%, U238 = 99.282%

Question 19

Purpose of the neutron reflector in reactor design State the primary function of installing a reflector region around the active reactor core.

Accepted Answer

Introduction / Context: A reflector region surrounds the active core to improve neutron economy. By scattering outward-moving neutrons back towards the fuel, a reflector reduces leakage and can increase multiplication, flatten flux, and allow smaller cores for the same reactivity. Given Data / Assumptions: A reflector is made of low-absorption, good-scattering material (graphite, beryllium, water, or steel depending on design). Steady-state operation is considered. The statement claims the reflector is used to reflect neutrons back into the active core. Concept / Approach: Neutron transport theory shows that adding a region with favorable scattering length and low capture cross section increases the probability that leaking neutrons re-enter the core. This produces the well-known “reflector savings” in criticality calculations, effectively reducing the required core size or fuel loading. Step-by-Step Solution: Identify desired effect: reduce leakage (neutrons escaping boundary).Add reflector with appropriate properties → more back-scatter and moderation.Result: more neutrons return to the core and contribute to fissions. Verification / Alternative check: Flux measurements show higher peripheral flux and improved core reactivity with reflectors, consistent with design calculations. Why Other Options Are Wrong: “False” or restrictive conditions ignore that reflectors are standard across many reactor types, not only fast or heavy-water designs. “Absorb all neutrons” contradicts the reflector selection principle of low absorption. Common Pitfalls: Assuming any material placed outside the core acts as a reflector; material selection is critical to achieve reflection rather than absorption. Final Answer: True

Question 20

Engine speed and knock tendency in petrol engines For a spark-ignition engine operating on a fixed-octane fuel, what is the qualitative effect of increasing engine speed on the tendency to knock?

Accepted Answer

Introduction / Context: Knock in a petrol (spark-ignition) engine is caused by auto-ignition of the end-gas ahead of the flame front. The octane rating of the fuel indicates resistance to this auto-ignition. Engine speed influences in-cylinder time scales, turbulence, and heat transfer, thereby affecting knock tendency. Given Data / Assumptions: Fuel octane number is fixed. Comparisons are made over a normal operating range, not at destructive overspeeds. Mixture strength and load are not drastically altered except due to normal volumetric efficiency effects. Concept / Approach: As engine speed increases, the absolute time available for end-gas to undergo pre-flame reactions before the flame front arrives becomes shorter (milliseconds decrease with higher rpm). Increased turbulence at higher speeds also accelerates flame propagation, reducing the residence time of end-gas at high temperature and pressure. Both effects reduce the probability of knock for the same octane fuel. Step-by-Step Solution: Higher rpm → shorter time per crank angle → less pre-flame reaction time.Higher turbulence → faster flame speed → end-gas consumed sooner.Net effect → decreased knock tendency with increasing speed for a given fuel. Verification / Alternative check: Ignition timing maps often advance at low–mid speeds and are retarded near knock limits at high load, but, all else equal, the shorter time scales at higher speeds give more knock margin for a fixed octane. Why Other Options Are Wrong: “Increases knocking” contradicts the time-scale argument. “No effect” ignores well-established combustion dynamics. Conditional statements about redline or WOT are too restrictive and not central to the basic relationship. Common Pitfalls: Confusing effects of increased load (which can promote knock) with effects of increased speed at similar load. Final Answer: Yes, increasing speed decreases knocking tendency

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