Question 1

Four-stroke diesel engine valve timing (inlet): In a conventional four-stroke compression-ignition (diesel) engine, which of the following inlet-valve timing pairs best represents typical practice for opening before top dead centre and closing after…

Accepted Answer

Introduction / Context: Valve timing in four-stroke diesel engines is deliberately advanced or retarded relative to ideal dead-centre positions to improve breathing, volumetric efficiency, and scavenging. Recognising typical inlet-valve opening (IVO) and inlet-valve closing (IVC) angles helps interpret timing diagrams and diagnose performance issues. Given Data / Assumptions: Standard single-intake-valve, four-stroke diesel engine. Angles are crankshaft degrees relative to top dead centre (TDC) and bottom dead centre (BDC). Objective is qualitative identification of typical timing, not an exact spec for a specific model. Concept / Approach: To fill the cylinder effectively, the inlet valve often opens slightly before TDC during the exhaust stroke overlap and remains open after BDC during the intake stroke. Opening early uses inertia of exiting gases to start flow; closing late takes advantage of inlet port momentum to pack more air at low-to-moderate speeds. Step-by-Step Solution: Typical diesel IVO: about 10°–25° before TDC (during overlap).Typical diesel IVC: about 30°–50° after BDC to exploit ram effect.Among the options, 20° BTDC opening and 40° ABDC closing fits the accepted range.Therefore, select the option with IVO ≈ 20° BTDC and IVC ≈ 40° ABDC. Verification / Alternative check: Standard timing diagrams in textbooks show inlet opening slightly before TDC and closing well after BDC for most naturally aspirated and turbocharged diesels at design speed ranges. Manufacturers vary exact angles, but the pattern remains consistent. Why Other Options Are Wrong: Opening after TDC and closing before BDC shortens effective intake duration, hurting volumetric efficiency. Exactly-at-dead-centre timing ignores flow inertia. Saying it may open and close anywhere is physically incorrect. An early open with early close (e.g., before BDC) misuses the ram effect. Common Pitfalls: Confusing SI and CI timing magnitudes; overlooking overlap; assuming idealised at-TDC/BDC events from simple schematics. Final Answer: Opens 20° before TDC and closes 40° after BDC

Question 2

Breeder reactors in nuclear engineering: Do special breeder reactors provide a net gain of fissionable (fissile) material over their operating cycle?

Accepted Answer

Introduction / Context: Breeder reactors are designed to “breed” more fissile material than they consume by converting fertile isotopes into fissile ones. Understanding this principle is central to fuel-cycle engineering and long-term nuclear resource utilisation. Given Data / Assumptions: Fertile isotopes include U-238 and Th-232. Fissile isotopes produced include Pu-239 from U-238 and U-233 from Th-232. Net gain means bred fissile mass exceeds fissile mass consumed over a defined period (breeding gain > 0). Concept / Approach: In a breeder, excess neutrons from fission are captured by fertile nuclei, transmuting them through beta decay chains into fissile isotopes. Blanket assemblies surrounding the core optimise absorption and recovery of bred material, enabling a breeding ratio ≥ 1. Step-by-Step Solution: Neutron economy: produce more neutrons per fission than required to sustain chain reaction.Direct surplus neutrons into fertile blankets (e.g., U-238 → U-239 → Np-239 → Pu-239).Recover bred fissile material through reprocessing.If bred amount > consumed fissile, there is a net gain (breeding). Verification / Alternative check: Fast breeder reactors (e.g., sodium-cooled) historically demonstrated breeding ratios above unity. Thermal breeders (thorium–U-233 cycles) can also approach unity with suitable moderators and reprocessing technology. Why Other Options Are Wrong: Limiting to fusion is incorrect; breeding is a fission-fuel-cycle concept. Heavy water moderates well but is not a prerequisite for breeding. “Only at zero power” contradicts the need for neutron flux to breed. Common Pitfalls: Confusing fissile with fertile; assuming all reactors are breeders; overlooking reprocessing needs to realise the bred fuel in practice. Final Answer: True

Question 3

Flywheel sizing comparison at equal rated power: Which engine type will require the heaviest flywheel for smooth operation, given the same 30 kW rating?

Accepted Answer

Introduction / Context: A flywheel stores rotational energy to smooth cyclic speed variation caused by non-uniform torque over an engine cycle. The required mass moment of inertia depends on the number of power strokes per revolution, torque pulsation severity, operating speed, and acceptable coefficient of fluctuation of speed. Given Data / Assumptions: All engines rated at 30 kW. Lower rpm increases required inertia for the same energy smoothing. Four-stroke has one power stroke per two revolutions per cylinder; two-stroke has one per revolution. Diesels generally exhibit higher compression and larger torque pulsations than comparable petrol engines. Concept / Approach: Energy fluctuation per cycle must be buffered by the flywheel: I * (omega_max^2 - omega_min^2) ≈ energy variation per cycle. For the same power, lower speed implies higher work per cycle and thus larger energy fluctuations. Fewer power strokes (four-stroke vs two-stroke) also increase fluctuation. Diesel torque waves are typically harsher than petrol, further increasing required smoothing capacity. Step-by-Step Solution: Compare speed: 750 rpm vs 1500 rpm → lower speed requires more inertia.Compare stroke type: four-stroke has half the firing frequency of two-stroke at same rpm.Compare fuel type: diesel tends to larger cyclic torque irregularity.Thus, four-stroke diesel at 750 rpm needs the heaviest flywheel. Verification / Alternative check: Textbook flywheel sizing problems consistently show I ∝ energy fluctuation / omega^2. Halving speed approximately quadruples the inertia needed for similar speed regulation, strengthening the conclusion. Why Other Options Are Wrong: Higher rpm options need less inertia. Two-stroke engines have more frequent power strokes, reducing fluctuation. Petrol engines have smoother pressure rise, reducing inertia demand compared to diesels. Common Pitfalls: Comparing only fuel type without considering rpm and stroke; ignoring that power equality does not imply equal per-cycle work. Final Answer: 30 kW four-stroke diesel engine at 750 rpm

Question 4

Moderator versus reflector materials in reactors: Is the statement “materials that make good neutron moderators do not make good neutron reflectors” correct?

Accepted Answer

Introduction / Context: In reactor physics, moderators slow down (thermalise) fast neutrons, while reflectors return leaking neutrons back into the core. Many practical materials can serve as both to varying degrees, and understanding this overlap is important in core design and shielding decisions. Given Data / Assumptions: Common moderators: light water, heavy water, graphite, beryllium. Common reflectors: graphite, beryllium, water, steel (less effective as moderator). Assessment is qualitative regarding scattering and absorption traits. Concept / Approach: Good moderators have high scattering cross-section and low absorption cross-section to efficiently slow neutrons. Good reflectors also rely on high scattering with low absorption to bounce neutrons back. Therefore, some excellent moderators (graphite, beryllium) are also excellent reflectors; water moderates well and reflects reasonably, albeit with higher absorption than heavy water or beryllium. Step-by-Step Solution: Identify material properties: high scattering, low absorption favorable to both moderation and reflection.Graphite/beryllium meet both criteria; they moderate and reflect effectively.Hence, the blanket statement that good moderators do not make good reflectors is incorrect.Conclusion: select “Incorrect”. Verification / Alternative check: Many reactor designs use graphite or beryllium as both moderator and reflector regions. Heavy water reactors also use D2O as moderator and reflector, confirming the overlap in function. Why Other Options Are Wrong: “Correct” contradicts empirical design. Conditional statements about fast reactors or heavy water are unnecessary; the general claim is false. Saying moderators are always reflectors is too absolute; some moderators reflect poorly depending on geometry and absorption. Common Pitfalls: Assuming mutually exclusive roles; forgetting that reflection is largely multiple scattering while moderation is energy loss per collision—both benefit from similar cross-section characteristics. Final Answer: Incorrect

Question 5

Typical quality of commercial diesel fuel: The usual cetane number range of diesel oil commonly available at retail pumps is approximately

Accepted Answer

Introduction / Context: Cetane number indicates the ignition quality of diesel fuel—higher values correspond to shorter ignition delay and smoother combustion in compression-ignition engines. Knowing typical ranges assists in diagnosing cold-start issues, knock, and emissions in field conditions. Given Data / Assumptions: Standard road diesel in many markets. Engines designed for CN around the mid-40s to low-50s. Premium or specialty diesels may vary, but the question targets “generally available”. Concept / Approach: Most on-road diesel specifications set minimum CN near 45–51 depending on jurisdiction and season. Practical retail fuels cluster roughly 40–55, ensuring reliable ignition without excessive advanced combustion phasing. Step-by-Step Solution: Relate common standards (e.g., EN 590, ASTM D975) to real-world pump values.Identify the broad everyday band encompassing regular and premium diesel.Select the range that captures typical availability: 40 to 55.Therefore, choose 40 to 55. Verification / Alternative check: OEM fuel recommendations and injector calibration maps are often built around CN in the mid-40s to ~50, corroborating this selection. Why Other Options Are Wrong: 20–40 are too low for modern on-road fuels; 60–80 represents specialty high-cetane blends not commonly available at standard pumps. Common Pitfalls: Confusing research octane number (for SI engines) with cetane number; assuming premium diesel universally reaches extremely high CN values. Final Answer: 40 to 55

Question 6

Typical quality of commercial petrol (gasoline): The general octane number range of petrol commonly available for road use is approximately

Accepted Answer

Introduction / Context: Octane number measures a spark-ignition fuel’s resistance to knock. Road gasoline is sold with octane ratings (often Research Octane Number or combined pump octane) tuned for typical passenger engines. Recognising common ranges aids in proper fuel selection and knock diagnostics. Given Data / Assumptions: Regular and premium petrol grades on retail forecourts. Octane presented as RON (common outside North America) or (R+M)/2 as “pump octane”. Question seeks a broad, commonly encountered range. Concept / Approach: Regular petrol is typically around RON 91–95; premium can be RON 95–99+ depending on market. The composite pump octane is slightly lower but still aligns within an approximate 80–100 span internationally for ordinary road fuels. Step-by-Step Solution: Identify regular vs premium brackets in multiple markets.Map these to a safe inclusive band for “generally available”.The 80 to 100 range captures regular to premium offerings.Thus, choose 80 to 100. Verification / Alternative check: Automaker owner’s manuals commonly recommend fuels within this band, and forecourt labels corroborate the values. Why Other Options Are Wrong: 20–80 are unrealistically low for modern road petrol; 100–120 exceeds typical pump grades and enters racing/special fuels territory. Common Pitfalls: Confusing RON with MON or (R+M)/2; overlooking regional labeling differences while the practical range remains similar. Final Answer: 80 to 100

Question 7

Air-standard (Otto) efficiency expression: For an ideal Otto cycle, the air-standard efficiency in terms of compression ratio r and specific heat ratio γ is

Accepted Answer

Introduction / Context: The air-standard efficiency of the Otto cycle relates thermal efficiency to compression ratio r and the ratio of specific heats γ. This canonical formula explains why higher compression ratios (subject to knock limits) yield better efficiency in spark-ignition engines. Given Data / Assumptions: Idealised air-standard analysis (working fluid is air, constant γ). Processes: two isentropic (compression/expansion) and two constant-volume heat exchange steps. No real losses (friction, heat transfer to walls, dissociation) considered. Concept / Approach: Air-standard analysis gives η_Otto = 1 - (T4 - T1)/(T3 - T2). Using isentropic relations for compression and expansion, temperature ratios collapse into r^(γ - 1), leading to the famous closed-form dependence on r and γ. Step-by-Step Solution: For isentropic compression: T2/T1 = r^(γ - 1).For isentropic expansion: T3/T4 = r^(γ - 1).Efficiency reduces to η = 1 - 1 / r^(γ - 1).Therefore select η = 1 - 1 / r^(γ - 1). Verification / Alternative check: Limiting behaviour: as r increases, 1 / r^(γ - 1) → 0 and η → 1, consistent with expectations (though real engines are knock-limited). For γ → 1, efficiency tends to zero, matching thermodynamic intuition. Why Other Options Are Wrong: Forms with 1 - r^(γ - 1) or 1 + 1 / r^(γ - 1) are algebraically incorrect. “None of these” is false because the correct closed form is present. r^(γ - 1) - 1 is not an efficiency and can exceed 1. Common Pitfalls: Forgetting the exponent is (γ - 1); mixing Otto and Diesel efficiency formulae; misapplying polytropic exponents. Final Answer: η = 1 - 1 / r^(γ - 1)

Question 8

Compression period in a four-stroke diesel engine: Identify the most typical crank-angle window for the compression stroke in a four-stroke CI engine (from intake-valve closure to near TDC).

Accepted Answer

Introduction / Context: The compression stroke in a four-stroke diesel begins once the intake valve closes and the piston moves upward, raising the temperature for auto-ignition. Understanding the real timing—offset from ideal dead centres—helps read timing diagrams and reason about performance and emissions. Given Data / Assumptions: Typical timing for a conventional diesel (exact values vary by design). Intake closes after BDC to utilise ram effect, so compression starts slightly after BDC. Fuel is injected close to TDC; ignition delay follows. Concept / Approach: Because the intake valve remains open after BDC (to benefit from air inertia), actual compression of a sealed charge starts only when the valve closes—typically several tens of crank degrees after BDC. Compression then continues nearly up to TDC; practical references sometimes cite end-of-compression a few degrees before TDC where injection may commence. Step-by-Step Solution: Identify IVC ≈ 30°–50° ABDC → compression begins there.Compression continues while piston rises toward TDC.End of compression is near TDC, commonly a few degrees BTDC for injection timing.Thus, “~40° after BDC to ~10° before TDC” is the most appropriate choice. Verification / Alternative check: Manufacturer timing charts confirm delayed IVC and near-TDC injection in many diesels. Exact numbers vary, but the qualitative window matches standard practice. Why Other Options Are Wrong: Starting before TDC or at TDC is inconsistent with the four-stroke sequence. “Anywhere” ignores valve timing constraints. Exact dead-centre start/end neglects reality of gas dynamics and injection timing. Common Pitfalls: Assuming compression begins exactly at BDC; forgetting intake-valve closing angle and overlap effects. Final Answer: Starts ~40° after BDC and ends ~10° before TDC

Question 9

Definition of overall (brake thermal) efficiency: What do we call the ratio of useful work obtained at the crankshaft in a given time to the fuel energy supplied during the same time?

Accepted Answer

Introduction / Context: Several efficiency metrics exist for internal combustion engines. Distinguishing overall (brake thermal) efficiency from mechanical and indicated efficiencies is essential for interpreting dynamometer tests and fuel economy. Given Data / Assumptions: Useful output measured as brake power at the crankshaft. Fuel energy based on mass flow multiplied by lower calorific value. Steady operation over the measurement period. Concept / Approach: Overall (brake thermal) efficiency relates shaft output to chemical input: η_overall = B.P. / (ṁ_f * LCV). It captures all losses from combustion irreversibility, heat rejection, incomplete expansion, and mechanical friction indirectly because only brake output appears in the numerator. Step-by-Step Solution: Define B.P. from dynamometer torque and speed.Compute fuel energy rate: ṁ_f * LCV.Form ratio: η_overall = B.P. / (ṁ_f * LCV).Identify this as overall (brake thermal) efficiency. Verification / Alternative check: Using brake specific fuel consumption, bsfc = ṁ_f / B.P., we have η_overall = 1 / (bsfc * LCV) with consistent units, confirming the same definition. Why Other Options Are Wrong: Mechanical efficiency = B.P. / I.P., comparing shaft to indicated power. Indicated thermal efficiency = I.P. / (ṁ_f * LCV). Volumetric efficiency concerns air induction, not energy conversion. Combustion efficiency quantifies heat release from fuel but not shaft delivery. Common Pitfalls: Mixing up brake and indicated quantities; using higher heating value instead of LCV without accounting for water vapour condensation assumptions. Final Answer: overall efficiency

Question 10

Production route for plutonium-239 (Pu-239): Which option correctly describes how Pu-239 is produced in nuclear fuel cycles?

Accepted Answer

Introduction / Context: Plutonium-239 is a fissile isotope important in both thermal and fast reactor fuel cycles. Understanding its production path is foundational in nuclear engineering and safeguards discussions. Given Data / Assumptions: Starting material: U-238 present in natural and enriched uranium fuel. Neutron irradiation occurs during reactor operation. Beta decay sequences transmute isotopes. Concept / Approach: Under neutron flux, U-238 captures a neutron to become U-239. U-239 is beta-unstable and decays to Np-239, which in turn beta-decays to Pu-239. The produced Pu-239 can fission with thermal neutrons, contributing to reactor power and enabling breeding in specific configurations. Step-by-Step Solution: Neutron capture: U-238 + n → U-239.Beta decay: U-239 → Np-239 + β⁻ + ν̄_e (half-life short).Beta decay: Np-239 → Pu-239 + β⁻ + ν̄_e.Hence, irradiation of U-238 yields Pu-239 via these steps. Verification / Alternative check: Spent fuel assays from light water reactors consistently show generated Pu isotopes from U-238 capture, confirming the pathway. Why Other Options Are Wrong: “Artificially” is too vague without the neutron-capture mechanism. Th-232 breeds to U-233, not Pu-239. Pu-239 is not a mined raw material. Fusion breeding concerns tritium, not plutonium. Common Pitfalls: Confusing thorium breeding (U-233) with plutonium breeding; assuming plutonium exists naturally in mineable quantities (only trace cosmogenic amounts occur). Final Answer: when U-238 is irradiated by neutrons (via U-239 → Np-239 → Pu-239)

Question 11

Thermal efficiency benchmark for spark-ignition engines In standard automotive practice, the brake thermal efficiency of typical petrol (gasoline) and gas (spark-ignition) engines is approximately what value under steady, well-tuned operating condit…

Accepted Answer

Introduction / Context: Brake thermal efficiency is a key performance metric for internal combustion engines. It represents the fraction of the chemical energy in the fuel that is converted into useful shaft work at the crankshaft. For spark-ignition engines (petrol and gas), understanding the typical efficiency band helps students interpret dyno data, compare technologies, and set realistic expectations for improvements such as turbocharging or hybridization. Given Data / Assumptions: Spark-ignition engines operating at steady load and speed. Conventional fuels: petrol (gasoline) or gaseous fuels (e.g., CNG). Well-maintained engine with modern but not exotic technology; no waste-heat recovery. Concept / Approach: Brake thermal efficiency = brake power output divided by the rate of fuel energy input. SI engines typically exhibit lower efficiency than CI engines due to throttling losses, lower compression ratios, and flame-speed/knock constraints. Historically, naturally aspirated SI engines achieve about 20–30% at best efficiency point; modern improvements can touch the low-to-mid 30% range, but a representative single figure for learning and quick estimates is around 30%. Step-by-Step Solution: Define BTE = P_brake / (m_dot_fuel * LHV).Identify known range for SI: roughly 20%–35% depending on design and operating point.Choose a reasonable representative value used in textbooks and exam problems: about 30%.Therefore, the closest option is 30%. Verification / Alternative check: Manufacturer and research maps often show best efficiency islands for SI engines near 30% BTE, with lower values at low load due to throttling. This corroborates the educational benchmark. Why Other Options Are Wrong: 15% is typical of very light load or old/poorly tuned engines, not a benchmark. 50% and 70% exceed even advanced SI engine capabilities; such figures are associated with large combined-cycle plants or fuel cells, not standard SI engines. Common Pitfalls: Confusing indicated efficiency with brake efficiency, or quoting peak lab results as typical road values. Real-world averages are lower than peak BTE at the best efficiency point. Final Answer: 30%

Question 12

Uranium fission chain reaction — how can a sustained chain be developed? For a reactor-grade uranium system, which measures help initiate and sustain a fission chain reaction so that the system becomes critical?

Accepted Answer

Introduction / Context: Nuclear reactors rely on neutron economy to maintain a self-sustaining chain reaction. Two powerful levers are fuel enrichment (increasing the fissile isotope U-235 fraction) and moderation (slowing down neutrons), both of which raise the probability that a neutron will induce fission rather than be absorbed or leak from the core. Given Data / Assumptions: Fuel contains uranium isotopes U-238 and U-235. Objective: achieve k-effective equal to 1 (criticality). Neutron losses occur by leakage and non-fission absorption. Concept / Approach: Criticality requires that the average number of neutrons from each fission that cause another fission equals one. Increasing U-235 content raises the macroscopic fission cross-section. Moderation slows down neutrons to thermal energies where the U-235 fission cross-section is much higher, greatly improving the likelihood of fission and reducing the required enrichment. Step-by-Step Solution: Recognize that U-235 is the primary fissile isotope in low-enriched uranium.Slower (thermal) neutrons have a larger probability of inducing U-235 fission.Therefore, enrichment (a) and moderation (b) both move the system toward k-effective = 1.Combining both is the standard approach in most power reactors. Verification / Alternative check: Pressurized and boiling water reactors use light-water moderation with low-enriched uranium; heavy-water reactors use stronger moderation allowing natural uranium; both exemplify how (a) and/or (b) achieve criticality. Why Other Options Are Wrong: Choosing only one factor ignores the complementary role of the other. “None of these” contradicts reactor design practice. Common Pitfalls: Confusing moderators with coolants or assuming enrichment alone is sufficient without considering leakage and absorption in structural materials. Final Answer: Both (a) and (b)

Question 13

Octane rating reference — what is the octane number assigned to normal heptane (C7H16)? In the octane number scale used for petrol knock resistance, normal heptane is assigned which reference value?

Accepted Answer

Introduction / Context: Octane number quantifies the knock resistance of petrol in spark-ignition engines. It is defined by comparing a test fuel's knock behavior to blends of two reference hydrocarbons: iso-octane and n-heptane. Understanding the endpoints of this scale is essential for interpreting fuel labels like RON and MON. Given Data / Assumptions: Standard CFR test engine procedure. Reference hydrocarbons are iso-octane (2,2,4-trimethylpentane) and normal heptane. Linear scale between the references by volume. Concept / Approach: On the octane scale, iso-octane is assigned a value of 100 for excellent knock resistance, while normal heptane is assigned 0 for very poor knock resistance. A test petrol's octane number equals the percentage by volume of iso-octane in a blend with n-heptane that matches the knock of the fuel under standard conditions. Step-by-Step Solution: Identify the octane scale endpoints.Recall: iso-octane = 100, n-heptane = 0.Therefore, the octane number of normal heptane is 0. Verification / Alternative check: Fuel testing protocols consistently define these endpoints; pump ratings (RON/MON) derive from this same reference concept. Why Other Options Are Wrong: 50 would indicate a blend equally split by volume with iso-octane. 100 is the rating for iso-octane, not heptane. 120 exceeds the reference scale and refers to performance numbers or boosted effective indices, not the basic octane scale. Common Pitfalls: Confusing octane number (knock resistance) with energy content or cetane number (diesel ignition quality). Final Answer: 0

Question 14

Preventing diesel knock — which measures are effective? Diesel knock is linked to a long ignition delay and an abrupt pressure rise. Which of the following measures help prevent or reduce knocking in compression-ignition engines?

Accepted Answer

Introduction / Context: Diesel knock occurs when a large fraction of fuel auto-ignites almost simultaneously after a prolonged delay, causing a steep pressure rise. Effective countermeasures aim to shorten the delay and smooth the heat release, thereby protecting components and improving NVH (noise, vibration, harshness). Given Data / Assumptions: Conventional compression-ignition engine with direct injection. Fuel of appropriate cetane number and normal operating temperatures. Mechanical integrity permits reasonable compression ratio and boost levels. Concept / Approach: Shorter ignition delay reduces the premixed fuel quantity at the start of combustion and lowers the initial heat-release spike. Higher compression ratio raises end-of-compression temperature and pressure, accelerating pre-flame chemistry and reducing delay. Boosting inlet pressure increases charge density and, often with intercooling, enables better mixture preparation and higher effective compression temperature, again cutting delay. Step-by-Step Solution: Identify the root cause: excessive premixed burn due to long delay.Method 1: reduce delay directly by fuel quality (higher cetane) or timing/atomization → option (a).Method 2: raise compression ratio (within design limits) to increase T and p at end of compression → option (b).Method 3: increase inlet pressure (turbo/supercharging) to raise density and temperature, improving ignition and mixing → option (c).Collectively, these justify option (d) All of these. Verification / Alternative check: Engine calibration practice uses higher cetane fuels, optimized compression ratios, and boost with suitable injection phasing to minimize knock while achieving emissions targets. Why Other Options Are Wrong: Choosing any single measure ignores the multi-factor nature of ignition delay; in practice, several levers are used together. Common Pitfalls: Confusing diesel knock with SI knock (detonation); remedies differ because combustion initiation mechanisms are different. Final Answer: All of these

Question 15

Harmful effects of detonation in spark-ignition engines — identify the best description Which option best summarizes why detonation (severe knock) is harmful to SI engines under load?

Accepted Answer

Introduction / Context: Detonation in spark-ignition engines is an abnormal combustion phenomenon where unburned end-gas auto-ignites and releases energy extremely rapidly. The resulting pressure waves can damage components and degrade efficiency. Understanding its effects underscores the importance of correct octane rating, ignition timing, and charge conditions. Given Data / Assumptions: Conventional petrol (gasoline) SI engine. Moderate to high load where end-gas temperatures are elevated. No special knock-control strategies assumed for the base explanation. Concept / Approach: Detonation increases the rate of heat transfer to chamber walls because violent pressure oscillations and high local temperatures intensify convective and conductive losses. It also disrupts the protective thermal boundary layer, raising metal temperatures. Finally, the shock-like pressure waves excite torsional vibrations and cause audible knock, leading to rough operation and potential fatigue failures. Step-by-Step Solution: Abnormal auto-ignition in end-gas → rapid, near-explosive heat release.Pressure oscillations at several kHz → higher instantaneous wall heat flux → lower indicated and brake efficiency.Turbulence and pressure waves strip boundary layers → elevated component temperatures and potential damage (piston crown, valves, head gasket).Crankshaft and bearings experience vibration from uneven torque impulses → rough running and durability risks. Verification / Alternative check: Dynamometer tests show reduced brake torque at the same fueling when detonation forces spark retard; thermal imaging and teardown inspections corroborate increased component distress under sustained knock. Why Other Options Are Wrong: Options (a), (b), and (c) each describe real, documented harms. Selecting only one understates the overall damage. Therefore, the comprehensive choice is correct. Common Pitfalls: Confusing mild, controlled combustion noise with detonation; only the latter has the damaging high-frequency pressure oscillations. Final Answer: All of the above

Question 16

Ignition delay in CI engines — relation to knocking Evaluate the statement: A long ignition delay period in a compression-ignition (diesel) engine produces a rapid pressure rise at the start of combustion and thereby promotes knocking.

Accepted Answer

Introduction / Context: Combustion phasing in CI engines has a premixed burn phase followed by diffusion-controlled burning. The amount of fuel accumulated before ignition is primarily decided by the ignition delay. Managing this delay is central to noise control and durability. Given Data / Assumptions: Direct-injection diesel at steady load. Injection starts before auto-ignition; ignition delay permits premixing. No exotic multi-pilot strategies assumed. Concept / Approach: When ignition delay is long, a larger portion of the injected fuel premixes with air before ignition. Once ignition starts, this premixed charge can burn very quickly, producing a sharp heat-release spike and a steep pressure rise rate. High pressure rise rates are directly linked with diesel knock intensity. Step-by-Step Solution: Longer delay → more premixed fuel mass.Premixed mass ignites almost simultaneously → rapid heat release.Rapid heat release → high dp/dθ and high acoustic excitation → audible knock.Therefore, the statement is correct. Verification / Alternative check: Raising cetane number (shorter delay) and using pilot injections to precondition the charge both reduce knock, demonstrating the causal link. Why Other Options Are Wrong: Answering 'No' contradicts well-established diesel combustion theory and measurement of pressure rise rates. Common Pitfalls: Confusing diesel knock with SI detonation; while mechanisms differ, both are associated with damaging pressure fluctuations. Final Answer: Yes

Question 17

Typical volumetric efficiency range of a well-designed naturally aspirated engine For a healthy, well-designed naturally aspirated engine at its best operating region, which range best represents the volumetric efficiency?

Accepted Answer

Introduction / Context: Volumetric efficiency quantifies how effectively an engine breathes: the actual mass of air inducted compared with the theoretical mass that would fill the displacement at ambient conditions. It strongly influences torque, power, and fuel economy. Given Data / Assumptions: Naturally aspirated four-stroke engine without boost. Operation near peak torque where intake tuning is favorable. Standard atmospheric conditions and proper air filtration. Concept / Approach: VE depends on intake/exhaust tuning, valve timing, port geometry, and engine speed. Well-optimized naturally aspirated engines often achieve 0.80–0.90 VE (80–90%) at their best point; some tuned racing engines can briefly exceed 100% due to dynamic ram effects, but for general engineering estimates, the 75–90% range is the accepted textbook band. Step-by-Step Solution: Define VE = actual air mass inducted / theoretical mass at ambient density.Consider tuned operation: favorable pressure waves raise cylinder filling.Typical NA street/industrial engines: VE commonly 0.75–0.90 at the sweet spot → choose 75–90%. Verification / Alternative check: Dyno airflow measurements and engine simulation outputs for well-designed NA engines consistently indicate VE in this band at peak torque. Why Other Options Are Wrong: 30–70% would describe severely restricted or off-design operation. Modern engines generally exceed 70% VE under favorable conditions. Common Pitfalls: Assuming VE is constant across the speed range; it varies significantly with RPM and valve timing. Final Answer: 75 to 90%

Question 18

Definition of brake power (BP) Brake power is defined as the mechanical power available where in an internal combustion engine?

Accepted Answer

Introduction / Context: Several power terms describe engine performance: indicated power (developed in-cylinder), brake power (delivered at the shaft), and friction power (the difference). Correctly distinguishing these terms is essential for testing and analysis. Given Data / Assumptions: Steady operation on a dynamometer. Mechanical losses include friction, pumping, and accessory drives. Power measured at the output shaft. Concept / Approach: Indicated power is derived from the pressure-volume work inside the cylinder. Mechanical losses reduce the available output. The net mechanical power at the crankshaft flange is brake power, typically determined by torque and speed measurements on a brake or dynamometer: BP = 2 * pi * N * T (with consistent units). Step-by-Step Solution: Measure torque at the output shaft using a brake/dyno load cell.Measure rotational speed N.Compute BP from BP = torque * angular speed.Recognize that BP excludes in-engine losses already accounted for by the difference from indicated power. Verification / Alternative check: Brake specific fuel consumption uses BP as the denominator; test standards explicitly define BP at the engine crankshaft or test rig output coupling. Why Other Options Are Wrong: Option (a) describes indicated power. Option (c) is not a standard measurement point and still suffers bearing losses; BP is defined at the output. Option (d) contradicts standard definitions. Common Pitfalls: Confusing indicated and brake power or mixing units when calculating BP from torque and RPM. Final Answer: At the crankshaft output

Question 19

Breeder reactor fundamentals — which statement is correct? Select the statement that correctly characterizes a fast breeder reactor used in nuclear power engineering.

Accepted Answer

Introduction / Context: Breeder reactors are designed to increase the amount of fissile material by converting fertile isotopes (such as U-238 or Th-232) into fissile isotopes (Pu-239 or U-233). Understanding their defining feature and the typical design choices helps clarify common misconceptions. Given Data / Assumptions: Fast breeder concept (minimal or no moderation). Use of fertile blanket surrounding the core. Coolant selection aimed at minimal neutron moderation. Concept / Approach: The hallmark of a breeder reactor is a breeding ratio greater than one: it generates more fissile material than it consumes. To favor breeding in U-238, fast neutrons are advantageous; therefore, fast breeder reactors avoid moderators and often employ coolants with low neutron moderation, such as liquid sodium or lead-bismuth eutectic. Fuel choices commonly include plutonium-based or mixed-oxide fuels rather than requiring highly enriched U-235 exclusively. Step-by-Step Solution: Identify defining feature: net production of fissile fuel → breeding ratio > 1.Design implication: avoid moderators to maintain a fast spectrum.Coolant choice: liquid sodium is common due to excellent heat transfer and low moderation.Fuel flexibility: can utilize plutonium or MOX with fertile blankets, not strictly highly enriched U-235. Verification / Alternative check: Historical fast breeder projects and literature describe sodium-cooled fast reactors with breeding blankets producing Pu-239 from U-238, exemplifying the principle. Why Other Options Are Wrong: Moderators are generally avoided in fast breeders (so (b) is false). Statement (c) is the opposite of practice since sodium is widely used as coolant. Statement (d) is incorrect because many breeders use plutonium or MOX fuels. Common Pitfalls: Confusing moderators with coolants and assuming all reactors require thermal moderation; breeder designs often deliberately maintain a fast spectrum. Final Answer: It produces more fissile fuel than it consumes during operation (net breeding).

Question 20

Effect of compression ratio on thermal efficiency of an internal combustion engine For the same cycle type and similar operating conditions, how does increasing the compression ratio of an I.C. engine affect its thermal efficiency?

Accepted Answer

Introduction / Context: Compression ratio is a dominant design parameter that influences efficiency in both spark-ignition (Otto) and compression-ignition (Diesel) engines. Higher compression generally leads to higher thermal efficiency, within the limits set by knock (SI) and mechanical stress/emissions (CI). Given Data / Assumptions: Cycle comparison within the same ideal cycle family (Otto or Diesel). No change in combustion phasing assumptions except those implied by higher compression. Materials and fuel quality are sufficient to tolerate higher compression without abnormal combustion. Concept / Approach: For the ideal Otto cycle, thermal efficiency = 1 − 1/(r^(gamma−1)); as compression ratio r increases, the term 1/(r^(gamma−1)) decreases, so efficiency rises. For the ideal Diesel cycle, increasing compression ratio also tends to raise efficiency for a given cut-off ratio. In real engines, SI efficiency gains are limited by knock; CI gains are limited by NOx and mechanical constraints. Step-by-Step Solution: Consider the ideal relation showing monotonic efficiency increase with r.Recognize real-world constraints but preserve the general trend.Therefore, increasing compression ratio increases thermal efficiency. Verification / Alternative check: Engine families offered in multiple compression ratios show improved part-load fuel economy and peak efficiency with higher r, provided fueling and timing are properly calibrated. Why Other Options Are Wrong: Stating decrease or no change contradicts both ideal cycle analysis and empirical data for similar designs. Common Pitfalls: Assuming that raising compression can be done indefinitely; knock limits in SI and emissions/stress limits in CI constrain practical implementation. Final Answer: Increase

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