Question 1

Brake thermal efficiency definition check In internal combustion engine testing, brake thermal efficiency is defined as the ratio of the heat equivalent of 1 kW·h of brake work to the heat supplied by the fuel per brake-power hour. Is this statement…

Accepted Answer

Introduction / Context: Brake thermal efficiency is a cornerstone metric in engine performance. It connects the useful brake work measured at the shaft with the chemical energy delivered by the fuel. Examiners often phrase its definition in several equivalent ways, so it is important to recognize whether a statement describes the same ratio in different words. Given Data / Assumptions: Brake power is the useful power at the crankshaft or output shaft. One kilowatt-hour corresponds to 3600 kJ of energy (heat equivalent of 1 kW·h). Fuel energy input per brake-power hour equals mass of fuel consumed in one hour * lower calorific value. Concept / Approach: Brake thermal efficiency, usually written as eta_bth, is defined by eta_bth = brake power output / fuel energy input rate. When considered over one hour at 1 kW brake output, the numerator is the heat equivalent of 1 kW·h, and the denominator is the chemical energy supplied by the fuel per brake-power hour. Thus the ratio described in the statement is indeed the brake thermal efficiency. Step-by-Step Solution: Let Q_out = heat equivalent of 1 kW·h = 3600 kJ.Let Q_in = m_f * CV over the same brake-power hour.Define eta_bth = Q_out / Q_in for 1 kW over one hour, which matches the statement. Verification / Alternative check: From the continuous form, eta_bth = BP / (m_dot * CV). Multiplying numerator and denominator by 3600 converts per-second units to per-hour units and leads to the same interpretation as the statement. Why Other Options Are Wrong: Options limiting the definition to two-stroke engines, gaseous fuels, or stoichiometric operation add conditions that are unnecessary; the definition is general. Common Pitfalls: Confusing indicated thermal efficiency (uses indicated power) with brake thermal efficiency (uses brake power), or mixing up mechanical efficiency with thermal efficiency. Final Answer: Yes

Question 2

Engineering requirements for a reactor moderator Beyond high slowing-down (moderating) power and low neutron absorption, a good moderator in nuclear reactors should also be…

Accepted Answer

Introduction / Context: Moderators reduce neutron energy to the thermal range where fission cross-sections are high for many fuels. While neutronics drives the primary choice, engineering requirements determine whether a material can survive and perform inside a real reactor environment. Given Data / Assumptions: Moderator must slow down neutrons effectively (high moderating ratio). Moderator should absorb as few neutrons as possible. It must survive thermal and radiation environments while transferring heat safely. Concept / Approach: Practical moderators (e.g., light water, heavy water, graphite) must be radiation-stable to avoid property degradation, corrosion resistant to maintain integrity against coolant/chemistry, and reasonably good at heat removal via adequate thermal conductivity to prevent hot spots and dimensional changes. Step-by-Step Solution: List non-neutronic requirements: radiation stability, corrosion resistance, thermal conductivity.Evaluate each: All are desirable to ensure lifetime performance and safety.Therefore, the best choice aggregates all listed properties. Verification / Alternative check: Historical choices—graphite (good thermal properties, radiation moderated design), heavy water (chemically managed systems), and pressurized water reactors (materials selection and chemistry control)—show these requirements in practice. Why Other Options Are Wrong: Choosing only one property ignores other critical constraints; failures often arise from corrosion or radiation damage rather than neutronics alone. Common Pitfalls: Assuming neutronics alone determines suitability; engineering survivability is equally decisive. Final Answer: all of these

Question 3

Reducing knock in spark ignition (SI) engines Which measures help reduce knocking tendency in SI engines under otherwise similar conditions?

Accepted Answer

Introduction / Context: Knock in SI engines arises from auto-ignition of the end-gas ahead of the flame front. It depends on pressure, temperature, and time available for pre-flame reactions. Control strategies modify one or more of these factors to reduce end-gas auto-ignition propensity. Given Data / Assumptions: Conventional SI engine with throttled or modern load control. Same fuel and compression ratio for the comparison. Focus on operating settings (timing and speed) rather than design changes. Concept / Approach: Retarding spark timing shifts the pressure rise closer to or slightly after top dead center, lowering end-gas temperature and pressure before auto-ignition can occur. Increasing engine speed shortens the total time available for end-gas reactions, also reducing knock likelihood for the same load and timing. Hence both measures mitigate knock. Step-by-Step Solution: Knock intensity correlates with end-gas temperature, pressure, and residence time.Retard spark → reduces peak pre-TDC pressures → end-gas less prone to auto-ignite.Increase speed → reduces residence time → fewer pre-flame reactions. Verification / Alternative check: Dynamometer tests show reduced knock counts when spark is retarded and when operating at higher rpm with constant load, until other limits (e.g., cycle-by-cycle variability) appear. Why Other Options Are Wrong: Advancing timing at low speed can aggravate knock due to higher end-gas pressures and temperatures. Common Pitfalls: Over-retarding damages efficiency and increases exhaust temperatures; mitigation must balance performance and durability. Final Answer: both (a) and (b)

Question 4

Valve timing in a four-stroke engine Approximately how long (in crank angle degrees) does the inlet valve remain open in a typical four-stroke I.C. engine with conventional timing overlap?

Accepted Answer

Introduction / Context: Valve timing diagrams show when intake and exhaust valves open and close relative to piston motion. Due to gas dynamics and inertia, valves open before and close after the ideal geometric points, producing an effective open duration greater than 180° for the intake process in a four-stroke engine. Given Data / Assumptions: Conventional naturally aspirated four-stroke spark-ignition engine. Typical timing advance/retard near design speed (some overlap around TDC). Representative textbook values are acceptable; exact timing varies with design. Concept / Approach: Because the intake valve opens slightly before the intake stroke begins (BTDC) and closes after the piston starts the compression stroke (ABDC), the crank-angle duration exceeds the geometric 180°. Typical combined advance and late closing sums to about 50° (e.g., 15–20° BTDC opening and 30–40° ABDC closing), giving roughly 230° total open duration. Step-by-Step Solution: Ideal intake stroke: 180° from TDC to BDC.Add opening advance (≈ 10–20°) + closing delay (≈ 30–40°).Total ≈ 180° + 50° = 230° (representative value). Verification / Alternative check: Manufacturer timing charts and classic examples routinely show intake durations in the 210–250° range, with 230° a common rounded teaching value. Why Other Options Are Wrong: 180° ignores advance/retard effects; 130° is far too short; 270° is unusually long for typical production engines. Common Pitfalls: Confusing cam duration (at a given lift) with valve-event crank angles; also, performance cams may differ significantly. Final Answer: 230°

Question 5

Basic thermodynamic cycle for petrol (spark-ignition) engines Identify the ideal cycle that best represents the operation of a conventional petrol engine.

Accepted Answer

Introduction / Context: Ideal cycles model real engines to permit analysis of efficiency limits and parameter trends. Recognizing the correct idealization for petrol engines is a foundational skill in thermodynamics and engine theory. Given Data / Assumptions: Conventional spark-ignition engine with nearly homogeneous mixture. Rapid heat addition approximated at constant volume (near top dead center). Compression and expansion taken as isentropic in the ideal model. Concept / Approach: The Otto cycle consists of two isentropic processes (compression and expansion) and two constant-volume heat-transfer processes (addition and rejection). This matches the idealized behavior of SI engines where combustion occurs quickly relative to piston motion, approximating heat addition at constant volume. Step-by-Step Solution: Map real SI engine events to ideal cycle: compression → heat addition at constant volume → expansion → heat rejection at constant volume.Compare with other cycles: Brayton has constant-pressure heat addition (gas turbines); Rankine is a vapor cycle; Stirling uses isothermal processes.Therefore select the Otto cycle. Verification / Alternative check: Efficiency trend with compression ratio in the Otto cycle mirrors observed SI engine behavior (higher compression ratio generally improves efficiency until knock limits). Why Other Options Are Wrong: Joule/Brayton applies to continuous-flow gas turbines; Rankine to steam plants; Stirling to external-combustion regenerative engines; Dual cycle models CI engines more closely. Common Pitfalls: Assuming real combustion is exactly constant-volume; it is only an approximation, but sufficient for the ideal model choice. Final Answer: Otto cycle

Question 6

Cylinder conditions: I.C. engine vs. steam engine Compared to a steam engine, the working pressure and temperature inside the cylinder of an internal combustion engine are…

Accepted Answer

Introduction / Context: Understanding relative operating conditions helps explain design choices such as materials, cooling, and lubrication. Internal combustion engines burn fuel directly inside the cylinder, while steam engines receive external heat in a boiler and expand steam at comparatively lower in-cylinder temperatures and pressures. Given Data / Assumptions: I.C. engines: peak cylinder pressures often 4–7 MPa (or more) and high combustion temperatures. Steam engines: cylinder pressures typically limited to boiler pressure and expansion, generally lower than peak I.C. cylinder pressures. Direct in-cylinder combustion produces very high local temperatures. Concept / Approach: Because chemical energy converts to thermal energy directly within the I.C. engine cylinder during a very short interval, both temperature and pressure peaks are significantly higher than in reciprocating steam engines. This necessitates robust cooling and high-performance lubricants in I.C. engines. Step-by-Step Solution: Identify heat addition mode: internal (I.C.) vs external (steam).Relate to peak conditions: direct combustion spikes pressure and temperature.Conclude I.C. engines experience very high in-cylinder P and T compared to steam engines. Verification / Alternative check: Empirical data from engine indicator diagrams show higher peak values in I.C. engines versus reciprocating steam engines operating at comparable mean effective pressures. Why Other Options Are Wrong: Choices indicating lower or similar values contradict the fundamental difference in heat addition and resulting thermodynamic states. Common Pitfalls: Confusing mean effective pressure with instantaneous peak pressure; the latter is the relevant comparison for material stress and temperature. Final Answer: very high

Question 7

Function of lubrication in I.C. engines Beyond reducing friction, does lubrication also help dissipate the heat generated at moving interfaces in an internal combustion engine?

Accepted Answer

Introduction / Context: Lubrication has multiple roles in engines: friction reduction, wear control, sealing, corrosion inhibition, and heat removal. Appreciating the thermal function clarifies why oil circuits include coolers and why viscosity and flow rate are carefully controlled. Given Data / Assumptions: Hydrodynamic and mixed lubrication regimes present in bearings and piston–cylinder interfaces. Pressurized oil circulation with sump or dry-sump designs. Heat generated by friction and by proximity to hot combustion surfaces must be removed. Concept / Approach: The oil film carries heat away from loaded surfaces (bearings, pistons, valve trains) and transports it to the sump and oil cooler. Thus, lubrication does dissipate frictional heat, in addition to reducing its generation by lowering the friction coefficient. Step-by-Step Solution: Recognize friction as a volumetric heat source in boundary layers.Oil absorbs this heat and convects it to cooler regions (sump, cooler).Therefore, lubrication both reduces and removes heat. Verification / Alternative check: Oil temperature rise across bearings and the presence of dedicated oil coolers on many engines confirm a heat-transport function. Why Other Options Are Wrong: Thermal removal is not restricted to air-cooled engines, synthetic oils, or low speeds; it is universal in pressurized lubrication systems. Common Pitfalls: Underestimating the importance of oil flow rate; insufficient flow increases bearing temperatures and reduces film thickness. Final Answer: True

Question 8

Propulsive efficiency of a turbojet (ideal expression) If V1 is the jet (exhaust) velocity and V0 is the flight (vehicle) speed, choose the correct ideal expression for the propulsive efficiency of a turbojet.

Accepted Answer

Introduction / Context: Propulsive efficiency measures how effectively a propulsion system converts jet kinetic energy into useful thrust power for flight. For an ideal turbojet with a single exhaust stream, the efficiency depends on the mismatch between jet velocity and flight speed. Given Data / Assumptions: Steady, one-dimensional jet with mass flow assumed constant. Ideal relationships (no losses, single exhaust stream). Vehicle speed V0 and jet speed V1 are colinear. Concept / Approach: Propulsive efficiency eta_p is defined as useful power (thrust * flight speed) divided by the rate of kinetic-energy increase given to the jet. Under ideal assumptions this reduces to eta_p = 2 * V0 / (V1 + V0). The closer the jet speed is to the flight speed, the higher the efficiency, which motivates high-mass-flow, low-excess-velocity designs in modern turbofans. Step-by-Step Solution: Thrust T ≈ m_dot * (V1 − V0).Useful power P_u = T * V0 = m_dot * (V1 − V0) * V0.Jet power added P_j ≈ 0.5 * m_dot * (V1^2 − V0^2) = 0.5 * m_dot * (V1 − V0) * (V1 + V0).eta_p = P_u / P_j = [m_dot * (V1 − V0) * V0] / [0.5 * m_dot * (V1 − V0) * (V1 + V0)] = 2 * V0 / (V1 + V0). Verification / Alternative check: Limit checks: if V1 → V0, eta_p → 1; if V0 → 0 (static jet), eta_p → 0. Both are physically sensible trends. Why Other Options Are Wrong: Other forms either invert the ratio, omit the factor of 2, or put V1 in the numerator, yielding incorrect limits (e.g., predicting high efficiency at static conditions). Common Pitfalls: Confusing propulsive efficiency with overall or thermal efficiency; the former concerns velocity matching, not fuel-to-jet energy conversion. Final Answer: 2 * V0 / (V1 + V0)

Question 9

Ignition delay in compression-ignition (CI) engines The ignition delay period in CI engines depends on which of the following factors?

Accepted Answer

Introduction / Context: Ignition delay in CI engines is the time between the start of injection and the onset of rapid heat release. It controls the fraction of premixed fuel and strongly influences pressure rise rates, noise, and emissions. Multiple physical and chemical factors interact to set the delay. Given Data / Assumptions: Chemical kinetics depend on temperature, pressure, and fuel properties. Air–fuel mixing quality depends on spray–air relative motion and turbulence. Residual gases and their pressure/temperature influence reactivity. Concept / Approach: Higher temperature and pressure at injection generally shorten delay by accelerating low-temperature oxidation chemistry. Fuel cetane quality and mixture strength affect reactivity, while good atomization and strong air motion promote faster mixing and vaporization. Residual gases alter oxygen concentration and temperature fields, modifying delay. Step-by-Step Solution: Relate thermodynamic state (T, p) to chemical reaction rates → shorter delay at higher T and p.Relate fuel property (cetane number) and mixture strength to ignition quality → higher cetane and proper equivalence ratio reduce delay.Relate spray–air dynamics (relative velocity, turbulence) to mixing/vaporization → improved mixing shortens delay.Account for residual gases → can dilute oxygen or raise temperature, affecting delay. Verification / Alternative check: Empirical correlations (e.g., Hardenberg–Hase) include pressure, temperature, and fuel cetane effects, consistent with the listed dependencies. Why Other Options Are Wrong: Any single-factor choice is incomplete; ignition delay is multi-factorial. Common Pitfalls: Focusing only on fuel cetane number; engine state and spray mixing are equally crucial. Final Answer: all of the above

Question 10

Interpreting octane number from a reference blend A fuel blend containing 65% iso-octane and 35% n-heptane by volume will have which standard rating?

Accepted Answer

Introduction / Context: Octane number defines a gasoline's knock resistance under specified test conditions by comparing it to blends of iso-octane (assigned 100) and n-heptane (assigned 0). Understanding this scale helps interpret fuel ratings and relate them to engine anti-knock performance. Given Data / Assumptions: Reference fuels: iso-octane (ON = 100) and n-heptane (ON = 0). Linear scale for primary reference fuels under standardized tests. Blend composition: 65% iso-octane, 35% n-heptane (by volume). Concept / Approach: By definition, the octane number of a gasoline equals the percentage of iso-octane in a reference blend that gives the same knock characteristics as the test fuel under the CFR engine method. Therefore, a 65/35 iso-octane/n-heptane blend has an octane number of 65. Step-by-Step Solution: Assign ON(iso-octane) = 100 and ON(n-heptane) = 0.Compute blend ON = 0.65 * 100 + 0.35 * 0 = 65.Therefore, the described mixture has octane number 65. Verification / Alternative check: Standard ASTM methods (e.g., RON/MON) use these primary reference fuels; the simple linear blend definition is the basis of the octane scale. Why Other Options Are Wrong: Cetane number applies to diesel fuels and is inverse in significance (higher cetane → shorter ignition delay). Octane 35 would correspond to a 35/65 blend, not 65/35. Common Pitfalls: Confusing octane with cetane; one measures knock resistance in SI engines, the other ignition quality in CI engines. Final Answer: octane number 65

Question 11

Compression-ignition (diesel) engines – why does part-load efficiency drop less than in spark-ignition? As the load on a compression-ignition engine decreases, its efficiency deteriorates less sharply (and may appear to “increase” relative to a thro…

Accepted Answer

Introduction / Context: When comparing part-load behavior of compression-ignition (CI) and spark-ignition (SI) engines, CI engines usually retain better efficiency at reduced loads. The key reason is how load is controlled. This question tests understanding of governing methods and their impact on pumping losses and thermal efficiency. Given Data / Assumptions: Engine type: four-stroke CI (diesel) versus SI as a reference. Load reduction from a medium operating point to lighter load. No exotic technologies (e.g., variable compression ratio or advanced turbo compounding) are assumed. Concept / Approach: CI engines operate unthrottled; fresh air is admitted freely and load is regulated by the injected fuel quantity. This is called qualitative governing. Since intake is not throttled, manifold pressure remains high and pumping work is small, so part-load efficiency remains relatively better. In contrast, SI engines often throttle the intake (quantitative governing), which raises pumping losses and reduces indicated efficiency at small loads. Step-by-Step Solution: Identify governing in CI: fuel-quantity control at nearly full air flow (no throttle).Relate governing to pumping work: unthrottled intake → low pumping losses.Connect to efficiency trend: smaller losses → less deterioration of efficiency as load drops.Therefore, the observed advantage at light loads is due to qualitative governing. Verification / Alternative check: Brake specific fuel consumption maps and pumping-loop p-V diagrams show that CI engines avoid the large negative pumping loop area typical of throttled SI engines at light loads. Why Other Options Are Wrong: Higher maximum temperature at light load is not the cause; peak temperatures usually fall with load. Quantitative governing refers to throttling, characteristic of SI engines, which hurts efficiency at part load. Hit-and-miss governing is for small constant-speed engines, not modern automotive diesels. Variable compression ratio is not the standard reason for the trend described. Common Pitfalls: Assuming CI engines “increase” absolute efficiency at light load. In practice, efficiency still drops, but much less than in throttled SI engines due to the absence of throttling losses. Final Answer: qualitative governing (varying the fuel quantity without throttling the air)

Question 12

Spark-ignition (petrol) engines – anti-knock additives The knocking tendency in spark-ignition engines can be reduced by adding anti-knock dopes such as tetra-ethyl lead (TEL) together with ethylene dibromide (as a scavenger).

Accepted Answer

Introduction / Context: Knock in spark-ignition (SI) engines is the spontaneous auto-ignition of unburned end-gas ahead of the flame front, producing pressure oscillations and potentially damaging the engine. Fuel chemistry strongly influences knock resistance, typically measured by octane rating. Given Data / Assumptions: Conventional SI engine with homogeneous charge. Use of historical anti-knock additives (TEL with ethylene dibromide scavenger). Focus on principle rather than modern emissions regulations. Concept / Approach: Anti-knock additives increase a fuel’s auto-ignition resistance by slowing pre-flame radical chain reactions in the end-gas. Tetra-ethyl lead was historically effective at raising octane number. Ethylene dibromide aided in scavenging lead deposits by forming volatile lead halides. Although modern fuels avoid lead for environmental reasons, the statement about their knock-reducing effect is correct. Step-by-Step Solution: Define knock: auto-ignition of end-gas leads to sharp pressure waves.Relate additive action: TEL interrupts radical pathways, delaying auto-ignition.Scavenger role: ethylene dibromide helps remove lead compounds to limit fouling.Outcome: higher octane rating → reduced knock tendency. Verification / Alternative check: Octane number tests (e.g., CFR engine) historically showed substantial octane increase with TEL addition, confirming reduced knock at given compression ratios and spark timing. Why Other Options Are Wrong: Applicability is specific to SI knock; diesel knock is a different phenomenon tied to ignition delay. Effect is not limited to very rich mixtures or a narrow rpm band. Common Pitfalls: Conflating modern unleaded gasoline practice with historical statements; environmental bans do not negate TEL’s anti-knock chemistry in principle. Final Answer: Agree

Question 13

Ignition systems – what does the distributor do in a coil-ignition I.C. engine? Select the primary function performed by the distributor in a conventional battery-coil ignition system for multi-cylinder petrol engines.

Accepted Answer

Introduction / Context: In coil-ignition systems used on traditional spark-ignition engines, the distributor is a key component that routes high-voltage pulses to the appropriate spark plug at the correct time. Understanding its role clarifies how ignition timing and cylinder sequencing are coordinated. Given Data / Assumptions: Battery-coil ignition with a single ignition coil. Mechanical distributor with rotor arm and cap (classic design). Engine has multiple cylinders and a defined firing order. Concept / Approach: The ignition coil steps up battery voltage to a high-tension (HT) pulse at each primary circuit interruption. The distributor rotor, synchronized with the camshaft, physically directs each HT pulse to the correct cylinder’s spark plug via the cap contacts. Ancillary mechanisms (centrifugal and vacuum advances) adjust timing, but the core task is controlled distribution of the spark. Step-by-Step Solution: Coil generates HT pulse → needs routing to one plug at a time.Distributor’s rotor aligns with cap contacts in firing order.Result: each cylinder receives a spark near the end of its compression stroke.Thus, the distributor’s primary function is to distribute the spark to the proper cylinder in sequence. Verification / Alternative check: Service manuals depict rotor and cap alignment marks, confirming the distributor’s routing role; timing lights verify correct sequencing and advance. Why Other Options Are Wrong: Mechanical power distribution, sensor current distribution, throttle actuation, and fuel-injection pressure regulation are unrelated to the distributor’s function. Common Pitfalls: Confusing the distributor’s routing role with timing adjustment; while it often houses advance mechanisms, the fundamental task is HT distribution. Final Answer: to distribute the high-tension spark to the correct plug in firing order

Question 14

Petrol (SI) engines – which description is correct? Choose the statement that correctly describes how a petrol (spark-ignition) engine prepares and ignites the charge.

Accepted Answer

Introduction / Context: Petrol engines are spark-ignition (SI) machines. Unlike diesel engines, which rely on compression ignition of a fuel spray, SI engines prepare a fuel–air mixture and then initiate combustion with a timed electric spark. Given Data / Assumptions: Conventional four-stroke petrol engine. Mixture preparation via carburettor or fuel injection (port or direct) depending on design. Ignition provided by an ignition system and spark plug. Concept / Approach: In SI engines, fuel is metered to achieve a target air–fuel ratio (often near stoichiometric at many conditions). The resulting homogeneous or stratified mixture is then ignited by an electric spark near the end of the compression stroke. This differs fundamentally from diesel operation, where auto-ignition occurs due to high temperature from compression. Step-by-Step Solution: Identify correct principle: mixture + spark → SI operation.Contrast with CI: diesel uses heat of compression to ignite injected fuel (not correct for petrol).Recognize variations in mixture preparation: carburetion is not the only method; injection systems are common.Therefore, the accurate single statement is that fuel and air are proportioned and ignited by a spark plug. Verification / Alternative check: Modern engines employ electronic fuel injection (EFI) and ignition control; the core SI principle remains mixture formation followed by spark ignition. Why Other Options Are Wrong: (a) Describes diesel CI behavior. (c) Overgeneralizes carburettors; port and direct injection are widely used. (d) Cannot be true because (a) and (c) are not universally correct. (e) Self-ignition would be knock or pre-ignition at normal petrol compression ratios, not normal operation. Common Pitfalls: Assuming carburettors are mandatory for SI engines; many use EFI. The essential feature is the spark-initiated flame. Final Answer: Fuel is proportioned with air to form a combustible mixture and ignited by a spark plug (spark ignition).

Question 15

Fuel rating for diesel – reference fuels In the cetane scale used for compression-ignition fuels, the straight-chain paraffin n-hexadecane (cetane, C16H34) is assigned what cetane number?

Accepted Answer

Introduction / Context: Diesel fuel ignition quality is rated by the cetane number, analogous to the octane number for petrol but measuring a very different property: readiness to auto-ignite (short ignition delay) under compression-ignition conditions. Given Data / Assumptions: Cetane scale is defined by two reference fuels. Test conditions follow standardized CI engine procedures. Ignition delay is the key metric mapped to cetane number. Concept / Approach: The cetane scale historically uses n-hexadecane (cetane, C16H34) as the high-ignition-quality reference with a cetane number of 100, and alpha-methylnaphthalene (or, in modern practice, isocetane) as the low-ignition-quality reference near 0. Real fuels are compared by matching ignition delay to mixtures of the reference fuels. Step-by-Step Solution: Identify reference: C16H34 is cetane → high ignition quality.Map to scale: by definition, cetane = 100 CN.Therefore, the assigned number for cetane is 100. Verification / Alternative check: Standard fuel testing literature and engine handbooks list cetane (n-hexadecane) at CN = 100, establishing the upper reference point of the scale. Why Other Options Are Wrong: 0 corresponds to the low-ignition-quality reference. 50 or 120 are not the defined reference values for cetane. 10 is arbitrary and incorrect. Common Pitfalls: Confusing octane and cetane concepts: high octane resists auto-ignition (good for SI), whereas high cetane favors easy auto-ignition (good for CI). Final Answer: 100

Question 16

Nuclear reactor control – role of control rods In thermal nuclear reactors, the primary function of the control rods in the control system is to:

Accepted Answer

Introduction / Context: Reactor power depends on the effective neutron multiplication factor. Control systems maintain this near unity during steady operation and adjust it during transients. Control rods are central to this function in many reactor types. Given Data / Assumptions: Thermal reactor using a moderator to slow neutrons. Control rods made of materials with high neutron absorption cross-section (e.g., boron, cadmium, hafnium). Objective: regulate reactivity safely. Concept / Approach: Control rods are inserted into or withdrawn from the core to change the net neutron population by absorbing neutrons. Insertion increases absorption, reduces reactivity, and lowers power; withdrawal does the opposite. While temperature and fuel consumption are indirectly affected, the direct function is neutron absorption. Step-by-Step Solution: Recognize control variable: reactivity via neutron economy.Identify actuator: control rods with strong absorbers.Insert rods → absorb more neutrons → reduce k_eff → power falls.Withdraw rods → absorb fewer neutrons → increase k_eff → power rises. Verification / Alternative check: Startup, shutdown, and scram procedures explicitly position control rods to achieve desired reactivity; in a scram, rods are rapidly inserted to terminate the chain reaction. Why Other Options Are Wrong: Fuel consumption is not directly metered by rods; it follows from power level. Coolant flow or temperature control is a separate system. Control rods do not chemically enhance moderation. Hence, “all of these” is incorrect. Common Pitfalls: Assuming rods are for temperature control; temperature changes result from reactivity changes but are not directly controlled by heat removal via rods. Final Answer: absorb excess neutrons to regulate the chain reaction

Question 17

Air motion in CI chambers – what is swirl? In compression-ignition engines, the term swirl refers to which characteristic motion of the gases within the combustion chamber?

Accepted Answer

Introduction / Context: Mixture formation and combustion in diesel engines rely on controlled air motion. Swirl, squish, and tumble are engineered flow patterns that enhance fuel–air mixing and influence ignition delay, heat release, and emissions. Given Data / Assumptions: Compression-ignition engine with an appropriate intake port and chamber design. Fuel injected as a high-pressure spray into hot, compressed air. Objective: rapid and uniform mixing for efficient combustion. Concept / Approach: Swirl is a coherent, rotary motion of the bulk charge about the cylinder’s axis, generally induced by helical or tangential intake ports and shaped chambers (e.g., toroidal bowls). It persists into the compression stroke and interacts with squish to intensify turbulence, promoting mixing of fuel with air. Step-by-Step Solution: Identify flow pattern: rotation around axis → swirl.Relate to purpose: improved atomization utilization and diffusion combustion.Outcome: better mixing reduces ignition delay variability and soot formation, aiding efficiency and emissions control. Verification / Alternative check: Flow bench tests and CFD visualize swirl number and swirl ratio; engine maps correlate optimal swirl with injection strategy and chamber shape. Why Other Options Are Wrong: Random motion is turbulence, not specifically swirl. Purely radial or purely axial motions describe other patterns, not swirl. “No organized motion” contradicts the deliberate design of swirl in many CI engines. Common Pitfalls: Confusing swirl with tumble (rotation about a transverse axis) or squish (radial squeeze near TDC). These may coexist but are distinct. Final Answer: a rotary, organized motion about the cylinder axis

Question 18

Boosting and performance – effect of supercharging For a given engine displacement, how does supercharging typically affect the power developed by the engine?

Accepted Answer

Introduction / Context: Supercharging (and turbocharging) increases the density of the intake charge so that more air (and proportionally more fuel) can be burned per cycle. This is a fundamental method for specific power enhancement in both SI and CI engines. Given Data / Assumptions: Same displacement and speed before and after supercharging. Boost pressure within mechanical and knock/combustion limits. Adequate intercooling or charge temperature management. Concept / Approach: Power is proportional to the mass of fuel burned per unit time and the efficiency of conversion. Supercharging raises manifold pressure, increasing trapped air mass. With proper fueling and control, this directly increases indicated mean effective pressure (IMEP) and brake power. Although there are additional losses (compressor work, heat), net power typically rises significantly. Step-by-Step Solution: Increase intake pressure → higher air density → more oxygen per cycle.Meter more fuel to maintain desired mixture or equivalence ratio.Higher charge → higher IMEP → higher brake power, subject to limits. Verification / Alternative check: Engine performance maps show strong correlation between boost and BMEP; turbocharged variants of the same base engine deliver substantially higher rated power. Why Other Options Are Wrong: No effect contradicts the purpose of supercharging. Pure decrease ignores the fact that compressor work is outweighed by increased fuel energy release in practical designs. Efficiency may rise or fall depending on design; it does not “always” reduce. Exhaust temperature effects are secondary; the primary impact is on charge mass and power. Common Pitfalls: Overlooking detonation/knock limits in SI engines and peak cylinder pressure/thermal limits in CI engines; careful calibration and intercooling are essential. Final Answer: increases power output by raising the mass of charge inducted

Question 19

Combustion phenomena in CI engines – find the incorrect statement Identify the wrong statement regarding combustion and knock in compression-ignition (diesel) engines.

Accepted Answer

Introduction / Context: Diesel knock is linked to ignition delay and the amount of fuel that accumulates before ignition. Managing delay and mixing is central to avoiding damaging pressure oscillations and high rates of heat release. Given Data / Assumptions: Conventional direct-injection CI engine without abnormal failures. Normal injection timing near top dead center. Focus on qualitative trends. Concept / Approach: A long ignition delay allows more fuel to accumulate and vaporize before ignition, creating a larger premixed fraction. When this mixture auto-ignites, the rapid heat release causes steep pressure rise and knock. Therefore, to reduce knock, designers seek a short delay: good atomization, appropriate injection timing, adequate air temperature, and enhanced mixing to transition quickly into diffusion burning. Step-by-Step Solution: Relate delay to accumulated premixed fuel: longer delay → more premixed mass → sharper pressure rise.Target for knock mitigation: shorter delay via higher charge temperature, optimized spray, and timing.Therefore, the statement advocating a long delay to prevent knock is wrong. Verification / Alternative check: Rate-of-heat-release analyses show the premixed spike grows with longer delay; calibrations that shorten delay reduce the spike and smooth pressure traces. Why Other Options Are Wrong or Right: (a) Correct: diesel knock manifests at the start of combustion after the delay. (b) Correct: absence of premixed charge during compression makes pre-ignition uncommon. (d) Generally correct for non-knocking operation; diffusion burning moderates pressure rise. (e) Clearly incorrect generically, but the uniquely wrong statement targeted by classic exams is the advocacy of a long delay in (c); hence (c) is selected as the wrong statement among the provided options. Common Pitfalls: Confusing SI knock (end-gas auto-ignition) with diesel knock (premixed spike after delay). Final Answer: To eliminate knock in CI engines, we want auto-ignition to occur as late as possible, so a long delay period is desirable.

Question 20

Operating cost comparison – petrol vs. diesel For similar output and duty, how does the running (operating) cost of a petrol engine compare with that of a diesel engine?

Accepted Answer

Introduction / Context: Running cost encompasses the fuel consumed per unit of useful work and the price of that fuel. Diesel engines generally achieve higher thermal efficiency and lower brake specific fuel consumption than spark-ignition petrol engines for comparable duty cycles. Given Data / Assumptions: Comparable displacement and load profiles. Conventional fuels: petrol (gasoline) vs. diesel fuel. Maintenance costs excluded; focus on fuel cost per work delivered. Concept / Approach: Diesel engines operate at higher compression ratios and avoid throttling losses, giving better efficiency, especially at part load. Consequently, for a given task, diesel engines consume less fuel energy. Even when fuel prices vary, the combination of lower specific consumption and often lower energy cost per unit can yield lower running cost for diesel. Thus, petrol engines typically have higher running costs for equivalent output. Step-by-Step Solution: Compare BSFC: diesel < petrol in g/kWh.Consider fuel price and energy content: diesel often competitive per kWh delivered.Result: cost per unit work for petrol > diesel → petrol has higher running cost. Verification / Alternative check: Fleet and generator operating data commonly show better fuel economy for diesel under steady loads; automotive comparisons reflect similar trends on highway cycles. Why Other Options Are Wrong: “Lower” contradicts typical BSFC differences. “Identical” ignores clear thermodynamic and mechanical distinctions. “Cannot be compared” is overly cautious; general engineering practice supports the comparison. Reversing the trend (diesel higher cost) does not reflect standard outcomes for similar duty. Common Pitfalls: Confusing capital/maintenance costs with running cost; while those can differ, the question specifically targets fuel-related running cost. Final Answer: higher

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