Question 1

Spark-ignition engine fundamentals: What is the term for ignition of the air–fuel charge by a hot spot inside the cylinder occurring before the timed spark event?

Accepted Answer

Introduction / Context:This question checks your understanding of abnormal combustion phenomena in spark-ignition (petrol) engines. Two terms are often confused: pre-ignition and detonation (knock). Distinguishing them is essential for correct diagnosis, calibration, and engine protection.Given Data / Assumptions:A spark-ignition engine uses a spark plug to ignite a premixed air–fuel charge at a scheduled crank angle.Hot spots can exist on valves, spark plug electrodes, carbon deposits, or sharp edges.We assume normal mixture strength and no forced-induction unless stated otherwise.Concept / Approach:Pre-ignition is ignition that starts before the intended spark because a local surface is hot enough to ignite the mixture. Detonation is a violent pressure rise that occurs after normal spark ignition when the unburned end-gas auto-ignites spontaneously, producing pressure waves (knock). Ignition delay refers to the time between spark discharge and the start of flame kernel growth, a separate concept.Step-by-Step Solution:Identify the event timing: ignition happens before the spark → not detonation.Identify the cause: a glowing/hot surface (valve head, plug, deposits) lights the charge → fits pre-ignition.Confirm terminology: early, uncontrolled start of combustion due to a hot spot = pre-ignition.Verification / Alternative check:Symptoms of pre-ignition include abnormally advanced pressure rise, severe overheating, and potential piston damage. Unlike knock, which often lessens when the throttle is eased, pre-ignition can persist and escalate with heat, validating the definition.Why Other Options Are Wrong:Detonation: Occurs after spark because end-gas auto-ignites; it is not the initial cause of ignition.Ignition delay: A time interval after the spark, not an early ignition event.Auto-ignition: Generic term for spontaneous ignition; here the specific pre-spark, hot-surface event is called pre-ignition.Common Pitfalls:Confusing knock noise with pre-ignition. Knock can sometimes be audible; pre-ignition may silently overheat components. Both are harmful, but mechanisms and remedies differ.Final Answer:pre-ignition

Question 2

Nuclear reactor kinetics: A reactor is said to be 'critical' when the neutron population in the core is ________ with time (i.e., power level is steady).

Accepted Answer

Introduction / Context:Reactor criticality is a foundational concept in nuclear power. It describes the balance between neutron production (via fission) and losses (via absorption and leakage). Understanding the terms subcritical, critical, and supercritical is essential for safe operation and control.Given Data / Assumptions:Thermal or fast reactor with steady-state consideration.Reactivity changes are small and control systems are active.We use the point-kinetics perspective for clarity.Concept / Approach:A reactor is critical when the effective multiplication factor keff = 1. At this condition, each generation of neutrons produces exactly one subsequent generation of the same size, so neutron population and hence reactor power remain constant over time.Step-by-Step Solution:Define keff: ratio of neutrons in successive generations.If keff > 1 → supercritical → neutron population grows with time.If keff If keff = 1 → critical → neutron population is constant (steady power).Verification / Alternative check:Examine reactor power trends: during long steady operation, heat removal (steam generation) balances fission heat production, indicating constant neutron flux and critical state.Why Other Options Are Wrong:Rapidly increasing → indicates supercritical conditions, not critical.Decreasing → subcritical operation.Reduced to zero → shutdown with no chain reaction.Common Pitfalls:Equating “critical” with “dangerous”. In nuclear engineering, “critical” simply means steady-state chain reaction, not instability.Final Answer:constant

Question 3

Compression-ignition (diesel) principle: In diesel engines, ignition occurs due to the high temperature attained at the end of compression without using a spark plug.

Accepted Answer

Introduction / Context:Diesel engines are compression-ignition (CI) machines. Unlike spark-ignition (SI) petrol engines, they do not use a spark plug to initiate combustion. Instead, fuel auto-ignites when injected into hot, highly compressed air.Given Data / Assumptions:Air is compressed to a high compression ratio (typically 15–25).Fuel is injected near the end of the compression stroke.No external spark source is provided.Concept / Approach:High compression raises the air temperature substantially (well above the auto-ignition temperature of diesel fuel). When finely atomized diesel is injected, it vaporizes and ignites after a short ignition delay. This fundamental mechanism defines CI operation.Step-by-Step Solution:Compression → temperature and pressure increase in trapped air.Near top dead centre, injector delivers diesel spray into hot air.Auto-ignition occurs → controlled combustion without a spark.Verification / Alternative check:Engines equipped with glow plugs still rely on auto-ignition; glow plugs only assist cold starting by preheating air or combustion chamber surfaces.Why Other Options Are Wrong:“Incorrect” would imply a need for a spark similar to petrol engines, which contradicts the CI definition and practical engine design.Common Pitfalls:Confusing glow plugs with spark plugs; glow plugs are heaters, not ignition triggers in normal operation.Final Answer:Correct

Question 4

Power plant economics: Which type of power plant generally offers the lowest running (operating) cost for producing electrical power, considering fuel and routine operation?

Accepted Answer

Introduction / Context:Operating (running) cost focuses on day-to-day expenses such as fuel, consumables, and routine operations. Capital cost and construction time are separate considerations. Knowing which plant type has the lowest running cost supports planning of base-load versus peak-load generation portfolios.Given Data / Assumptions:Running cost emphasizes fuel and regular operation, not initial capital or long-term financing.Hydro plants use water head as the energy source; fuel cost is essentially zero.Steam (coal), gas turbines (gas/jet fuel), and nuclear all have ongoing fuel-related costs and other operating expenses.Concept / Approach:Hydroelectric plants convert potential energy of stored water into electricity. Because there is no purchased fuel, their running cost is generally the lowest. Thermal and nuclear plants require continuous fuel supply and associated handling, making their operating cost higher even if fuel prices vary by region and time.Step-by-Step Solution:Identify cost categories: capital vs. running.Compare fuels: hydro (none) vs. coal/gas/uranium (non-zero ongoing cost).Conclude: hydro has the lowest running cost for typical systems.Verification / Alternative check:Utility cost breakdowns routinely show minimal fuel expense for hydro units. Their primary recurring costs are operations, maintenance, and water management, which are modest per kWh compared to fossil or nuclear fuels.Why Other Options Are Wrong:Steam power plant: Coal procurement, handling, and emissions control add significant ongoing expense.Gas turbine plant: Fuel cost is substantial; often used for peaking due to high operating cost per kWh.Nuclear plant: While fuel volume is small, fuel cycle, operations, and regulatory requirements raise running costs compared with hydro.Common Pitfalls:Conflating operating cost with total cost of ownership; hydro may have high capital cost but still wins on running cost.Final Answer:hydro electric power plant

Question 5

Engine abnormal sounds: The loud, sharp, pulsating noise observed within an internal combustion engine cylinder due to rapid pressure rise of end-gas is called ________.

Accepted Answer

Introduction / Context:Engines can exhibit abnormal combustion that creates distinctive noises. Recognizing whether the sound is detonation (knock) or something else is vital for protecting the engine and improving performance.Given Data / Assumptions:A spark-ignition engine operating under load.End-gas may auto-ignite after the spark-initiated flame begins to propagate.Mechanical noise is distinguished from combustion-induced knock.Concept / Approach:Detonation (knock) is spontaneous combustion of the unburned end-gas ahead of the flame front, creating strong pressure waves that ring the combustion chamber. This causes a characteristic metallic, pulsating knock. Turbulence is a flow property aiding flame speed; pre-ignition is early ignition due to hot spots; supercharging increases intake density and is not a noise phenomenon.Step-by-Step Solution:Identify symptom: loud, pulsating “ping/knock” during combustion.Relate to cause: auto-ignition of end-gas after spark → detonation.Exclude others: turbulence is beneficial flow; pre-ignition occurs before spark timing; supercharging is a boosting method.Verification / Alternative check:Knock sensors detect characteristic vibration frequencies linked to resonant modes of the cylinder, confirming the detonation mechanism.Why Other Options Are Wrong:Turbulence: Enhances combustion stability, not a noise source by itself.Pre-ignition: Earlier-than-spark ignition; may not create the same ringing knock signature.Supercharging: A hardware/operating condition, not a noise type.Common Pitfalls:Assuming any loud engine noise is “knock.” Mechanical issues (e.g., bearings) can also be noisy; diagnosis should consider timing and load correlation.Final Answer:detonation

Question 6

Neutron moderation in reactors: Slowing down fast neutrons is desirable because slow (thermal) neutrons are generally more effective at inducing fission in common reactor fuels.

Accepted Answer

Introduction / Context:Most commercial reactors (e.g., light-water reactors) are thermal reactors that employ moderators to slow neutrons. Understanding why thermal neutrons are preferred illuminates core design, material selection, and safety.Given Data / Assumptions:Fuel contains fissile isotopes such as U-235 or Pu-239.Moderator materials (water, heavy water, graphite) are present in thermal reactors.Goal: maximize probability that a neutron causes fission rather than escaping or being captured non-productively.Concept / Approach:Fission cross-sections of many fissile nuclides are much higher for thermal (slow) neutrons than for fast neutrons. Moderation increases the likelihood of fission per neutron, enabling a sustained chain reaction with lower enrichment and more compact cores.Step-by-Step Solution:Fast neutrons born from fission have high kinetic energy.Elastic scattering in a moderator reduces neutron energy toward thermal equilibrium with the medium.At thermal energies, the microscopic fission cross-section is higher → more effective at triggering fission.Verification / Alternative check:Cross-section data tables show orders-of-magnitude increase in fission probability for U-235 at thermal energies versus MeV energies, affirming the desirability of moderation in thermal reactors.Why Other Options Are Wrong:“Disagree” ignores the fundamental cross-section behavior that underpins thermal reactor operation.Common Pitfalls:Assuming all reactors are thermal. Fast reactors avoid moderation intentionally, but the statement specifically addresses why slowing down is desirable in typical thermal designs.Final Answer:Agree

Question 7

Brake power measurement with a rope brake: Assume a rope brake dynamometer around a drum. Using effective radius Re = (D + d) / 2, the brake power (B.P.) is ________ (use W = dead load, S = spring balance reading, N = rpm).

Accepted Answer

Introduction / Context:Brake power (B.P.) is the usable power delivered at the engine shaft. A rope brake dynamometer is a simple device to measure torque via a dead weight and a spring balance around a brake drum. Getting the sign and effective radius right is key to accurate power calculation.Given Data / Assumptions:Rope brake around a drum of diameter D with rope diameter d.Dead load W (N) hangs on one side; spring balance reads S (N) on the other.Effective radius Re = (D + d) / 2 (in metres).Shaft speed N (rpm).Concept / Approach:Torque T equals net circumferential force times effective radius. The net force causing braking is (W − S). Power equals torque times angular speed. Angular speed in rad/s is 2 * pi * N / 60. Combine to obtain B.P.Step-by-Step Solution:Compute net force: F = W − S.Effective radius: Re = (D + d) / 2.Torque: T = F * Re.Angular speed: omega = 2 * pi * N / 60.Brake power: B.P. = T * omega = (2 * pi * N / 60) * (W − S) * Re.Verification / Alternative check:Dimensional check: N is rpm, so 2 * pi * N / 60 gives s^-1; multiplying by torque (N·m) yields watts, confirming consistency.Why Other Options Are Wrong:Using (W + S) overestimates torque; the opposing forces act in opposite directions so net is W − S.Dividing by Re instead of multiplying inverts torque dimensionally.Both incorrect sign and inverse radius compound the error.Common Pitfalls:Forgetting rope thickness in effective radius or misreading the spring balance direction. Also, ensure steady state (no acceleration) so that net torque equals measured braking torque.Final Answer:B.P. = (2 * pi * N / 60) * (W - S) * Re

Question 8

Benefits of supercharging: Compared with a naturally aspirated engine delivering the same power, a supercharged engine can be ________ in mass and infrastructure requirements, and may show reduced lubricating oil consumption per kWh.

Accepted Answer

Introduction / Context:Supercharging increases the density of the intake charge, enabling higher specific power from the same displacement. Comparing engines at equal power output clarifies practical benefits in weight, installation, and operating metrics.Given Data / Assumptions:Comparison is at equal delivered shaft power.Supercharged engine uses a compressor (mechanical or exhaust-driven turbo) to increase manifold pressure.Foundation size relates to engine mass and envelope for the same power.Concept / Approach:By increasing air mass flow per cycle, a supercharged engine can achieve the target power with smaller displacement. A smaller, lighter engine and correspondingly reduced foundation requirements follow. Oil consumption per unit energy often improves because bearing loads scale with absolute torque at a given size, and modern boosted engines are optimized for sealing and oil control.Step-by-Step Solution:Boost raises intake density → more fuel burned efficiently per cycle.For a fixed power target, select a smaller engine → lighter mass.Smaller engine footprint → smaller foundations/supports.Optimized boosted operation often reduces specific lubricating oil consumption.Verification / Alternative check:Industry practice shows downsized, turbocharged engines replacing larger naturally aspirated units for equal power with lower mass and compact installations, supporting the statements.Why Other Options Are Wrong:Each individual statement is generally true at equal power; hence choosing anything less than “all of these” omits benefits.Common Pitfalls:Confusing absolute oil use with specific (per kWh) consumption; boosted engines may use similar total oil at high load but less per unit energy compared with a larger NA engine of equal power.Final Answer:all of these

Question 9

Fuel types and knock tendency in spark-ignition engines: Arrange the following in order of decreasing knock tendency (i.e., from most prone to knock to least prone): paraffins, naphthenes, aromatics.

Accepted Answer

Introduction / Context:In spark-ignition (SI) engines, a fuel’s resistance to knock is characterized by its octane rating. Different hydrocarbon families have different inherent tendencies to knock, informing gasoline formulation and additive strategies.Given Data / Assumptions:Hydrocarbon families considered: paraffins (alkanes), naphthenes (cyclo-alkanes), aromatics.We rank by knock tendency (inverse of octane quality).Operating conditions typical of SI engines.Concept / Approach:Generally, normal paraffins have the poorest knock resistance (lowest octane), naphthenes are intermediate, and aromatics often show the highest octane numbers (best knock resistance). Therefore, from most knocking to least: paraffins → naphthenes → aromatics.Step-by-Step Solution:Define “decreasing knock tendency” = moving toward better knock resistance.Assign relative tendencies: paraffins (high knock), naphthenes (moderate), aromatics (low knock).Create ordered list accordingly: paraffins, naphthenes, aromatics.Verification / Alternative check:Reference fuel data show higher octane for many aromatics (e.g., toluene) and lower for straight-chain alkanes (e.g., n-heptane as low-octane reference), confirming the trend.Why Other Options Are Wrong:Any order placing aromatics before paraffins (as more knocking) contradicts typical octane behavior.Orders swapping naphthenes and aromatics overlook the superior knock resistance of many aromatics.Common Pitfalls:Assuming all compounds in a family behave identically; branching and specific structure matter, but the family trend remains broadly valid.Final Answer:paraffins, naphthenes, aromatics

Question 10

Comparative efficiency: The thermal efficiency of a typical two-stroke cycle engine is generally ________ that of a four-stroke cycle engine of similar size and technology.

Accepted Answer

Introduction / Context:Two-stroke and four-stroke engines differ in gas exchange, combustion timing, and scavenging. These differences affect indicated and brake thermal efficiencies. Understanding the comparative trends is important for engine selection and application.Given Data / Assumptions:Similar displacement class and technology level.Conventional crankcase-scavenged two-stroke versus poppet-valve four-stroke.No direct-injection stratified-charge two-stroke improvements assumed.Concept / Approach:Traditional two-stroke engines suffer from short-circuiting (fresh charge lost during scavenging), poorer volumetric efficiency control, and higher mixing losses. Four-strokes separate intake and exhaust strokes, enabling better gas exchange and more complete combustion, typically yielding higher thermal efficiency.Step-by-Step Solution:Consider scavenging: two-strokes overlap intake/exhaust → charge loss.Combustion quality: four-strokes have more stable combustion over wider loads.Mechanical efficiency: although two-strokes have one power stroke per revolution, losses from scavenging reduce overall efficiency.Verification / Alternative check:Brake-specific fuel consumption (BSFC) data commonly show higher BSFC (worse efficiency) for traditional two-strokes compared with comparable four-strokes.Why Other Options Are Wrong:Equal to / greater than: Only in specialized modern two-strokes (e.g., DI with advanced scavenging) might parity be approached; the general case is lower efficiency.Common Pitfalls:Confusing power density (two-strokes deliver more frequent power strokes) with efficiency. More frequent power strokes do not guarantee better fuel economy.Final Answer:less than

Question 11

Compression-ignition (diesel) engines: purpose of a masked (shrouded) inlet valve In air-standard diesel engines, a masked or shrouded inlet valve partially blocks one side of the port. What is the primary design objective of providing this masking …

Accepted Answer

Introduction / Context:Diesel (compression-ignition) engines rely on high-quality air–fuel mixing immediately after injection. Since fuel is injected late in the cycle, strong in-cylinder air motion at the end of compression is crucial. A masked or shrouded inlet valve is a classic hardware feature used to tailor swirl, one of the organized large-scale motions that enhance mixing and combustion efficiency.Given Data / Assumptions:Four-stroke CI engine with poppet valves.Masked (partially shrouded) intake valve redirects the incoming air jet.Goal is improved mixture formation and cleaner combustion.Concept / Approach:Swirl is an organized, bulk angular motion of the entire air mass about the cylinder axis. By covering a sector of the intake valve curtain, the valve mask blocks one flow path and encourages the remaining curtain flow to enter tangentially, imparting angular momentum. This produces a primary, cylinder-scale vortex that persists into compression and supports rapid atomization and evaporation of the injected fuel.Step-by-Step Solution:Identify the challenge: short time available after fuel injection demands intense air–fuel interaction.Engineer the intake: partial shrouding biases the flow direction, creating tangential entry.Resulting motion: large-scale swirl is established and maintained through compression.Combustion benefit: enhanced mixing shortens ignition delay and reduces smoke.Verification / Alternative check:Engines without masks often rely on port geometry (tangential/helical) to generate swirl. Flow-bench swirl number measurements and CFD confirm that masking primarily increases swirl rather than total mass flow.Why Other Options Are Wrong:Enhance flow rate: masking usually reduces peak flow coefficient slightly.Control overall air flow: throttling is not used in diesels; load is set by fuel.Induce only secondary turbulence: masking targets organized swirl, not just small-scale eddies.Reduce valve seat temperatures: that is a cooling/material issue, not the aerodynamic purpose.Common Pitfalls:Confusing swirl (bulk rotation) with turbulence (random eddies). Both aid mixing, but masking is aimed at swirl creation, while turbulence is addressed by piston bowl shape and squish.Final Answer:induce primary swirl in the cylinder during induction

Question 12

Four-stroke petrol engine: direction of piston motion during the suction (intake) stroke Consider a conventional four-stroke spark-ignition engine. During the suction stroke, the inlet valve opens and the fresh charge is drawn into the cylinder as t…

Accepted Answer

Introduction / Context:The intake (suction) stroke is the first of the four events in a four-stroke spark-ignition cycle: intake, compression, power, and exhaust. Understanding piston motion and valve events during intake is foundational for grasping volumetric efficiency, manifold tuning, and throttle response.Given Data / Assumptions:Four-stroke petrol engine with poppet intake and exhaust valves.Throttle controls manifold pressure; spark timing is irrelevant to the intake stroke.We use the standard sign convention for crank angles: TDC at the top, BDC at the bottom.Concept / Approach:On the intake stroke, the inlet valve opens (typically slightly before TDC). As the piston travels downward from TDC toward BDC, cylinder volume increases and its pressure drops below the intake manifold pressure. This pressure differential draws the fresh air–fuel mixture into the cylinder. Therefore, the piston moves downward, not upward, during suction.Step-by-Step Solution:At end of exhaust, inlet valve begins to open near TDC.Piston motion: TDC → BDC (downward), increasing cylinder volume.Pressure effect: cylinder pressure Intake closure occurs slightly after BDC to exploit inertia of the incoming charge.Verification / Alternative check:Valve-timing diagrams and PV plots show the intake stroke associated with decreasing pressure and increasing volume, confirming downward piston travel during suction.Why Other Options Are Wrong:“True” contradicts the kinematics of the four-stroke cycle.Two-stroke engines do not have a discrete suction stroke in the same sense; scavenging/intake occurs differently.Cold start and throttle position affect pressure levels, not piston direction.Common Pitfalls:Confusing the intake and compression strokes: compression is the upward motion from BDC to TDC after the intake stroke.Final Answer:False

Question 13

Typical compression ratio for spark-ignition (petrol) engines Within standard automotive practice (unboosted SI engines), which range best represents the usual compression ratio adopted for petrol engines?

Accepted Answer

Introduction / Context:Compression ratio (CR) is the key geometric parameter influencing thermal efficiency, knock tendency, and emissions in spark-ignition engines. While modern engines may push slightly beyond historical values with direct injection and knock control, the classical range remains informative for fundamentals.Given Data / Assumptions:Conventional, naturally aspirated petrol engines without extreme knock-mitigation or high boost.Regular fuel octane ratings, typical passenger-car operation.Comparison based on geometric compression ratio.Concept / Approach:Theoretical efficiency of the Otto cycle increases with CR, but real engines are limited by knock (auto-ignition). Historically, SI engines have used CR values around 6–10. Direct injection, cooled EGR, and high-octane fuels can allow somewhat higher CR, but the foundational range taught in thermodynamics courses is 6–10.Step-by-Step Solution:Relate CR to ideal cycle efficiency: higher CR → higher efficiency.Constraint: SI knock risk limits CR for a given fuel and cooling.Canonical range in textbooks: 6 to 10 → choose this option.Verification / Alternative check:Engine specifications across decades show many unboosted SI engines with CR within 7.5–10.5; teaching examples standardize 6–10 for simplicity.Why Other Options Are Wrong:10–15 and higher ranges are typical of diesels or specially optimized SI with DI and high octane; 15–25 and 25–40 suit CI engines or special cycles.Common Pitfalls:Confusing geometric CR with effective CR under valve timing effects; the geometric number still falls in the indicated range for typical SI engines.Final Answer:6 to 10

Question 14

Definitions of efficiencies in I.C. engines The ratio of indicated thermal efficiency to the corresponding air-standard (ideal cycle) efficiency is termed as which engine performance metric?

Accepted Answer

Introduction / Context:Separating real-engine losses from ideal-cycle limits helps diagnose where performance is being lost. Relative efficiency compares the indicated thermal efficiency of the real engine to the theoretical air-standard cycle efficiency under similar compression ratio and conditions.Given Data / Assumptions:Indicated thermal efficiency accounts for in-cylinder conversion of fuel energy to indicated work.Air-standard efficiency is derived from the corresponding ideal cycle (e.g., Otto) at the same compression ratio.Comparison excludes mechanical losses (that is handled by mechanical efficiency).Concept / Approach:Relative efficiency = eta_indicated / eta_air-standard. It expresses how closely the real combustion and heat-transfer processes approach the ideal assumptions. It is sometimes called efficiency ratio. It does not include drivetrain or accessory losses.Step-by-Step Solution:Identify numerator: indicated thermal efficiency (cylinder-level).Identify denominator: ideal air-standard cycle efficiency.Take the ratio → definition of relative efficiency.Verification / Alternative check:Textbook performance maps typically report mechanical efficiency separately as BP/IP, while relative efficiency benchmarks indicated performance against the ideal limit.Why Other Options Are Wrong:Mechanical efficiency compares BP to IP; overall efficiency compares BP to fuel energy; volumetric efficiency compares actual to theoretical air charge; isentropic efficiency applies to turbomachinery stages.Common Pitfalls:Assuming relative efficiency includes friction. It does not; friction is outside indicated processes.Final Answer:relative efficiency (efficiency ratio)

Question 15

Determinants of firing order in multi-cylinder I.C. engines On what design factors does the firing order primarily depend when laying out a multi-cylinder internal combustion engine?

Accepted Answer

Introduction / Context:The firing order determines vibration characteristics, crankshaft loading, exhaust pulse timing, and acoustics. Selecting an order is a multidisciplinary compromise among mechanical balance, torsional dynamics, and gas-exchange tuning.Given Data / Assumptions:Multi-cylinder engine design problem.Crankshaft geometry fixed by cylinder arrangement and bank angle.Goal: smooth torque delivery, acceptable NVH, and good breathing.Concept / Approach:Because each firing event imposes forces and produces exhaust pulses, the spatial cylinder arrangement, crankpin phasing, and total cylinder count jointly constrain feasible firing sequences. Designers test candidate orders via torsional vibration analysis and gas-dynamic simulations to achieve even firing intervals and desirable pulse pairing.Step-by-Step Solution:Start with cylinder arrangement → defines crankshaft layout options.Consider crankshaft phasing → limits feasible firing spacing.Select order that balances forces/moments and optimizes exhaust pulse timing.Verification / Alternative check:Common orders (e.g., 1-3-4-2 for inline-4) arise from these constraints and are validated by NVH testing and durability calculations.Why Other Options Are Wrong:Any single factor alone cannot fully dictate firing order; it is a combined optimization. Drivetrain layout is not a primary determinant.Common Pitfalls:Assuming firing order is chosen arbitrarily or solely for sound; it fundamentally affects mechanical stresses and smoothness.Final Answer:all of these

Question 16

Terminology check: relative efficiency of an I.C. engine State whether the following definition is correct: “The relative efficiency of an internal combustion engine is the ratio of its indicated thermal efficiency to the corresponding air-standard …

Accepted Answer

Introduction / Context:Naming conventions matter when comparing real engines to ideal cycles. Relative efficiency (also called efficiency ratio) normalizes indicated performance to the theoretical air-standard limit at the same basic cycle conditions.Given Data / Assumptions:We compare indicated thermal efficiency to an ideal Otto/Diesel cycle efficiency.Air-standard model uses simplified assumptions (ideal gas, no pumping or heat losses).Applies to both SI and CI engines.Concept / Approach:Relative efficiency = eta_indicated / eta_air-standard. It evaluates how closely in-cylinder thermodynamic processes emulate the idealized cycle, independent of mechanical losses. It is a dimensionless ratio less than 1 for real engines.Step-by-Step Solution:Define indicated thermal efficiency.Define air-standard efficiency for the same compression ratio and cycle.Form the ratio to obtain relative efficiency → statement is correct.Verification / Alternative check:Engine texts list mechanical efficiency (BP/IP) and overall efficiency (BP/fuel energy) separately from relative efficiency, confirming the definition’s correctness.Why Other Options Are Wrong:It is not limited to one engine type; and it does not depend only on compression ratio, since combustion phasing, heat losses, and burn duration also matter.Common Pitfalls:Mixing up relative efficiency with mechanical or overall efficiency; they capture different loss mechanisms.Final Answer:Correct

Question 17

Basic hardware of a four-stroke diesel engine A typical automotive/compression-ignition cylinder head contains which of the following devices?

Accepted Answer

Introduction / Context:Compression-ignition engines operate with air induction and direct fuel injection near the end of compression. The cylinder head must therefore accommodate air exchange components and the high-pressure injector hardware.Given Data / Assumptions:Four-stroke diesel with poppet valves.Direct injection or unit injector/rail-based injector located in the head.No spark plug is used in normal diesel operation.Concept / Approach:Diesel combustion requires: (1) an inlet valve for fresh air, (2) a high-pressure fuel injector (often called fuel injection valve/nozzle assembly) to deliver finely atomized fuel into hot compressed air, and (3) an exhaust valve for burned gases. This hardware mix enables non-premixed, diffusion-dominated combustion after auto-ignition.Step-by-Step Solution:Identify air-path components: inlet and exhaust valves.Identify fuel-path component: injector/nozzle (sometimes termed an injection valve).Confirm absence of spark plug in CI engines.Verification / Alternative check:Cutaway diagrams of modern diesels show two or four valves plus a centrally located injector; glow plugs may be present for cold starting but are not ignition sources in normal running.Why Other Options Are Wrong:Premixed upstream fueling (option b) applies to SI carbureted engines; option c includes a spark plug, not used in normal CI; option d misplaces gas-exchange; ports in the piston are not used; two-stroke engines may use ports in the liner, not in the piston.Common Pitfalls:Calling the injector “not a valve.” The injector assembly incorporates a spring-loaded needle valve; many texts refer to it as a fuel injection valve.Final Answer:An inlet (intake) valve, a fuel injection valve/nozzle assembly, and an exhaust valve

Question 18

Who controls air–fuel ratio in a petrol engine? In a conventional spark-ignition petrol engine (non-GDI), which component regulates the air–fuel ratio supplied to the cylinders under steady running?

Accepted Answer

Introduction / Context:The air–fuel ratio (AFR) directly affects power, efficiency, emissions, and drivability. In traditional petrol engines without electronic fuel injection, the carburettor meters fuel using the pressure drop created by intake air and provides mixture control across operating regimes.Given Data / Assumptions:Carbureted SI engine (not GDI/port-EFI).Throttle controls airflow; carburettor circuits meter fuel accordingly.Engine speed and load vary during normal operation.Concept / Approach:A carburettor uses a venturi to create a pressure differential that draws fuel from jets into the airstream, with dedicated circuits (idle, main, power, accelerator pump, choke) to maintain suitable AFR under different conditions. While a governor may control speed on stationary engines by adjusting throttle, the metering of fuel relative to air in a carbureted SI engine is the carburettor’s function.Step-by-Step Solution:Identify system type: carbureted SI.Relate AFR control to venturi/jet metering behavior.Select the component that meters fuel: carburettor.Verification / Alternative check:Service manuals describe adjustments (jets, mixture screws, choke) on carburettors for AFR tuning; EFI replaces carburettors with injectors controlled by ECUs, not relevant here.Why Other Options Are Wrong:Diesel-type injectors belong to CI engines; governors manage speed, not direct AFR metering; turbochargers alter air density, not the metering of fuel per se.Common Pitfalls:Equating speed control with mixture control; they are related but not identical.Final Answer:carburettor

Question 19

Definition: indicated power in an internal combustion engine Which term denotes the power developed inside the engine cylinder based on the pressure–volume work (before mechanical losses), as obtained from indicator diagrams or equivalent calculatio…

Accepted Answer

Introduction / Context:Engine performance is partitioned into indicated power (IP), brake power (BP), and friction power (FP). Distinguishing these helps quantify mechanical efficiency and identify loss sources.Given Data / Assumptions:Indicated power is derived from in-cylinder pressure data (indicator diagram or IMEP).Brake power is the net shaft output measured by a dynamometer.Friction power = IP − BP, representing mechanical and pumping losses.Concept / Approach:IP reflects the thermodynamic work produced by combustion acting on the piston during a cycle. It precedes drivetrain and accessory losses. BP is always less than IP in real engines due to friction and pumping work. Therefore, the correct term for “power actually developed by the engine cylinder” is indicated power.Step-by-Step Solution:Define IP via PV integral or IMEP * displacement * cycles per second.Define BP via torque * angular speed at the crankshaft.Relate: IP = BP + FP → IP denotes in-cylinder developed power.Verification / Alternative check:Classic Morse test and modern in-cylinder pressure transducers provide independent pathways to estimate or measure IP.Why Other Options Are Wrong:“Theoretical power” is not standard terminology; “actual power” often refers to BP; “friction power” is a loss term, not developed power.Common Pitfalls:Confusing BMEP with IMEP; IMEP corresponds to indicated work per cycle, while BMEP relates to brake work.Final Answer:indicated power

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