Question 1

Definition check — internal vs. external combustion: Engines in which the chemical energy of the fuel is released by combustion taking place inside the engine cylinder itself are called internal combustion engines. State whether this statement is co…

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Introduction / Context:This question distinguishes internal combustion (IC) engines from external combustion (EC) prime movers. In IC engines, fuel burns inside the working cylinder, directly raising the pressure and temperature of the working fluid that pushes the piston or drives a turbine. In EC systems such as steam power plants, fuel burns outside the engine, and heat is transferred to a separate working fluid (water/steam).Given Data / Assumptions:The statement claims: if combustion occurs inside the cylinder, the engine is an internal combustion engine.Typical IC examples: petrol (spark-ignition) engines, diesel (compression-ignition) engines, gas turbines.Typical EC examples: steam engines and steam turbines where fuel burns in a boiler/furnace.Concept / Approach:The defining criterion is the location of heat release relative to the working fluid. If the working fluid is the combustion products themselves and combustion occurs within the power-producing component, the engine is IC. If combustion heats a separate fluid in a heat exchanger (boiler), it is EC. Therefore, the statement is accurate by definition.Step-by-Step Solution:Identify power-producing element: cylinder (piston engine) or turbine stage.Check where fuel burns: inside cylinder/combustor (IC) or in a separate furnace (EC).Conclude that combustion inside the cylinder defines an IC engine → statement is correct.Verification / Alternative check:Consider a diesel engine: air is compressed in the cylinder, fuel is injected and burns in the same space; the expanding gases perform work on the piston. This satisfies the IC definition. Conversely, a steam engine’s piston is driven by steam generated in a boiler where fuel burns externally, so it is EC.Why Other Options Are Wrong:False: Contradicts the standard definition.True only for petrol/diesel: Both petrol and diesel are IC; limiting the definition to one subtype is incorrect.Common Pitfalls:Confusing the presence of a carburettor or injector with the IC/EC classification; the classification depends on where combustion occurs, not on the fuel metering device.Final Answer:True

Question 2

Four-stroke petrol (spark-ignition) engine — suction stroke pressure: During the intake (suction) stroke, the pressure inside the cylinder is ________ atmospheric pressure.

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Introduction / Context:The intake or suction stroke draws fresh air–fuel mixture into a petrol engine cylinder. Understanding the pressure level relative to atmosphere explains how throttling, volumetric efficiency, and breathing losses arise in spark-ignition engines.Given Data / Assumptions:Conventional naturally aspirated four-stroke petrol engine with a throttle plate.Intake valve open, piston moving downward during suction.No supercharger or turbocharger raising manifold pressure.Concept / Approach:During the intake stroke, the descending piston increases cylinder volume. Because the throttle and intake system cause a pressure drop, the instantaneous cylinder pressure is slightly below atmospheric, creating the differential that draws mixture into the cylinder. This sub-atmospheric manifold/cylinder pressure is typical of throttled SI engines, especially at part load.Step-by-Step Solution:At intake start, intake valve opens and piston descends.Flow restriction (venturi, throttle, ports) causes pressure in the intake manifold to drop below atmospheric.Cylinder tracks manifold pressure → cylinder pressure is below atmospheric during most of the suction stroke.Verification / Alternative check:Manifold vacuum gauges on petrol engines read below atmospheric during idle/part load, confirming sub-atmospheric intake/cylinder pressure on the suction stroke.Why Other Options Are Wrong:Equal to: Only an ideal, frictionless, unthrottled flow would approach this; practical engines show vacuum.Above: Requires boost (supercharger/turbo) or intake ram effects, not assumed here.Common Pitfalls:Confusing petrol SI with diesel CI; diesels often operate unthrottled, but cylinder pressure during intake is still close to atmospheric unless boosted.Final Answer:below

Question 3

Turbo-prop vs. turbo-jet turbines: The power turbine of a turbo-prop engine (which must drive a propeller) is generally ________ the turbine of a turbo-jet engine.

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Introduction / Context:Gas-turbine layouts differ depending on whether thrust is produced by jet exhaust (turbo-jet) or by a propeller driven through a turbine and gearbox (turbo-prop). The fraction of enthalpy drop extracted by the turbine determines its size and work output.Given Data / Assumptions:Turbo-jet: turbine extracts only enough power to drive the compressor and accessories; most exhaust kinetic energy provides thrust.Turbo-prop: turbine must also deliver shaft power for a propeller via reduction gearing.Similar overall compressor pressure ratios for a given era of design (conceptual comparison).Concept / Approach:A turbo-prop diverts a larger enthalpy drop in the turbine to produce substantial shaft power. More stage work extraction generally requires additional turbine stages and/or larger flow areas and blade heights to handle the work and mass flow at acceptable tip speeds. Therefore, the turbine section of a turbo-prop is typically larger (physically and in stage count) than that of a comparable turbo-jet.Step-by-Step Solution:Identify required turbine work: compressor drive + propeller power in turbo-prop > compressor-only in turbo-jet.More turbine work → larger stage count/diameter/flow path.Conclude: turbo-prop turbine is generally bigger than turbo-jet turbine.Verification / Alternative check:Cutaway comparisons show turbo-props often have a free power turbine downstream of the gas generator turbine, increasing overall turbine hardware compared with pure turbo-jets.Why Other Options Are Wrong:Same as / smaller than: Conflicts with the added propeller power requirement.Identical blade count but smaller diameter: Neither blade count nor diameter is fixed; both scale with design targets.Common Pitfalls:Assuming nozzle thrust dominates in all gas turbines; for turbo-props, shaft power is primary, so more energy is extracted in the turbine.Final Answer:bigger than

Question 4

Otto cycle fuel introduction — is external fuel injection required? State whether the following is correct: Engines working on the ideal Otto cycle do not require high-pressure fuel injection by an external source into the cylinder (mixture is prepa…

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Introduction / Context:The ideal Otto cycle models a homogeneous, premixed charge that is compressed and then ignited by a spark. Historically, spark-ignition engines used carburettors to meter fuel upstream; modern SI uses port or direct injection but still does not rely on diesel-like high-pressure injection to trigger ignition.Given Data / Assumptions:Otto cycle = constant-volume heat addition applied to a premixed homogeneous charge.Ignition source is a spark, not auto-ignition by compression.“External source injection” here refers to diesel-style high-pressure in-cylinder injection being essential for operation.Concept / Approach:In the classical Otto framework, fuel is introduced and mixed with air before the intake valve, and the cylinder draws in the mixture. Thus, no high-pressure, in-cylinder “external” injection system is required to achieve combustion. While many modern SI engines use injection (port or DI) for mixture preparation and control, ignition still requires a spark and does not depend on compression ignition or mandatory diesel-type injection hardware.Step-by-Step Solution:Recall Otto assumptions: premixed, homogeneous charge at intake.Recognize spark ignition provides the heat addition at (approximately) constant volume near TDC.Conclude: diesel-like external high-pressure injection is not required by the ideal Otto concept → statement is correct.Verification / Alternative check:Carburetted engines fully satisfy the ideal Otto premise. Port-fuel-injected SI engines still premix before ignition; direct-injection SI times fuel delivery but relies on a spark, not compression-induced auto-ignition.Why Other Options Are Wrong:No: Implies injection is required for Otto-cycle operation, which is not inherent.Conditional options: The ideal cycle definition is general and not limited to specific hardware era.Common Pitfalls:Confusing “fuel injection present” with “fuel injection required for ignition.” SI uses a spark to initiate combustion; the presence of injectors does not change the fundamental ignition mode.Final Answer:Yes

Question 5

Misfueling scenario: If diesel fuel is used in a conventional petrol (spark-ignition) engine, what will happen?

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Introduction / Context:Using the wrong fuel type undermines the ignition method and spray/volatility characteristics for which an engine is designed. Petrol engines require a volatile, high-octane fuel that forms a combustible mixture and ignites by spark at the right timing, whereas diesel fuel is less volatile and intended for compression ignition with injection.Given Data / Assumptions:Conventional SI petrol engine with spark ignition and either carburetion or port injection.Diesel fuel substituted for petrol in the tank.No hardware changes (no high-pressure injection or glow assistance).Concept / Approach:Diesel has lower volatility and higher auto-ignition tendency at high temperature but resists forming a proper combustible vapor mixture at typical SI intake conditions. The spark may fail to ignite large droplets; plugs foul with unburned fuel; the engine may cough briefly and stall. Pre-ignition or “explosion” is highly unlikely under these conditions; the usual outcome is failure to start or poor running followed by stalling.Step-by-Step Solution:Assess mixture formation: diesel does not vaporize readily in SI intake.Assess ignition: spark cannot reliably ignite poorly vaporized diesel droplets.Outcome: engine fails to run (or runs very poorly and stops).Verification / Alternative check:Workshop experience confirms petrol engines misfueled with diesel typically do not start or stall quickly due to plug fouling and lack of proper vaporization/ignition.Why Other Options Are Wrong:Run more efficiently / at high speed: Opposite of observed behavior.Explode: Unfounded; diesel in an SI engine lowers, not raises, knock risk during attempted operation.Common Pitfalls:Confusing misfueling cases (diesel in petrol vs. petrol in diesel). Petrol in a diesel can cause severe knock-like noise and damage; diesel in a petrol mainly causes no-start/poor running.Final Answer:not run

Question 6

Packaging comparison: Does a two-stroke cycle engine generally occupy a larger floor area than a comparable four-stroke cycle engine?

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Introduction / Context:Two-stroke engines fire once per revolution, delivering higher power density for a given displacement in simple single-cylinder comparisons. Their simpler valve systems (ports rather than poppet valves) and fewer mechanical parts often allow more compact packaging than four-stroke counterparts.Given Data / Assumptions:Engines of similar output within the same class.Conventional naturally aspirated, non-opposed-piston designs.Focus on basic footprint/floor area rather than height or auxiliaries.Concept / Approach:For comparable power, a two-stroke can employ smaller displacement or fewer cylinders than a four-stroke, reducing the crankcase length and accessory count. Although large marine slow-speed two-strokes are tall, their footprint relative to power remains favorable. Therefore, as a rule of thumb, two-strokes do not need larger floor area than four-strokes of similar output.Step-by-Step Solution:Compare power-per-displacement: two-stroke > four-stroke (one power stroke every 360° vs. 720°).For the same power, a two-stroke can be smaller in swept volume and cylinder count.Smaller displacement and simpler mechanisms → reduced footprint.Verification / Alternative check:Motorcycle and handheld tool engines demonstrate compact two-stroke packaging. Large marine two-strokes also achieve high power per unit footprint compared with four-stroke diesels of similar power.Why Other Options Are Wrong:Yes: Contradicts general power-density reality.Conditional options: Specific applications vary, but the general engineering comparison stands.Common Pitfalls:Confusing floor area with overall height; some two-strokes can be tall, yet still compact in footprint for a given power.Final Answer:No

Question 7

Volumetric efficiency application (single cylinder gas engine): A gas engine has a swept volume of 300 cm³ and a clearance volume of 25 cm³. If the volumetric efficiency is 0.88 and mechanical efficiency is 0.90, determine the actual volume of fresh…

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Introduction / Context:Volumetric efficiency measures how effectively the cylinder is filled with fresh charge compared to its swept volume. It is central to intake tuning, breathing analysis, and performance estimation in both SI and CI engines.Given Data / Assumptions:Swept volume Vs = 300 cm³.Clearance volume Vc = 25 cm³ (not directly used in this basic computation).Volumetric efficiency η_v = 0.88.Mechanical efficiency η_m = 0.90 (irrelevant for volumetric calculation).Concept / Approach:By definition (at a specified reference state), η_v = (actual volume of fresh charge inducted per cycle) / (swept volume). Therefore, the inducted mixture volume per stroke equals η_v * Vs. Mechanical efficiency relates brake to indicated power and does not enter this calculation.Step-by-Step Solution:Write the relation: η_v = V_actual / Vs.Rearrange: V_actual = η_v * Vs.Insert values: V_actual = 0.88 * 300 cm³ = 264 cm³.Verification / Alternative check:If η_v were 100%, the engine would induct 300 cm³ per stroke. Since η_v is 88%, the result must be slightly less than 300 cm³, which 264 cm³ satisfies.Why Other Options Are Wrong:248 cm³ / 252 cm³: Do not match 0.88 * 300.286 cm³: Exceeds 0.95 of the swept volume and is inconsistent with η_v = 0.88.Common Pitfalls:Confusing volumetric efficiency (based on swept volume) with total cylinder volume (swept + clearance). Also, mechanical efficiency is unrelated to inducted volume.Final Answer:264 cm³

Question 8

Typical brake specific fuel consumption (BSFC) for a petrol engine: The approximate specific fuel consumption per brake power hour for a conventional petrol (SI) engine is

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Introduction / Context:Brake specific fuel consumption (BSFC) expresses how many kilograms of fuel are required to produce one brake power hour. Recognizing typical BSFC values is useful for quick performance and economy estimates.Given Data / Assumptions:Conventional naturally aspirated petrol engine at or near best-efficiency operating point.Representative, rounded values rather than exact manufacturer data.Fuel quality and engine technology typical of textbook examples.Concept / Approach:Petrol SI engines typically show BSFC in the neighborhood of 0.25 kg per brake power hour at best efficiency, with values rising at idle or far from optimal load/speed. Diesel engines, by contrast, often achieve lower BSFC (around 0.2 kg/BP·h or less) due to higher compression ratio and lean operation.Step-by-Step Solution:Recall typical SI BSFC range: about 0.25–0.30 kg/BP·h.Select the commonly cited nominal value near the optimum: 0.25 kg/BP·h.Confirm alternatives are either too optimistic (0.2 kg typical of diesel) or representative of off-optimum operation (0.3–0.35 kg).Verification / Alternative check:Dynamometer maps show minima in the 230–300 g/kWh range. Converting 0.25 kg/BP·h using 1 BP = 0.746 kW gives a similar magnitude in g/kWh, reinforcing the selection.Why Other Options Are Wrong:0.2 kg: More representative of diesel engines at good operating points.0.3 kg and 0.35 kg: Possible at non-optimal loads/speeds but not the typical “about” best value taught in fundamentals.Common Pitfalls:Confusing metric (g/kWh) with imperial (lb/bhp·h) units; always check the basis.Final Answer:0.25 kg

Question 9

End-of-compression pressure (petrol engine): The approximate cylinder pressure at the end of compression in a spark-ignition petrol engine is

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Introduction / Context:Estimating the pressure at the end of compression is useful for sanity checks on indicator diagrams and for diagnosing compression-related faults. Typical values depend on compression ratio, charge temperature, and valve timing.Given Data / Assumptions:Conventional petrol engine with compression ratio roughly 8:1–11:1.Near-ambient intake conditions and throttled operation typical of SI.No boost or abnormal heating.Concept / Approach:A simplified adiabatic compression estimate from about 1 bar to end-of-compression yields pressures on the order of 10–12 bar for common SI ratios and polytropic indices between isothermal and adiabatic. Hence, 10 bar is a representative textbook value for “approximately” the end-of-compression pressure in SI engines.Step-by-Step Solution:Assume initial pressure ~1 bar (throttled manifold somewhat lower at part load).Apply polytropic compression with exponent between 1.3 and 1.4.For CR near 9–10, the resulting p2 commonly falls around ~10 bar in SI practice.Verification / Alternative check:Compression testers for petrol engines often report pressures roughly 9–14 bar depending on engine and test conditions, aligning with the 10 bar nominal figure.Why Other Options Are Wrong:20–35 bar: More typical of higher compression diesels or boosted cases; beyond standard SI expectations.Common Pitfalls:Confusing cranking compression gauge readings with true thermodynamic end-of-compression under hot, running conditions; however both are of similar order for healthy SI engines.Final Answer:10 bar

Question 10

Combustion mode in compression-ignition (diesel) engines: Select the correct description of the in-cylinder combustion process.

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Introduction / Context:Diesel (compression-ignition) combustion differs fundamentally from spark-ignition combustion. Understanding whether combustion is homogeneous or heterogeneous reveals why diesel engines emphasize spray formation, air motion, and mixing control.Given Data / Assumptions:Fuel is injected directly into hot compressed air near the end of the compression stroke.No premixed, fully homogeneous air–fuel charge exists before injection.Auto-ignition occurs after a short ignition delay as mixing proceeds.Concept / Approach:Diesel combustion is a diffusion-controlled process: liquid fuel is atomized into droplets, vaporizes, mixes with air locally, and burns where the mixture is within flammability limits. This inherently non-uniform mixture distribution makes the process heterogeneous. Some initial premixed burning can occur after ignition delay, but the sustained burn is mixing-controlled and spatially non-uniform.Step-by-Step Solution:Recognize that fuel is injected into air, not premixed uniformly.Note presence of distinct zones: liquid core, vapor, rich/lean pockets, and flame sheets.Conclude that the overall combustion process is heterogeneous (diffusion flame dominated).Verification / Alternative check:Measured heat release rates show an initial premixed spike followed by a longer diffusion burn, consistent with a heterogeneous mixing/combustion field.Why Other Options Are Wrong:Homogeneous: Better describes ideal SI with premixed charge (though even SI can be stratified in some modes).Both / none: Ambiguous; diesel is predominantly heterogeneous by standard classification.Common Pitfalls:Equating any premixed fraction in diesel with an overall homogeneous process; the dominant, sustained phase is mixing-controlled and heterogeneous.Final Answer:hetrogeneous

Question 11

Spark-ignition (petrol) engine basics In a conventional petrol (spark-ignition) engine, the combustible air–fuel charge inside the cylinder is initiated by an electric discharge produced by the spark plug. Is this statement correct?

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Introduction / Context:Petrol engines are also called spark-ignition (SI) engines because combustion is started by an electrical spark. Recognizing how ignition is initiated is fundamental for understanding timing, knock control, and the differences between SI and compression-ignition (diesel) engines.Given Data / Assumptions:Engine uses a premixed homogeneous air–fuel charge (gasoline/petrol).A spark plug is installed in the combustion chamber.Normal operating conditions without abnormal misfire.Concept / Approach:In SI engines, the ignition system (coil, plug, and controls) creates a high-voltage pulse that jumps the spark plug gap at a precisely chosen crank angle. The spark kernel grows into a flame front that propagates through the mixture. This is distinct from diesel engines, where fuel auto-ignites due to high temperature from compression and no spark plug is required for normal operation.Step-by-Step Solution:Identify engine type: petrol → spark-ignition principle applies.Ignition system generates a spark across the plug electrodes at set timing.Spark ignites the charge; therefore, the statement is correct.Verification / Alternative check:Workshop manuals and engine wiring diagrams always include a spark plug in gasoline engines; diagnostic trouble codes for misfire often relate to ignition components, confirming the essential role of the spark plug.Why Other Options Are Wrong:Limiting ignition to two-stroke, cold start, or high speed is incorrect; sparks are required throughout operation at virtually all loads and speeds in SI engines.Common Pitfalls:Confusing glow plugs (used in some diesels for cold starts) with spark plugs; glow plugs do not provide a spark and are not used in petrol engines.Final Answer:Yes

Question 12

Preventing knock in spark-ignition engines To reduce knocking tendency in the end-gas region far from the spark plug, which properties should that charge region ideally have?

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Introduction / Context:Knock in spark-ignition (SI) engines results from auto-ignition of the unburned end-gas ahead of the flame front. Reducing end-gas reactivity is central to knock control, protecting the engine and improving drivability and efficiency.Given Data / Assumptions:Premixed SI combustion initiated at the spark plug.End-gas conditions determine auto-ignition risk.Typical fuels with finite octane rating.Concept / Approach:Auto-ignition depends on temperature, pressure, and time for pre-flame chemistry. Lower density implies less mass and lower pressure rise; lower temperature slows chemistry; longer ignition delay (higher octane behavior) gives more time for the flame to consume the end-gas before it auto-ignites. Hence all three listed attributes reduce knock likelihood.Step-by-Step Solution:Relate knock to end-gas auto-ignition kinetics.Low temperature → slower reaction rates → delayed auto-ignition.Low density → lower local pressure/temperature rise → reduced reactivity.Long ignition delay → flame can arrive before spontaneous ignition.Verification / Alternative check:Empirical controls that create these conditions include charge cooling (intercoolers, water injection), retarded spark (reduces pre-TDC compression heating), and using higher-octane fuel (increases ignition delay).Why Other Options Are Wrong:Choosing only one factor ignores the multi-parameter nature of auto-ignition. Residual gases can both cool and dilute but also reduce oxygen; taken alone, it is not the complete answer.Common Pitfalls:Assuming lean mixtures always reduce knock; at high load, excessively lean operation can raise end-gas temperature due to slower burning and may not help.Final Answer:All of these

Question 13

Spark plug service data Typical spark plug electrode gap used in passenger automobiles is approximately within which range (in millimetres)?

Accepted Answer

Introduction / Context:The spark plug gap influences the voltage required to produce a reliable spark and the size of the initial flame kernel. Manufacturers specify a gap window to balance ignition reliability and plug life across operating conditions.Given Data / Assumptions:Modern passenger-car petrol engines with conventional plug designs.Normal ignition systems capable of 20–40 kV (or more) output.No special racing or ultra-lean applications.Concept / Approach:A common service range taught in automotive courses is approximately 0.4–0.9 mm. Smaller gaps ease firing at low voltage but can weaken the kernel; larger gaps improve kernel size but demand higher voltage and may misfire under high-pressure conditions. Electronic ignition and fine-wire electrodes permit the upper end of the range in many vehicles.Step-by-Step Solution:Identify mainstream recommended ranges from service literature.Compare with options: 0.4–0.9 mm fits typical OE specifications.Therefore select 0.4–0.9 mm as the representative range.Verification / Alternative check:Owner’s manuals and workshop data often specify values such as 0.7–0.9 mm or 0.6–0.8 mm; these lie within the broader 0.4–0.9 mm window, validating the choice.Why Other Options Are Wrong:Very small gaps (0.1–0.3 mm) are atypical; 0.6–1.8 mm is too wide at the upper end and would require impractically high voltage in-cylinder.Common Pitfalls:Gapping without accounting for cylinder pressure: boosted engines may need slightly smaller gaps to prevent blowout.Final Answer:0.4 to 0.9 mm

Question 14

Breeding in nuclear reactors A reactor that converts fertile material into fissile isotopes is properly called a breeder reactor, not a “regenerative” reactor. Is the statement using the term regenerative reactor correct?

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Introduction / Context:Terminology matters in nuclear engineering. Reactors designed to create more fissile material than they consume perform a process known as breeding, and the accepted term is breeder reactor.Given Data / Assumptions:Fertile nuclides such as U-238 or Th-232 can capture neutrons to form fissile nuclides (Pu-239 or U-233).Design objective is net production of fissile material (breeding ratio ≥ 1).Term “regeneration” is not the standard for this concept in reactor physics.Concept / Approach:Breeder reactors place fertile material in regions with adequate neutron flux so that neutron capture and subsequent decays produce fissile isotopes. The word “regenerative” is occasionally used loosely in other engineering fields but is not the precise nuclear term here.Step-by-Step Solution:Define the function: convert fertile → fissile via neutron capture.Identify the accepted name: breeder reactor.Conclude the provided term “regenerative reactor” is incorrect.Verification / Alternative check:Standard reactor engineering texts and glossaries consistently use “breeder” for such designs (e.g., fast breeder reactors and thorium-based thermal breeder concepts).Why Other Options Are Wrong:Limiting correctness to thermal or fast types misses the terminology issue; the naming is independent of spectrum.Common Pitfalls:Confusing breeder (produces fissile material) with regenerative feed heating in power plants—completely different concepts.Final Answer:No

Question 15

Comparing two-stroke and four-stroke engines Which of the following statements about two-stroke versus four-stroke engines is incorrect?

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Introduction / Context:Two-stroke (2S) and four-stroke (4S) engines differ in how often a power stroke occurs and in their mechanical complexity. Misconceptions are common regarding power output, mass, bulk, and cost trends.Given Data / Assumptions:Engines compared at similar displacement and speed.Conventional naturally aspirated designs.General market trends for cost: diesel vs petrol.Concept / Approach:In principle, a simple crankcase-scavenged two-stroke engine has one power stroke per revolution, whereas a four-stroke has one power stroke every two revolutions. Thus, for the same size and speed, the 2S can develop up to about twice the power (ignoring scavenging and thermal differences), not the 4S. Two-strokes also tend to be lighter and more compact for a given power. In common practice, diesel engines are more expensive than similar petrol engines due to higher compression ratios, injection hardware, and structural strength.Step-by-Step Solution:Evaluate (a): Reversed; 2S can produce roughly twice the firing frequency.Evaluate (b): Reversed; 2S are typically lighter/more compact for a given output.Evaluate (c): Reversed; diesels usually cost more than petrol engines.Therefore (d) is correct: all statements (a)–(c) are wrong.Verification / Alternative check:Textbook comparisons and manufacturer data support higher specific power and lower weight for simple 2S designs, and higher purchase price for diesels due to injection systems and robustness.Why Other Options Are Wrong:Option (e) ignores that (b) and (c) are also wrong.Common Pitfalls:Assuming modern emissions-compliant 2S designs always keep the historical power advantage; scavenging and aftertreatment can alter practical results, but the conceptual comparison holds.Final Answer:All of the above statements are wrong.

Question 16

Reflector performance in reactor physics Reflectivity of a reflector is defined as the fraction of neutrons that, after entering the reflector, are returned (scattered back) into the reactor core. Is this definition correct?

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Introduction / Context:Neutron reflectors surround many reactor cores to reduce neutron leakage, improving economy and flattening the flux. The metric used to describe how effectively a reflector returns neutrons is its reflectivity.Given Data / Assumptions:Reflector is a non-fueled region adjacent to the core.Neutrons migrate into the reflector via diffusion.Some neutrons scatter back; others are absorbed or escape.Concept / Approach:Reflectivity is indeed defined as the fraction of neutrons entering the reflector that return to the core. High-scattering, low-absorption materials (e.g., graphite, heavy water, beryllium) tend to have higher reflectivity. This improves neutron economy by reducing the required fuel loading for criticality or by boosting the effective multiplication factor.Step-by-Step Solution:Define neutron balance at the core–reflector interface.Reflectivity R = neutrons returned to core / neutrons entering reflector.Confirm the provided statement matches this definition.Verification / Alternative check:Diffusion-theory solutions with reflected boundary conditions show increased flux near the edge and an effective albedo that captures this “return fraction,” consistent with the verbal definition.Why Other Options Are Wrong:Limiting correctness to a specific material or fuel enrichment misunderstands that reflectivity is a property/metric applicable to any reflector material.Common Pitfalls:Confusing reflectivity with moderator effectiveness; a reflector can also moderate, but its primary role here is to return neutrons, not necessarily slow them to thermal energy.Final Answer:Agree

Question 17

Fuel detonation tendency (octane perspective) Among the following, which fuel detonates most readily (i.e., has the poorest knock resistance in an SI engine)?

Accepted Answer

Introduction / Context:Detonation (knock) in SI engines is inversely related to octane rating. The octane scale is anchored by two reference fuels: iso-octane and n-heptane, which set the extremes of knock resistance used for fuel testing and calibration.Given Data / Assumptions:Octane number of iso-octane is defined as 100.Octane number of normal heptane is defined as 0.Comparative evaluation under standard test conditions.Concept / Approach:A fuel with a lower octane number detonates more readily under compression and heat. Since normal heptane has the lowest octane in the set, it will knock first. Iso-octane is highly resistant to knock. Aromatics like benzene/toluene and alcohols typically have relatively high octane indices compared with n-heptane.Step-by-Step Solution:Identify reference endpoints on the octane scale.Compare listed fuels’ typical knock resistance.Select the poorest knock resistance: normal heptane.Verification / Alternative check:Standard ASTM methods (RON/MON) use blends of iso-octane and n-heptane to match knock behavior; n-heptane’s designation as 0 confirms its propensity to detonate readily.Why Other Options Are Wrong:Iso-octane, benzene/toluene, and alcohols have comparatively higher octane ratings; they resist knock better than n-heptane.Common Pitfalls:Assuming aromatics always promote knock due to soot formation; octane rating, not soot, dictates knock tendency in SI engines.Final Answer:Normal heptane

Question 18

Operating controls that reduce SI knocking Which single adjustment will reliably decrease knocking tendency in a spark-ignition engine (all else equal)?

Accepted Answer

Introduction / Context:Engine knock control uses multiple levers. Some adjustments reduce end-gas temperature/pressure or shorten residence time, lowering the chance of auto-ignition. Understanding which operational change helps is critical for diagnosis and calibration.Given Data / Assumptions:Conventional SI engine with fixed compression ratio.Same load and fuel unless otherwise stated.We compare single parameter changes.Concept / Approach:Retarding spark timing delays peak pressure closer to or just after top dead center, reducing end-gas temperature and pressure before spontaneous ignition can occur. Conversely, advancing timing, raising intake temperature, increasing compression ratio, or lowering fuel octane all increase knock propensity by elevating end-gas reactivity or pressure–temperature history.Step-by-Step Solution:Consider the end-gas: reduce pre-flame heating and pressure → reduce knock.Retard timing → lower pre-TDC compression heating → longer safe delay.Other listed changes all worsen end-gas conditions, increasing knock likelihood.Verification / Alternative check:Knock sensors and closed-loop ignition control in modern ECUs routinely retard timing upon knock detection, proving its effectiveness as a first-line mitigation.Why Other Options Are Wrong:Advancing timing, higher compression, higher intake temperature, or lower octane each increases end-gas reactivity or load, thus promoting knock.Common Pitfalls:Over-retarding reduces efficiency and increases exhaust temperature; it is a mitigation, not a cure for fundamental design/fuel mismatches.Final Answer:Retarding the ignition timing

Question 19

Power strokes in a two-stroke engine For a two-stroke cycle engine, the number of working (power) strokes per minute is __________ the engine speed in revolutions per minute (r.p.m.).

Accepted Answer

Introduction / Context:Cycle timing directly determines how often an engine delivers a power stroke. This affects specific power and helps explain why two-stroke engines achieve higher firing frequency than four-strokes of the same speed.Given Data / Assumptions:Ideal two-stroke cycle: one power stroke per crank revolution.Engine speed = N r.p.m.No misfires and steady operation.Concept / Approach:In a two-stroke engine, the cycle completes in two strokes (one revolution). Therefore, each revolution includes a power event. Consequently, the number of power strokes per minute equals the rotational speed in revolutions per minute.Step-by-Step Solution:Two-stroke: 1 cycle = 1 revolution.Each cycle includes a power stroke.Hence power strokes per minute = N.Verification / Alternative check:By contrast, a four-stroke requires two revolutions per cycle, yielding N/2 power strokes per minute—useful as a check against the two-stroke result.Why Other Options Are Wrong:Half, twice, or four times do not match the one-power-stroke-per-revolution nature of two-strokes.Common Pitfalls:Confusing “firing events” with “cylinder count” in multi-cylinder engines. Total engine firings per minute equal per-cylinder firings multiplied by the number of cylinders.Final Answer:equal to

Question 20

Effect of intake air temperature on engine efficiency In internal combustion engines operating at fixed hardware (same compression ratio and calibration), increasing the intake air temperature will generally __________ the overall efficiency.

Accepted Answer

Introduction / Context:Intake air temperature influences charge density, knock tendency, and pumping losses. Knowing the direction of its effect on efficiency helps interpret intercooling benefits and hot-weather performance changes.Given Data / Assumptions:Hardware unchanged (same compression ratio and combustion system).Throttle and calibration adjust to maintain load.Comparison over typical operating conditions (SI focus, but principles also affect CI engines).Concept / Approach:Hotter intake air is less dense, so less oxygen mass enters for a given manifold pressure. To achieve the same torque, the throttle must open further in SI engines, but the lower charge density typically reduces volumetric efficiency and can increase propensity to knock (forcing spark retard), both of which reduce thermal efficiency. In turbocharged engines, intercooling (cooling the intake air) commonly increases efficiency by improving charge density and allowing more optimal spark or fueling.Step-by-Step Solution:Raise intake temperature → reduce air density at the same pressure.Lower trapped mass → lower indicated efficiency or spark must be retarded to avoid knock.Net effect: overall efficiency tends to decrease.Verification / Alternative check:Practical data: intercoolers in turbo/supercharged engines measurably improve brake specific fuel consumption by cooling the intake; the inverse (heating) degrades it.Why Other Options Are Wrong:“Increase” contradicts general observations; “no effect” would only hold in an ideal Otto analysis where efficiency depends solely on compression ratio, ignoring real-world volumetric, timing, and knock constraints.Common Pitfalls:Confusing ideal-cycle dependence (efficiency vs compression ratio only) with real-engine behavior where volumetric efficiency and knock play crucial roles.Final Answer:decrease

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