Question 1

Spark-ignition (petrol) engines – identify the component that does not typically belong Among the following components commonly discussed for fuel–air preparation and ignition in spark-ignition engines, which one is NOT characteristically associated…

Accepted Answer

Introduction / Context: This question checks your understanding of the classic hardware of spark-ignition (SI) engines used for mixture preparation and ignition. While modern SI engines may use electronic fuel injection, historically the canonical SI setup included a carburettor, an ignition coil, a distributor, and spark plugs. Recognizing which component is not inherently tied to the traditional SI configuration prevents confusion between SI and compression-ignition (diesel) technologies. Given Data / Assumptions: Focus on the traditional SI engine layout as taught in foundational automotive engineering. Components considered: ignition coil, spark plug, carburettor, fuel injector, distributor. Goal: pick the item that does not characteristically belong to the classic SI set. Concept / Approach: Conventional SI engines: the ignition coil steps up battery voltage; the distributor times and routes the high tension; spark plugs ignite the premixed charge; and a carburettor meters fuel into the incoming air. Fuel injectors are more closely associated historically with diesel engines (which inject fuel directly into compressed hot air). Although many modern SI engines use port or direct fuel injection, the question contrasts traditional SI versus CI hardware. Step-by-Step Solution: List items that are core to classic SI: ignition coil, spark plug, carburettor, distributor.Identify the outlier in the classic context: fuel injector.Therefore, select “Fuel injector.” Verification / Alternative check: Historical textbooks and service manuals show carburetted SI systems with ignition coil, distributor, and spark plugs; fuel injection appears primarily in diesel and later-generation SI systems, not in the earliest/typical examples. Why Other Options Are Wrong: Ignition coil: essential to step up voltage for spark. Spark plug: necessary to initiate combustion in SI. Carburettor: traditional SI mixture device before EFI. Distributor: routes and times spark in classic systems. Common Pitfalls: Assuming that because modern SI uses injectors, injectors are “the” SI hallmark. The question context is traditional SI hardware, where injectors were not standard. Final Answer: Fuel injector

Question 2

Nuclear fuel forms – solid fuel shapes used to build fuel elements Solid reactor fuel can be fabricated into small shapes and assembled into fuel elements. In practice, which form(s) are used?

Accepted Answer

Introduction / Context: Nuclear power reactors employ solid fuel in standardized shapes that fit the core lattice, allow coolant flow, and meet structural and thermal limits. Understanding common fuel forms helps connect fuel fabrication, core design, and thermal-hydraulic performance. Given Data / Assumptions: We are discussing solid fuels (e.g., ceramic UO2). Common shapes include pellets, pins/rods, and plates. The question asks which of the listed forms are used to assemble fuel elements. Concept / Approach: Many reactor types use different fuel geometries: Light Water Reactors (LWRs) use cylindrical pellets stacked inside long cladding tubes forming pins/rods; these pins are assembled into fuel assemblies. Research and some heavy-water or gas-cooled reactors use plate-type fuel. Hence, multiple forms are valid and used in real designs. Step-by-Step Solution: Identify prevalent forms: pellets (LWRs), pins/rods (assemblies), plates (research reactors, certain designs).Since each listed shape is used in some reactor type, choose the inclusive option.Therefore, “All of these” is correct. Verification / Alternative check: Fuel assembly diagrams for PWR/BWR show pellets and pins; material test reactors often use plate fuel with aluminum cladding matrices. Why Other Options Are Wrong: Plates, pellets, and pins alone are incomplete; all exist in practice. “Only spheres” refers to pebble-bed designs, which are not among the listed forms. Common Pitfalls: Equating “pellet” with “fuel element”; pellets are inside cladding tubes that form fuel pins, which are then built into assemblies. Final Answer: All of these

Question 3

Diesel (CI) vs. petrol (SI) engines at rated load A diesel engine is ________ in efficiency compared to a petrol engine when both run at rated load.

Accepted Answer

Introduction / Context: Comparing thermodynamic efficiencies of compression-ignition (diesel) and spark-ignition (petrol) engines is a staple topic in automotive engineering. Diesels typically achieve higher brake thermal efficiency under rated load conditions. Given Data / Assumptions: Both engines operate at rated load and similar displacement class. Standard fuels: diesel vs. petrol. Focus on brake thermal efficiency, not transient or idle behavior. Concept / Approach: Diesel engines run at higher compression ratios and avoid throttling the intake air, reducing pumping losses. The diesel cycle’s ideal efficiency rises with compression ratio, and practical combustion with lean mixtures further improves fuel economy. Consequently, diesels generally consume less fuel energy per unit output at rated load than petrol engines. Step-by-Step Solution: Higher compression ratio → higher theoretical efficiency.Unthrottled intake → lower pumping work at load.Lean operation and high energy conversion → higher brake thermal efficiency.Therefore, select “more efficient.” Verification / Alternative check: BSFC charts show diesel engines typically in the 190–230 g/kWh range at rated points, versus higher values for comparable SI engines. Why Other Options Are Wrong: Equally efficient: contradicts empirical data. Less efficient: opposite of standard results. Random or “only at idle” are not meaningful or accurate comparisons. Common Pitfalls: Mixing operating cost with capital cost or emissions constraints; the question targets efficiency at rated load. Final Answer: more efficient

Question 4

Spark-ignition engines – typical ignition delay period In conventional petrol (spark-ignition) engines, the ignition delay (chemical delay before noticeable pressure rise) is typically of the order of:

Accepted Answer

Introduction / Context: Ignition delay in SI engines refers to the short time interval between spark discharge and the onset of rapid flame development/pressure rise. Understanding its magnitude helps in calibrating ignition timing and avoiding knock or misfire. Given Data / Assumptions: Conventional gasoline SI engine with homogeneous charge. Normal operating temperature and mixture near stoichiometric. We focus on the chemical delay, not total combustion duration. Concept / Approach: After the spark ignites a small kernel, a brief chemical-kinetic delay occurs before a self-sustaining flame front establishes. Typical ignition delays in SI engines are on the order of milliseconds, often around 1 millisecond (0.001 s), depending on pressure, temperature, and mixture. Step-by-Step Solution: Estimate typical delay scale from engine-speed–time scales (milliseconds).Common reference values: ~0.5–2 ms, often approximated as 0.001 s.Hence, choose “0.001 second.” Verification / Alternative check: Optical diagnostics and pressure traces confirm sub-2 ms ignition kernel development under standard conditions in many SI engines. Why Other Options Are Wrong: Higher values (0.003–0.010 s) are too long for typical SI delay at normal conditions. 0.002 s can occur in adverse conditions, but 0.001 s is the usual textbook order-of-magnitude. Common Pitfalls: Confusing SI ignition delay (milliseconds) with diesel ignition delay, which can be longer and strongly load/temperature dependent. Final Answer: 0.001 second

Question 5

Radiation shielding – most commonly used bulk material Considering practicality, cost, and performance in power plant shielding, which material is most commonly used for radiation shielding structures?

Accepted Answer

Introduction / Context: Radiation shielding protects personnel and equipment by attenuating neutrons and photons (gamma). While different materials excel for specific radiation types, large-scale power plant shielding prioritizes availability, structural capability, and cost. Given Data / Assumptions: Application: nuclear power plant bulk shielding (biological shield, containment structures). Radiation types: fast/thermal neutrons and gamma rays. Constraints: cost, constructability, mechanical strength. Concept / Approach: Concrete is the most widely used bulk shielding material in nuclear plants. Its hydrogenous content (from water) helps moderate and attenuate neutrons, while its density provides gamma attenuation. It is inexpensive, pourable into complex forms, and structurally robust. Lead is excellent for gamma shielding in limited thickness but is heavy, expensive, and unsuitable for huge structures. Graphite and polymers have specialized uses but are not the dominant bulk choice. Step-by-Step Solution: List candidate materials and typical uses.Match to large-scale plant shielding needs → concrete satisfies most criteria.Therefore, select “Concrete.” Verification / Alternative check: Plant drawings and specifications show thick concrete biological shields around the reactor vessel and spent fuel pools. Why Other Options Are Wrong: Lead: effective per thickness for gamma, but impractical and costly for bulk structural shields. Carbon/graphite: used as moderator/reflector in some designs, not as prevailing bulk shielding. “All equally” and “polyethylene only” are inaccurate generalizations. Common Pitfalls: Assuming lead must be most common due to high Z; cost and constructability make concrete the standard bulk shield. Final Answer: Concrete

Question 6

Neutron reflectors – factors affecting reflectivity The effectiveness (reflectivity) of a neutron reflector in a reactor depends primarily on which of the following?

Accepted Answer

Introduction / Context: Neutron reflectors are placed around a reactor core to reduce neutron leakage, improve utilization, and flatten power distributions. Reflector performance depends on several interacting factors that influence scattering and absorption. Given Data / Assumptions: Steady reactor operation. Reflector materials such as graphite, beryllium, water, or steel. Interest in overall reflectivity (return of neutrons into the core). Concept / Approach: Reflectivity is governed by (1) material properties (scattering cross section, absorption cross section, atomic mass), (2) geometry and thickness (more mean free paths increase return probability), and (3) neutron energy (fast vs. thermal scattering behavior). Therefore, it is not a single-parameter phenomenon. Step-by-Step Solution: Consider material: low absorption, high scattering improves reflection.Consider geometry: sufficient thickness and proper shape minimize leakage paths.Consider energy: moderation and back-scatter depend on neutron energy spectrum.Therefore, “All of these” control reflectivity. Verification / Alternative check: Core design calculations and diffusion/transport theory solutions show reflector savings dependent on both material buckling and reflector thickness. Why Other Options Are Wrong: Any single factor alone is incomplete; real reflectivity arises from their combination. “None” is clearly incorrect. Common Pitfalls: Overlooking neutron energy effects; scattering angles and lethargy change per collision matter to return probability. Final Answer: All of these

Question 7

Diesel fuel rating – low reference on the cetane scale Alpha-methylnaphthalene (C11H10) historically defines the low end of the cetane scale. Its assigned cetane number is:

Accepted Answer

Introduction / Context: The cetane number rates a diesel fuel’s tendency to auto-ignite easily (short ignition delay). The scale is anchored by two reference fuels that set the endpoints of ignition quality for standardized testing. Given Data / Assumptions: Reference high: n-hexadecane (cetane) = 100. Historical reference low: alpha-methylnaphthalene = 0 (modern low reference may use isocetane, CN 15, but the classic exam convention remains 0 for alpha-methylnaphthalene). Question targets the historical definition commonly used in textbooks. Concept / Approach: Alpha-methylnaphthalene exhibits very poor ignition quality with long ignition delay, establishing the zero point of the original cetane scale. Real diesel fuels are compared by matching their ignition delay to blends of the reference fuels. Step-by-Step Solution: Recall reference endpoints.Match material to endpoint: alpha-methylnaphthalene → low end.Assigned number: 0. Verification / Alternative check: Standards literature notes the historical use of alpha-methylnaphthalene (CN 0) and the later substitution of isocetane (CN 15) for practical reasons; exam problems typically retain CN 0 for alpha-methylnaphthalene. Why Other Options Are Wrong: 100 is the value for cetane (n-hexadecane), the high reference fuel. 50, 120, or 15 are not the classic assigned value for alpha-methylnaphthalene in this context. Common Pitfalls: Confusing updated testing practices with the classical exam convention; the question explicitly names alpha-methylnaphthalene. Final Answer: 0

Question 8

Purpose of reflectors in nuclear reactors In power reactors, installing a reflector layer around the core primarily ________ neutron leakage.

Accepted Answer

Introduction / Context: Neutron reflectors surround the active core region to improve neutron economy. By scattering escaping neutrons back into the fuel region, reflectors reduce leakage and can enhance reactivity and fuel utilization. Given Data / Assumptions: A reflector made of suitable material (e.g., graphite, water, beryllium) is placed outside the core. Steady-state reactor operation is assumed. Interest: net effect on neutron leakage. Concept / Approach: Reflectors present additional scattering paths beyond the core boundary, increasing the probability that outward-moving neutrons are redirected inward before being lost. This effectively decreases net leakage and provides reflector savings in core design (allowing a smaller critical core or flatter flux). Step-by-Step Solution: Consider boundary without reflector → higher leakage probability.Add reflector → more back-scattering and moderation.Therefore, leakage decreases with an effective reflector. Verification / Alternative check: Diffusion theory solutions show reduced neutron current out of the boundary when reflector regions are included; measured flux maps confirm higher edge flux and lower leakage. Why Other Options Are Wrong: Increase/no effect contradict typical reflector behavior. “Randomly changes” is not consistent with predictable transport physics. Reversing flow “permanently” is not meaningful in this context. Common Pitfalls: Assuming any material works as a reflector; absorption cross section matters. Poor reflector materials could reduce benefits, but proper reflectors decrease leakage. Final Answer: decreases

Question 9

Air-standard cycle efficiency – dominant parameter For idealized air-standard analyses of internal combustion engine cycles, the thermal efficiency depends primarily on which of the following?

Accepted Answer

Introduction / Context: Air-standard cycle models (Otto, Diesel, Dual) provide insight into how design parameters influence theoretical efficiency. Among many variables, compression ratio is the key determinant of thermal efficiency in these idealized cycles. Given Data / Assumptions: Air-standard assumptions: working fluid is air with constant properties. Comparing ideal thermal efficiencies, not real-engine indicated or brake efficiencies. Same cycle type unless specified otherwise. Concept / Approach: For the Otto cycle, efficiency = 1 − 1/(r^(γ−1)), directly showing dependence on compression ratio r. Diesel and dual cycles also exhibit strong efficiency increases with r, though with additional dependence on cut-off ratio. Fuel type and speed do not appear explicitly in the ideal efficiency expressions. Step-by-Step Solution: Write the ideal Otto efficiency form: η = 1 − 1/(r^(γ−1)).Observe r as the controlling parameter; fuel identity and speed are absent.Therefore, select “Compression ratio.” Verification / Alternative check: Plotting η versus r for γ ≈ 1.4 shows strong monotonic increase; similar trends hold for Diesel/Dual with their respective parameters. Why Other Options Are Wrong: Fuel type and speed affect real engines but not the idealized air-standard thermal efficiency. Chamber color and ambient humidity are irrelevant in the ideal model. Common Pitfalls: Confusing real-world knock limits (which cap allowable compression ratio) with the ideal mathematical dependence. Final Answer: Compression ratio

Question 10

Meaning of a higher octane number for SI fuel A higher octane number for spark-ignition (petrol) fuel primarily indicates that the fuel has:

Accepted Answer

Introduction / Context: The octane number rates a gasoline’s knock resistance in spark-ignition engines. Knock arises from premature auto-ignition of the end-gas ahead of the flame front, producing damaging pressure oscillations. A higher octane rating means the fuel better resists auto-ignition under compression and temperature. Given Data / Assumptions: Standard test methods (RON/MON) assess knock resistance under controlled conditions. Target attribute: resistance to auto-ignition, not energy content. We compare qualitative implications of octane number. Concept / Approach: High-octane fuels lengthen the chemical induction time (ignition delay) of the end-gas under engine conditions, allowing the flame to consume the mixture before spontaneous ignition can occur. This enables higher compression ratios or more advanced spark timing without knock. Octane rating does not directly correlate with heating value, flash point, or volatility across fuels. Step-by-Step Solution: Define octane: measure of knock resistance.Relate to chemistry: longer ignition delay in end-gas → less tendency to auto-ignite.Implication: permits higher performance settings with reduced knock risk. Verification / Alternative check: Engine calibration practice advances spark and/or compression ratio with higher octane fuels, consistent with increased knock margin (longer delay to auto-ignition). Why Other Options Are Wrong: Heating value is largely independent of octane rating. Flash point and volatility specifications are separate properties. Sulfur content is an impurity limit, unrelated to octane rating. Common Pitfalls: Assuming “premium” fuel has more energy; performance gains are from knock resistance, not higher calorific value. Final Answer: a longer ignition delay (greater resistance to auto-ignition)

Question 11

Overall (Brake Thermal) Efficiency – Definition for Heat Engines For an internal combustion engine, overall efficiency is defined as the ratio of useful work obtained at the crankshaft over a given time to the total fuel energy supplied in the same …

Accepted Answer

Introduction / Context: Overall efficiency, often called brake thermal efficiency, is a cornerstone metric for heat engines. It compares the useful shaft output delivered at the crankshaft to the chemical energy supplied by the fuel during the same duration. This provides a practical measure of how effectively an engine converts fuel energy into mechanical work that can drive a vehicle or generator. Given Data / Assumptions: Useful output is brake power measured at the crankshaft via a dynamometer. Energy input is fuel mass flow multiplied by lower calorific value over the same time. Steady-state operation during the measurement interval. Concept / Approach: By definition, overall efficiency evaluates the entire energy conversion chain inside the engine, including combustion, thermodynamic cycle, and mechanical transmission to the shaft. It therefore uses brake power in the numerator, not indicated power. Using the supplied fuel energy in the denominator ensures the metric accounts for all losses including incomplete combustion, heat rejection, and frictional losses between piston and crankshaft. Step-by-Step Solution: Let W_out be crankshaft work during time t; let E_in be fuel energy supplied in time t.Overall efficiency: eta_overall = W_out / E_in.In rate form: eta_overall = B.P. / (m_dot_f * LCV).Since crankshaft work and supplied fuel energy are measured over the same time, the given definition is correct. Verification / Alternative check: Brake specific fuel consumption provides the inverse relationship: bsfc = fuel flow / brake power. Using bsfc, eta_overall can be cross-checked as eta_overall = 1 / (bsfc * LCV) with consistent units, confirming the same concept. Why Other Options Are Wrong: Limiting the definition to diesel or full load is unnecessary; it applies to any heat engine and at any load. Supercharging does not alter the definition, only the values. Common Pitfalls: Confusing overall efficiency with indicated thermal efficiency (which uses indicated power) or with mechanical efficiency (which is B.P. / I.P.). Another pitfall is mixing time bases; numerator and denominator must correspond to the same interval. Final Answer: True

Question 12

Spark-Ignition Fuels – Tendency Toward Detonation (Knock) Which of the following fuels exhibits the least tendency to detonate in a typical spark-ignition engine under the same operating conditions?

Accepted Answer

Introduction / Context: Detonation, or knock, is uncontrolled auto-ignition of the end-gas ahead of the flame front in spark-ignition engines. Fuel octane quality indicates resistance to this auto-ignition. Understanding which fuels resist knock is vital for compression ratio selection, ignition timing, and overall engine calibration. Given Data / Assumptions: Comparison is at equal compression ratio, mixture strength, and intake temperature. Reference octane scale uses iso-octane (100) and normal heptane (0). Alcohols can have high octane, but the standard reference for zero knock tendency in the scale is iso-octane. Concept / Approach: Iso-octane (2,2,4-trimethylpentane) is the 100-point standard on the octane scale and represents high knock resistance. Normal heptane defines the 0-point and has a very strong knock tendency. Aromatics like benzene and toluene have complex effects; while toluene boosts octane in blends, iso-octane remains the canonical answer for least detonation tendency in basic theory questions because it anchors the octane rating at 100 under standard test conditions. Step-by-Step Solution: Identify octane reference fuels: iso-octane (100), n-heptane (0).Rank knock resistance: iso-octane > benzene/toluene (context dependent) > n-heptane.Select the option with the least tendency to detonate: iso-octane. Verification / Alternative check: Standard CFR (Cooperative Fuel Research) engine tests define the octane number scale. Under those conditions, iso-octane resists knock by definition, confirming the choice. Why Other Options Are Wrong: Normal heptane knocks readily (octane 0). Benzene and toluene can contribute to octane in blends but are not the test standard for least tendency. Alcohols have high octane, but the question expects the canonical reference fuel answer. Common Pitfalls: Confusing research octane number with motor octane number, and overlooking that the multiple-choice convention typically looks for the reference fuel iso-octane as the least knock-prone option. Final Answer: Iso-octane

Question 13

Octane Scale – Standard Value for Iso-octane (C8H18) On the primary reference fuel octane scale, what octane number is assigned to iso-octane (2,2,4-trimethylpentane, C8H18)?

Accepted Answer

Introduction / Context: The octane number quantifies a spark-ignition fuel’s resistance to knock. It is defined by comparing a test fuel to blends of two primary reference fuels in a standard CFR engine. Knowing the assigned numbers for these references is basic and critical for interpreting fuel specifications at the pump and for engine calibration. Given Data / Assumptions: Primary reference fuels: iso-octane and normal heptane. Standard test conditions and procedures (RON or MON) apply. Octane number is an index, not a direct physical property. Concept / Approach: By definition, iso-octane is assigned an octane number of 100, while normal heptane is assigned 0. A test fuel that behaves like a blend of X percent iso-octane and (100 − X) percent n-heptane will have an octane number X. Thus, iso-octane anchors the high end of the traditional scale. Step-by-Step Solution: Recall reference points: ON(iso-octane) = 100, ON(n-heptane) = 0.Therefore, the assigned value for iso-octane is 100.Choose the corresponding option. Verification / Alternative check: Fuel standards and engine testing literature consistently cite iso-octane as the 100 point, confirming this textbook value. Why Other Options Are Wrong: Values 0 or 50 contradict the defined scale; 120 exceeds the classic primary reference fuel range; 95 is a common pump octane for commercial fuels, not the reference assignment. Common Pitfalls: Confusing research octane number (RON) with pump octane (R+M)/2; the reference assignments remain the same for both testing methods. Final Answer: 100

Question 14

Reactor Physics – Fuel Enrichment and Core Compactness Consider a thermal nuclear reactor. If the fuel is richer in fissionable material (higher fraction of fissile atoms), can the critical reactor core be made more compact compared to fuel with a l…

Accepted Answer

Introduction / Context: Reactor core size and geometry are governed by neutron economy. The balance between neutron production, absorption, and leakage determines whether a system reaches criticality. Fuel enrichment—raising the fraction of fissile isotopes like U-235—directly influences this balance and therefore affects how compact a core can be while remaining critical and controllable. Given Data / Assumptions: Thermal reactor with a suitable moderator (e.g., light water). Comparison at similar power density constraints and materials. Goal is to understand critical size trends with fissile content. Concept / Approach: Increasing fissile atom density raises the probability that a thermalized neutron causes fission rather than being captured non-productively or leaking out. As the macroscopic fission cross-section grows, the required neutron path length to sustain the chain reaction shrinks. This permits a reduction in geometric buckling requirements and allows a smaller core to achieve k_eff = 1 with acceptable margins, provided heat removal and structural limits are respected. Step-by-Step Solution: Recognize that critical size depends on production vs. losses (absorption + leakage).Higher fissile density increases neutron production rate per unit volume.With better neutron economy, a smaller core can remain critical.Therefore, a more compact reactor is feasible with richer fissile fuel. Verification / Alternative check: Practical designs move from natural uranium (large cores with specific moderators) to low-enriched uranium (smaller cores). Research reactors with high enrichment achieve very compact cores, illustrating the principle. Why Other Options Are Wrong: Moderator removal in thermal reactors would worsen moderation; enrichment does not eliminate the need for moderation. Fast reactors are a different regime; the statement applies broadly, not exclusively to fast systems. Leakage certainly changes with size; enrichment helps counteract leakage by boosting fission probability, so saying leakage is unaffected is incorrect. Common Pitfalls: Ignoring thermal-hydraulic limits: even if criticality allows a compact core, heat flux and material limits may constrain minimum size. Criticality alone does not set all design dimensions. Final Answer: Agree

Question 15

Air–Fuel Mixture Terminology – What Is the Stoichiometric Ratio? Choose the best definition of the stoichiometric ratio used in internal combustion engines.

Accepted Answer

Introduction / Context: The stoichiometric air–fuel ratio is fundamental for engine calibration and emissions. It is the proportion at which exactly enough oxygen is supplied to completely oxidize the fuel, leaving no excess air and no unburnt fuel. For gasoline, this is approximately 14.7–15:1 by mass; for diesel it is much higher but engines usually operate lean of stoichiometric. Given Data / Assumptions: Definition is mass based unless explicitly stated otherwise. Complete combustion to CO2 and H2O is assumed. No allowance for dissociation or residuals in the simplified definition. Concept / Approach: Combustion chemical equations relate fuel molecules to required oxygen molecules. Because fuel metering and emissions limits are typically treated on a mass basis, the conventional definition of stoichiometric ratio in engines is by weight. Volumetric ratios can vary widely with temperature and pressure and are less standard for defining stoichiometry in engine practice. Step-by-Step Solution: Write balanced equation for a representative fuel (e.g., C8H18) and determine O2 requirement.Convert oxygen requirement to equivalent air mass using oxygen fraction in air.Compute air mass required per unit fuel mass to obtain the stoichiometric value.Conclude that the definition is the chemically correct air–fuel ratio by weight. Verification / Alternative check: Closed-loop control with lambda sensors targets lambda = 1, corresponding to the mass-based stoichiometric ratio for the given fuel. This operational practice confirms the mass-based definition. Why Other Options Are Wrong: Volume-based statements are not standard for stoichiometric definition in engines. Maximum efficiency and maximum power typically occur at mixtures different from stoichiometric. Cold start enrichment is a separate strategy and not a definition. Common Pitfalls: Mixing up lambda (relative ratio) with the absolute AFR; confusing stoichiometric with best economy or best power mixtures. Final Answer: chemically correct air–fuel ratio by weight

Question 16

Diesel (Compression-Ignition) Intake Process – What Is Drawn In During Suction? During the suction stroke of a conventional diesel engine, which working fluid is inducted through the intake valve into the cylinder?

Accepted Answer

Introduction / Context: Compression-ignition engines differ fundamentally from spark-ignition engines in the way fuel and air enter the cylinder. Recognizing what is drawn in during the suction stroke is critical for understanding diesel combustion, emissions formation, and turbocharging strategies. Given Data / Assumptions: Conventional four-stroke diesel engine with direct fuel injection. Fuel injection occurs near the end of the compression stroke. EGR may be present in small fractions but does not replace the fundamental intake of fresh air. Concept / Approach: In diesels, only air is inducted during the intake stroke. The piston then compresses this air to a high temperature and pressure. Near top dead center, injectors spray atomized fuel directly into the hot, compressed air, where it auto-ignites. There is no premixed air–fuel charge drawn in during suction, unlike carburetted petrol engines. Step-by-Step Solution: Intake valve opens → cylinder fills primarily with fresh air.Compression raises air temperature well above fuel auto-ignition temperature.Fuel is injected late in compression and ignites without a spark.Therefore, during suction the correct answer is air only. Verification / Alternative check: Cycle diagrams and timing charts confirm that fuel injection is a compression event, not an intake event. Any EGR is mixed with the incoming air but fresh air remains the primary inducted medium. Why Other Options Are Wrong: Diesel only or a diesel–air premix is incorrect; diesels do not intake premixed fuel. Exhaust-only statements do not describe a working induction process. Common Pitfalls: Confusing diesel operation with spark-ignition or with two-stroke gasoline port-injection strategies. Also, assuming EGR replaces air rather than being a small, controlled fraction of the intake mixture. Final Answer: air only

Question 17

Engine Governing Methods – Which Applies to Compression-Ignition (Diesel) Engines? Compression-ignition engines control load and speed primarily using which governing method?

Accepted Answer

Introduction / Context: Governing is the method by which engine output is regulated to meet load demands while maintaining speed. Different engine types achieve governing differently depending on how the charge is prepared and ignited. Understanding the correct approach for diesel engines helps explain their superior part-load efficiency and control strategies. Given Data / Assumptions: Compression-ignition engine with direct injection of fuel into air. Airflow is largely unthrottled; load is controlled by fuel quantity. Governor adjusts injected fuel mass per cycle to control speed. Concept / Approach: Quantitative governing varies the total quantity of fuel injected while the amount of air is essentially constant for a given speed and boost. This is the hallmark of diesel operation. In contrast, qualitative governing (varying mixture strength) is typical of early gas engines and some SI systems, while hit-and-miss is associated with small, old stationary engines where firing cycles are skipped to control speed. Step-by-Step Solution: Note that diesel engines induct only air and inject fuel later.Therefore, output is controlled by changing injected fuel mass per cycle.This is quantitative governing.Select the corresponding option. Verification / Alternative check: Governor linkages or electronic control units adjust injection duration or rail pressure to vary fuel per stroke, directly demonstrating quantitative governing in practice. Why Other Options Are Wrong: Qualitative governing varies mixture quality, not applicable to diesel intake. Hit-and-miss is obsolete for modern diesels. A combination is not the standard diesel approach. Common Pitfalls: Confusing throttle-based SI control with diesel control; assuming diesels regulate air mass the same way as gasoline engines. Final Answer: quantitative governing (varying quantity of fuel admitted)

Question 18

Comparative Thermal Efficiency – Petrol (SI) Versus Diesel (CI) Engines In general, how does the thermal efficiency of a petrol (spark-ignition) engine compare with that of a diesel (compression-ignition) engine of similar size and technology?

Accepted Answer

Introduction / Context: Thermal efficiency indicates how much of the fuel’s chemical energy is converted into useful shaft work. Comparing spark-ignition (SI) and compression-ignition (CI) engines reveals key design trade-offs that drive vehicle choice, fuel economy, and emissions outcomes. Given Data / Assumptions: Similar displacement and technology level (e.g., naturally aspirated or similarly boosted). Comparable operating conditions. No hybridization or unusual cycles. Concept / Approach: Diesel engines typically achieve higher thermal efficiency than petrol engines due to several factors: higher compression ratios enabled by auto-ignition, lean operation which reduces throttling losses at part load, higher ratio of specific heats during expansion, and better effective expansion utilization. Petrol engines are knock-limited and often operate throttled at part load, which increases pumping work and lowers efficiency. Step-by-Step Solution: Note CI engines have higher compression ratio → higher cycle efficiency.Diesels operate with excess air → lower pumping losses and lower peak temperatures for a given load.Therefore, diesel thermal efficiency exceeds that of petrol under comparable conditions.Conclusion: petrol is less efficient than diesel in general. Verification / Alternative check: Typical brake thermal efficiency ranges: modern light-duty SI about 0.30–0.36; comparable CI about 0.38–0.45. These empirical values support the stated trend. Why Other Options Are Wrong: “Same” and “cannot be compared” ignore consistent empirical and theoretical trends. “More” contradicts both cycle analysis and data. Idle is not a meaningful benchmark for efficiency comparison. Common Pitfalls: Conflating specific power or transient response with efficiency; they are distinct metrics. Also, turbocharging can raise both engines’ efficiencies but does not reverse the general ranking. Final Answer: less

Question 19

Engine Governing Methods – Which Applies to Spark-Ignition Engines? Spark-ignition engines are primarily governed by which of the following methods under traditional control systems?

Accepted Answer

Introduction / Context: Governing methods describe how an engine’s output is regulated. Historically, gas-fuelled and early carburetted spark-ignition engines have used different strategies from diesels because fuel and air are premixed before entering the cylinder. Understanding the classic textbook distinction clarifies multiple-choice questions that contrast SI and CI control philosophies. Given Data / Assumptions: Conventional SI engines with carburetion or gaseous fuel mixers. No electronic fuel injection strategies considered for the basic definition. Comparative framing versus diesel governing (quantitative). Concept / Approach: In classic terminology, SI engines are said to employ qualitative governing, in which the mixture quality (fuel–air ratio) is varied to control power while the charge quantity per cycle is approximately maintained. By contrast, diesels employ quantitative governing by changing the amount of fuel injected into roughly constant air mass. Although modern throttle-based SI systems also vary charge quantity, the conventional exam answer associates SI with qualitative governing and CI with quantitative governing. Step-by-Step Solution: Recall textbook pairing: SI → qualitative; CI → quantitative.Recognize that early gas engines altered mixture strength to regulate output.Select option describing qualitative governing for SI engines.Note: contemporary control may blend methods, but the classical definition persists in exam contexts. Verification / Alternative check: Historic governing mechanisms on gas engines varied fuel admission relative to air, changing mixture richness while keeping volume similar, illustrating qualitative governing. Why Other Options Are Wrong: Hit-and-miss is associated with early small engines that skip firing cycles. Pure quantitative governing characterizes diesel engines. A combination can occur in modern systems but is not the standard textbook association sought here. Common Pitfalls: Projecting modern electronic throttle control onto historical definitions and missing the conventional exam pairing SI–qualitative versus CI–quantitative. Final Answer: qualitative governing (varying mixture strength at nearly constant quantity)

Question 20

Ignition Timing and Knock – Effect of Advancing Spark in Petrol Engines In a spark-ignition petrol engine, what is the effect on knocking tendency if the spark timing is advanced (spark occurs earlier before top dead centre)?

Accepted Answer

Introduction / Context: Ignition timing is a powerful lever in spark-ignition engines. Advancing or retarding the spark changes the pressure–temperature history of the unburned end-gas and therefore the likelihood of knock. Understanding this relationship is vital for engine calibration, particularly under high load where knock limits torque and compression ratio. Given Data / Assumptions: Constant compression ratio and fuel octane rating. Load high enough that knock is a limiting factor. No active knock-control interventions assumed. Concept / Approach: Advancing the spark starts combustion earlier in the compression stroke. This increases the time the end-gas spends at elevated pressure and temperature before the flame front arrives. The longer residence at high temperature raises the chance of auto-ignition, thereby increasing knock tendency. Retarding the spark reduces the end-gas exposure and typically suppresses knock but may reduce efficiency and torque if excessive. Step-by-Step Solution: Advance spark → earlier heat release → higher pressure and temperature sooner.End-gas is subjected to severe conditions for longer.Probability of auto-ignition rises.Therefore, advancing spark increases knocking tendency. Verification / Alternative check: Knock sensors in modern engines detect onset of knock and automatically retard timing to eliminate it, reflecting the strong positive relation between spark advance and knock intensity at a given fuel octane. Why Other Options Are Wrong: “Not affect” is incorrect because timing strongly affects knock. “Decrease” reverses the known trend. “First decrease then increase” lacks basis under fixed conditions. “Eliminate entirely” is unrealistic; spark advance cannot remove knock by itself. Common Pitfalls: Confusing MBT (minimum advance for best torque) timing with knock-limited timing; at high load, MBT may be beyond the knock limit, requiring retard to avoid knock even if torque drops slightly. Final Answer: increase

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