Question 1

Engine components — name and purpose of the piston's lower cylindrical portion In a reciprocating engine piston, what is the name of the lower cylindrical portion that helps guide the piston and aids heat dissipation to the cylinder wall?

Accepted Answer

Introduction / Context: Accurate identification of piston features is important for assembly and diagnostics. The lower cylindrical section plays both mechanical and thermal roles. Given Data / Assumptions: Conventional aluminium alloy piston with ring pack near the crown. Piston operates within a lubricated cylinder liner. Normal petrol or diesel engine architecture. Concept / Approach: The piston skirt is the lower cylindrical surface below the ring pack. It stabilizes the piston, controls side thrust, and provides area for heat transfer to the cylinder wall and then to the coolant. Proper clearance and lubrication at the skirt reduce scuffing and noise. Step-by-Step Solution: Locate regions: crown (top), ring belt, pin bosses, and skirt (bottom).Identify function: skirt guides the piston and dissipates heat by conduction to the liner.Relate to maintenance: correct skirt clearance prevents slap and seizure. Verification / Alternative check: Workshop manuals and piston drawings label the lower cylindrical region as the “skirt.” Why Other Options Are Wrong: Crown: top surface exposed to combustion. Pin boss: localized bosses supporting the wrist pin. Connecting rod: separate component linking piston to crank. Oil control expander: part of ring assembly, not the piston body. Common Pitfalls: Misattributing ring-related oil control to the skirt; overlooking the need for proper piston-to-bore clearance. Final Answer: piston skirt

Question 2

PFI systems — why the electric fuel pump primes for about two seconds at key-on In a programmed fuel injection (PFI) system, the in-tank or in-line electric fuel pump typically runs for about two seconds at key-on (before cranking). What is the purp…

Accepted Answer

Introduction / Context: Electronic fuel-injected engines need stable rail pressure for accurate metering at start. The ECU briefly energizes the fuel pump at key-on to prepare the system. Given Data / Assumptions: PFI system with electric pump, fuel rail, pressure regulator or returnless control. ECU controls the pump relay/driver. Engine is not yet cranking. Concept / Approach: The short prime builds pressure in the fuel line and rail, purges vapour pockets, and ensures that injectors deliver the correct mass of fuel at first crank. Without priming, extended cranking could be required as pressure slowly builds. Step-by-Step Solution: ECU detects key-on event and energizes the pump for about 2 seconds.Pump raises line/rail pressure to the target setpoint.When cranking begins, injectors already see correct pressure; start quality improves. Verification / Alternative check: Fuel pressure gauge connected to the rail shows rapid rise to specification at key-on before cranking. Many OEM service manuals describe this prime routine. Why Other Options Are Wrong: Diagnostics: not the primary reason; fault checks occur simultaneously by ECU. Pump warm-up: unnecessary; pumps are cooled and lubricated by fuel during normal operation. Choke effect: obsolete carburetor concept; EFI meters fuel precisely. Immediate canister purge: occurs when warm and running, not at key-on. Common Pitfalls: Failed pump relay or immobilizer logic prevents priming; leaking injectors or bad check valves cause pressure bleed-down leading to long cranks. Final Answer: pressurise the fuel system before the engine is started

Question 3

Spark-ignition (petrol) engines — contents drawn during the suction (intake) stroke During the suction stroke of a conventional petrol engine with a carburettor or port fuel injection, what enters the cylinder?

Accepted Answer

Introduction / Context: This question probes your understanding of the intake process in spark-ignition (SI) petrol engines. Knowing what actually enters the cylinder during the suction stroke is foundational to combustion, emissions, and performance concepts. Given Data / Assumptions: Petrol (gasoline) engine operating on the Otto principle. Mixture preparation by a carburettor or by port fuel injection upstream of the intake valve. Throttle controls mass flow and manifold pressure. Concept / Approach: In a typical SI engine, fuel is vaporised and mixed with air before entering the cylinder. In carburetted systems, the venturi draws fuel into the airstream. In port fuel injection, the injector sprays fuel into the intake port where it mixes with incoming air; the intake valve then admits this premixed charge. This differs from compression-ignition Diesels, which ingest air only on the intake stroke and inject fuel directly into hot compressed air near the end of compression. Step-by-Step Solution: Identify the engine type: petrol SI with upstream mixture formation.During the suction stroke, piston descends and intake valve opens.Manifold depression draws in the prepared air–fuel mixture.Therefore, the correct entry to the cylinder is air and fuel together. Verification / Alternative check: Direct-injection petrol engines (GDI) may inject fuel into the cylinder; however, many still admit air (sometimes air only in specific stratified modes) and then add fuel during the intake or compression event. The mainstream historical and educational baseline for “petrol engine” intake is air–fuel mixture via carburettor or PFI. Why Other Options Are Wrong: Only fuel: impossible; combustion requires oxygen and the engine cannot ingest fuel alone. Only air: that is the Diesel intake case, not the classic petrol carburetted/port-injected case. None of these: incorrect because a clear correct choice is available. Common Pitfalls: Confusing SI with CI operation, or assuming all modern engines are GDI and thus always draw “air only.” Even with GDI, the intake stroke contains air and often some fuel addition depending on the strategy. Final Answer: air and fuel

Question 4

Wheel alignment basics — which parameter is adjusted by changing tie-rod length? Identify the alignment setting that is directly altered by modifying the effective tie-rod length on a steering linkage.

Accepted Answer

Introduction / Context: Wheel alignment is defined by camber, caster, and toe. Service technicians commonly adjust tie-rod length during alignment. Recognising which parameter this affects is essential for accurate vehicle handling and tyre-wear control. Given Data / Assumptions: Conventional rack-and-pinion or recirculating-ball steering with adjustable tie rods. MacPherson strut or double wishbone suspension (typical passenger vehicles). No adjustable upper mounts or special cam bolts unless noted. Concept / Approach: Tie rods connect the steering rack (or center link) to the steering knuckle arms. Changing the effective tie-rod length rotates the knuckles slightly inward or outward relative to the vehicle’s centerline, thus altering toe angle. Camber and caster are set primarily by suspension geometry, cam bolts, shims, or slotted mounts; steering gear ratio is fixed by the gear design. Step-by-Step Solution: Identify the linkage component adjusted in alignments: tie rod(s).Understand the effect: shorter/longer tie rod pulls/pushes the knuckle → changes toe-in or toe-out.Conclude that adjusting tie-rod length directly affects toe. Verification / Alternative check: Alignment procedures universally specify toe adjustment via inner/outer tie-rod threads while camber/caster require different hardware or shims. Post-adjustment steering wheel centring also involves equalising left and right tie-rod changes. Why Other Options Are Wrong: Camber: typically altered using cam bolts, slotted strut towers, or shims, not tie-rod length. Caster: altered with shims, adjustable arms, or subframe position, not tie-rod length. Steering gear ratio: a design characteristic of the gearbox/rack; not field-adjustable. Common Pitfalls: Adjusting only one tie rod causes off-centre steering wheels and unequal toe. Always split adjustments symmetrically when possible. Final Answer: Toe

Question 5

Engine valvetrain layout — mounting orientation of the camshaft relative to the crankshaft In conventional automotive engines, how is the camshaft mounted with respect to the crankshaft?

Accepted Answer

Introduction / Context: Valve timing in four-stroke engines is delivered by a camshaft driven from the crankshaft through a chain, belt, or gear train. Knowing the relative orientation is key to understanding timing drives and packaging. Given Data / Assumptions: Cam-in-block (pushrod) or overhead cam designs. Cam drive via timing belt, timing chain, or gear train. Standard passenger-vehicle engine layouts. Concept / Approach: To maintain consistent phasing and compact packaging, the camshaft’s axis runs parallel to the crankshaft’s axis. The drive system then transmits rotation at a fixed 1:2 ratio (cam rotates at half the crank speed in four-stroke engines) without requiring skewed bevels or right-angle drives. Step-by-Step Solution: Identify the required kinematic relation: cam speed = crank speed / 2.Parallel shafts allow simple sprockets, pulleys, or gears to achieve the ratio.Hence, the camshaft is mounted parallel to the crankshaft. Verification / Alternative check: Inspection of typical timing covers and valve covers shows the camshaft running longitudinally along the engine, aligned with the crank. Even in DOHC heads, both cams remain parallel to the crankshaft. Why Other Options Are Wrong: Perpendicular or inclined: would require complex bevel gear sets and create packaging challenges; not standard practice. None of these: a clear correct configuration exists. Common Pitfalls: Confusing cam orientation with valve angles; valves can be splayed while the camshaft remains parallel to the crankshaft. Final Answer: parallel to the crankshaft

Question 6

Thermal cycles — relative efficiency of Otto and Diesel cycles at the same maximum pressure and temperature For equal peak (maximum) pressure and maximum temperature limits, which ideal cycle is more thermally efficient?

Accepted Answer

Introduction / Context: Otto and Diesel cycles are idealised models for spark-ignition and compression-ignition engines, respectively. Comparing their efficiencies under matching constraints reveals why SI engines can be thermally competitive despite lower compression ratios in practice. Given Data / Assumptions: Air-standard analysis with constant specific heats (idealised comparison). Both cycles limited to the same maximum pressure and maximum temperature. Compression and expansion are isentropic; heat addition follows the ideal processes for each cycle. Concept / Approach: For the same peak pressure and temperature, the Otto cycle (heat addition at constant volume) produces a larger pressure rise during heat addition than the Diesel cycle (heat addition partly at constant pressure). This leads to a higher average temperature of heat addition relative to exhaust heat rejection, improving thermal efficiency. Step-by-Step Solution: Recognise constraint: identical Pmax and Tmax for both cycles.In the Otto cycle, heat is added at constant volume → greater pressure rise for the same temperature limit.In the Diesel cycle, part of heat addition occurs at constant pressure → lower peak pressure rise for the same Tmax.Hence, the Otto cycle attains higher thermal efficiency under the stated constraints. Verification / Alternative check: At the same compression ratio, Otto is also more efficient than Diesel. The present constraint (same Pmax and Tmax) similarly favours Otto due to the thermodynamic path of heat addition and resulting mean temperatures. Why Other Options Are Wrong: Diesel more efficient: not under equal Pmax and Tmax; its constant-pressure portion reduces efficiency here. Equally efficient or none: contradict thermodynamic comparisons of the ideal cycles. Common Pitfalls: Confusing real-engine data (influenced by friction, pumping, and mixture control) with ideal cycle results. Final Answer: Otto cycle is more efficient than Diesel cycle

Question 7

Primary role of the suspension system in road vehicles What is the basic function of an automotive suspension under real-world road inputs?

Accepted Answer

Introduction / Context: The suspension isolates the vehicle body from uneven road inputs and maintains tyre contact for control and comfort. Understanding this primary function is crucial to grasping ride, handling, and safety fundamentals. Given Data / Assumptions: Conventional suspension with springs and dampers (shock absorbers). Normal operation on varied road surfaces. Chassis and body need isolation and control. Concept / Approach: Springs store energy from bumps, while dampers dissipate it to prevent oscillations. Suspension geometry keeps wheels oriented appropriately but its core purpose is to absorb and manage vertical and longitudinal impacts so the tyres stay planted and the body experiences reduced acceleration. Step-by-Step Solution: Identify road inputs: bumps, potholes, undulations.Springs compress/extend to accommodate displacement.Dampers control spring motion to limit oscillation and improve stability.Therefore, the suspension’s basic function is absorbing vibration and impact forces. Verification / Alternative check: Ride and handling tests correlate reduced sprung-mass acceleration and maintained tyre contact patch with effective suspension design. Why Other Options Are Wrong: Steering force delivery: primarily a steering system function (assist units, geometry). Wheel alignment constancy: a design goal but not the basic function; geometry and bushings help here. Autocorrect oversteer: stability control/driver inputs address this, not the suspension alone. Common Pitfalls: Assuming dampers “absorb energy” alone; springs store and dampers dissipate. Both are needed. Final Answer: absorb vibration and impact forces from the road surface

Question 8

Brake dynamics — consequence of locking the front wheels under braking If the front wheels of a vehicle lock during heavy braking, what is the most likely outcome?

Accepted Answer

Introduction / Context: Wheel lock-up changes tyre forces and drastically affects directional control. Understanding the difference between front-wheel and rear-wheel lock-up is fundamental to safe braking and ABS benefits. Given Data / Assumptions: Dry or low-μ surface without ABS assistance. Front wheels lock before rears (common in front-heavy cars without proper proportioning). Vehicle travelling straight before lock-up. Concept / Approach: When a tyre locks, it transitions from static to kinetic friction, losing lateral cornering capability. Locked front wheels cannot generate the lateral force required for steering, so the vehicle plows straight ahead (understeer to the extreme). Rear lock-up, in contrast, can induce a spin because the rear loses lateral stability first. Step-by-Step Solution: Front wheels lock → no lateral grip at the steered axle.Steering input has little effect because tyres are sliding.Vehicle maintains approximate heading and continues straight, reducing steering control. Verification / Alternative check: ABS systems are designed to prevent lock-up and preserve steerability by modulating brake pressure, illustrating how steer control is lost during lock-up. Why Other Options Are Wrong: Very long stopping distance: not necessarily “extremely long”; the main issue is loss of steerability. Vehicle spins around due to front lock: spins are more associated with rear lock-up. Rear tyres skid: the premise states front-wheel lock, not rear. Common Pitfalls: Pumping brakes rapidly on ABS-equipped cars is counterproductive; maintain firm pressure and let ABS work. Final Answer: driver loses control over the steering, and the vehicle continues moving in its current direction

Question 9

Engine power definitions — power developed inside the cylinder What is the term for the power calculated from the pressure–volume work inside an engine cylinder before mechanical losses?

Accepted Answer

Introduction / Context: Engine performance analysis distinguishes between gross work in the cylinder and net output at the crankshaft. Understanding these terms is vital for interpreting test results and diagnosing losses. Given Data / Assumptions: Pressure in cylinder measured (conceptually) via indicator diagrams or transducers. Mechanical losses (friction, pumping, accessory drives) exist between cylinder and output shaft. Concept / Approach: Indicated power (IP) is derived from the integrated P–V work of the working cycle multiplied by engine speed and number of cylinders. Brake power (BP) is the usable output measured at the crankshaft or dynamometer. Friction power (FP) is the difference: FP = IP − BP. Step-by-Step Solution: Define IP as the gross power developed by gases on the piston.Recognise BP as measured output after mechanical losses.Therefore, the power developed inside the cylinder is indicated power. Verification / Alternative check: From the relation, FP = IP − BP and Mechanical efficiency = BP / IP, it follows that IP represents in-cylinder work rate. Why Other Options Are Wrong: Brake power: output after losses, not in-cylinder development. Frictional power: a loss term, not a developed power. Common Pitfalls: Assuming dynamometer power equals combustion work; it is lower due to friction, oil pump, water pump, valve train, and sealing losses. Final Answer: indicated power

Question 10

Compression ratio sensitivity — effect of increasing clearance volume in I.C. engines If the clearance volume of an internal combustion engine is increased (with other factors constant), how is the compression ratio affected?

Accepted Answer

Introduction / Context: Compression ratio strongly influences efficiency, power, and knock tendency. Understanding the geometric definition reveals how design changes alter this critical parameter. Given Data / Assumptions: Swept volume Vs fixed. Clearance volume Vc is changed by head gasket thickness, piston crown shape, or combustion chamber design. Definition of compression ratio: r = (Vs + Vc) / Vc. Concept / Approach: From the definition, increasing Vc (denominator and part of numerator) reduces the ratio because Vc grows faster relative to the total volume at bottom dead center. Step-by-Step Solution: Let r = (Vs + Vc) / Vc.If Vc increases while Vs remains constant, the denominator increases.Therefore, r decreases. Verification / Alternative check: Numerical example: Vs = 500 cm^3, Vc = 50 cm^3 → r = (500 + 50)/50 = 11.Increase Vc to 60 cm^3 → r = 560/60 ≈ 9.33, which is lower. Why Other Options Are Wrong: Increase or be doubled: contradicts the formula. Remain constant: impossible if Vc changes and Vs is fixed. Common Pitfalls: Confusing clearance volume with swept volume; only Vc change here affects the ratio inversely. Final Answer: decrease

Question 11

Anti-roll (stabilizer) bar — primary function in a front suspension system What is the main purpose of a front stabilizer (anti-roll) bar on a passenger vehicle?

Accepted Answer

Introduction / Context: Stabilizer (anti-roll) bars are torsion springs linking left and right suspension members. They play a pivotal role in lateral dynamics and cornering feel. Given Data / Assumptions: Front stabilizer bar connected via end links to control arms or struts. Vehicle negotiating corners causing body roll. Other springs and dampers present as usual. Concept / Approach: When the vehicle corners, the outside suspension compresses and the inside extends. The stabilizer bar twists and resists the difference in vertical travel, reducing roll angle and improving tyre contact distribution. It also moderates transient weight transfer across the axle. Step-by-Step Solution: Cornering induces differential wheel travel.Bar experiences torsion proportional to roll gradient.Result: reduced body roll and more controlled suspension movement. Verification / Alternative check: Handling tests show lower roll angles and crisper response with appropriately sized anti-roll bars, albeit with trade-offs in single-wheel bump compliance. Why Other Options Are Wrong: Increase load capacity: governed by springs, chassis, and tyres, not primarily by the anti-roll bar. Softer ride: anti-roll bars can actually decrease single-wheel ride softness by coupling wheels. All of the above: overbroad; only roll control is correct. Common Pitfalls: Oversizing bars can cause inside-wheel lift and oversteer/understeer balance issues; tuning front-to-rear bar stiffness is critical. Final Answer: control suspension movement and body roll

Question 12

Shock absorbers (dampers) — energy pathway during suspension motion In an automobile suspension, what is the primary function of the damper (shock absorber) with respect to energy?

Accepted Answer

Introduction / Context: Springs and dampers work together to manage ride comfort and stability. Clarifying each component’s role prevents common misconceptions about “absorbing” vs “dissipating” energy. Given Data / Assumptions: Coil or leaf spring plus hydraulic damper. Vehicle experiencing bumps and rebounds. Hydraulic damper with valves and orifices. Concept / Approach: Springs store potential energy when compressed or extended and then release it. Dampers convert the suspension’s kinetic energy into heat by forcing oil through restrictions, thereby dissipating energy and reducing oscillations. Without damping, the system would oscillate excessively after a disturbance. Step-by-Step Solution: Road input excites the sprung mass → spring stores energy.Damper resists motion with velocity-proportional force.Hydraulic losses convert mechanical energy to heat → energy is dissipated. Verification / Alternative check: Temperature rise in dampers after rough-road driving demonstrates energy conversion to heat. Damping coefficients are tuned to balance comfort and control. Why Other Options Are Wrong: Absorb energy: ambiguous; the damper transforms and dissipates, not store. Release or increase energy: contrary to the physical role of a damper. Common Pitfalls: Assuming stiffer dampers “support” weight; springs support static load, dampers control motion rate. Final Answer: dissipate the energy

Question 13

Automotive powertrain basics What is the primary purpose of the transmission in an automobile? Choose the most accurate statement that reflects its role in matching engine characteristics to road demand.

Accepted Answer

Introduction / Context: The transmission is a critical component of the drivetrain. While many learners say it “changes speed,” the deeper purpose is to transform the engine’s torque and speed to suit road load, grade, and acceleration needs. Understanding this distinction improves diagnosis and design choices. Given Data / Assumptions: Engine delivers useful power over a limited speed band. Vehicle requires high wheel torque to start and climb, and lower torque at cruise. Transmission provides a set of ratios (including reverse and sometimes overdrive). Concept / Approach: Power at a given instant is approximately the same (minus losses) across the drivetrain: power = torque * angular speed. Changing gear ratio multiplies torque at the wheels and inversely affects wheel speed. The transmission therefore primarily varies wheel torque to meet tractive demand; vehicle speed then results from the torque–resistance balance. Step-by-Step Solution: Identify the need: high launch torque, moderate cruise torque.Transmission selects a low gear → high torque multiplication at the wheels.As speed increases, higher gears reduce multiplication → efficient cruising.Therefore, the essential function is to vary torque at the driven wheels. Verification / Alternative check: Dyno or data-logging shows wheel torque changing dramatically across gears even when engine torque stays similar; vehicle speed follows from torque overcoming drag and grade. Why Other Options Are Wrong: “Vary speed” is a by-product, not the core purpose. “Vary power” is incorrect; power is largely conserved aside from losses. “None of these” and “improve octane” are irrelevant to transmissions. Common Pitfalls: Confusing engine power curve with wheel power; ignoring final-drive ratio; assuming more gears always mean faster cars—gearing must match the engine’s torque band. Final Answer: to vary the torque at the wheels

Question 14

Vehicle emissions control What does a modern three-way catalytic converter do to the exhaust gases passing through it?

Accepted Answer

Introduction / Context: The three-way catalytic converter (TWC) is the centerpiece of spark-ignition emission control. Knowing exactly which pollutants it targets improves understanding of diagnostics and regulations. Given Data / Assumptions: Gasoline stoichiometric combustion with lambda control via oxygen sensor. Converter coated with precious metals (e.g., platinum, palladium, rhodium). Operating above light-off temperature. Concept / Approach: Three-way refers to simultaneous treatment of three pollutant groups: carbon monoxide (CO), unburned hydrocarbons (HC), and oxides of nitrogen (NOx). Oxidation reactions convert CO and HC to CO2 and H2O; reduction reactions convert NOx to N2 (with some N2O traces minimized by control strategies). Step-by-Step Solution: Engine control maintains lambda ~1 for optimal TWC efficiency.Oxidation: CO + O2 → CO2; HC + O2 → CO2 + H2O.Reduction: NOx → N2 + O2 (simplified overall).Thus, the TWC removes CO, HC, and NOx, not N2, H2O, or CO2. Verification / Alternative check: Tailpipe measurements with an analyzer show sharp drops in CO, HC, NOx when the converter is active and at proper air–fuel ratio. Why Other Options Are Wrong: Exhaust gas recirculation (EGR) recirculates gases; the TWC does not. It does not specifically “burn fuel vapour only.” N2, H2O, and CO2 are desired products, not pollutants to reduce. Particulate filtration is for diesel/DGPF systems, not a standard TWC role. Common Pitfalls: Running rich or lean degrades TWC efficiency; misfires overheat and melt the substrate; delayed O2 sensor response causes excess NOx or HC slip. Final Answer: converts CO and HC to CO2 and H2O and reduces NOx to harmless gases

Question 15

Tyre construction — general categories Identify the two general types of tyres commonly used on passenger vehicles.

Accepted Answer

Introduction / Context: Tyres can be classified by whether they contain an inner tube that holds the air or integrate the air seal within the carcass. Recognizing this helps in puncture repair and rim selection. Given Data / Assumptions: Modern passenger cars predominantly use tubeless radial tyres. Older vehicles or certain off-road applications may use tube-type tyres. Rims must be compatible with the tyre type. Concept / Approach: Tube-type tyres rely on a separate inner tube for air retention. Tubeless tyres use an airtight liner bonded to the tyre and an air-tight bead against the rim to maintain pressure, improving safety and reducing heat. Step-by-Step Solution: Identify mainstream categories: tube-type and tubeless.Relate to service: tubeless allows puncture sealing from outside; tube-type requires tube repair or replacement.Select the option that lists both: “tube type and tubeless”. Verification / Alternative check: Vehicle manufacturer specifications and tyre sidewall markings (e.g., “Tubeless”) confirm the categorization. Why Other Options Are Wrong: “Solid” tyres are industrial; not paired with tubeless in standard road cars. “Air and pneumatic” is redundant terminology. “Split-rim and drop centre” are rim types, not tyre categories. Common Pitfalls: Mounting tubeless tyres on damaged rims may cause slow leaks; using tubes inside tubeless tyres is a temporary fix and alters heat management. Final Answer: tube type and tubeless

Question 16

Valve timing drive The timing belt (toothed belt) is wrapped around the camshaft pulley and which other engine pulley?

Accepted Answer

Introduction / Context: Correct valve timing synchronizes camshaft rotation with the crankshaft so that intake and exhaust events occur at the right piston positions. The timing belt is the common drive method in many engines. Given Data / Assumptions: Four-stroke engine: camshaft rotates at half crankshaft speed. Toothed belt prevents slip between pulleys. Some layouts route the belt around an idler and tensioner and may also drive a water pump. Concept / Approach: The camshaft must be phase-locked to the crankshaft. Therefore, the belt necessarily connects the camshaft pulley to the crankshaft timing pulley. Ancillary pulleys (water pump, balance shafts) can be driven but are not the fundamental pair. Step-by-Step Solution: Identify primary requirement: cam timing tied to crank angle.Primary pulleys: camshaft and crankshaft timing pulley.Select the option naming the crankshaft timing pulley. Verification / Alternative check: Service manuals show timing marks on both cam and crank pulleys; alignment confirms their direct relationship. Why Other Options Are Wrong: Distributors on older engines are driven from the cam, not by the belt directly in most designs. Fan and alternator are driven by accessory belts, not the timing belt. Water pump may be belt-driven but is not the essential mating pulley. Common Pitfalls: Incorrect belt tension or misaligned timing marks causes poor running or engine damage in interference engines; always lock the crank and cam before removal. Final Answer: timing belt drive (crankshaft) pulley

Question 17

Steering system force path Select the correct order in which the driver’s effort at the steering wheel is transmitted to turn the front wheels.

Accepted Answer

Introduction / Context: Understanding the order of steering components helps in diagnosing play, vibration, and alignment issues. The steering system translates driver torque into a controlled change in wheel toe angles. Given Data / Assumptions: Conventional rack-and-pinion or recirculating-ball system. Basic links: steering wheel, steering shaft (column), gearbox (rack-and-pinion or gear box), tie rods, steering knuckles, wheels. Concept / Approach: The driver applies torque at the steering wheel. The steering shaft conveys this to the gearbox, which translates or amplifies motion. Output links and tie rods transmit linear motion to the steering arms on the knuckles, pivoting the wheels about kingpin/ball joints. Step-by-Step Solution: Start at the steering wheel.Mechanical link: steering shaft to gearbox.Gearbox output creates linear motion or angular motion at a pitman arm/rack.Tie rods carry this motion to steering knuckles.Knuckles rotate the front wheels to a new steer angle. Verification / Alternative check: With the vehicle raised, rocking the steering wheel reveals the same order of motion through components. Why Other Options Are Wrong: Option a misorders gearbox and shaft. Option c incorrectly places knuckles before tie rods. Option d begins with tie rods, which cannot be driven directly by the wheel. Option e introduces “rack housing” before the shaft, which is incorrect. Common Pitfalls: Misattributing free play to alignment when it is actually within the steering column joints or rack inner tie-rod ends; always check the sequence for looseness. Final Answer: steering wheel → steering shaft → steering gearbox → tie rod → steering knuckle → front wheels

Question 18

Ignition systems — accuracy and control Which ignition system offers the most accurate and consistent spark timing in a modern spark-ignition engine?

Accepted Answer

Introduction / Context: Accurate ignition timing maximizes torque, reduces emissions, and prevents knock. Different systems vary in precision, stability, and adaptability to engine conditions. Given Data / Assumptions: Comparison among magneto, breaker-point, and ECU-controlled systems. Engine speed and load vary continuously; sensors provide feedback. Concept / Approach: ECU-controlled ignition uses sensors (crank/cam position, MAP/MAF, knock, temperature) and mapped algorithms to schedule spark events with high angular accuracy. Solid-state drivers and coil-per-plug minimize energy loss and dwell variability over the rev range. Step-by-Step Solution: Determine requirement: precise, adaptive timing.ECU reads position and load → computes advance.Igniter triggers coils accurately even at high rpm.Therefore, ECU ignition is most accurate. Verification / Alternative check: Scope traces of coil primary show tight timing scatter with ECU systems; mechanical points show more drift due to wear and bounce. Why Other Options Are Wrong: Magnetos are robust but timing is mechanically fixed or limited. Breaker-point systems suffer from dwell and cam-follower wear. Hybrid without ECU lacks closed-loop adaptability. Manual advance cannot react continuously to transient conditions. Common Pitfalls: Faulty sensors (e.g., CKP, CMP) or poor grounds can still degrade timing accuracy; ensure proper shielding and calibration. Final Answer: electronic control unit (ECU)–controlled ignition

Question 19

Diesel four-stroke valve timing (typical) For a four-stroke Diesel engine, which of the following intake valve timing pairs is most representative of practice?

Accepted Answer

Introduction / Context: Valve timing is advanced/retarded to improve cylinder filling and scavenging. Knowing typical ranges helps read timing diagrams and diagnose cam phasing faults. Given Data / Assumptions: Conventional naturally aspirated or mildly boosted four-stroke Diesel. Values vary by design but fall within common ranges. Cam phasing aims at volumetric efficiency and smooth operation. Concept / Approach: Intake valves often open slightly before TDC (end of exhaust stroke) to use scavenging and close after BDC during the intake stroke to leverage inertia of the incoming charge. Representative numbers: open 10°–25° BTDC, close 20°–60° ABDC. Step-by-Step Solution: Evaluate options against typical ranges.Option b (20° BTDC open, 35° ABDC close) lies comfortably within norms.Option a (exact TDC/BDC) ignores real-world gas dynamics.Option c (late open, early close) reduces breathing. Verification / Alternative check: Manufacturer timing charts confirm early opening and late closing for intake in many Diesel engines. Why Other Options Are Wrong: Opening exactly at TDC wastes scavenging benefits. Late opening and early closing restrict mass flow. Extreme values like 60°/60° are atypical for standard road Diesels. Common Pitfalls: Assuming one exact value fits all engines; cam wear or incorrect belt timing can shift these points and cause hard starting or smoke. Final Answer: opens about 20° before TDC and closes about 35° after BDC

Question 20

Fuel quality for compression-ignition (Diesel) engines Which hydrocarbon type provides the best ignition quality (highest cetane) for CI engines?

Accepted Answer

Introduction / Context: Diesel engines rely on auto-ignition. The fuel’s ignition quality is measured by cetane number. Understanding which molecular structures promote quick ignition aids in blending and troubleshooting. Given Data / Assumptions: Cetane number correlates inversely with ignition delay. Fuel families include paraffins, naphthenes, aromatics, and olefins. Combustion conditions typical of road Diesels. Concept / Approach: Straight-chain (normal) paraffins have high cetane numbers and ignite readily. Aromatics and highly branched paraffins generally have lower cetane and ignite more slowly, increasing ignition delay and noise. Step-by-Step Solution: Identify molecular structure that favours short ignition delay.Normal paraffins (e.g., n-cetane) are the reference for high cetane quality.Therefore select straight chain paraffins. Verification / Alternative check: Standardized tests (CFR engine) show normal paraffins score higher cetane than aromatics and branched isomers. Why Other Options Are Wrong: Aromatics and polyaromatics tend to low cetane and higher soot. Branched paraffins reduce cetane compared with their normal isomers. Olefins’ behavior is variable and not as favourable as straight paraffins. Common Pitfalls: Confusing octane (spark-ignition knock resistance) with cetane (compression-ignition quality); additives can raise cetane but do not change base chemistry. Final Answer: straight chain paraffins

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