Question 1

Clutch design practice — a road vehicle clutch is usually sized to transmit a design torque equal to which proportion of the maximum engine torque, to ensure reliability and avoid slip under transient loads?

Accepted Answer

Introduction / Context: The clutch must transmit engine torque without slip during launches, gear changes, and transient load spikes. Designers include a safety margin above the quoted peak engine torque to account for manufacturing tolerances, friction coefficient variation, heat, wear, and dynamic factors. The question asks for the typical design proportion relative to maximum engine torque. Given Data / Assumptions: Conventional dry friction clutch in a passenger vehicle or light commercial vehicle. Variable friction coefficient with temperature and wear is expected in service. Transient torque spikes can exceed rated engine torque momentarily. Concept / Approach: Clutch torque capacity T is proportional to normal load, mean radius, number of friction surfaces, and friction coefficient. To prevent slip under worst-case conditions and to provide service life, a margin of about 1.25 to 1.5 times the maximum engine torque is common. Among the given options, 150 percent is the recognized textbook value that captures this safety factor succinctly, ensuring that even with reduced friction due to fade or contamination the clutch still holds torque. Step-by-Step Solution: 1) Identify peak engine torque as the base load case.2) Apply a design margin to accommodate friction variability and dynamics.3) Select a factor near 1.5 to cover severe service and aging.4) Conclude that a clutch is usually designed for about 150 percent of maximum engine torque. Verification / Alternative check: Engineering handbooks cite design targets in the range of 1.25 to 1.5, with 1.5 widely used for robust service, corroborating the selection. Why Other Options Are Wrong: Equal to maximum torque leaves no margin and risks slip. Eighty percent is clearly inadequate. “None of these” is not applicable since 150 percent is a standard guideline. Common Pitfalls: Ignoring friction coefficient drop at elevated temperature; forgetting that clutch wear reduces clamp load over time, which the design margin counters. Final Answer: 150 per cent of the maximum engine torque

Question 2

Engine fundamentals — identify the standard firing order commonly used in an in-line four-cylinder internal combustion engine for smoothness and balance.

Accepted Answer

Introduction / Context: The firing order of a multi-cylinder engine determines the sequence in which cylinders ignite. Proper selection minimizes vibration, balances primary and secondary forces and couples, and smooths torque delivery to the crankshaft. The in-line four-cylinder configuration is ubiquitous, and its conventional firing order is widely taught in automotive engineering courses and service manuals. Given Data / Assumptions: Four cylinders in a single in-line bank using a four-stroke cycle. Uniform 180-degree crank throws between adjacent cylinders. Objective of even power strokes and acceptable balance without complex shafts. Concept / Approach: For a four-stroke, four-cylinder engine, one cylinder fires every 180 degrees of crank rotation. A firing order of 1-3-4-2 spaces power impulses evenly and yields satisfactory balance with typical crankshaft layouts. Alternatives can introduce undesirable torsional oscillations or uneven exhaust pulsing. The 1-3-4-2 sequence also simplifies exhaust manifold design by pairing cylinders that are out of phase to reduce interference. Step-by-Step Solution: 1) Note that four power strokes occur every 720 degrees, so spacing is 180 degrees.2) Choose an order that distributes firing between front and rear to reduce rocking couples.3) Verify that 1-3-4-2 satisfies even spacing and acceptable balance with standard crankpin arrangement.4) Confirm common industry practice and service documentation reference the same order. Verification / Alternative check: Many OEM specifications and textbooks list 1-3-4-2 as the canonical firing order for in-line fours, validating the selection. Why Other Options Are Wrong: 1-2-3-4 and 1-2-4-3 produce poor balance and exhaust interference. 1-3-2-4 disrupts smoothness and can raise torsional vibration. Common Pitfalls: Assuming firing order is arbitrary; overlooking the effect on crankshaft torsion, NVH, and manifold tuning. Final Answer: 1-3-4-2

Question 3

Ignition system practice — the typical spark plug gap between the central electrode and the ground electrode in a conventional petrol engine is approximately:

Accepted Answer

Introduction / Context: The spark plug gap is the distance the ignition system must arc across to ignite the compressed air-fuel mixture. The correct gap balances reliable ignition at various pressures with coil energy limits and erosion considerations. Too small a gap can lead to weak ignition; too large a gap can misfire under high cylinder pressure, especially during acceleration or cold starts. Given Data / Assumptions: Conventional gasoline engine with standard ignition energy levels. No specialty racing or extended-spark systems are assumed. General workshop settings rather than vehicle-specific extremes. Concept / Approach: Typical automotive spark plug gaps fall around 0.5 to 0.9 mm depending on ignition system capability, combustion chamber design, and emissions strategy. A commonly taught nominal value for standard plugs and mid-range systems is about 0.5 mm, which provides reliable ignition margin without excessive coil demand. While some modern systems specify larger gaps closer to 0.7–0.9 mm, the option set provided places 0.5 mm as the most representative textbook value. Step-by-Step Solution: 1) Identify the role of the gap in determining breakdown voltage and spark kernel size.2) Balance ignition energy versus required voltage under compression.3) Select a mid-range value appropriate for general purpose engines.4) Conclude that approximately 0.5 mm is a standard practical setting given the options. Verification / Alternative check: Service manuals often list gaps from 0.5 to 0.8 mm; older breaker-point systems commonly used around 0.5 mm, reinforcing the choice here. Why Other Options Are Wrong: 0.2 mm risks weak spark kernel and fouling tolerance; 1 mm or 1.5 mm demand high voltage that many systems cannot supply under load, risking misfire. Common Pitfalls: Setting the gap without considering coil capability and boost or high compression effects; forgetting that erosion widens the gap over service life. Final Answer: 0.5 mm

Question 4

Engine management sensors — the oxygen (O₂) sensor provides feedback to the control unit primarily about which combustion parameter for closed-loop mixture regulation?

Accepted Answer

Introduction / Context: Modern engines use sensors to control mixture, spark, and emissions. The oxygen sensor, mounted in the exhaust, is a key element of closed-loop fuel control. By measuring oxygen content in exhaust gases, the controller adjusts injector pulse width to maintain stoichiometry for three-way catalyst efficiency, or targets richer or leaner mixtures as needed for power or economy. Given Data / Assumptions: Closed-loop operation with a narrowband or wideband oxygen sensor. Electronic control unit capable of interpreting sensor voltage or lambda signal. Catalyst system that benefits from stoichiometric combustion around lambda equal to one. Concept / Approach: The oxygen sensor output is directly related to lambda, which is the ratio of actual to stoichiometric air-fuel ratio. A narrowband sensor toggles rapidly around lambda equal to one, while a wideband sensor quantifies deviation above or below stoichiometry. The control unit uses this signal to correct fuel delivery, thereby controlling the actual air-fuel ratio. It does not measure intake air temperature or flow speed directly, nor does it indicate exhaust volume; those are handled by other sensors such as intake air temperature, mass air flow, and manifold pressure sensors. Step-by-Step Solution: 1) Recognize that oxygen content in exhaust is a proxy for mixture richness or leanness.2) Map sensor output to lambda and therefore to air-fuel ratio.3) Apply fuel trims based on feedback to maintain the target mixture.4) Achieve emissions and efficiency goals through continuous closed-loop correction. Verification / Alternative check: Control strategies in OEM documentation describe short-term and long-term fuel trim derived from oxygen sensor feedback, confirming that air-fuel ratio is the parameter being controlled. Why Other Options Are Wrong: Air temperature and air flow speed come from separate intake sensors. Exhaust gas volume is not directly used for mixture control and is influenced by many variables. Common Pitfalls: Assuming sensor voltage indicates absolute oxygen concentration without context; confusing narrowband switching behavior with wideband proportional output. Final Answer: air-fuel ratio

Question 5

Ride comfort and masses — for improved vehicle ride comfort over typical road inputs, which mass property should be minimized to reduce harshness transmitted to the body?

Accepted Answer

Introduction / Context: Vehicle ride comfort depends on how the suspension isolates the body from road irregularities. The system can be modeled with sprung mass (body supported by springs) and unsprung mass (wheels, tyres, hubs, a portion of suspension links and brakes). Lowering the unsprung mass reduces the force transmitted to the sprung mass when the wheels follow high-frequency road inputs. Given Data / Assumptions: Standard passive suspension with springs and dampers, no active control assumed. Road inputs contain significant high-frequency content that primarily affects the unsprung assembly. Goal is to improve comfort without changing primary spring rates or damping drastically. Concept / Approach: The unsprung mass moves almost in phase with the road at higher frequencies; the force transmitted through the spring is roughly proportional to the product of unsprung mass and its acceleration. Minimizing unsprung mass lowers this force and improves isolation. Reducing sprung mass does not necessarily improve comfort, and very low overall vehicle mass can increase sensitivity to disturbances. Therefore, engineering practice prioritizes lightweight wheels, tyres, brake hardware, and sometimes inboard brakes to cut unsprung mass for better ride and grip over rough surfaces. Step-by-Step Solution: 1) Separate masses into sprung and unsprung components.2) Recognize that high-frequency road input excites unsprung resonance and transmits force to the body.3) Reduce unsprung mass to lower transmissibility and peak acceleration of the body.4) Validate through design choices such as lightweight wheels and aluminium suspension parts. Verification / Alternative check: Quarter-car models show that decreasing unsprung mass reduces body acceleration and tyre hop, aligning with accepted ride-handling trade-offs. Why Other Options Are Wrong: Minimizing sprung mass is not a primary path to comfort and can raise natural frequency. Minimizing total vehicle mass is driven by efficiency, not ride comfort. Therefore “all of these” is not correct. Common Pitfalls: Confusing handling objectives with comfort; assuming heavy wheels improve stability while they actually increase impact harshness. Final Answer: unsprung mass is kept minimum

Question 6

Lubrication faults — if engine coolant leaks into the crankcase and contaminates the engine oil, what visible change most typically appears on the dipstick or under the filler cap during inspection?

Accepted Answer

Introduction / Context: Cross-contamination between the cooling and lubrication systems can occur due to head gasket failure, cracked heads or blocks, or cooler leaks. Coolant mixing with oil severely degrades lubrication, accelerates bearing damage, and can lead to catastrophic failure. Early detection through visual checks is therefore vital during maintenance. Given Data / Assumptions: Presence of ethylene glycol or similar coolant entering the oil sump. Engine has been run, causing emulsification of oil and coolant. Inspection is via dipstick, filler cap, or under-cap surfaces. Concept / Approach: When water-based coolant emulsifies with engine oil, the mixture forms a light brown or cream coloured milky sludge. This appearance is very different from normal darkening of oil due to soot and oxidation. Although some foam can appear during operation, persistent milky emulsions are characteristic of coolant intrusion and often accompanied by rising oil level, sweet odour, and possibly white exhaust vapour on startup if coolant also enters the combustion chamber. Step-by-Step Solution: 1) Withdraw dipstick and observe colour and texture of the lubricant.2) Identify creamy or milky appearance indicating emulsion.3) Corroborate with other signs such as unexplained coolant loss or overfull oil level.4) Plan immediate diagnosis and repair to prevent bearing damage. Verification / Alternative check: Workshops confirm coolant contamination with laboratory analysis showing glycol presence and elevated water content, validating the visual cue. Why Other Options Are Wrong: Foam can result from aeration and is transient; it is not diagnostic of coolant contamination. Black oil is typical of normal use in Diesel engines or long intervals and does not indicate coolant. “None of these” is incorrect because milky oil is the well known indicator. Common Pitfalls: Attributing condensation under the cap after many short trips to major leaks; always confirm with a pressure test and oil analysis if in doubt. Final Answer: appears milky

Question 7

Valve train kinematics — in an overhead-valve or overhead-cam engine, the motion imparted by the rotating cam lobes is transmitted to the valves primarily through which component?

Accepted Answer

Introduction / Context: The valve train converts camshaft rotation into precise valve lift and timing so that intake and exhaust events occur when required. Depending on the engine layout, this motion can be transmitted through lifters, pushrods, and rocker arms, or directly by cam followers and finger rockers in overhead-cam designs. Understanding which component delivers motion to the valve helps in diagnostics and design discussions. Given Data / Assumptions: A typical production engine with either overhead valve (cam in block) or overhead camshaft architecture. Cam lobe displacement needs leverage and direction change to depress the valve. Terminology distinguishes actuators from passive structural elements like stems. Concept / Approach: Rocker arms act as levers that translate and often amplify the cam or follower motion to open the valve against spring force. In pushrod engines, the cam lifts a tappet and pushrod which then moves the rocker arm. In many OHC engines, a finger follower or rocker directly contacts the cam lobe and the valve stem. Pistons do not transmit valve motion, and the camshaft pulley merely drives the cam via a belt or chain. Valve stems are parts of the valves themselves and are moved by the rocker, not by the cam directly in most common arrangements that use rockers. Step-by-Step Solution: 1) Identify cam lobe generating lift via follower or tappet.2) Trace the motion path through pushrod or direct rocker in OHC designs.3) Recognize the rocker arm lever applies force to the valve stem tip.4) Valve spring returns the valve when the lobe rotates off peak lift. Verification / Alternative check: Exploded diagrams and service manuals for both OHV and OHC engines label the rocker arm as the element that transmits motion to the valve in the majority of designs that are not direct-acting bucket tappet types. Why Other Options Are Wrong: Pistons control cylinder pressure, not valve motion. Camshaft pulley only provides drive timing. Valve stems are the receiving members, not the transmitting lever. Common Pitfalls: Confusing direct-acting bucket tappets with rocker systems; even there, the bucket is a follower, not a pulley or piston. Final Answer: rocker arms

Question 8

Electronic fuel injection control in modern automotive engines: Is the following statement accurate—‘‘In a fuel-injection system, the Electronic Control Module (ECM) computes the optimum fuel-injection volume for the current engine condition from se…

Accepted Answer

Introduction / Context: Electronic fuel injection replaced carburetors by metering fuel precisely according to engine operating conditions. The Electronic Control Module (ECM), also called the Engine Control Unit (ECU), reads multiple sensors and commands injectors to deliver an amount of fuel that achieves the target air–fuel ratio while meeting power, emissions, and drivability goals. The statement claims the ECM calculates the optimum injection quantity and schedules it at the appropriate time into the intake manifold—this describes manifold/port fuel injection behavior. Given Data / Assumptions: Multi-point or single-point port fuel injection (PFI/MPI) with injectors spraying into the intake manifold runners. Standard automotive sensors: Mass Air Flow (MAF) or Manifold Absolute Pressure (MAP), Throttle Position, Engine Coolant Temperature, Intake Air Temperature, Oxygen/Lambda sensor, Crank/Cam position, etc. Closed-loop feedback during part-load and open-loop maps at cold start/WOT. Concept / Approach: The ECM estimates intake air mass (air-charge) and computes required fuel mass using a target λ (lambda). It then converts fuel mass to injector pulse width, compensating for battery voltage, fuel pressure, injector dead-time, wall-wetting, and temperature. Timing of injection is synchronized to crank/cam signals so fuel arrives before intake-valve opening for good vaporization and mixture preparation. Step-by-Step Solution: 1) Measure air and load: MAF or speed-density (MAP, RPM) gives air-charge estimate.2) Compute fuel mass: mfuel = (mair/AFRtarget) with enrichment/trim factors.3) Command injector: Pulse width = f(mfuel, injector flow rate, voltage, temp); schedule relative to intake event. Verification / Alternative check: Closed-loop corrections from the oxygen sensor trim the base calculation to maintain stoichiometry (≈14.7:1 for gasoline) at cruise; during transients the ECM uses acceleration enrichment and wall-film models. All align with the statement. Why Other Options Are Wrong: Incorrect / fixed volume: modern ECUs continuously calculate and adjust fueling. Only DI: direct injection sprays into the cylinder; the statement explicitly references injection into the intake manifold—PFI, not DI. Only at WOT: computation occurs under all loads and modes. Common Pitfalls: Confusing PFI (manifold injection) with DI (in-cylinder injection). Overlooking compensation tables that refine the base pulse width. Final Answer: Correct

Question 9

Fuel quality index for compression-ignition engines: The cetane rating (number) of standard automotive diesel fuel is typically in which approximate range?

Accepted Answer

Introduction / Context: Cetane number (CN) quantifies the ignition quality of diesel fuel: higher CN indicates shorter ignition delay and smoother combustion in compression-ignition (C.I.) engines. Appropriate CN is critical for cold starting, noise, emissions, and overall drivability. This question asks for the typical order of the cetane rating used in road-going diesel engines and standards worldwide. Given Data / Assumptions: Automotive on-road diesel fuel for light- and heavy-duty engines. Reference to common specifications (e.g., minimum CN ≈ 40–51 depending on region and standard). Normal ambient conditions; not specialized high-cetane additives for testing only. Concept / Approach: Most market diesel fuels target a cetane number in the mid-40s to low-50s to balance ignition quality, cost, and emissions control. While regional regulations may set different minimum CN values, the typical classroom/MCQ answer for ‘‘order of’’ CN is about 45. Values like 70 or 90 correspond to reference fuels or highly treated blends not representative of standard pump diesel, whereas 25 would be far too low, causing hard starting and noisy combustion. Step-by-Step Solution: 1) Identify the metric: cetane number reflects ignition delay.2) Recall typical road-fuel targets: mid-40s to low-50s.3) Select the closest offered value to the typical range → 45. Verification / Alternative check: Engine manufacturers often calibrate for CN ≥ 45 for acceptable start and combustion stability. Seasonal and regional variations exist but rarely approach 70–90 in regular retail fuels. Why Other Options Are Wrong: 25: too low; would lengthen ignition delay excessively. 70, 90: unrealistically high for commercial pump diesel; used as reference numbers or special blends. 35: below typical modern standards. Common Pitfalls: Confusing cetane (diesel) with octane (gasoline), which behaves oppositely regarding knock resistance. Final Answer: 45

Question 10

Automotive fuels—CNG storage: Natural gas in a compressed natural gas (CNG) cylinder onboard a vehicle is typically stored at approximately what rated pressure at standard reference conditions?

Accepted Answer

Introduction / Context: Compressed natural gas (CNG) vehicles store methane fuel in high-pressure cylinders. The nominal storage pressure defines cylinder design, filling protocols, safety devices, and refueling equipment. Many exam syllabi reference a standard nominal value used for classroom calculations and safety understanding. Given Data / Assumptions: On-vehicle CNG cylinders rated for typical automotive service. Reference temperature around 15–20°C for nominal rated pressure. Familiarity with refueling station practices that may temporarily exceed nominal to compensate for cooling/settling. Concept / Approach: The commonly cited nominal storage pressure for automotive CNG cylinders is about 200 bar at reference temperature. While filling systems may reach ~200–220 bar depending on station and temperature to ensure usable capacity after temperature equalization, the textbook figure used for design familiarity is 200 bar. Higher values like 250–300 bar pertain to different standards or overfills not used as the typical classroom nominal. Step-by-Step Solution: 1) Identify the standard nominal rating used in teaching: ≈200 bar.2) Recognize real-world variation with temperature and fast fill; still, nominal remains 200 bar.3) Choose 200 bar from the options. Verification / Alternative check: Automotive training materials and safety guides frequently quote 200 bar nominal; station fast-fill practices may list 200–220 bar compensated fills but do not change the nominal reference. Why Other Options Are Wrong: 220 bar: possible as a compensated fill end-point; not the standard nominal reference in many curricula. 250, 300 bar: exceed typical passenger-vehicle cylinder nominal ratings for common systems. 150 bar: too low for full capacity and range expectations. Common Pitfalls: Confusing nominal rated pressure with station delivery peaks or hot-soak equalized pressures. Final Answer: 200 bar

Question 11

Vehicle resistance calculations: A truck weighs 63 500 N and has a coefficient of rolling resistance of 0.018. What is the rolling resistance force acting on the truck (in newtons)?

Accepted Answer

Introduction / Context: Rolling resistance is a fundamental road-load component that opposes vehicle motion. It arises from tire deformation, hysteresis, and road interaction, and it is typically modeled as proportional to the vehicle's normal load via a coefficient of rolling resistance. This problem applies the basic formula to compute the rolling resistance force for a specified truck weight and coefficient—an essential step in tractive-effort and fuel-economy analyses. Given Data / Assumptions: Vehicle weight W = 63 500 N (acts as normal force on level ground, neglecting aerodynamic lift/squat). Coefficient of rolling resistance fr = 0.018 (dimensionless). Level road; speed effects on fr neglected. Concept / Approach: The rolling resistance force on level ground is modeled as Frr = fr × W. This linear relation is widely used in first-order vehicle dynamics. Units remain in newtons because fr is dimensionless and W is in newtons. Step-by-Step Solution: 1) Write the formula: Frr = fr × W.2) Substitute values: Frr = 0.018 × 63 500 N.3) Compute: 0.018 × 63 500 = 1 143 N.4) Check units: coefficient is dimensionless → result in newtons. Verification / Alternative check: Estimate: 0.02 × 63 500 = 1 270 N; since 0.018 is 10% less than 0.02, reduce by ≈127 N → ≈1 143 N. This mental-math check confirms the exact calculation. Why Other Options Are Wrong: 114.3 N and 11.43 N / 1.143 N: each is off by one or more powers of ten—a common decimal-placement error. 1.143 kN: although numerically equivalent to 1 143 N, the option set expects units in newtons; selecting kN would be mismatched unless explicitly allowed. Common Pitfalls: Mistaking vehicle mass (kg) for weight (N) and introducing an extra g factor. Applying grade resistance or aerodynamic drag to a pure rolling-resistance question. Final Answer: 1143 N

Question 12

Engine bearings selection: Which type of bearing is typically used for crankshaft main bearings and connecting-rod big-end bearings in automotive engines?

Accepted Answer

Introduction / Context: Crankshaft main journals and connecting-rod big ends operate under high cyclic loads and need compact, quiet, and durable bearings with excellent conformability and damping. The bearing type chosen dictates oil supply, clearances, wear behavior, and rebuild practices. Understanding why engines favor one bearing type over rolling-element alternatives is central to engine design and service. Given Data / Assumptions: Four-stroke automotive engines with pressurized lubrication systems. High surface speeds at the journal with periodic load reversals. Need for compact package and low noise/vibration. Concept / Approach: Engines employ plain (journal) bearings—typically bi-metal or tri-metal shells (steel backing with copper/lead/aluminum-tin overlays). With hydrodynamic lubrication, a wedge of oil supports the journal without metal-to-metal contact at speed, offering high load capacity, good damping, and manufacturability with replaceable shells. Rolling-element bearings would be bulkier for equivalent capacity, noisier, and less tolerant of misalignment and debris in this application. Step-by-Step Solution: 1) Identify operating regime: high RPM journals favor hydrodynamic films.2) Map to bearing type: plain shells create oil wedge → high capacity and damping.3) Conclude: crankshaft mains and big ends use plain bearings. Verification / Alternative check: Exploded engine diagrams and overhaul manuals show replaceable half-shell bearings in main saddles and rod caps, confirming the standard practice. Why Other Options Are Wrong: Ball, needle, taper roller: used in some motorcycle/specialty engines but not typical for automotive crank journals due to size, noise, and impact-load sensitivity. Hydrostatic thrust: different function (axial load) and supply complexity; not used for mains/rods. Common Pitfalls: Assuming rolling bearings must be ‘‘better’’; in hydrodynamic regimes, plain bearings excel. Overlooking the role of oil viscosity, clearance, and alignment in film formation. Final Answer: Plain bearings

Question 13

Clutch engagement basics: The flywheel and pressure plate clamp (bind) the clutch disc between them so that the engine and the transmission input shaft rotate together when engaged. Is this statement correct?

Accepted Answer

Introduction / Context: A friction clutch connects and disconnects engine torque to the gearbox. Understanding which components clamp the clutch disc and how torque is transmitted is foundational for diagnosing slip, chatter, and engagement issues. The statement references the flywheel, pressure plate, and driven disc—key elements of a typical single-plate dry clutch. Given Data / Assumptions: Single-plate dry clutch with diaphragm spring pressure plate. Friction facings bonded to both sides of the clutch disc. Release mechanism disengages the pressure plate when the pedal is pressed. Concept / Approach: In the engaged state, the diaphragm spring pushes the pressure plate against the clutch disc, clamping it against the flywheel. Friction between the flywheel face and pressure plate face with the disc facings transmits torque from the engine to the transmission input shaft via the disc hub and splines. Thus, flywheel + pressure plate bind the disc to couple engine and gearbox. Step-by-Step Solution: 1) Pedal released → diaphragm spring loads the pressure plate.2) Disc is clamped between flywheel and pressure plate → friction lock.3) Engine torque passes through disc hub splines to the gearbox input shaft. Verification / Alternative check: Service manuals show that pressing the pedal retracts the pressure plate via the release bearing, unclamping the disc and allowing relative slip—confirming that clamping is the mechanism for engagement when the pedal is released. Why Other Options Are Wrong: No / free-wheeling: contradicts clutch operation; free-wheeling occurs only when disengaged. Only multi-plate / only at idle: the described mechanism applies to single-plate designs and all speeds within torque capacity. Common Pitfalls: Confusing release-bearing action (pedal down) with engagement (pedal up). Ignoring that inadequate clamp load or worn facings cause slip despite the intended clamping. Final Answer: Yes

Question 14

Valve timing in four-stroke petrol engines: The approximate valve overlap (simultaneous opening of intake and exhaust valves around top dead center) is generally about how many crankshaft degrees?

Accepted Answer

Introduction / Context: Valve overlap is the period near top dead center (TDC) between the exhaust and intake strokes when both valves are open. Overlap improves scavenging and charge exchange at certain speeds but can hurt idle quality if excessive. Textbook values provide a sense of typical design choices for naturally aspirated petrol engines. Given Data / Assumptions: Conventional small/medium petrol engines without extreme race cams. Factory street-tuned camshafts prioritizing drivability and emissions. Approximate values suitable for academic MCQs. Concept / Approach: Production engines often use modest overlap, commonly around a few tens of crank degrees. While performance and racing cams may show 50–80° or more overlap, classroom references typically cite ≈30° as a representative figure for general gasoline engines, balancing idle stability and mid-range performance. Step-by-Step Solution: 1) Define overlap: simultaneous open period around exhaust TDC.2) Recognize production compromise: modest overlap to control emissions and idle.3) Choose the commonly taught approximate value: 30°. Verification / Alternative check: Valve-timing diagrams in textbooks frequently show intake opening slightly before TDC and exhaust closing slightly after TDC, summing to roughly 20–40°, often simplified to 30° for MCQs. Why Other Options Are Wrong: 60°, 90°, 120°: typical of aggressive performance profiles; not representative of standard road engines in foundational questions. 15°: lower than common production baselines; some engines may approach this, but 30° is the standard teaching value. Common Pitfalls: Confusing crank degrees with cam degrees (camshaft rotates at half crank speed). Assuming a single fixed value across all engines—here we choose the widely accepted approximate figure. Final Answer: 30°

Question 15

Valve train maintenance: Why is it necessary to maintain specified valve clearances (lash) in an overhead-valve or overhead-cam engine?

Accepted Answer

Introduction / Context: Valve lash is the small clearance between cam-operated components (cam and tappet, rocker tip and valve stem) when the valve is fully closed and the cam is on its base circle. Correct lash is vital for sealing, durability, and precise timing. This question asks for the primary reason engines specify and adjust valve clearances. Given Data / Assumptions: Mechanical (solid) lifter systems or adjustable rocker arms; hydraulic tappets auto-compensate but still rely on design lash concepts. Hot running conditions elongate valves and change geometry. Clearance measured cold unless specified otherwise by the OEM. Concept / Approach: Valves, particularly exhaust valves, expand (lengthen) as temperature rises. Lash provides room for this thermal growth so that, at operating temperature, the valve still seats fully for compression sealing and heat transfer to the seat. Insufficient or zero lash can hold a valve slightly open when hot, causing loss of compression, burning, and rapid failure. Excessive lash leads to noisy operation, impact loading, and altered valve timing (reduced effective duration/lift). Step-by-Step Solution: 1) Recognize thermal expansion of the valve train during operation.2) Provide a calibrated clearance at cold state to accommodate growth.3) Result: valves fully seat at temperature, maintaining sealing and cooling. Verification / Alternative check: Service manuals specify different cold/hot lash values; exhaust lash is often slightly larger than intake due to higher temperatures, confirming the role of thermal expansion. Why Other Options Are Wrong: Reduce sliding resistance: lubrication, not lash, primarily controls sliding friction. Increase valve speed: excessive lash impairs timing and durability. Make crankshaft smoother: lash does not directly smooth crank rotation. Hydraulic lifters only: hydraulics self-adjust; the principle applies generally to account for expansion. Common Pitfalls: Setting lash too tight after a valve grind; hot running then holds the valve off its seat. Ignoring OEM specified hot vs cold measurement conditions. Final Answer: allow for lengthening (thermal expansion) of the valves due to combustion heat

Question 16

Road–tire interaction: The torque effectively available at the contact patch between the driving wheels and the road surface (used to propel the vehicle) is commonly termed what?

Accepted Answer

Introduction / Context: Vehicle performance analyses translate engine torque into a usable pushing (or pulling) capability at the tire–road interface. After accounting for transmission ratios, driveline losses, and wheel radius, the resulting torque at the wheel generates a longitudinal force at the contact patch. The name of this propulsive capability is a core term in vehicle dynamics and traction studies. Given Data / Assumptions: Driven axle transmitting torque through tires to a level surface. No wheel slip beyond the traction limit; static friction applies. Losses (driveline, aerodynamic, gradient, rolling resistance) are considered separately. Concept / Approach: The tractive effort is the longitudinal force available at the road contact patch to accelerate the vehicle or overcome resistances. It is related to wheel torque T by F = T/Rwheel (neglecting tire deformation effects). ‘‘Brake effort’’ refers to retarding force under braking. ‘‘Clutch effort’’ is not a vehicle-dynamics term; it refers to pedal or clamping force in the clutch assembly. Step-by-Step Solution: 1) Start with engine torque multiplied by gear and final-drive ratios.2) Subtract driveline losses to get wheel torque.3) Convert wheel torque to longitudinal force at the road: F = T/R → the tractive effort. Verification / Alternative check: Performance charts plotting ‘‘tractive effort vs. speed’’ show how gear ratios and power limit the available force, validating the definition. Why Other Options Are Wrong: Brake effort: decelerating force, opposite of propulsion. Clutch effort: unrelated to road force. None of these / tractive resistance: the correct standard term is tractive effort; tractive resistance is the opposing sum of road loads. Common Pitfalls: Confusing wheel torque with tractive effort; wheel radius must be considered. Ignoring traction limits; available friction caps the usable tractive effort. Final Answer: tractive effort

Question 17

Wheel alignment geometry: What is the primary adverse effect observed when a vehicle has excessive positive or negative camber beyond specification?

Accepted Answer

Introduction / Context: Camber is the inward or outward tilt of the wheel relative to vertical, viewed from the front of the vehicle. Proper camber improves handling and tire contact during cornering. However, camber that is too positive or too negative causes the tire to run on one shoulder, degrading wear life and grip in straight-ahead driving. Recognizing the symptom helps technicians prioritize alignment corrections. Given Data / Assumptions: Production vehicles with small camber angles (near zero) specified for even tread contact. Tires inflated and rotated correctly; no significant toe error masking camber effects. Suspension bushings and components in serviceable condition. Concept / Approach: Excessive camber shifts the load path to the inner or outer tread shoulder. The concentration of contact pressure elevates local wear, producing uneven tyre wear patterns (e.g., inner-edge feathering for excessive negative camber). While camber can contribute to steering feel and alignment torque, the most prominent practical consequence for drivers is accelerated, uneven tread wear and reduced tire life. Step-by-Step Solution: 1) Identify the geometry error: camber magnitude exceeds OEM spec.2) Predict contact patch shift: load moves to one shoulder.3) Observe tire condition over time: uneven shoulder wear develops. Verification / Alternative check: Tire-wear diagnostic charts clearly link camber errors to inner/outer edge wear patterns, distinct from ‘‘saw-tooth’’ toe wear or cupping from shocks. Why Other Options Are Wrong: Too much traction: excessive camber can reduce straight-line traction by shrinking the effective contact area. Hard steering / alignment torque only: camber affects these, but they are not the primary or most visible consequence. Immediate bearing failure: bearings see different loads, but failure is not the primary near-term symptom of modest camber error. Common Pitfalls: Misdiagnosing wear caused by toe or pressure issues; verify all alignment angles and tire pressures. Assuming symmetrical wear across an axle when only one side has bent components. Final Answer: uneven tyre wear

Question 18

Intake system fundamentals — what is the primary function of the intake manifold in a multi-cylinder spark-ignition or compression-ignition engine, considering mixture formation and cylinder-to-cylinder distribution?

Accepted Answer

Introduction / Context: The intake manifold is the network of runners and plenums that supplies the engine's cylinders with the air (or air–fuel mixture in carbureted/port-injected layouts) required for combustion. Its geometry strongly influences volumetric efficiency, throttle response, emissions, and smoothness. This question focuses on the single most important function among several secondary benefits often attributed to the manifold. Given Data / Assumptions: Multi-cylinder internal combustion engine operating across a speed/load range. Modern engines may use electronic fuel injection; older designs may mix fuel upstream. Manifold includes a plenum and individual runners of tuned lengths and diameters. Concept / Approach: The core task is cylinder-to-cylinder distribution. If each cylinder does not receive an equivalent mass of air (and, where applicable, equal mixture quality), the engine will exhibit imbalance, roughness, power loss, and increased emissions. Manifold design (runner cross-section, length, taper, and plenum volume) aims to equalize dynamic pressure waves and minimize maldistribution across cylinders over the expected operating envelope. Step-by-Step Solution: 1) Identify the need: every cylinder must inhale approximately the same air mass per cycle.2) Provide a common plenum to buffer throttle pulsations and enable pressure equalization.3) Route matched runners of appropriate length/diameter so flow resistance and wave tuning are comparable cylinder to cylinder.4) Result: more uniform filling, smoother operation, and balanced combustion. Verification / Alternative check: Datalogs of manifold absolute pressure (MAP) and cylinder-wise lambda show that well-designed manifolds reduce cylinder variability. Flow bench and CFD studies also validate distribution improvements with tuned runners and adequate plenum sizing. Why Other Options Are Wrong: Promotes mixture formation — possible in carbureted systems, but not the primary function. Reduces intake noise — a useful side effect (the plenum acts like a silencer), yet secondary. Cools intake air — cooling is mainly via charge-air coolers or heat shielding, not the manifold itself. Common Pitfalls: Confusing mixture quality with distribution; overemphasizing acoustic silencing over airflow equality; ignoring that modern port/direct injection decouples fuel mixing from manifold function. Final Answer: distributes intake air equally to the cylinders

Question 19

Occupant protection — within a vehicle's supplementary restraint system (SRS), which component is the most widely used and provides primary restraint in crashes?

Accepted Answer

Introduction / Context: Supplementary restraint system (SRS) is an umbrella term for passive safety devices designed to work together to mitigate occupant injury in a crash. While airbags get visual attention, the primary and most widely used restraint remains the seat belt, which restrains occupant motion and manages crash energy via controlled load limiting and pre-tensioning. Given Data / Assumptions: SRS includes seat belts, airbags, sensors, pretensioners, and control modules. Seat belts are worn on every trip (compliance assumed), across all seating positions. Airbags supplement but do not replace belt restraint. Concept / Approach: Seat belts provide primary restraint by preventing occupant ejection, controlling deceleration loads on the chest/pelvis, and properly positioning occupants for effective airbag deployment. Airbags deploy only in certain crash severities/directions and are explicitly designed to supplement belts. Brakes and steering are active control systems for avoiding crashes, not SRS components. Step-by-Step Solution: 1) Identify SRS purpose: reduce injury during crash events.2) Determine which device is universally present and engaged: the seat belt.3) Recognize airbags deploy selectively and rely on occupants being belted for optimal protection.4) Conclude seat belt is the most commonly used and primary SRS restraint. Verification / Alternative check: Regulatory and epidemiological data consistently show belts as the single most effective in-vehicle safety device for injury reduction in most crash modes, with airbags providing additional benefit when used in combination. Why Other Options Are Wrong: Brake — not an SRS component; it is an active system for speed control. Airbag — crucial but supplemental; not engaged on every trip and not effective without belts. Steering — part of vehicle control, not restraint. Common Pitfalls: Assuming airbags alone provide sufficient protection; neglecting seat belt usage which is foundational to SRS performance. Final Answer: seat belt

Question 20

Engine lubrication — evaluate the statement: “A baffle plate is fitted inside the oil pan to prevent the oil from splashing when subjected to vibration, acceleration, braking, and cornering during vehicle operation.”

Accepted Answer

Introduction / Context: The oil pan (sump) stores and returns engine oil to the pump pickup. During vehicle maneuvers or on rough roads, rapid changes in acceleration can cause oil to surge or aerate, risking momentary oil starvation. Engineers add internal baffles and windage trays to control fluid motion and ensure a stable oil supply. Given Data / Assumptions: Wet-sump lubrication with a pickup strainer located in the pan. Vehicle experiences longitudinal and lateral accelerations and vibration. Baffle plates and trap doors may be present near the pickup area. Concept / Approach: Baffle plates partition the sump into regions that slow oil slosh, keeping oil pooled near the pickup. They may include one-way trap doors that allow oil to flow toward the pickup under acceleration while restricting reverse flow. Windage trays reduce oil whipped by the rotating crankshaft, lowering aeration. Together, these features stabilize oil delivery and prevent pressure drop spikes that can damage bearings and valvetrain components. Step-by-Step Solution: 1) Identify dynamic conditions: braking, cornering, and bumps displace oil in the pan.2) Install baffles/trap doors to limit free surface motion and retain oil near the pickup.3) Reduce aeration and cavitation risk at the pump inlet to maintain pressure.4) Achieve consistent lubrication even under spirited driving or off-road use. Verification / Alternative check: Oil pressure logs show fewer transient drops with baffled pans. Motorsport and off-road vehicles commonly upgrade to baffled or dry-sump systems precisely for this reason. Why Other Options Are Wrong: False — contradicts standard wet-sump design practice and the known function of baffles and windage control hardware. Common Pitfalls: Confusing baffles (slosh control) with cooling fins (heat rejection); assuming only racing engines need baffles—production vehicles also benefit for durability. Final Answer: True

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