Advanced Automobile interview questions

125 questions with answers (AUT-301 to AUT-425). Try each one aloud before reading the answer.

Engine Fundamentals

AUT-301 · What displacement does a four-stroke engine need to make 100 kW at 6000 rpm with BMEP of 12 bar?

P = BMEP·Vd·N/120, so Vd = 120·P/(BMEP·N) = 120×100 000/(1.2×10⁶×6000) ≈ 1.67×10⁻³ m³. About 1.7 L is needed.

AUT-302 · What are the trade-offs of engine downsizing with turbocharging?

A smaller engine has lower friction and pumping losses at part load, which improves cycle fuel economy and CO₂. Trade-offs are knock limits (needing a lower CR or retarded timing), turbo lag, enrichment at high load and higher thermal and mechanical stress. Real-world gains are often smaller than test-cycle gains.

AUT-303 · A petrol engine has a BSFC of 240 g/kWh. What is its brake thermal efficiency? (LHV = 43 MJ/kg)

η = 3600/(BSFC × LHV in MJ/kg) = 3600/(240×43) ≈ 0.349, so about 35%.

AUT-304 · Why do hybrid engines often use an Atkinson or Miller cycle?

Late (or early) intake valve closing makes the effective compression ratio smaller than the expansion ratio. This cuts pumping loss and extracts more work, giving high efficiency but low power density. The electric motor makes up the low-end torque, so the trade-off works well in hybrids.

AUT-305 · Why do some inline four-cylinder engines have balance shafts?

In an inline-4, primary forces cancel, but secondary inertia forces, at twice crank speed, add up and cause vibration that grows with displacement and rpm. Two counter-rotating shafts at 2× crank speed (Lanchester balancer) cancel this force.

AUT-306 · An engine overheats only at idle in traffic but is fine on the highway. What is the likely cause?

Highway ram air hides the problem, so suspect low airflow at idle: a failed radiator fan, fan relay or sensor, a slipping viscous fan clutch, a missing fan shroud or a clogged radiator exterior. A stuck thermostat or weak pump would usually overheat at speed as well.

AUT-307 · How can engine friction be reduced in design?

Use low-tension piston rings, roller cam followers, low-viscosity oil, coated pistons and liners, offset crankshafts, variable-flow oil and coolant pumps, smaller bearings and electric accessories. Together these improve part-load BSFC.

AUT-308 · How does cylinder deactivation improve fuel economy?

At light load, valves on some cylinders are held closed and fuel is cut. The remaining cylinders work at higher load with a wider throttle opening, which reduces pumping loss. The challenges are NVH and smooth transitions.

AUT-309 · What do blue, white and black exhaust smoke indicate?

Blue smoke means oil burning (worn rings, valve stem seals or turbo seals). Persistent white smoke means coolant burning (head gasket); white smoke from a diesel can also be unburnt fuel at cold start. Black smoke means a rich mixture or poor combustion (faulty injectors, restricted air filter, overfuelling).

AUT-310 · What does the heat balance of a typical IC engine look like at full load?

Roughly a third of fuel energy becomes brake work (about 25–40%), with the rest split between coolant and exhaust (each roughly 25–35%), plus a smaller share to radiation, oil and friction. Waste heat recovery targets the exhaust share.

AUT-311 · Calculate the indicated power of a 4-cylinder, four-stroke engine (bore 80 mm, stroke 90 mm) at IMEP 10 bar and 3000 rpm.

Vd = 4 × (π/4)×0.08²×0.09 ≈ 1.81×10⁻³ m³. IP = IMEP·Vd·N/120 = 10⁶ × 1.81×10⁻³ × 3000/120 ≈ 45.2 kW.

Fuel Systems & Combustion

AUT-312 · What is HCCI and why is it hard to commercialise?

Homogeneous charge compression ignition auto-ignites a premixed lean charge, giving diesel-like efficiency with very low NOx and soot. It is hard to commercialise because ignition timing has no direct trigger, the load range is narrow (knock at high load, misfire at low load) and HC/CO are high. Spark-assisted variants are used to control it.

AUT-313 · How is a turbocharger matched to an engine using the compressor map?

Plot the engine's airflow and pressure ratio across its speed–load range on the compressor map. The operating line must stay right of the surge line, left of the choke region and within the speed limit, ideally through the high-efficiency islands at key operating points.

AUT-314 · A naturally aspirated 2.0 L engine makes 100 kW. Roughly what boost pressure ratio is needed for 150 kW at the same speed?

Power scales with air mass, so about 1.5× density is needed. With an effective intercooler, the pressure ratio is about 1.5 plus margin for higher charge temperature and back-pressure, so about 1.6–1.7. Knock may also force lower efficiency.

AUT-315 · What are the effects of raising diesel injection pressure above 2000 bar?

Finer atomisation and faster mixing reduce soot and allow more EGR for NOx control, improving the NOx–PM trade-off. The costs are higher pump parasitic power, more combustion noise without pilot shots, and tighter tolerances and filtration needs.

AUT-316 · What are the effects of running petrol engines on ethanol blends such as E20?

Ethanol raises octane and charge cooling, allowing more spark advance, but has about one-third less energy per litre than petrol, so volumetric fuel economy drops. Fuel system materials (elastomers, metals) must be compatible, and cold-start calibration may need changes.

AUT-317 · A GDI engine shows high particulate emissions and intake valve deposits. What can be done?

Use a gasoline particulate filter, higher injection pressure, multiple injections, better spray targeting to avoid wall wetting, and warm-up strategies. Valve deposits form because fuel no longer washes the valves; dual (port + direct) injection solves this.

AUT-318 · How does a closed-loop knock control strategy work?

The knock sensor signal is filtered and windowed per cylinder around the combustion angle and compared with a learned noise threshold. On knock, the ECU retards timing on that cylinder in a step, then advances it slowly. Persistent knock triggers enrichment or boost reduction.

AUT-319 · What changes when a petrol engine is converted to CNG?

CNG has a high octane rating (allowing more advance or higher CR in dedicated engines) and gives lower CO and HC. Power drops because gaseous fuel displaces intake air and gives no evaporative cooling. Slow flame speed needs more advance, and dry gas can cause valve seat wear.

AUT-320 · What are the main challenges of hydrogen IC engines?

Hydrogen's wide flammability and low ignition energy cause pre-ignition and backfire. NOx forms near stoichiometric, so lean operation is used. Low volumetric energy density needs direct injection or boosting, and hydrogen embrittlement and storage are problems. There is essentially no CO₂ or CO from the fuel.

AUT-321 · Long-term fuel trim is +20% at idle but near 0% at high load. What is the likely fault?

An unmetered air leak, such as a vacuum hose, intake gasket or PCV leak. It is a large fraction of airflow at idle and negligible at high flow. If the trim were positive across all loads, suspect low fuel pressure, clogged injectors or a MAF under-reading.

Clutch, Gearbox & Transmission

AUT-322 · Design a single-plate clutch for 250 N·m with a 1.3 service factor, μ = 0.3, R₁ = 120 mm and R₂ = 80 mm. Find the clamp load and peak pressure.

Design torque = 325 N·m, Rm = 0.1 m, n = 2. W = T/(μ·n·Rm) ≈ 5.42 kN. Uniform wear: C = W/(2π(R₁ − R₂)) ≈ 21.6 kN/m, so p_max = C/R₂ ≈ 270 kPa. This is acceptable for organic linings.

AUT-323 · How much heat does a clutch absorb during a vehicle launch?

With constant engine speed and constant clutch torque, the slip energy equals the vehicle's kinetic energy at lock-up. For 1500 kg locking up at 3 m/s, that is ½×1500×3² ≈ 6.8 kJ. Gradients, trailers and repeated starts raise it sharply, which limits clutch thermal capacity.

AUT-324 · Choose the overall top-gear ratio for 180 km/h at 6000 rpm with a 0.31 m rolling radius.

v = 50 m/s. Wheel speed = 50/(2π×0.31) × 60 ≈ 1540 rpm. Overall ratio = 6000/1540 ≈ 3.9. Max speed is often set near or slightly above peak-power rpm, where power meets road load.

AUT-325 · Find the first-gear overall ratio to climb a 30% grade (m = 1500 kg, Crr = 0.015, T = 200 N·m, η = 0.9, r = 0.3 m).

θ = 16.7°, sin θ = 0.287. Required F = 1500×9.81×(0.287 + 0.015×0.958) ≈ 4.43 kN. Ratio = F·r/(T·η) = 4430×0.3/180 ≈ 7.4. The driven axle load must still support this force without wheelspin.

AUT-326 · How would you choose between AT, CVT and DCT for a compact city car?

A CVT gives smooth, efficient engine operating points and low cost but has a torque limit and "rubber-band" feel. A DCT gives fast, efficient shifts but dry versions overheat in creep traffic. A torque-converter AT is smooth and durable in traffic but heavier. Duty cycle, torque, cost and market preference decide.

AUT-327 · What happens in the torque phase and inertia phase of an automatic upshift?

In the torque phase, torque transfers from the off-going clutch to the on-coming clutch at constant speed, and output torque dips. In the inertia phase, engine speed is pulled down to the new ratio. Engine torque reduction (spark retard) during this phase reduces the output torque spike and clutch energy.

AUT-328 · An AMT car creeps jerkily in traffic. How would you fix it?

Re-learn the clutch kiss point and the clutch torque-versus-position map, check actuator hysteresis and position sensor, and improve slip control (closed-loop on engine–input speed difference). Adding creep torque ramping and thermal protection for repeated slip also helps.

AUT-329 · Where are power losses in a manual gearbox, and how can efficiency be improved?

Losses come from gear mesh friction (each mesh is roughly 98–99% efficient), oil churning, bearing and seal friction. Improvements: low-viscosity oil with a controlled level or dry sump, low-friction bearings and seals, superfinished teeth and fewer meshes in the most-used gears.

AUT-330 · How is a torque converter matched to an engine?

The converter capacity factor K = N/√T defines how much torque it absorbs at a given impeller speed. Overlay the converter curve on the engine torque curve: the intersection gives the stall speed. Choose K so stall falls near peak torque for performance, or lower for economy and towing.

AUT-331 · What is a Ravigneaux gear set and why is it used?

A compound planetary set with two sun gears, short and long planets on one carrier and one ring gear. It gives four or more ratios in a compact unit with few parts, which is why it is common in automatic transmissions.

Driveline & Differential

AUT-332 · Why are crown wheel and pinion tooth counts chosen as a "hunting" ratio such as 41/11?

If the counts share no common factor, each pinion tooth meets every crown wheel tooth in turn. This spreads wear evenly and avoids repeating defects. Here 41/11 ≈ 3.73.

AUT-333 · How does a torque-vectoring differential work?

It uses clutches with overdrive or underdrive gear sets, or two motors in EVs, to actively send more torque to the outer wheel in a corner. This creates a yaw moment that reduces understeer without braking losses.

AUT-334 · What are the trade-offs between on-demand and permanent AWD?

On-demand systems (an electronically controlled coupling engaging the secondary axle) are light and efficient but react to slip or predict it. Permanent AWD with a centre differential gives constant traction and stable handling but has more mass and drag losses. Disconnecting systems try to combine both.

AUT-335 · Find the whirling speed of a 1.5 m steel propeller shaft tube (OD 76 mm, 2 mm wall).

I ≈ 3.19×10⁻⁷ m⁴, A ≈ 4.65×10⁻⁴ m², EI ≈ 6.7×10⁴ N·m², ρA ≈ 3.65 kg/m. ωc = (π²/L²)√(EI/ρA) ≈ 4.39×135 ≈ 594 rad/s ≈ 5670 rpm. If maximum shaft speed is about 5000 rpm, use a two-piece shaft, larger diameter or CFRP.

AUT-336 · A Torsen differential has a torque bias ratio of 3:1. One wheel can support only 100 N·m. What is the maximum total torque?

The high-grip wheel can receive up to 3 × 100 = 300 N·m, so the axle can deliver up to 400 N·m in total. An open differential would deliver only 200 N·m.

AUT-337 · How large is the speed fluctuation of a single Hooke's joint at 15°?

Output speed varies between ω·cos β and ω/cos β, which is 0.966ω to 1.035ω, about ±3.5%, twice per revolution. The resulting torsional excitation is why joint angles are limited and joints are phased.

AUT-338 · What design challenges does an integrated e-axle face?

Packaging the motor, inverter and reduction gear in one housing creates shared cooling and lubrication needs. Other challenges are high motor speed (often above 15 000 rpm) causing gear whine and bearing stress, electromagnetic noise without engine masking, and shaft voltage damaging bearings (needing insulated bearings or grounding rings).

AUT-339 · A RWD truck clunks when taking up drive. What would you check?

Excessive backlash in the crown wheel and pinion or differential gears, worn universal joints, worn slip-yoke splines, loose propeller shaft bolts, worn engine or gearbox mounts, and leaf spring U-bolts or bushings allowing axle wind-up.

AUT-340 · What is driveline shuffle, and how do EVs suppress it?

A low-frequency (a few Hz) fore-aft oscillation after a sudden throttle tip-in. It comes from driveline torsional compliance working against vehicle inertia. EVs suppress it with active damping in motor torque control, which shapes or counteracts torque based on measured speed oscillation.

AUT-341 · Size a solid half shaft. Engine 250 N·m, first overall ratio 13, η = 0.9, open differential, allowable shear 300 MPa.

Torque per shaft = 250×13×0.9/2 ≈ 1460 N·m. d = (16T/(πτ))^(1/3) = (16×1460/(π×3×10⁸))^(1/3) ≈ 29 mm. Check against the traction-limited torque and add a shock factor for clutch dumps.

Suspension

AUT-342 · What does the quarter-car model reveal about ride and road holding?

It has a sprung mass on suspension spring and damper, and an unsprung mass on tyre stiffness. It shows two modes: body bounce (about 1–1.5 Hz) and wheel hop (about 10–15 Hz). Softer springs improve comfort, but damping chosen for body control conflicts with damping for minimum tyre load variation.

AUT-343 · Find the spring rate for a 350 kg sprung corner mass at 1.3 Hz, with motion ratio 0.9.

Wheel rate = (2πf)²·m = (8.17)²×350 ≈ 23.4 kN/m. Spring rate = wheel rate / MR² = 23.4/0.81 ≈ 28.8 kN/m. Tyre stiffness in series lowers the ride rate slightly.

AUT-344 · Why is rear ride frequency often set slightly higher than front?

On a bump, the front wheels are disturbed first. A slightly stiffer rear lets the rear catch up so the body bounces rather than pitches. This is the Olley "flat ride" principle; typical rear frequencies are about 10–20% higher.

AUT-345 · Find the damping coefficient for ζ = 0.3 at a corner with k = 23.4 kN/m and m = 350 kg.

Critical damping cc = 2√(k·m) = 2√(23 400×350) ≈ 5.72 kN·s/m. c = 0.3×5720 ≈ 1.72 kN·s/m at the wheel. Divide by MR² to get the damper's own coefficient.

AUT-346 · How does roll stiffness distribution affect handling balance?

The axle with more roll stiffness takes more lateral load transfer. Because of tyre load sensitivity, it loses more grip. A stiffer front anti-roll bar increases understeer; a stiffer rear bar shifts toward oversteer.

AUT-347 · Find the roll angle at 0.5g for m = 1500 kg, roll arm 0.4 m and Kφ = 80 kN·m/rad.

φ = m·ay·h/(Kφ − m·g·h) = 1500×4.905×0.4/(80 000 − 5886) ≈ 2943/74 114 ≈ 0.040 rad ≈ 2.3°. That is a roll gradient of about 4.5°/g.

AUT-348 · Estimate the wheel-hop frequency (mu = 40 kg, ks = 25 kN/m, kt = 200 kN/m).

f = (1/2π)√((ks + kt)/mu) = (1/2π)√(225 000/40) = 75/(2π) ≈ 11.9 Hz. Heavier unsprung mass, for example in-wheel motors, lowers this and increases tyre load variation.

AUT-349 · What is skyhook damping?

A semi-active control idea in which the damper force is made proportional to absolute body velocity, as if attached to a fixed "sky". A semi-active damper approximates it by switching to high damping when body velocity and relative velocity have the same sign. It reduces body motion without the usual harshness trade-off.

AUT-350 · How does a heavy floor battery change suspension design for an EV?

Higher sprung mass needs stiffer springs and bushes for the same frequencies and stronger links. A low CG cuts roll, which allows softer anti-roll bars. In-wheel motors would raise unsprung mass and hurt ride and grip, which is why most EVs use inboard motors.

AUT-351 · What is kinematic and compliance (K&C) testing?

A rig test that measures how wheel camber, toe, track and wheelbase change with suspension travel and roll (kinematics), and under applied lateral, longitudinal and aligning forces (compliance). It is used to validate and tune handling models.

AUT-352 · A van bottoms out when fully loaded but is harsh when empty. What solutions exist?

Use progressive-rate or dual-rate springs, longer progressive bump stops, helper leaves, load-sensing or air self-levelling suspension, and position-sensitive dampers. Each lets the suspension be soft when empty and support the high load.

Steering & Wheel Alignment

AUT-353 · What is Ackermann steering geometry?

Steering geometry in which the inner wheel turns more sharply than the outer, so all wheels roll about a common centre on concentric circles without tyre scrub. The condition is cot δo − cot δi = w/L, where w is the king-pin track and L the wheelbase.

AUT-354 · For L = 2.6 m, w = 1.4 m and δi = 35°, what outer wheel angle gives true Ackermann?

cot δo = cot 35° + 1.4/2.6 = 1.428 + 0.538 = 1.967, so δo ≈ 27.0°.

AUT-355 · Why do race cars often use reduced Ackermann or parallel steer?

At high lateral acceleration the outer wheel carries most of the load and works at a large slip angle. Less Ackermann increases the inner wheel slip angle to use its share of grip and reduces low-speed geometry effects that do not apply at the limit.

AUT-356 · How is an EPS assist curve calibrated?

Assist torque is mapped against driver torque and vehicle speed: high gain when parking and low gain at speed for weighting and stability. Damping, inertia compensation, active return and friction compensation are added to tune on-centre feel and reduce kickback.

AUT-357 · What are the key design requirements for steer-by-wire?

Removing the mechanical column needs fail-operational redundancy (dual sensors, motors, power and ECUs, ASIL D), a feedback actuator to recreate road feel, and fault handling. Benefits include variable ratio, packaging freedom and integration with automated driving.

AUT-358 · How does four-wheel steering improve both manoeuvrability and stability?

At low speed the rear wheels steer opposite to the front (counter-phase), reducing turning radius. At high speed they steer in phase by a small angle, reducing yaw overshoot and body slip in lane changes.

AUT-359 · A car pulls to the left on a flat road. How would you diagnose it?

Swap the front tyres side to side: if the pull reverses, it is tyre conicity. Otherwise check pressures, side-to-side caster and camber, ride height, brake drag (hot wheel), wheel bearing preload and thrust angle.

AUT-360 · What causes steering wheel shimmy at a certain speed?

Usually wheel and tyre imbalance or radial runout exciting the wheel's steer mode at its resonance, typically around 80–110 km/h. It is amplified by worn tie rod ends, ball joints, bushes or weak dampers. Brake judder from disc thickness variation appears only during braking.

AUT-361 · How do pneumatic trail and aligning torque warn the driver of front grip loss?

Pneumatic trail falls as slip angle rises, because the lateral force centroid moves forward in the contact patch. Aligning torque peaks before maximum lateral force, so steering lightens just before the front tyres saturate.

AUT-362 · A car has 36° maximum road wheel angle and a 15:1 steering ratio. How many turns is it lock to lock?

Steering wheel angle = 36 × 15 = 540° each side. Lock to lock = 1080° = 3 turns. Lowering the ratio to 12:1 gives 2.4 turns but more parking effort without assist.

AUT-363 · What is active front steering?

A planetary gear set in the steering column, driven by an electric motor, adds a superimposed angle to the driver's input. This gives variable ratio with speed and allows yaw-stabilising steering corrections, while keeping a mechanical link.

Braking Systems

AUT-364 · A 1500 kg car stops from 100 km/h. Estimate each front disc's temperature rise if the front takes 70% of the energy (8 kg discs, c = 460 J/kg·K).

KE = ½×1500×27.8² ≈ 579 kJ. Each front disc takes 0.35×579 ≈ 203 kJ. ΔT = 203 000/(8×460) ≈ 55 K for one stop, ignoring cooling and pad share. Repeated stops accumulate heat.

AUT-365 · How do you find the ideal front brake force fraction?

Ideal front fraction = (static front load + transfer)/total = lr/L + z·h/L, where z is deceleration in g. For lr/L = 0.6, h/L = 0.2 and z = 0.8: 0.6 + 0.16 = 0.76. Real fixed-ratio systems sit below this ideal curve so the front axle locks first.

AUT-366 · Size a front caliper for 1g braking. m = 1500 kg, 70% front, tyre radius 0.3 m, effective disc radius 0.13 m, pad μ = 0.4, line pressure 100 bar.

Force per front wheel = 1500×9.81×0.7/2 ≈ 5.15 kN. Torque ≈ 1545 N·m. Clamp force = 1545/(2×0.4×0.13) ≈ 14.9 kN. Piston area = 14 900/10⁷ ≈ 1.49×10⁻³ m², about a 44 mm single piston in a floating caliper.

AUT-367 · How does ABS control logic decide when to release pressure?

The ECU estimates a vehicle reference speed from all wheel speeds and computes each wheel's slip and deceleration. If wheel deceleration exceeds a threshold, for example around −1.5 g, or slip exceeds a limit, it holds and then dumps pressure. It reapplies when the wheel re-accelerates, cycling around peak μ.

AUT-368 · How does ABS handle split-μ braking?

Fronts are controlled individually, but yaw moment build-up is delayed by limiting how fast the high-μ side pressure rises, so the driver can counter-steer. Rears use select-low control: both rears are limited to the low-μ wheel's pressure for stability.

AUT-369 · How are regenerative and friction braking blended in a brake-by-wire EV?

The pedal acts as a demand sensor, and the brake controller fills the total deceleration demand with motor regen first. Friction brakes make up the shortfall when regen is limited (full battery, cold battery, low speed, ABS events, rear stability). Accurate blending is needed so pedal feel stays constant.

AUT-370 · A car has steering wheel vibration only when braking from high speed. What is the cause?

Disc thickness variation (DTV), which causes torque variation and judder. It usually starts from lateral runout due to hub face corrosion, poor fitting or uneven wheel nut torque. Runout causes uneven pad contact, wear and deposits. Measure runout and DTV, clean the hub, machine or replace discs and bed in pads.

AUT-371 · How can disc brake squeal be reduced?

Squeal is a friction-induced coupled vibration, often 1–16 kHz. Countermeasures: chamfers and slots on pads, shims with damping layers, pad lubrication on abutments, changing disc or caliper modal frequencies, and different friction formulations. Complex eigenvalue FE analysis predicts unstable modes.

AUT-372 · How would you design a brake fade test?

Perform repeated stops from high speed at fixed deceleration and short intervals, logging disc and fluid temperatures, pedal force and deceleration. Then do recovery stops. The design passes if pedal force rise and deceleration loss stay within targets and the fluid stays below its boiling point.

AUT-373 · Why do long articulated vehicles need relay valves and electronic braking systems (EBS)?

Air signals take time to travel the length of a long pipe, so rear axles and trailers would brake late, causing jack-knife risk. Relay valves near the chambers, using local reservoirs, and EBS with electrical control signals cut response time and allow ABS and stability control on each axle.

AUT-374 · The brake pedal is still spongy after conventional bleeding on an ABS car. What next?

Air may be trapped in the ABS modulator's internal circuits. Use a scan tool to run the modulator bleed routine, which cycles the valves and pump while bleeding. Also check for soft hoses, caliper slide problems and master cylinder bypass.

Chassis, Body & Tyres

AUT-375 · A car hits a rigid barrier at 56 km/h with 0.6 m of crush. What is its average deceleration?

v = 15.6 m/s. a = v²/(2s) = 242/(1.2) ≈ 202 m/s² ≈ 20.6 g. Occupant loads also depend on the shape of the deceleration pulse and restraint ride-down, so crush zones aim for a square-wave pulse.

AUT-376 · How would you compare steel, aluminium and CFRP for lightweighting a body?

Compare cost per kg saved, manufacturing rate, joining, crash behaviour and repairability. AHSS gives modest weight savings at low cost. Aluminium saves much more weight at higher cost. CFRP saves the most but has slow cycle times, high cost and brittle crash behaviour. Mixed-material design uses each where it works best.

AUT-377 · What body design challenges come from integrating an EV battery pack?

The pack needs side-impact and pole protection (sill structures, crash-absorbing frames), underbody impact and debris protection, sealing and venting for thermal events, and good torsional stiffness. Cell-to-pack or cell-to-body designs use the pack as a structural floor member.

AUT-378 · A torque of 10 kN·m twists a body 0.5° between the axle lines. What is its torsional stiffness?

K = T/θ = 10 000/0.5 = 20 000 N·m/deg (20 kN·m/deg). Modern passenger car bodies are often in the tens of kN·m/deg.

AUT-379 · What is the tyre "magic triangle"?

The trade-off between rolling resistance, wet grip and wear life. Improving one usually costs another. Silica-filled tread compounds with silane coupling cut hysteresis at rolling frequencies while keeping it high at grip frequencies, improving both RR and wet grip.

AUT-380 · How do EV tyre requirements differ from ICE tyre requirements?

EVs need a higher load capacity for battery mass (often HL/XL ratings), low rolling resistance for range, durable tread for high instant torque, and low noise because there is no engine masking. Some EV tyres have acoustic foam inside.

AUT-381 · What joining methods are used for mixed steel–aluminium bodies?

Self-piercing rivets, flow-drill screws, clinching and structural adhesives, often combined. Resistance spot welding of steel to aluminium directly is avoided because brittle intermetallics form. Galvanic corrosion is controlled with sealants, coatings and isolation.

AUT-382 · How is a front bumper beam and crash box system designed?

The beam spreads load across the front. Crash boxes fold progressively in low-speed impacts, absorbing energy and protecting the main rails, so the car can be repaired cheaply. In high-speed crashes they trigger rail crush in a controlled sequence. Thickness, section and folding initiators are tuned with FE simulation.

AUT-383 · How is chassis durability validated?

Road load data is recorded on proving-ground surfaces with wheel force transducers and strain gauges. It is used for fatigue damage calculations and multi-axial rig tests that compress the vehicle's life into weeks. FE fatigue predictions are correlated with the rig results.

AUT-384 · Estimate the dynamic aquaplaning speed of a tyre inflated to 220 kPa.

Horne's empirical formula (from aircraft tyres) gives v (knots) ≈ 9√p (psi). 220 kPa ≈ 32 psi, so v ≈ 51 knots ≈ 94 km/h. Worn tread, deep water and wide tyres lower the real onset speed.

Vehicle Dynamics

AUT-385 · Compare traction-limited acceleration for RWD and FWD. lf/L = lr/L = 0.5, h/L = 0.2, μ = 0.9.

RWD: a/g = μ·(lf/L)/(1 − μ·h/L) = 0.45/0.82 ≈ 0.55 g. FWD: a/g = μ·(lr/L)/(1 + μ·h/L) = 0.45/1.18 ≈ 0.38 g. Load transfer favours the rear axle.

AUT-386 · Calculate the understeer gradient and characteristic speed. Wf = 8800 N, Wr = 5900 N, Cαf = Cαr = 2500 N/deg per axle, L = 2.6 m.

K = 8800/2500 − 5900/2500 ≈ 3.52 − 2.36 = 1.16 deg/g. Characteristic speed = √(57.3·L·g/K) = √(57.3×2.6×9.81/1.16) ≈ 35.5 m/s ≈ 128 km/h.

AUT-387 · What is steady-state yaw rate gain?

r/δ = v/(L + K·v²/(57.3·g)), with K in deg/g. For an understeering car the gain peaks at the characteristic speed and then falls. For an oversteering car it rises without bound toward the critical speed.

AUT-388 · What is the bicycle (single-track) model?

A linear 2-DOF model that lumps each axle's tyres into one, with lateral velocity and yaw rate as states. It predicts steady-state cornering, understeer gradient, yaw response and stability, and is the basis of ESC reference models.

AUT-389 · How does ESC limit its yaw rate reference?

The desired yaw rate from the bicycle model is capped at about μ·g/v, because the road cannot support more lateral acceleration. Excessive body slip angle or slip rate also triggers intervention, through individual wheel braking and engine torque cut.

AUT-390 · How are road load coefficients found from a coastdown test?

The vehicle coasts in neutral from high speed while speed versus time is logged in both directions on a level road. Deceleration × effective mass gives road load, fitted as F = A + B·v + C·v². A is mostly rolling resistance and C mostly aerodynamic. These coefficients are set on dynamometers for emission and range tests.

AUT-391 · Estimate the energy consumption of an EV at 90 km/h. m = 1800 kg, Crr = 0.009, Cd = 0.28, A = 2.3 m², drivetrain efficiency 88%.

v = 25 m/s. Fr ≈ 159 N, Fd = 0.5×1.2×0.28×2.3×625 ≈ 242 N, total ≈ 401 N. Wheel power ≈ 10.0 kW; battery power ≈ 11.4 kW. Energy ≈ 11.4/90×100 ≈ 12.7 kWh/100 km, before HVAC and auxiliary loads.

AUT-392 · What is lift-off oversteer?

Lifting the throttle suddenly mid-corner transfers load forward. This raises front grip and cuts rear grip, and engine braking on driven wheels adds rear longitudinal slip. The rear slip angle grows and the car tightens its line or spins. It is most pronounced in FWD and mid-engined cars.

AUT-393 · Show how lateral load transfer reduces axle grip. μ = 1.2 − 0.0001·Fz (N), 4000 N per tyre statically, 2000 N transfer.

Static: each tyre gives 0.8×4000 = 3200 N, total 6400 N. With transfer, the outer tyre at 6000 N gives 0.6×6000 = 3600 N and the inner at 2000 N gives 1.0×2000 = 2000 N, total 5600 N. The axle loses about 12% of its grip.

AUT-394 · How would you reduce drag on an EV to extend range?

Use a smooth sealed underbody, active grille shutters (EVs need less cooling air), a tapered tail with a sharp trailing edge, aero wheels and air curtains, flush handles, cameras instead of mirrors where allowed, and a lower ride height at speed. Drag is about half or more of road load at highway speed.

AUT-395 · An SUV fails the ISO 3888-2 obstacle-avoidance (moose) test. What would you change?

Increase roll stiffness and damping to reduce roll and wheel lift, lower the CG, increase track, use stiffer-sidewall tyres, retune ESC for earlier intervention and roll-over mitigation, and reduce rear compliance oversteer so the rear stays stable through the second lane change.

Automotive Electronics & ECUs

AUT-396 · What is the CAN bus?

Controller Area Network: a robust, multi-master, differential two-wire serial bus. Nodes broadcast ID-tagged frames, bitwise arbitration gives the lowest ID (highest priority) access without collisions, and CRC, acknowledgement, bit monitoring and error counters give strong error detection and fault confinement.

AUT-397 · Two CAN nodes start transmitting IDs 0x100 and 0x0FF at the same time. Which wins and why?

0x0FF wins. In arbitration, a dominant 0 overwrites a recessive 1. Both send the MSB of the ID first: at bit 8 node 0x100 sends 1 and 0x0FF sends 0. Node 0x100 reads back a 0, stops transmitting and retries later.

AUT-398 · Estimate CAN bus load. 500 kbit/s; 20 messages every 10 ms and 30 every 100 ms, each about 125 bits with stuffing.

Messages per second = 2000 + 300 = 2300. Bits per second ≈ 2300×125 = 287.5 kbit/s. Load ≈ 57%. Many designs keep load well below saturation to protect latency of low-priority frames.

AUT-399 · How does CAN fault confinement work?

Each node has a transmit and a receive error counter. Below 128 the node is error-active and sends active error flags. Above 127 it becomes error-passive and sends passive flags. If the transmit counter exceeds 255 the node goes bus-off and stops transmitting until recovery, so a faulty node cannot jam the bus.

AUT-400 · Why are vehicles moving to zonal E/E architectures with Automotive Ethernet?

Many separate ECUs and long dedicated harnesses are replaced by a few high-performance central computers linked over Ethernet (100BASE-T1, 1000BASE-T1) to zonal controllers that serve local I/O. This cuts harness weight and cost and enables software-defined features and OTA updates.

AUT-401 · How is an ASIL assigned under ISO 26262?

Hazard analysis and risk assessment rates each hazardous event for Severity (S0–S3), Exposure (E0–E4) and Controllability (C0–C3). The combination gives QM or ASIL A to D. For example, unintended full braking at speed is typically rated high. The ASIL sets the required development rigour and hardware metrics.

AUT-402 · How is vehicle cybersecurity addressed?

ISO/SAE 21434 defines cybersecurity engineering and threat analysis (TARA). UNECE R155 requires a certified cybersecurity management system and R156 a software update management system. Technical measures include secure boot, SecOC message authentication, gateway firewalls, HSMs and intrusion detection.

AUT-403 · Several ECUs intermittently log "lost communication" U-codes. How would you diagnose it?

With the battery disconnected, measure resistance between CAN_H and CAN_L: about 60 Ω is normal, 120 Ω means a missing terminator and near 0 Ω a short. Scope both lines for amplitude, noise and reflections. Check connectors, shielding and ground offsets, and unplug nodes one by one to find one dragging the bus.

AUT-404 · What is a torque-based engine control architecture?

All torque requests (driver pedal, cruise, ESC, transmission shifts, idle) are expressed as torque, arbitrated and then converted into air, fuel and spark targets. This decouples functions and makes powertrain coordination modular.

AUT-405 · What is hardware-in-the-loop (HIL) testing of an ECU?

The real ECU is connected to a real-time simulator that models the vehicle and plant, emulates sensors and loads actuators. It allows automated, repeatable testing of functions, diagnostics and fault injection (open or short circuits) before vehicle prototypes exist.

Electric & Hybrid Vehicles

AUT-406 · What does a battery management system (BMS) do?

It monitors every cell's voltage and temperature and the pack current, and estimates SOC, SOH and available power. It enforces protection limits (over- and under-voltage, over-current, temperature), balances cells, controls contactors and pre-charge, monitors isolation and commands thermal management.

AUT-407 · How does regenerative braking work, and what limits it?

The inverter runs the traction motor as a generator, producing braking torque and returning kinetic energy to the battery. It is limited by motor and inverter torque, battery charge-power acceptance (high SOC, cold cells), very low speed, and stability on only the driven axle. Friction brakes cover the rest.

AUT-408 · Compare series, parallel and power-split hybrids.

Series: the engine only drives a generator, and the motor drives the wheels; it is efficient in city driving but double conversion costs efficiency on the highway. Parallel: engine and motor both drive the wheels mechanically. Power-split (for example Toyota THS): a planetary gear divides engine power between a mechanical path and a generator–motor electrical path, acting as an eCVT.

AUT-409 · What is the difference between passive and active cell balancing?

Passive balancing bleeds excess charge from higher cells through resistors as heat. It is simple and cheap but slow and wasteful. Active balancing transfers charge between cells using capacitors, inductors or transformers; it is efficient and can raise usable capacity, but costs more.

AUT-410 · How is thermal runaway propagation in a battery pack mitigated?

Use thermally stable chemistries (for example LFP), heat barriers such as mica or aerogel between cells, venting channels that direct gases away from the cabin, cooling plates, cell spacing, fused cell connections and early detection with gas or pressure sensors. Regulations increasingly require warning time before hazards reach occupants.

AUT-411 · Estimate the average power needed for an 1800 kg EV to reach 100 km/h in 8 s.

KE = ½×1800×27.8² ≈ 694 kJ. Average power = 694/8 ≈ 87 kW at the wheels, plus road load and losses. Because motors are torque-limited at low speed, peak power is higher, roughly 120 kW or more.

AUT-412 · What are the benefits and challenges of an 800 V EV architecture over 400 V?

At equal power, current halves, so I²R losses fall, cables are lighter and DC fast charging can be faster. Challenges: semiconductors with higher voltage ratings (SiC), insulation and creepage design, motor insulation stress from fast dv/dt, and compatibility with 400 V chargers (boost converter or pack switching).

AUT-413 · How is battery SOC estimated accurately?

Coulomb counting integrates current but drifts with sensor offset. OCV–SOC lookups correct it at rest but fail under load and on flat LFP curves. Model-based observers such as an extended Kalman filter with an equivalent-circuit cell model fuse both and adapt to temperature and ageing.

AUT-414 · How much energy can regeneration recover in one stop from 100 km/h for an 1800 kg EV?

KE ≈ 694 kJ ≈ 0.19 kWh. Subtract road load, motor, inverter and battery losses; with roughly 60–70% recovery to the battery, about 0.12–0.13 kWh is stored. Frequent stop-go driving is where regeneration matters most.

AUT-415 · Why are SiC MOSFETs used in EV traction inverters?

Silicon carbide has lower switching and conduction losses than Si IGBTs, especially at part load and high voltage. It allows higher switching frequency and junction temperature, and gives a few percent more drive-cycle efficiency. The costs are higher price and EMI and dv/dt management.

AUT-416 · Why is DC fast charging power reduced in cold weather?

At low temperature, cell resistance rises and lithium intercalation slows. High charge current risks lithium plating on the anode, which causes permanent capacity loss and internal-short risk. The BMS limits current until the pack is heated, which is why pre-conditioning before fast charging matters.

Emissions, Regulations & Vehicle Safety

AUT-417 · What aftertreatment layout does a BS-VI light-duty diesel typically use, and why that order?

DOC → DPF → urea dosing → SCR → ammonia slip catalyst. The DOC oxidises CO and HC and makes NO₂ for passive DPF regeneration and fast SCR. The DPF traps soot. SCR reduces NOx, and the ASC removes excess NH₃. Some designs combine SCR coating on the DPF (SCRoF) to heat it faster.

AUT-418 · What are the SAE J3016 levels of driving automation?

L0 no automation, L1 driver assistance (steering or speed), L2 partial automation (both, with the driver supervising), L3 conditional automation (the system drives in its operating domain and the driver must take over on request), L4 high automation (no takeover needed within the domain) and L5 full automation everywhere.

AUT-419 · Why do ADAS systems fuse radar, camera and lidar?

Radar measures range and relative speed well in all weather but has low resolution. Cameras classify objects and read lanes and signs but are degraded by glare, darkness and range estimation. Lidar gives precise 3-D shape but costs more and suffers in heavy rain or fog. Fusion gives redundancy and confidence.

AUT-420 · How does SOTIF (ISO 21448) differ from ISO 26262?

ISO 26262 addresses hazards from E/E system malfunctions (random hardware and systematic faults). SOTIF addresses hazards when the system works as designed but its intended function is insufficient, for example sensor limitations or misjudged scenarios. It also covers reasonably foreseeable misuse. Both are applied to ADAS.

AUT-421 · How do fleet-average CO₂ or fuel economy regulations influence vehicle design?

Manufacturers must meet a sales-weighted average target, often mass-adjusted, so they mix in hybrids and EVs (often with super-credits), and adopt downsizing, low-RR tyres, aerodynamics, start-stop and lightweighting. Off-cycle technologies may earn credits under some regimes.

AUT-422 · An AEB system closes at 50 km/h on a stationary car 30 m ahead. When must it brake fully at 0.8g?

v = 13.9 m/s, so TTC = 30/13.9 ≈ 2.2 s. Stopping distance = 13.9²/(2×0.8×9.81) ≈ 12.3 m. Allowing system latency and brake build-up time, full braking must start no later than about 14–15 m (TTC ≈ 1 s). Warnings come earlier.

AUT-423 · How is a car front end designed for pedestrian protection?

Leave clearance between the bonnet and hard engine parts for head impact deformation, or use an active pop-up bonnet. Use a soft bumper fascia and a lower stiffener to control leg bending, and energy-absorbing bonnet leading edges. Stiff areas such as the A-pillars and cowl are hard to soften, so some cars add external airbags there.

AUT-424 · A diesel delivery fleet in city use suffers repeated DPF blockages. What would you recommend?

Short trips never reach active regeneration temperature, or regenerations are interrupted. Options: scheduled highway runs or service-triggered regeneration, regeneration strategies that work at low load (intake throttling, late post-injection, burner), low-ash oil, checking for faults raising soot (EGR, injectors, sensors), or moving the duty to CNG or EV.

AUT-425 · How does OBD monitor catalytic converter efficiency?

It compares upstream and downstream oxygen sensor signals. A healthy catalyst stores oxygen, so the downstream signal stays fairly steady while the upstream sensor switches. As oxygen storage capacity degrades, the downstream signal starts to follow the upstream one. A switch-ratio or storage-capacity threshold sets a DTC (for example P0420).

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