Question 1

Lubricants — Which organization defines engine oil viscosity grades? Engine oil viscosity grades such as 5W-30 or 15W-40 are established by which standards body?

Accepted Answer

Introduction / Context: Viscosity grade numbers on engine oils (e.g., 0W-20, 10W-40) indicate flow characteristics at cold and hot temperatures. These grades are standardized for consistency across brands and markets. Given Data / Assumptions: Multigrade engine oils for passenger and commercial vehicles. Common retail labels and owner’s manual references. Concept / Approach: The Society of Automotive Engineers (SAE) defines the J300 viscosity classification specifying kinematic and cranking viscosity ranges at various temperatures. The American Petroleum Institute (API) defines performance categories (e.g., SP, CK-4), not the viscosity numbers themselves. ATF and GVW are unrelated to viscosity grading; ISO viscosity numbers apply mostly to industrial lubricants, not automotive engine oils. Step-by-Step Solution: Identify the standard for viscosity grades → SAE J300.Recognize API's role → service categories and performance, not the grade numbers.Select the organization: SAE. Verification / Alternative check: Owner's manuals list recommended SAE grades; oil product data sheets cite compliance with SAE J300. Why Other Options Are Wrong: ATF/GVW: not standards bodies or relevant specifications for engine oil viscosity.API: performance/service classification, not viscosity grade definitions.ISO: industrial oils mainly, different numbering scheme. Common Pitfalls: Confusing API SP/CK-4 with SAE grade; both appear on oil labels but indicate different attributes. Final Answer: society of automotive engineers (SAE)

Question 2

Compression ratio calculation — Using swept and clearance volumes An engine has a clearance volume of 100 cm³ and a swept volume of 800 cm³. What is its compression ratio?

Accepted Answer

Introduction / Context: The compression ratio is a key geometric parameter affecting thermal efficiency, knock tendency, and combustion speed. It is computed directly from the cylinder's swept and clearance volumes. Given Data / Assumptions: Swept volume Vs = 800 cm³. Clearance volume Vc = 100 cm³. Single cylinder equivalent; ratio is independent of cylinder count. Concept / Approach: Compression ratio CR is defined as total volume at bottom dead centre divided by clearance volume at top dead centre: CR = (Vs + Vc) / Vc This uses plain arithmetic with volumes in consistent units. Step-by-Step Solution: Compute total volume at BDC: Vs + Vc = 800 + 100 = 900 cm³.Divide by clearance volume: 900 / 100 = 9.Express as a ratio: 9:1. Verification / Alternative check: Reverse check: Vc = (Vs + Vc) / CR → 900 / 9 = 100 cm³, matching the given. Why Other Options Are Wrong: 7:1 or 8:1: would imply smaller Vs or larger Vc than provided.10:1 or 11:1: would require either larger Vs or smaller Vc. Common Pitfalls: Forgetting to add swept and clearance volumes before dividing.Mixing units (e.g., cm³ and mm³); here units are consistent. Final Answer: 9:1

Question 3

Automotive clutches — vibration control in a single dry-plate clutch In a single dry-plate clutch used in vehicles, which component primarily absorbs torsional (twist) vibrations between engine and gearbox?

Accepted Answer

Introduction / Context: A single dry-plate clutch must transmit torque smoothly while isolating the driveline from engine firing pulses and gear tooth disturbances. Understanding which element damps twist fluctuations (torsional vibrations) is crucial for diagnosing shudder, rattle, and drivetrain NVH. Given Data / Assumptions: Clutch is a typical automotive single dry-plate type. Driven (friction) plate includes a splined hub that fits the gearbox input shaft. Springs may exist in two places: cushion springs under linings and torsional coil springs in the hub. Concept / Approach: Torsional vibrations are angular fluctuations of speed. The component that provides angular compliance and damping between engine flywheel/cover and transmission input is the set of coil springs in the driven plate hub. Cushion springs serve a different purpose: they soften axial engagement to reduce judder but do not meaningfully isolate twist. Step-by-Step Solution: Identify vibratory mode: torsional (twist) about the shaft axis.Locate element with angular compliance: hub-mounted coil springs.Function: store and release energy and, with friction washers, dissipate energy to damp oscillations.Conclusion: hub torsion springs absorb torsional vibrations. Verification / Alternative check: Exploded views and service manuals show the driven plate hub containing multiple coil springs and often friction shims serving as a torsional damper. Why Other Options Are Wrong: Cushion springs (B): cushion axial clamp engagement; not primary torsional dampers. Central hub only (C): a rigid hub transmits, not absorbs, torsion. Clutch pedal (D) and rubber pad (E): driver interface parts; no torsional damping in the torque path. Common Pitfalls: Confusing engagement smoothness (axial) with torsional isolation (angular). A clutch can engage smoothly yet still transmit torsional rattle if hub springs are weak or seized. Final Answer: Coil springs integrated in the driven plate hub (torsional springs)

Question 4

Automotive body engineering — when is frameless (unitized/monocoque) construction economical? Compared to a separate frame (body-on-frame), frameless or unitized construction becomes cost-effective when produced in __________ numbers.

Accepted Answer

Introduction / Context: Automotive bodies may be built as body-on-frame or frameless (unitized/monocoque). Economic viability depends on tooling, manufacturing processes, and scale. This question probes production economics rather than structural behavior. Given Data / Assumptions: Frameless structures need large body dies and extensive spot-welding/jig investments. Body-on-frame allows more flexibility with smaller tooling for low volume. Concept / Approach: High initial investment in press tools and body-in-white welding fixtures is amortized over many units. Therefore, the unit cost decreases markedly only when volume is high. Low volume cannot justify the tooling cost, favoring frame-based designs. Step-by-Step Solution: Identify major cost: dies, fixtures, robots → high fixed cost.Variable cost advantage of lighter unitized bodies realized over many vehicles.Amortization model: average cost = fixed cost/quantity + variable cost.As quantity increases, fixed cost per unit drops → economical at large numbers. Verification / Alternative check: Industry practice shows mass-market cars use unitized bodies, while niche trucks/buses and specialty vehicles often remain body-on-frame. Why Other Options Are Wrong: Small or prototype runs (A/C/D): tooling cannot be recovered; per-unit cost is high. Seasonal batches (E): volume uncertainty still undermines tooling amortization. Common Pitfalls: Confusing structural efficiency (stiffness/weight) with production economics. A unitized body may be structurally superior yet not economical at tiny volumes. Final Answer: Large numbers (high-volume mass production)

Question 5

Fuels and combustion — chemical formula of iso-octane (the primary high-octane reference fuel) What is the correct molecular formula for iso-octane?

Accepted Answer

Introduction / Context: Octane rating benchmarks knocking tendency using two reference fuels: n-heptane (low resistance to knock) and iso-octane (2,2,4-trimethylpentane, high resistance). Knowing iso-octane’s formula helps connect fuel chemistry to engine behavior. Given Data / Assumptions: Iso-octane is an isomer of octane. Hydrocarbons follow C_n H_(2n+2) for alkanes (saturated). Concept / Approach: For an alkane with n = 8 carbon atoms, the general formula is C8H18. Iso-octane is one isomer with branching that improves knock resistance but retains the same molecular formula as straight-chain octane. Step-by-Step Solution: Recognize iso-octane as an alkane → use C_n H_(2n+2).For n = 8: H = 2*8 + 2 = 18.Therefore, formula = C8H18. Verification / Alternative check: Chemical data sheets list iso-octane (2,2,4-trimethylpentane) as C8H18 with molecular weight about 114.23 g/mol. Why Other Options Are Wrong: C7H17 or C7H18: seven-carbon species, not octane. C6H18: violates alkane formula (for n=6, H=14). C8H16: corresponds to an alkene (unsaturated), not iso-octane. Common Pitfalls: Confusing structural isomerism (changes in bonding arrangement) with empirical/molecular formulas. All octane isomers share C8H18. Final Answer: C8H18

Automobile Engineering Questions