RRB ALP Mechanic Motor Vehicle Trade — Complete CBT-2 Study Guide

• Petrol (SI — Spark Ignition) engines use a spark plug to ignite a pre-mixed air-fuel charge; Diesel (CI — Compression Ignition) engines rely on heat of compression alone to ignite injected fuel — this is the fundamental difference between the two engine families used in motor vehicles. • Four-stroke cycle (most common in vehicles): Intake, Compression, Power, Exhaust — each stroke is one piston travel (up or down), completing the full cycle in two crankshaft revolutions. • Engine cooling in vehicles is typically liquid (water/coolant) cooled via a radiator, water pump, and thermostat, though some smaller/older vehicles use air cooling with cooling fins. • Firing order (the sequence in which cylinders receive their power stroke) is engineered to balance engine vibration and ensure smooth power delivery — a common 4-cylinder firing order is 1-3-4-2.

• Clutch engages/disengages engine power from the transmission, allowing smooth gear changes and vehicle starting from rest; a friction clutch uses a pressure plate to press a friction disc against the flywheel when engaged. • Manual transmission requires the driver to select gears via a gear lever and clutch pedal; Automatic transmission uses a torque converter (a fluid coupling) instead of a mechanical clutch, and shifts gears automatically based on speed/load. • Gear ratio determines the trade-off between torque and speed — a lower (numerically higher) gear ratio gives more torque/acceleration but lower top speed for a given engine RPM, and vice versa for a higher gear. • Differential allows the two drive wheels to rotate at different speeds while cornering (outer wheel travels a longer path than the inner wheel) — essential for smooth turning without tire scrubbing/wear.

• Drum brakes use brake shoes that press outward against the inside of a rotating drum; Disc brakes use a caliper that squeezes brake pads against a rotating disc (rotor) — disc brakes generally dissipate heat better and provide more consistent stopping power, especially under repeated hard braking. • Hydraulic braking system uses brake fluid (incompressible) to transmit pedal force to all four wheel brakes simultaneously and proportionally, based on Pascal's law. • ABS (Anti-lock Braking System) prevents wheel lock-up during hard braking by rapidly modulating brake pressure at each wheel individually, maintaining steering control and typically shortening stopping distance on most surfaces. • Brake fade (temporary loss of braking effectiveness) occurs from excessive heat build-up during prolonged/repeated hard braking, causing the brake fluid to vaporize or the friction material to lose grip — a known risk on long descents.

• Suspension system absorbs road shocks/vibrations (via springs and dampers/shock absorbers) to maintain ride comfort and keep tires in contact with the road for safe handling. • Independent suspension allows each wheel to move vertically independent of the others (better ride/handling, common on modern cars' front axle); Dependent (rigid axle) suspension links both wheels on an axle together (simpler, more common on trucks/heavy vehicles). • Steering system converts rotary motion of the steering wheel into the angular turning of the front wheels; Rack-and-pinion steering is the most common modern mechanism due to its simplicity and direct feel; Power steering (hydraulic or electric) reduces the physical effort needed to turn the wheel. • Wheel alignment (camber, caster, toe angles) affects tire wear, handling stability, and steering feel — incorrect alignment causes uneven/premature tire wear and can pull the vehicle to one side.