Engine performance: torque and power
Torque is the twisting force the engine gives to its shaft, measured in newton-metres (N·m). It decides how strongly the car pulls from a stop or up a hill. Power is how fast the engine does work, in kilowatts (kW): power = torque × turning speed. Power decides top speed and how quickly the car speeds up at higher speeds.
An engine has a performance curve: torque is best in the middle rpm range, and power peaks at a higher rpm. Engine size (displacement, in litres or cc), the number of cylinders, a turbocharger and fuel injection all change the curve. Efficiency is the share of fuel energy that becomes useful work; a petrol engine turns only about 25 to 35 percent into motion, and the rest leaves as heat.
Engine auxiliary systems
- Fuel system: tank, pump, filter and injectors. The injector sprays a fine mist of fuel in the right amount at the right time.
- Intake and exhaust: an air filter cleans the air going in. Burnt gas leaves through the exhaust pipe, a catalytic converter (turns harmful gases into less harmful ones) and a silencer (cuts noise).
- Cooling system: a water pump pushes coolant through the engine, then through the radiator where air cools it. A thermostat keeps the temperature right (about 90 °C).
- Lubrication system: a pump moves oil from the oil pan to moving parts. Oil cuts friction and wear, carries away some heat and keeps parts clean. The filter catches dirt.
- Ignition and starting (petrol): a spark plug lights the mixture; the battery and starter motor turn the engine to start it.
Car body and accessories
Old cars had a separate steel frame with a body bolted on. Most cars today use a monocoque (unibody): the body shell itself is the frame, which is lighter and stronger. A modern body has three zones: a front crumple zone and rear crumple zone that squash and soak up crash energy, and a rigid passenger cell that stays intact. Body panels also shape the air flow (streamlining) to cut drag, and use steel, aluminium or plastic.
Accessories add comfort and safety: lights, wipers, horn, mirrors, seat belts, seats, air conditioning, windows, instrument panel, and audio or navigation.
Driving and performance: suspension, steering, tyres
Suspension joins the wheels to the body with springs and dampers (shock absorbers). The spring takes a bump; the damper stops it bouncing again and again. Result: tyres stay on the road and the cabin stays smooth.
Steering turns the front wheels through a steering rack when you turn the wheel. Many cars add power steering (hydraulic or electric) so it is light to turn. On a bend the two front wheels turn by slightly different angles.
Tyres are the only parts touching the road. Their grip depends on tread, rubber, air pressure and road wetness. Running resistance = rolling resistance + air drag + slope; the engine must beat it. Fuel economy is distance per litre (km/L) or litres per 100 km: smooth driving, correct tyre pressure and low weight improve it.
Key formulas and definitions
- Power (kW) = torque (N·m) × rpm × 2π ÷ 60 ÷ 1000
- Efficiency = useful work ÷ fuel energy × 100%
- Fuel economy (km/L) = distance ÷ fuel used
- Running resistance = rolling + air drag + slope
- Key terms: torque, power, radiator, catalytic converter, monocoque, crumple zone, damper
Worked examples
1. An engine gives 120 N·m at 3000 rpm. Find the power in kW.
Power = 120 × 3000 × 2π ÷ 60 = 37 699 W ≈ 37.7 kW.
2. A car travels 150 km using 10 litres. Find the fuel economy.
150 ÷ 10 = 15 km/L.
3. Fuel has 100 units of energy. An engine gives 30 units of work. Find its efficiency.
30 ÷ 100 × 100 = 30%. The other 70 units leave as heat.
4. Why is the oil pump needed even though oil sits in the oil pan?
The moving parts at the top of the engine are far from the pan. The pump pushes oil up to them, so every part gets a thin film.
5. A car with mileage 12 km/L covers 480 km. How many litres does it need?
480 ÷ 12 = 40 litres.
6. Torque is the same at two speeds, 2000 rpm and 4000 rpm. How do the powers compare?
Power = torque × speed, so at 4000 rpm the power is double that at 2000 rpm.
Common mistakes
- Mixing up torque and power. Torque is twisting force; power is how fast work is done.
- Thinking the radiator makes the coolant hot. It cools it by giving heat to the air.
- Thinking a stiff, rigid body is the safest. A body that never crumples passes the whole shock to people.
- Believing suspension only gives comfort. It also keeps tyres in contact for grip and steering.