Petrol powers a car through a process called internal combustion. In simple terms, the engine mixes petrol with air, compresses it, and ignites it. The tiny, controlled explosions create high-pressure gas that pushes pistons, converting chemical energy into mechanical motion that turns the wheels.
The heart of the system is the internal combustion engine (ICE). Here’s a step-by-step breakdown of the four-stroke cycle, also known as the Otto cycle:
- Intake Stroke: The intake valve opens, and the piston moves down, drawing a mixture of air and atomized fuel from the fuel injectors into the cylinder.
- Compression Stroke: Both valves close, and the piston moves back up, compressing the air-fuel mixture. This makes the subsequent explosion more powerful.
- Combustion Stroke: The spark plug generates a spark, igniting the compressed mixture. The rapid expansion of burning gases forces the piston down with tremendous force. This is the "power stroke" that delivers energy to the crankshaft.
- Exhaust Stroke: The exhaust valve opens, and the moving piston pushes the spent gases out of the cylinder.
This cycle repeats hundreds of times per minute in each cylinder. The up-and-down motion of the pistons is converted into rotational motion by the crankshaft, which then sends power through the transmission and driveshafts to the wheels.
Supporting this process are several key systems:
- Fuel System: A fuel pump draws petrol from the tank, sends it through a filter, and delivers it under high pressure to the fuel injectors, which spray a fine mist into the cylinders.
- Ignition System: The battery and alternator provide electricity to the spark plugs at precisely the right moment to trigger combustion.
- Exhaust System: This system safely channels hot, toxic gases away from the engine and out the tailpipe, using a catalytic converter to reduce harmful emissions.
The efficiency of this energy conversion is a key metric. Only about 20-30% of the energy in petrol actually moves the car; the rest is lost as heat. Here's a typical energy distribution for a standard petrol car:
| Energy Loss Component | Approximate Percentage of Total Fuel Energy |
|---|
| Useful Energy (Wheel Power) | 22% |
| Engine Losses (Heat) | 38% |
| Idling at Stop/Neutral | 17% |
| Drivetrain Losses | 5% |
| Accessories (A/C, etc.) | 3% |
| Aerodynamic Drag | 3% |
| Rolling Resistance | 5% |
| Braking Losses | 7% |
| Source: Adapted from U.S. Department of Energy data | |
