A useful way to read this: focus on what the technology changes for a real driver, not just the specification number. Exact equipment and service requirements can vary by engine, model year and market.
The four strokes
A conventional four-stroke petrol engine uses intake, compression, power and exhaust strokes. During intake, the intake valve opens and the piston moves down, drawing air into the cylinder. During compression, the valves close and the piston moves upward.
Near the top of the compression stroke, the spark plug produces an electrical spark. Combustion rapidly increases pressure and pushes the piston down during the power stroke. Finally, the exhaust valve opens and the piston moves upward to push combustion gases out.
Air and fuel control
Older petrol engines often relied on simpler mechanical or electronic fuel systems. Modern engines use electronically controlled injection and a large number of sensors. The ECU can adjust fuel quantity, ignition timing, throttle behaviour and other parameters based on engine speed, load, temperature and oxygen measurements.
This control helps the engine balance performance, fuel consumption and emissions rather than operating at one fixed setting.
Naturally aspirated and turbo petrol engines
A naturally aspirated petrol engine draws air into the cylinders without a turbocharger. A turbocharged petrol engine compresses intake air before it reaches the cylinders. The turbo arrangement allows more air to enter and can increase torque and power from a smaller displacement.
Turbo petrol engines also create additional heat and pressure, so cooling and lubrication become especially important.
Valves and timing
The intake and exhaust valves have to open at the right time. Modern engines may use variable valve timing to change when valves open or close depending on engine speed and load. Some systems can also vary valve lift.
This is one of the reasons a modern engine can be reasonably efficient during gentle driving and still produce strong output when the driver asks for it.
What the driver should care about
For normal ownership, the important lesson is that an engine is a system. Oil, coolant, spark plugs, air filtration, fuel quality and electronic control all affect how well the basic combustion process works. When one part is neglected, the effect can appear somewhere else.
For your specific vehicle
This guide is general information. For exact specifications, service intervals, fluid requirements and repair procedures, use the owner's manual or official service information for your vehicle.
Most conventional petrol engines use four basic piston strokes: intake, compression, power and exhaust. During intake, the piston moves down while the intake valve allows an air-fuel mixture or air into the cylinder. Compression raises the pressure before ignition. The spark plug then starts combustion, pushing the piston down during the power stroke. Finally, the exhaust valve opens and the piston pushes spent gases out. Valve timing and fuel control are carefully managed so this sequence happens thousands of times per minute.
Older engines relied more heavily on mechanical systems. Modern petrol engines use sensors and an engine control unit to adjust fuel delivery, ignition timing, throttle position and sometimes valve timing. Oxygen sensors help the system understand combustion conditions in the exhaust, while temperature and pressure sensors provide additional information. The result is not simply more electronics for their own sake; it is tighter control over combustion under changing loads and temperatures.
Two four-cylinder petrol engines can have very different personalities. Compression ratio, displacement, turbocharging, valve timing, injection system, gearing and software all change the way the engine produces power. This is why looking only at cylinder count is not enough when comparing cars. The engine is part of a larger system, and the transmission and vehicle weight can change what the driver actually feels.
When comparing petrol engines, focus on the whole operating range rather than one impressive specification. Ask how the engine responds in traffic, how it behaves when the car is loaded, and how much power is available during highway acceleration. Turbocharging, valve timing and fuel injection can change the character of an otherwise familiar four-cylinder layout. Service history matters too. The correct oil, spark plugs, filters and coolant specification are part of the engine's design, not optional extras. If a specification looks unusual, check the exact owner's manual or manufacturer documentation for the model and engine code. That simple habit avoids many internet-based assumptions.