Engine basics · Petrol and diesel
What happens when you press the accelerator?
Your right foot is asking the engine for more power. Here is what actually happens inside a modern petrol car—from the pedal and engine computer to the fuel, spark, pistons and wheels—and how a diesel car differs.
Published and updated: October 23, 2026 · Typical modern combustion-engine cars; systems vary
In a modern petrol car, pressing the accelerator sends a torque request to the engine computer. It generally opens the air throttle and adjusts fuel injection and spark timing. Combustion pushes pistons, turning the crankshaft and ultimately driving the wheels.
What happens in a petrol car? Six steps
Your foot moves the pedal
A position sensor measures how far you press. In most modern petrol cars, the pedal sends an electrical signal instead of pulling the throttle open by a cable.
The ECU interprets your request
The engine control unit (ECU) decides how much torque to deliver. It considers engine speed, temperature, available traction and other demands.
The throttle admits more air
An electric motor adjusts a butterfly valve in the intake. Opening it typically lets the engine draw in more air. In a turbocharged engine, boost can also change.
Fuel injection is adjusted
The ECU commands the injectors to meter fuel for the incoming air. Petrol may enter the intake port or be injected directly into the cylinder, depending on the engine.
A spark starts combustion
The spark plug ignites the compressed air–petrol mixture at a carefully chosen time. Expanding gases push the piston down and turn the crankshaft.
Torque reaches the wheels
The crankshaft sends torque through the transmission and drivetrain to the driven wheels. The car speeds up when driving force exceeds the forces resisting motion.
What actually happens inside the cylinder?
A typical four-stroke petrol engine repeats four events: intake (air enters), compression (the piston moves up), power (the spark ignites the mixture and expanding gas pushes the piston down) and exhaust (burned gases leave). A cylinder does not fire merely because you press the pedal: combustion is already repeating while the engine runs, and the ECU adjusts the operating conditions to produce more torque.
No. On most modern cars it is an electronic request for engine torque. The engine computer chooses how to meet that request, including the throttle opening and fuel delivery. Older petrol cars could use a mechanical cable to operate the throttle.
Why might engine RPM rise before the car speeds up?
Engine speed (RPM) describes how fast the crankshaft rotates. Vehicle speed depends on the gearbox ratio, wheel speed and the forces acting on the car. An automatic transmission may select a lower gear when you press hard (a kickdown), raising engine RPM to access more available power. If you press the accelerator while parked in neutral, engine RPM can rise without driving the wheels.
What changes if the petrol engine has a turbo?
The same pedal-to-ECU process applies, but the ECU also manages boost and related controls. A turbocharger uses exhaust energy to compress incoming air, allowing the engine to burn more fuel when operating conditions permit. There can be a delay while boost builds, often called turbo lag.
What happens when you press the accelerator in a diesel car?
In a modern diesel, the pedal also sends a torque request to the ECU. The main difference is how the engine makes more power: the ECU primarily adjusts the amount and timing of diesel fuel injected into hot, highly compressed air. The diesel fuel ignites because of the heat of compression—there is no spark plug firing the mixture in normal operation. A turbocharger and other air-management controls help supply enough air when additional fuel is requested.
Many diesels do have air-control or throttle-like valves, for example to help emissions control or shut down smoothly, but these are generally not the main method of controlling torque as in a conventional spark-ignition petrol engine.
Petrol vs diesel: what is different?
| Question | Petrol (gasoline) | Diesel |
|---|---|---|
| What does the pedal do? | Sends a torque request to the ECU | Sends a torque request to the ECU |
| How is power mainly adjusted? | Intake air through the throttle, with matching fuel and ignition control | Fuel quantity and injection timing, coordinated with available air |
| How does fuel burn? | A spark plug starts combustion of an air–petrol mixture | Fuel ignites in hot compressed air (compression ignition) |
| Where is fuel injected? | Into an intake port or directly into the cylinder, depending on design | Typically directly into the cylinder on modern road diesels |
| What does a turbo do? | Provides extra air when boost is available | Provides extra air when boost is available; common in modern diesels |
| What ultimately moves the car? | Combustion turns the crankshaft; the transmission and drivetrain deliver torque to the wheels | |
Petrol: mainly controls incoming air, then matches fuel and spark. Diesel: mainly meters the fuel injected into compressed hot air. Both use computer-controlled systems to meet the driver's torque request.