What happens when you press the accelerator

It’s easy to think the accelerator pumps fuel into the engine. In a petrol car it really controls air.

An engine makes power by burning fuel with air, and the fuel has to be matched to the air for a clean burn. So to make more power, the engine needs to breathe more air. The accelerator controls the throttle, a valve in the air intake. Press harder and the throttle opens wider, more air rushes in, the engine adds fuel to match, and each bang in the cylinders gets bigger.

In older cars a cable ran from the pedal straight to the throttle. Almost every car now uses drive-by-wire: the pedal is just a sensor, and the engine computer, called the ECU, works the throttle with a small electric motor. Throttle bodies and electronic throttles looks at how it works in detail.

Move the pedal slider, then step through the chain with the numbered buttons. Each one highlights the part doing the work.

1. Pedal. A sensor on the pedal measures how far you’ve pressed it and sends that to the ECU as an electrical signal.
2. ECU. The engine computer reads the pedal and decides how far to open the throttle. It doesn’t copy your foot exactly: it opens the throttle gently over the first part of the pedal’s travel, which makes the car easier to drive smoothly.
3. Throttle. A motor turns the throttle plate, a disc inside the intake pipe. Edge-on to the air it is wide open; across the pipe it is nearly shut. It never closes completely, because the engine needs a little air to idle.
4. Fuel. Sensors measure the air coming in, and the ECU has the injectors spray in fuel to match: about 14.7 times as much air as fuel, by weight, called the air-fuel ratio. Flat out, that’s only about 40 milligrams of fuel each time a cylinder fills.
5. Power. More air and fuel make a bigger bang on every power stroke, so the pistons push harder on the crankshaft. That extra torque reaches the wheels, the car speeds up, and the revs climb.

The whole chain takes a fraction of a second. At 3,000 rpm, every cylinder fills 25 times a second, so the engine responds within a few turns of the crankshaft.

The next lessons look more closely at how fuel gets into the engine and how the engine gets its air.

The engine responds almost instantly, but a few things shape what you actually feel:

• The car takes time to speed up. More power is a bigger push, but a car weighs well over a tonne, so it gathers speed gradually.
• Turbos take a moment. On a turbocharged engine, the extra boost builds as the turbo spins up, so the full push can arrive a moment later.
• Automatics may change down. Press hard in an automatic and it drops a gear or two, called kickdown, to put the engine where it makes more power.
• Drive modes change the pedal. Sport mode often opens the throttle more for the same pedal movement, so the car feels sharper. Eco mode does the opposite.
• Lifting off cuts the fuel. Take your foot off at speed, still in gear, and the throttle shuts and the injectors stop completely. The engine is turned by the wheels instead, and the drag slows the car: that’s engine braking.

For safety, if you press the brake and the accelerator together, most modern cars ignore the accelerator and let the brakes win.

Diesel engines work the other way round. Most let in plenty of air all the time and control their power by how much fuel they inject, so in a diesel the pedal mainly decides how much fuel goes in. They need extra air to burn cleanly, so they always run with more air than the fuel needs.

Electric cars have no air or fuel to manage. The pedal tells the motor controller how much electric current to send to the motor, and the motor’s torque arrives almost instantly. That’s why electric cars feel so eager when you press the pedal.