How an engine gets air

An engine is really an air pump. Every time a cylinder breathes in, it fills with air, and the fuel is added to match. The more air it can get in, the more fuel it can burn and the more power it makes. That’s why getting air in is the hard part of making power, and why so many modifications are about helping an engine breathe.

The parts that bring the air in are called the intake:

• Air filter. A box with a pleated paper element that traps dust.
• Airflow sensor. Measures how much air is going in, so the ECU can add the right amount of fuel.
• Throttle. A valve that decides how much air gets past, controlled by the accelerator.
• Intake manifold. A set of pipes that shares the air out to every cylinder.
• Intake valves. Doors in the top of each cylinder that open to let the air in.

It takes a lot of air. Each litre of petrol needs around 9,000 litres of air to burn it, and a 2.0-litre engine flat out at 3,000 rpm swallows about 50 litres of air every second.

Step along the intake with the numbered buttons, and try different pedal positions.

1. Air filter. Air usually comes in through a duct from behind the grille, where it’s cooler, and passes through the filter. Dust acts like sandpaper inside an engine, so the filter matters. It’s replaced at a service every year or two.
2. Airflow sensor. A heated wire or film in the airflow cools down as air rushes past it; the faster the air, the more it cools. The ECU reads that as grams of air per second and adds fuel to match. Some engines work it out from the pressure in the manifold instead.
3. Throttle. The plate turns with the accelerator, opening the way for more air.
4. Intake manifold. A chamber, called the plenum, feeds a pipe, called a runner, to each cylinder.
5. Intake valve. As the piston moves down on its intake stroke, the valve opens. The falling piston leaves low pressure in the cylinder, and the air behind pushes in to fill it.

Nothing actually sucks the air in. The air around us is pressing on everything at about 1 bar, and when the piston makes room, that pressure pushes air into the space. That’s also why an engine without a turbo can never fill its cylinders completely: it can only rely on the air pressure outside. The turbo and supercharger lesson shows how forced induction gets past that.

Watch the dots in the demo. Before the throttle they’re at outside pressure. After it, at part throttle, they spread out: the pistons are trying to take in more air than the throttle lets through, so the pressure in the manifold drops. The thinner air has to move faster to deliver the same amount.

With your foot off the accelerator, the pressure in the manifold of a petrol engine can fall to a fifth of the air outside. Mechanics call this manifold vacuum, even though it’s only a partial one.

Engineers put that vacuum to work. The brake servo, the round drum behind the brake pedal, uses it to multiply the force of your foot, which is why the brake pedal goes hard if you press it with the engine off.

Diesels have no throttle in the way, so their manifold sits near outside pressure, and electric cars have no manifold at all. Both use a small electric or engine-driven vacuum pump to work the brake servo instead.

How well an engine fills its cylinders decides how much power it can make. A few things make a big difference:

• Cold air. Cold air is denser, so every fill holds more of it: about 3% more for every 10°C cooler. Engines often feel a little stronger on a cold, crisp day.
• Altitude. The air gets thinner as you climb, by about 1% for every 100 metres, and an engine without a turbo loses power to match.
• Short, smooth paths. Every bend, narrow section and dirty filter makes it harder for air to get through. Engine designers tune the length and shape of the manifold so the air arrives at each valve at the right moment, as Intake manifolds explains.
• Forced induction. A turbo or supercharger squeezes extra air in, so an engine can fill its cylinders with more air than the outside pressure alone could push in.

Once the air and fuel have burnt, the engine has to get the exhaust out again, which is the subject of the next lesson.