Why forced induction makes more power

Naturally aspirated vs turbocharged vs supercharged showed the three ways to fill an engine with air. This lesson looks at why pushing air in makes so much more power, and why it isn’t quite as simple as twice the pressure for twice the power.

An engine makes power by burning fuel, and fuel needs air to burn: about 14.7 times its own weight in air, as Air-fuel ratios explained. Getting more fuel in is easy, because the injectors can squirt far more than the engine could ever burn. Air is the limit. However much air gets into each cylinder sets how much fuel it can burn, and so how much power it makes.

What counts is the weight of the air, not its volume. Each cylinder has a fixed size, so an engine can take in at most its own size in air every two turns of the crank. A 2.0-litre engine at 6,000 rpm fills itself 50 times a second, and all that air weighs only about 0.12 kg (0.3 lb).

To fit more air into the same space, you have to make it denser. There are two ways to do that:

• Squeeze it, so the same space holds more. That’s what a turbo or supercharger does, and the extra pressure is boost.
• Cool it, because cold air is denser than warm air. That’s what an intercooler does.

A boost gauge reads the pressure above the air around us, which is already about 1 bar (15 psi) at sea level. So 1 bar (15 psi) of boost means about 2 bar (29 psi) in all: twice the pressure.

The demo follows the air from outside the car, through a compressor and an intercooler, into one cylinder at the bottom of its intake stroke. Each dot in the cylinder stands for an eighth of what it holds with no boost. The graph shows the engine’s power at 6,000 rpm for each amount of boost: the Mustard line is the setting you’ve picked, and the grey line the other one.

Start with the slider at 0. The compressor isn’t doing anything, so the cylinder fills with air at normal pressure and the engine makes 129 kW (173 bhp), the same 2.0-litre engine as in the earlier lesson.

Now set 1 bar (15 psi) of boost and watch the dots in the pipes and cylinder pack closer together:

1. The compressor squeezes the air to 2 bar (29 psi) in all, twice the pressure around us. On its own that would mean twice the air.
2. Squeezing heats it. The air leaves the compressor at about 108°C (226°F), against 20°C (68°F) outside. Hot air is thinner, so it doesn’t pack in as well.
3. The intercooler cools it back down to about 42°C (108°F). Each cylinder now holds about 1.85 times the air, and the engine makes about 238 kW (319 bhp).

Press No intercooler and the cylinder loses dots. The air goes in at 108°C (226°F), so each cylinder holds only about 1.5 times the air, and the engine makes about 197 kW (264 bhp). The heat has cost almost half the gain.

The gap grows as the boost rises. At 1.5 bar (22 psi) the air leaves the compressor at about 140°C (284°F). With an intercooler each cylinder holds 2.25 times the air; without one, only 1.75 times. Air that hot would also make the engine knock, as the caption warns.

The model is simplified: it assumes the ECU adds fuel to match the air, and leaves out smaller losses such as the pressure the intercooler costs.

Pushing air into a smaller space takes work, and that energy ends up in the air as heat. Pump up a bicycle tyre and the end of the pump gets warm for the same reason.

Even a perfect compressor would heat air to about 84°C (183°F) while squeezing it to 2 bar (29 psi). Real ones aren’t perfect. A good road-car compressor is about 70 to 78% efficient, and the work it wastes goes into the air as extra heat too, which is how it reaches about 108°C (226°F). A compressor pushed past its best range, too small for the job or spinning too fast, heats the air even more for the same boost.

Heat matters because density falls as temperature rises. Near room temperature, every 3°C (37°F) hotter makes air about 1% thinner. That’s why the ECU measures the temperature of the air going in, as well as how much there is, before it works out the fuel.

Air squeezed into the engine is called the charge, and an intercooler is a charge-air cooler: a radiator for the intake air. There are two main kinds.

• Air-to-air. The squeezed air runs through a finned core in front of the radiator, and the air rushing past the car cools it. It’s simple and works well at speed, but the pipes to the front of the car and back are long.
• Air-to-water, often called a chargecooler. A small core sits right by the engine, often inside the intake manifold, and coolant from its own small radiator takes the heat away. The air path is short, so boost builds a little faster, but there are more parts.

A typical intercooler takes 70 to 85% of the extra heat back out. It costs a little pressure, as the air has to squeeze through the core, so the turbo has to make slightly more boost to make up for it. Intercoolers goes into them in more detail.

There are other ways to cool the charge too. In a direct-injection engine, the fuel evaporates inside the cylinder and cools the air as it does. A few cars, such as the BMW M4 GTS, sprayed a fine mist of water into the intake to do the same.

Boost on its own does nothing: the extra air only makes power because the ECU adds fuel to match. It measures the air coming in, with an airflow sensor or a pressure and temperature sensor in the intake manifold, and works out how much fuel it needs.

The fuel system has to keep up. Boosted engines need bigger injectors, and direct-injection ones a fuel pump that can reach very high pressures. At full boost, many petrol engines run richer than 14.7:1 on purpose, as rich as 12:1, because the spare fuel soaks up heat as it evaporates and helps protect the pistons and the turbo.

If more boost means more power, why not run 3 bar (44 psi) on everything? Because every extra bit of air brings more pressure and heat with it, and something reaches its limit.

• Knock. Boost raises both the pressure and the temperature of the charge before it’s even squeezed by the piston. As Octane: what it actually means explained, the hotter and harder the end of the charge is squeezed, the more likely it is to explode on its own. So boosted petrol engines use a lower compression ratio, often about 9.5 to 10.5:1 against 11 to 13:1 for a naturally aspirated one, fire the spark a little later, and do best on higher-octane fuel.
• Strength. Peak pressure in the cylinder rises with boost, and the pistons, con-rods, bearings and head gasket all take it. Boosted engines get tougher pistons, often with oil jets spraying their undersides to cool them.
• Heat. Burning more fuel makes more heat, in the pistons, the valves and the exhaust, and the cooling system has to carry it away.
• The compressor itself. Every compressor has a range where it works well. Push it past that and it mostly heats the air instead of packing more in.

Diesels don’t knock in the same way, because their fuel is only injected when it should burn. So they can run far more boost, often 1.5 bar (22 psi) to 2.5 bar (36 psi), and are limited more by the strength of the engine and by smoke.

The air gets about 1% thinner for every 100 metres you climb. Over a pass in the Alps at about 2,000 metres, a naturally aspirated engine loses around a fifth of its power, because each cylinder fills with thinner air and there’s nothing it can do about it.

A turbo engine loses far less. Its compressor can spin faster and squeeze harder, bringing the pressure in the intake back up to what the ECU asks for. That’s why turbochargers were first used on aircraft in the 1930s and 1940s, to keep engines making power high in the sky. At the top of the Pikes Peak hill climb in Colorado, where the air is about 40% thinner, turbo cars have a big advantage.

A belt-driven supercharger, which turns at a fixed speed for the engine speed, loses power with altitude much like a naturally aspirated engine.

At full boost, a 2.0-litre engine with 1.85 times the air behaves rather like a 3.7-litre one. So why not just build the bigger engine?

Because most of the time you don’t need full power. A small engine has fewer or smaller parts to rub against each other, less weight to carry and, at part throttle, a throttle that’s open wider, so it wastes less effort sucking air past it. Off boost, it burns fuel like the small engine it is.

The catch is that on boost it burns fuel like the big engine, or a little more if it’s running rich. That’s why a small turbo engine can do very well in official economy tests, which are fairly gentle, but less well for drivers who use the boost a lot.

• Heat soak. Sitting in traffic on a hot day, the intercooler, the pipes and the intake all warm up. The air going in is hotter and thinner, and the ECU fires the spark later to avoid knock, so the first hard acceleration feels flat until fresh air cools things down.
• The wrong fuel. Using lower-octane fuel than the engine needs makes it knock more readily. The ECU hears this through its knock sensors and fires the spark later to protect the engine, so you lose power and use more fuel. If knock control fails, knock can crack pistons.
• Boost leaks. A split hose or loose clamp lets squeezed air escape, so the cylinders get less of it. The car feels flat and may hiss under load. If the engine measures air with an airflow sensor, air it measured is escaping, so it also runs rich, and a diesel may puff black smoke.
• A dirty or damaged intercooler. Oil mist from the engine’s breather or a worn turbo can coat the inside of the intercooler and pipes. Stones and minor bumps can bend its fins, or crack it, as it sits right at the front of the car.
• Overboost. If the system that controls boost fails, the pressure can climb too high. The ECU sees it, cuts the power to protect the engine and stores a fault code such as P0234, turbo overboost.