Naturally aspirated vs turbocharged vs supercharged

An engine makes power by burning fuel with air. Adding more fuel is easy; the hard part is getting more air in. How an engine fills its cylinders with air is called its induction, and there are three main kinds.

• Naturally aspirated. The pistons suck air in on their own as they move down. Each cylinder can only fill at the pressure of the air around the car, so the engine’s size sets how much it can burn.
• Turbocharged. A turbo uses the engine’s own exhaust gas to spin a fan that squeezes extra air into the cylinders.
• Supercharged. A supercharger does the same squeezing, but it is driven by a belt from the crankshaft.

Squeezing air into the engine is called forced induction, and the extra pressure it makes is boost. With 1 bar (15 psi) of boost, each cylinder holds roughly twice the air, so it can burn twice the fuel and make close to twice the power.

Pick an engine type and press Floor it. The engine underneath is the same in all three; only the way it gets its air changes. Watch the boost gauge, and how closely the air dots are packed going into the cylinder.

Naturally aspirated. The gauge never moves. As each piston falls it pulls in air, but nothing pushes extra in, so the cylinder fills once over. These engines respond instantly and smoothly, and their power builds steadily all the way to the redline. Many drivers love that for its precision and sound, but you need a bigger engine to make more power.

Turbocharged. A turbo is two fans on one shaft. In the exhaust, hot gas, close to 1,000°C (1,832°F) in a petrol engine, spins the turbine at well over 100,000 rpm. On the intake side, the compressor on the other end of the shaft squeezes fresh air into the engine. Floor it and watch the gauge: boost takes a moment to build, because the exhaust has to spin the turbine faster first. That wait is turbo lag. Drop the revs below about 2,000 and there isn’t enough exhaust gas to spin the turbo hard at all.

Supercharged. The supercharger is a compressor driven by a belt from the crankshaft, so it spins in step with the engine. Boost arrives the instant you press the throttle, with no lag. The catch is that the engine has to spend some of its own power turning it. How superchargers work goes into the different kinds.

At 6,000 rpm in the demo, the same engine makes about 129 kW (173 bhp) naturally aspirated, 180 kW (241 bhp) supercharged and 227 kW (304 bhp) turbocharged. The turbo wins because it runs on energy that would otherwise go out of the exhaust.

A few decades ago most petrol cars were naturally aspirated. Now nearly every new car has a turbo, and almost every diesel has had one for years.

The reason is efficiency. A small turbo engine can make the power of a bigger naturally aspirated one when you need it, but when you’re cruising, with little boost, it burns fuel like the small engine it is. That’s why a 1.0-litre three-cylinder turbo can replace an old 1.6. It’s called downsizing.

Turbos have other advantages too. They lose less power at altitude, where the air is thin, and modern small turbos spool up so quickly that lag is barely noticeable in normal driving.

Superchargers are rarer now. Their instant response and their whine suit big performance cars, but driving them costs fuel. A few engines have used both: a supercharger for low revs and a turbo for high revs.

Forced induction adds parts that spin very fast and run very hot, so there is more to go wrong.

• Turbo failure. The turbo’s shaft runs on a thin film of engine oil. Old or low oil, or switching off straight after a hard drive while the turbo is glowing hot, can cook that oil and wear the bearings. Signs are blue smoke, a siren-like whine, oil in the intake pipes and a loss of power. Gentle driving for the last minute or so of a journey lets it cool.
• Boost leaks. A split hose or loose clamp between the turbo and the engine lets boost escape. The car feels flat, may hiss under load, and often puts the engine warning light on.
• Supercharger wear. The drive belt can slip or squeal, and worn bearings make the normal whine louder and rougher.
• More heat and pressure. Forced induction puts more strain on pistons, head gaskets and the cooling system, which is why boosted engines are built tougher and are fussier about oil and fuel quality.

Forced induction is where tuners find big gains, because the engine’s air supply can be turned up.

• Remapping a turbo engine raises the boost. It is the cheapest way to add power to a modern car, often 20 to 30 per cent, but it adds heat and strain to every part.
• A bigger turbo can flow more air for more power at the top end, but it is heavier and slower to spin up, so lag gets worse. Bigger isn’t always better.
• Upgraded intercoolers keep the squeezed air cooler, so the engine can hold its power on hot days and hard runs.
• Adding a turbo or supercharger to a naturally aspirated engine is a big job. The engine usually needs stronger internals, more fuel and a new map to survive the extra pressure.