How superchargers work

A supercharger is an air pump driven by the engine itself, usually by a belt from the crankshaft. Like a turbo, it squeezes extra air into the cylinders so they can burn more fuel. Naturally aspirated vs turbocharged vs supercharged showed the big difference: because the engine drives it directly, a supercharger responds instantly, but the engine has to spend some of its own power turning it.

There are two families:

• Positive-displacement superchargers, the Roots and the twin-screw, trap a fixed amount of air on every turn and push it into the engine.
• Centrifugal superchargers fling air outwards with a fast-spinning wheel, just like a turbo’s compressor.

This lesson looks at how each one works. The next one, Turbo vs supercharger, compares them with turbos.

The demo shows the same 2.0-litre engine as earlier lessons, floored, with a supercharger. A belt from the crank pulley drives it, and it pushes the air down through a chargecooler to the engine. Beside it is the boost at each engine speed: the Mustard line is the supercharger you’ve picked, and the grey line another kind to compare.

It starts with a Roots supercharger at 2,000 rpm. Its pulley is smaller than the crank’s, so it turns 2.4 times as fast, at 4,800 rpm. Each turn pushes the same amount of air through, so even this low down it makes 0.53 bar (7.7 psi). Without it the engine would make 36 kW (48 bhp); with it, 47 kW (63 bhp). Slide up the revs and boost stays nearly flat, reaching 0.7 bar (10 psi) at the top. At 6,000 rpm the engine makes 186 kW (249 bhp), against 129 kW (173 bhp) without it.

The readouts also show the cost. Squeezing the air takes work, and the engine pays for it through the belt: about 16 kW (21 bhp) at 6,000 rpm. That work ends up as heat, so the air leaves the supercharger at about 106°C (223°F).

Press Twin-screw. It makes the same boost, but heats the air less, to about 88°C (190°F), so it takes only about 13 kW (17 bhp) to drive, and the engine makes 193 kW (259 bhp).

Press Centrifugal. Its boost rises with the square of its speed, so at 2,000 rpm there’s barely any, 0.07 bar (1 psi), even though belt and gears spin it at nearly 17,000 rpm. It catches up at the top, spinning at over 50,000 rpm. That suits a car driven hard high up the revs, but it feels much like a bigger naturally aspirated engine rather than a muscular one.

The model is simplified; real superchargers are sized and geared for each engine.

The Roots blower is the oldest design, patented in 1860 by the brothers Philander and Francis Roots to blow air into furnaces. Two rotors, each with two to four lobes, turn the opposite way to each other inside a close-fitting case, geared together so they never touch. Air comes in on one side, gets trapped in the pockets between the lobes and the case, and is carried round the outside to the other side.

The Roots doesn’t squeeze the air inside itself. It carries air across at normal pressure and pushes it into a manifold that’s already full of boosted air, which rushes back into each pocket as it opens. That’s wasteful, so a Roots heats the air more than the others, especially at high boost.

It’s simple, tough and gives strong boost from low revs. It’s the “blower” sticking up through the bonnet of a drag racer, and modern versions with twisted rotors, made by Eaton, sat under the bonnet of the Mini Cooper S of the 2000s, Audi’s 3.0 TFSI V6 and Jaguar’s supercharged V8s.

A twin-screw supercharger, also called a Lysholm after the Swedish engineer who developed it in the 1930s, has two long rotors twisted like screws: one with fat lobes and one with deep grooves. As they turn, air trapped between them is carried along the rotors into a smaller and smaller space, so it’s squeezed before it comes out.

Squeezing inside the supercharger wastes less work than a Roots, so the air comes out cooler and it takes less power to drive. It gives the same strong low-down boost. The catches are cost, because the rotors have to be machined very precisely, and a louder whine. The 2005 Ford GT and Mercedes-AMG’s supercharged 5.4-litre V8 both used twin-screws.

A centrifugal supercharger is the compressor half of a turbo, driven by a belt instead of a turbine. Its wheel has to spin very fast, often 50,000 rpm or more, so a set of step-up gears inside it, or on some designs, such as Rotrex, smooth rollers, multiplies the speed of its pulley.

Because its boost rises with the square of its speed, it makes little at low revs and most at the top, the opposite of a Roots. It’s compact, efficient and easy to fit, so it’s a favourite for aftermarket kits on engines that weren’t built with a supercharger, from the Toyota GT86 to big American V8s.

Most supercharged engines cool the squeezed air before it goes in. Many put a chargecooler straight under the supercharger, in the V of the engine, as Audi and Jaguar did, so the air path stays short.

A supercharger turns whenever the engine does, which costs fuel even when you don’t need boost. So positive-displacement superchargers usually have a bypass valve. When you’re cruising, there’s a vacuum behind the throttle, and the valve opens a path from the supercharger’s outlet back to its inlet. The air just circulates, the rotors spin freely, and they take very little power. Press the accelerator and the valve shuts, and the boost is there at once.

Some go further and fit an electromagnetic clutch in the pulley, like an air-conditioning compressor’s, which disconnects the supercharger altogether when it isn’t needed.

A few engines have used both a supercharger and a turbo, called twincharging: the supercharger for instant boost at low revs, and the turbo taking over higher up. VW’s 1.4 TSI “Twincharger” from 2005 and Volvo’s 2.0-litre T6 engines both did this.

The newest idea is the electric supercharger: a compressor spun by an electric motor instead of a belt. It can reach full speed in a fraction of a second, whatever the engine is doing. It needs a lot of electricity, so it usually comes with a 48-volt electrical system. Audi’s SQ7 diesel used one in 2016 to fill in before its turbos woke up, and Mercedes-AMG’s “53” models use one with their straight six.

• The belt. A worn or loose belt slips under load, so boost drops and you may hear a squeal, especially when cold or wet. If it snaps, boost goes, and on many engines the same belt drives the alternator and water pump, so you lose those too.
• Bearings and gears. A supercharger always whines a little, but a growl, rattle or much louder whine means worn bearings, gears or the coupling inside. Many need their own oil changed now and then. The Mini Cooper S of the 2000s is well known for a rattle when its supercharger’s oil is neglected.
• A sticking bypass valve. Stuck open, the supercharger never squeezes the air, so the car feels flat. Stuck shut, it squeezes all the time, so the car uses more fuel. A split vacuum hose to the valve’s actuator often causes it.
• A failed chargecooler pump. The air going in gets hot, and the ECU cuts power to protect the engine, as on a turbo engine with a chargecooler.

• A smaller supercharger pulley is the classic supercharger upgrade. It spins the supercharger faster for the same engine speed, so it makes more boost everywhere. It needs a remap to add fuel and adjust the spark, and the belt may start to slip, so it’s often paired with a stronger belt or tensioner. More boost also means hotter air and more power to drive it.
• A bigger chargecooler or better pump keeps the extra heat in check, especially on a track.
• A supercharger kit for a naturally aspirated engine, usually centrifugal, is one of the simplest ways to add forced induction, because it bolts to the front of the engine with no changes to the exhaust. The engine’s internals and fuel system still have to cope with the extra air.