The 125-kW (168 hp) four-cylinder 1.4 TSI delivers maximum torque of 240 Nm (177 lb-ft) with a combined cycle fuel consumption of 7.2 liters/100km (32.7 mpg US).

By contrast, the current 2.0-liter GT FSI offers 110 kW (148 hp) of power, 200 Nm (148 lb-ft) of torque, and 7.8 l/100km (30 mpg US) fuel consumption.

VW will also preview a new Touran using the TSI engine. The Touran 1.4 TSI is a lower-performance version, offering 103 kW (138 hp), and is due to launch in Germany in the first quarter of 2006.

Combustion in an engine is limited not by the amount of fuel that can be injected but by the amount of air an engine can suck in to mix with that fuel. Forcing compressed air into the engine allows more fuel to be burned, resulting in higher output than from engines of the same size, but that are naturally aspirated.

Both superchargers and turbochargers are designed to provide that compressed air to boost output, but they do so quite differently. A supercharger is a compressor driven by a belt, chain or gears—and hence can react quickly to start-up or low-speed acceleration. A turbocharger uses the otherwise wasted heat energy of the exhaust gases to spin the fans that compress air entering the engine’s intake manifold.

But superchargers put a parasitic load on the engine, and turbochargers can have a lag time (“turbo lag”) as the fans come up to speed.

Combining the two (compound, or dual charging) provides the best of both. In the new combined system, the mechanical compressor operates at low engine speeds to increase low-end power. At middle revs, the turbocharger will engage. Once the turbocharger reaches sufficient speed to provide boost, a clutch will disengage the supercharger, which will then be bypassed.

That’s tricky to accomplish well, but is made more approachable by the increasing sophistication of engine management software and systems.

In and of themselves, neither turbochargers or superchargers increase fuel economy. It’s the opposite—they are designed to support increased fuel burn, resulting in more power.

But the additional power enabled by a super-, turbo- or compound-approach enables the use of a smaller engine for the same application. In other words, even though the charging systems burn more fuel, by applying them in a smaller (downsized) engine, the automakers can deliver equivalent (or even better) power compared to a larger system, and as a result see a net gain in fuel economy.

That’s exactly what you see in the comparative output of the two Golf platforms above. The 1.4-liter TSI outperforms the 2.0-liter FSI, while using less fuel.

We’ll do more on super-, turbo- and compound-charging following the Frankfurt show.