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Turbos top superchargers as fuel saver

Wheels contributor Gerry Malloy explains the difference between turbocharger and supercharger.

  • The image of cars in a showroom

“Turbocharged!”

The term is ubiquitous in advertising for everything from energy drinks to computers.

But what does it really mean?

In other cases the term is typically just hyperbole, but in an automotive context it has a real meaning. A turbocharged engine is one equipped with a turbocharger, or a “turbo” in the vernacular.

Once used predominantly in race cars and associated solely with the ability to generate additional power, turbos are now embraced by engineers as a fuel-saving technology, helping them achieve strict fuel-economy targets without sacrificing vehicle performance.

There is nothing magic about a turbocharger. It is simply a type of supercharger, which means it is an air compressor that forces air into an engine’s cylinders under higher-than-atmospheric pressure (often called boost).

In unboosted (sometimes called normally aspirated) internal-combustion engines, air is typically drawn into each cylinder by the suction force of the piston moving down on the intake stroke with the intake valve(s) open.

Squeezing more air and corresponding fuel into the combustion chamber with use of a supercharger means more fuel can be burned and thus more energy released during combustion, resulting in greater power and/or efficiency.

What makes a turbocharger different from other types of supercharger is how it is driven — by exhaust gases, rather than by some mechanical connection such as gears or belts. Because of that differentiation, the term supercharger is now usually applied just to mechanically-driven compressors, while turbochargers are identified by their own name.

A turbocharger operates on the same principle as an old-time water wheel. Exhaust gases flowing past the blades of a turbine cause the shaft on which they are mounted to rotate, just as water flowing over a water wheel rotates the shaft of a mill-wheel.

The other end of that turbocharger shaft rotates another turbine wheel, in a separate sealed housing, which compresses air en route to the engine’s intake system.

Because a supercharger is mechanically driven, some of the engine’s power output is consumed to drive it. Not so with a turbocharger, which is driven by exhaust-gas energy that is normally wasted, thus enhancing efficiency.

In a mechanically-driven supercharger, the level of boost created is directly proportional to engine speed: the higher the speed, the greater the boost.

With a turbocharger, the level of boost generated is proportional to exhaust gas flow, not engine speed. The higher the load on the engine, the greater the boost.

The corollary to that statement is that at low load there is very little, if any, boost. Which is precisely why turbochargers are becoming a preferred means for reducing fuel consumption.

Either type of supercharging permits engineers to adopt smaller and lighter engines to achieve a given level of peak power output.

The advantage of turbocharging is that it generates boost and uses additional fuel only when there is sufficient engine load, and thus exhaust-gas flow, to require additional power. In other words, it provides extra power — and uses extra fuel — only on demand.

In other circumstances, it operates like a conventional engine, with the fuel-consumption advantages of the smaller engine size it enables.

The disadvantage of a mechanically-driven supercharger is that fuel is used to generate boost continuously, even when it is not needed.

The advantage of supercharging, however, is that, because of its direct mechanical relationship, boost is immediately available for acceleration. With a turbocharger, it takes some time for the turbine wheel to build up speed and that may result in a delayed response (turbo-lag) when the driver steps on the accelerator pedal.

Turbo-lag was a major problem when turbochargers enjoyed their first round of mainstream popularity a couple of decades ago.

The bigger the turbo, the greater the turbo-lag. (It takes longer to get a big wheel turning than a small one.)

One approach to that problem is the use of twin turbos — two small turbos instead of one big one. But there have been other significant developments in the interim, including variable-geometry turbochargers.

Those developments, in conjunction with electronic controls, have all but eliminated turbo-lag in most modern applications.

If current trends continue, turbochargers may become such an accepted piece of engine equipment that marketers would no longer feel the need to proclaim that a vehicle is “Turbocharged!”

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