Turbocharging exemplifies that everything has a downside

By T. Uncle

From a car driver's point of view, turbocharging can be a double-edged sword.

On one side, it can provide an almost miraculous surge of "free" performance; on the other, it can leave a performance vacuum in which the driver is left, foot to the floor and red- faced, watching pedal-powered cyclists accelerate away at the traffic lights.

Like all devices that appear to give something for nothing, the turbocharger extracts a cost.

It delivers exceptional power for a given engine size, but it also has a negative effect on the way that power is delivered, especially at low speeds.

How does a turbocharger work? And what causes this erratic behavior?

More importantly, what are carmakers able to do to maximize the good, while minimizing the bad?

Basically, a turbocharger is a gas pump. On one side is a turbine driven by exhaust gases being expelled by the engine, on the other a turbine that draws in the fresh air needed for combustion, then compresses it before delivering it to the engine.

The result: a greater volume of air in the combustion chamber, and a more intensive combustion of the fuel-air mix that is required to set the whole reciprocating mass in motion.

A turbocharged engine in many ways behaves like something much larger than its stated capacity would suggest.

The power that can be wrung out of a forced-induction engine is well beyond what can be managed with an engine using normal atmospheric pressure to fill the cylinders with a mixture of air and fuel.

Where it is uncommon for even a highly efficient engine to produce more than, say, 50kW per liter, a turbo engine stretched to its maximum delivers many times that.

The best example can be found in Formula One cars from the 1980s which used turbocharging to wring well over 500kW per liter out of their otherwise conventional engines.

BMW, for example, achieved such figures out of a four-cylinder engine that was developed from its road-going 3-Series range.

But while the dreaded turbo "lag" -- the time spent for the exhaust-powered turbine to wind up sufficiently to create meaningful boost pressure -- was not so much of a problem for Formula One race cars handled by drivers familiar with narrow power bands, it was, in many cases, a source of potential embarrassment on the road.

Early turbo road cars, such as the Saab 900 that was the first production turbo, tended to suffer from inexcusable low-speed lag.

Under certain engine speeds, the turbine was simply not delivering enough boost to produce even the power of a conventionally aspirated engine of the same capacity.

This was partly brought about by the need to lower the engine's normal compression ratio to accommodate the stress placed on the internals by the extra boost, and also by the fact that the turbine's inertia was not quickly overcome by the slowly building speed of the exhaust gas.

A smaller turbo would allow the turbine speed to build more quickly, but then it would not have the capacity to continue delivering a strong boost as speeds rose.

Engine designers convinced of the potential of turbocharging continued, however, working on the problem and various techniques, including twin turbochargers -- one small and one large, working in sequence -- to minimize the lag.

And it must be said that today, those who remain faithful to the concept have more or less ironed out virtually all the bugs.

This has been done by constant refinement of the size of the turbocharger relative to what it is intended to do, more accurate engine management, the use of "intercoolers" to lower the temperature and increase the density of the charge while maintaining cooler operating temperatures and careful matching of the engine's exhaust system to the needs of the turbocharger.

Today, the "low-boost" turbo engine is gaining popularity with many European carmakers, including Audi, Saab and Volvo.

These engines manage to deliver impressive low-speed response, though still not quite up to "atmospheric" engines, with plenty of the midrange power that results in fast, responsive on-road performance.

No more the embarrassment of waiting at the lights for the turbo to wind up.

Another good thing has happened too: turbo engines, with their higher-than-normal under-hood temperatures were once prone to destroying themselves, particularly in hot climates.

Today, through developments in metallurgy and the use of features such as water-cooled turbine bearings, the forced- induction engine is equally as reliable as a normally aspirated design.

The turbo itself has the same life expectancy as other engine parts, such as the alternator or starter motor.

And low-boost turbo engines are used less to provide high performance than to extract sufficient power from small capacity engines than would normally by used in a given size of car.

Saab, for example, uses only turbo-powered engines in its new 9-5 range, while other carmakers such as Audi and Volvo are also aiming at the more conservative, less performance-oriented end of the market with their low-pressure turbo engines.

Insurance companies, who once placed high premiums on turbo- powered cars, are also starting to recognize this new style of turbo and are downscaling their rates accordingly.