Turbocharging and supercharging page 2 |
Turbocharging consists of a pair of turbines in separate housings but on a common shaft. One turbine is driven by exhaust pressure; the other serves as an air compressor. Aside from the divided housing, a turbocharger is similar to a jet engine, and shares its distinctive whistle. The act of compressing air creates heat, hot air is less dense and less dense air in the inlet means that the effective amount of air entering the engine also decreases, so many turbocharged engines include an intercooler. Before it reaches the engine, the compressed air is ducted through a radiator-like device at the front of the car that cools the air and increases its density. Turbochargers make use of power (exhaust pressure) that would otherwise be wasted. As a result, they produce power gains with relatively little parasitic loss.
Turbochargers wait patiently in the background until the revs rise and the accelerator is hit hard. When they come alive, they deliver a wallop that's unlike anything else. The chief disadvantage is turbo lag. The turbocharger requires sufficient exhaust pressure to spin the turbines and create boost. It has little effect at low RPMs or light throttle. When you tromp on the accelerator, there's a slight delay before the boost builds, and then the power comes on like a herd of stampeding buffalo, this aspect is most evident, especially in cars like the Tata Indigo TDi and the Indica turbo; the power is just not there below 2200rpm, but beyond that point, the power just piles on.
Turbochargers rely on the engine's oil system for lubrication. When the engine stops, the oil stops flowing, but the turbo is still spinning, often at tens of thousands of RPM, and can literally cook itself in the still oil. Some engines have auxiliary pumps that oil the turbo bearings after the engine is shut off, but most, need to be "driven down": three to five minutes of idling or gentle driving before the engine is shut down.
When the engine has been idling or at low speed for a while, the turbo is not spinning or is spinning very slowly because there is very little exhaust leaving the engine. When the throttle is opened, the engine produces more exhaust, which spins the turbo faster. A faster spinning turbo means more air and fuel is being blown into the engine, therefore even more exhaust is being produced, which makes the turbo spin even faster and so on. This cycle is known as turbo "spool-up", which feels like a sudden surge in engine power and appears on your boost guage as a sudden increase in pressure. The time before the surge, when the turbo is spooling up but the engine doesn't have much power yet, is called turbo lag. A large turbocharger can produce more air flow and pressure, but will have more lag because of its increased size. A small turbocharger will have a smaller amount of lag, but will not be able to move as much air. This is explained in more detail is the sections below.
A supercharger on the other hand is an air pump that is driven by the engine (usually by a belt. Most superchargers will provide anywhere from 5 to 15 psi of pressure (or boost). Blower designs vary, but most sit directly on top of the intake manifold. The chief advantage of a supercharger is that it's always on, if the engine is running, the supercharger is supplying boost.
Add a supercharger to your car, and you'll feel as if it's grown a bigger engine. There's no lag and no sudden bursts of power-just smooth, consistent thrust. It's the smooth power delivery that makes superchargers popular for high-end drag racing, where an unpredictable power strike can make things pretty hairy.
Superchargers add power, but they also require power to run - often as much as 5 to 10 ponies that could be going to the wheels. And while some may call their power delivery "even", others would call it "docile". Turbochargers are like hot-blooded thoroughbreds, while superchargers are even-tempered mares. Which is more likely to get your heart a-pumpin'?
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![Turbo Impeller](javascript:openwin("largephoto.asp?imagename=turbo_imp.jpg&imagetitle=Turbo Impeller"))