Turbochargers Vs Superchargers
A turbocharger is generally more desirable than a supercharger unless outright power is required. Turbochargers offer increased power without the same decrease in fuel economy. In both a turbo- and a supercharged engine, power is increased by providing air under pressure to the engine's cylinders. This allows an increased amount of fuel to be burnt, producing more power. However, this inevitably increases fuel consumption. A supercharger is driven directly from the engine and thus its boost output is directly related to engine speed. A turbocharger is more directly controlled by the pressure of the exhaust gases which, as well as increasing with engine speed, also vary significantly with engine load.
When a diesel engine is put under a load there is greater resistance to the expansion of combustion gases in the cylinder. This increases combustion pressure and temperature which, in turn, increases the pressure and temperature of the exhaust gases. A turbodiesel engine under a heavy load will thus drive its turbocharger at a greater speed than if the same engine is run at the same rpm under little or no load.
This has the effect that a turbocharger delivers boost, thus increasing power (and fuel consumption) only when such a power increase is demanded by putting the engine under a heavy load. A turbodiesel-powered vehicle accelerating from rest, for example, will put its engine under a heavy load, thus causing high boost pressures to be delivered by the turbocharger. This is detected by the fuel injection system which delivers more fuel to provide more power. Once the vehicle reaches a constant speed and constant engine rpm load decreases significantly, the pressure of the exhaust gases through the turbo drop, boost and fuel delivery decrease, thus lowering fuel consumption to near the same levels as a naturally aspirated diesel engine. If, say, the vehicle starts climbing a gradient, the engine load increases and the turbocharger and fuel system provide more power. Extra fuel is delivered only when needed.
A supercharger delivers near-constant boost pressures, and so fuel consumption suffers. Superchargers have the advantage of having no boost threshold (an rpm level below which a turbocharger does not operate effectively) and almost no lag. Superchargers only need to be connected to the engine's intake system, thus making installation easier and reducing to some extent the increase in internal temperatures that occurs with turbocharging.
Even in engines operating under a constant load (such as electrical generators), turbochargers have advantages over superchargers. The main advantage is that a turbocharger does not "rob" power from the engine to the same extent that a supercharger does. A supercharger takes power directly from the engine's crankshaft to drive it- large units can draw up to 10% of the engine's total power when at full boost, although of course, they provide a power increase much greater than this. Turbochargers are driven by the engine's exhaust gases. A smaller power loss is caused by the turbocharger's turbine restricting the flow of exhaust gases and increasing back-pressure. In a gasoline engine this power-loss is much more pronounced. It is commonly referred to as turbo-lag and is experienced at lower engine speeds. However, since these speeds are where a diesel is most efficient, the turbo spools (spins) very quickly and lag is almost non-existent. The diesel's torque output is increased and a broader range of engine speeds can be used.
Read more about this topic: Turbodiesel