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Turbocharger vs. Supercharger

By Unknown - February 05, 2018

Unlike the turbocharger, the supercharger is mechanically driven by the engine. Belts, chains, shafts, and gears are common methods for running a supercharger, putting a mechanical load on a machine. For example, at one stage of the Rolls-Royce Merlin engine with a single-speed supercharger, the supercharger will use approximately 150 horsepower (110 kilowatts) of engine power. But the benefits are greater, because of the 150 hp (110 kW) power to drive the supercharger, the engine will generate an additional 400 horsepower, resulting in a net profit of 250 hp (190 kW). From here the main weakness of the supercharger is seen, because the engine must withstand the net output power of the engine plus power to drive the supercharger.

Another disadvantage of most superchargers is that adiabatic efficiency is lower than that of a turbocharger (especially a root-modeled supercharger). Adiabatic efficiency is a measure of the compressor's ability to compress air without adding additional heat to the air. The compression process always generates heat as a by-product of the process; however, a more efficient compressor produces less heat. Root-model supercharger produces excess heat into the air rather than a turbocharger. Thus, for the same volume and air pressure, the air turbocharger is cooler, and as a result more dense, it contains more oxygen molecules, and ultimately generates more potential power than the supercharger air. In practical applications the difference between the two can be dramatic, with turbochargers often producing 15% to 30% higher power, solely on adiabatic efficiency differences.

In comparison, the turbocharger does not place a direct mechanical load on the engine, although the turbocharger puts the exhaust gas pressure back on the engine, increasing pumping losses. This is more efficient, because it uses the energy wasted from the exhaust gases to drive the compressor. Unlike the supercharger, the main disadvantage of the turbocharger is the so-called "lag" or "spool time". This is the time between demand for increased power (throttle opened) and turbocharger provides increased intake pressure, thus increasing power.

Throttle lag occurs because the turbocharger relies on the accumulation of exhaust gas pressure to drive the turbine. In an output variable system such as a car engine, exhaust gas pressure at idle, low engine speed, or low throttle is usually not enough to drive the turbine. Only when the engine reaches sufficient speed, the turbine part begins to spool up, or spins fast enough to produce intake pressure above atmospheric pressure.

The combination of turbocharger and supercharger can eliminate the disadvantages of both. This technique is called twincharger.

Related article :

Supercharger : How dose it work ?

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