In contrast to turbochargers, superchargers are mechanically driven by the engine. Belts, chains, shafts, and gears are common methods of powering a supercharger, placing a mechanical load on the engine. For example, on the single-stage single-speed supercharged rollls-royce merlin engine, the supercharger uses about 150 horsepower (110 kw). Yet the benefits outweigh the costs; for the 150 hp (110 kW) to drive the supercharger the engine generates an additional 400 horsepower, a net gain of 250 hp (190 kW). This is where the principal disadvantage of a supercharger becomes apparent; the engine must withstand the net power output of the engine plus the power to drive the supercharger.
Another disadvantage of some superchargers is lower adiabatic efficiency as compared to turbochargers. Adiabatic efficiency is a measure of a compressor's ability to compress air without adding excess heat to that air. The compression process always produces heat as a byproduct of that process; however, more efficient compressors produce less excess heat. Roots superchargers impart significantly more heat to the air than turbochargers. Thus, for a given volume and pressure of air, the turbocharged air is cooler, and as a result denser, containing more oxygen molecules, and therefore more potential power than the supercharged air. In practical application the disparity between the two can be dramatic, with turbochargers often producing 15% to 30% more power based solely on the differences in adiabatic efficiency.
By comparison, a turbocharger does not place a direct mechanical load on the engine (however, turbochargers place exhaust back pressure on engines, increasing pumping losses). This is more efficient because it uses the otherwise wasted energy of the exhaust gas to drive the compressor. In contrast to supercharging, the primary disadvantage of turbocharging is what is referred to as "lag" or "spool time". This is the time between the demand for an increase in power (the throttle being opened) and the turbocharger(s) providing increased intake pressure, and hence increased power.
Throttle lag occurs because turbochargers rely on the build up of exhaust gas pressure to drive the turbine. In variable output systems such as automobile engines, exhaust gas pressure at idle, low engine speeds, or low throttle is usually insufficient to drive the turbine. Only when the engine reaches sufficient speed does the turbine section start to spool up, or spin fast enough to produce intake pressure above atmospheric pressure.
A combination of an exhaust-driven turbocharger and an engine-driven supercharger can mitigate the weaknesses of both. This technique is called twincharging.
In the case of electro motive diesel two-stroke engines, the mechanically-assisted turbocharger is not specifically a twincharger, as the engine uses the mechanical assistance to charge air only during starting. Once started, the engine uses true turbocharging. This differs from a turbocharger that uses the compressor section of the turbo-compressor only during starting, as a two-stroke engines cannot naturally aspirate, and, according to SAE definitions, a two-stroke engine with a mechanically-assisted compressor during starting is considered naturally aspirated.
