Turbo Vs Superchargers
In contrast to turbochargers, superchargers are not powered by exhaust gases but driven by the engine mechanically. Belts, chains, shafts, and gears are common methods of powering a supercharger. A supercharger places a mechanical load on the engine to drive. For example, on the single-stage single-speed supercharged Rolls-Royce Merlin engine, the supercharger uses up about 150 horsepower (110 kW). Yet the benefits outweigh the costs: For that 150 hp (110 kW), 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 internal hardware of the engine must withstand the net power output of the engine, plus the 150 horsepower to drive the supercharger.
Another notable disadvantage of superchargers is lower adiabatic efficiency as compared to turbochargers. Adiabatic efficiency is a measure of a compressors ability to compress air without adding excess heat to that air. The compression process always produces heat as a bi-product of that process, however, more efficient compressors produce less excess heat. The most common and widely used forms of superchargers tend to use compressors that impart significantly more heat to the air then their turbocharged counterparts. 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.
In comparison, a turbocharger does not place a direct mechanical load on the engine. It is more efficient because it uses the otherwise wasted potentialand kinetic 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 horsepower.
This lag occurs because turbochargers rely on the build up of exhaust gas pressure to spin the turbine section of the turbocharger that compresses the air. In variable output systems such as automobile engines, exhaust gas pressure at idle or lower engine speeds 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 standard atmospheric pressure. When the engine reaches a sufficient speed, the turbine itself lags behind as it responds to the increase in exhaust gas pressure.
Engineers have mitigated this problem in modern ball bearing turbochargers that use high quality production methods and materials that reduce turbine spooling time. Other notable disadvantages of turbochargers are the inherent increase in complexity over a supercharged system, and cost associated with that complexity.
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's two-stroke engines, the mechanically-assisted turbocharger is not specifically a twincharger, as the engine uses the mechanical assistance to create charge air only during starting. Thereafter, 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 engine, such as EMD's, cannot naturally aspirate, and, according to SAE definitions, a two-stroke engine with a mechanically-assisted compressor during starting is considered naturally aspirated.