of this, variable-geometry turbochargers often have reduced lag, a lower boost threshold, and greater efficiency at higher engine speeds. The benefit of variable-geometry turbochargers is that the optimum aspect ratio at low engine speeds is very different from that at high engine speeds. Electrically-assisted turbochargers An electrically-assisted turbocharger combines a traditional exhaust-powered turbine with an electric motor, in order to reduce turbo lag. This differs from an electric supercharger, which solely use an electric motor to power the compressor. Compressor The compressor draws in outside air through the engine's intake system, pressurises it, then feeds it into the combustion chambers (via the inlet manifold). The compressor section of the turbocharger consists of an impeller, a diffuser and a volute housing. The operating characteristics of a compressor is described by the compressor map. Ported shroud Some turbochargers use a "ported shroud", whereby a ring of holes or circular grooves allows air to bleed around the compressor blades. Ported shroud designs can have greater resistance to compressor surge and can improve the efficiency of the compressor wheel. Center hub rotating assembly The center hub rotating assembly (CHRA) houses the shaft that connects the turbine to the compressor. A lighter shaft can help reduce turbo lag. The CHRA also contains a bearing to allow this shaft to rotate at high speeds with minimal friction. Some CHRAs are water-cooled and have pipes for the engine's coolant to flow through. One reason for water cooling is to protect the turbocharger's lubricating oil from overheating. Supporting components The simplest type of turbocharger is the free floating turbocharger. This system would be able to achieve maximum boost at maximum engine revs and full throttle, however additional components are needed to produce an engine that is driveable in a range of load and rpm conditions. Additional components that are commonly used in conjunction with turbochargers are: Intercooler - a radiator used to cool the intake air after it has been pressurised by the turbocharger Water injection - spraying water into the combustion chamber, in order to cool the intake air Wastegate - many turbochargers are capable of producing boost pressures in some circumstances that are higher than the engine can safely withstand, therefore a wastegate is often used to limit the amount of exhaust gases that enters the turbine Blowoff valve - to prevent compressor stall when the throttle is closed Turbo lag and boost threshold Turbo lag