Video: The Science Of Boost

By design, reciprocating engines are air pumps. They compress the aspirated air-fuel charge, ignite it, convert this expansion of hot gases into mechanical energy, and then expel the cooler, lower pressure gases. The amount of energy converted is determined by the pressure exerted on its pistons by combustion and the length of its expansion cycle. By increasing how aggressively a given mass of air-fuel charge is compressed, higher combustion pressures are achieved, allowing more energy to be extracted and thus creating more mechanical power output.

ROOTS SUPERCHARGER In 1859 two brothers Philander Higley Roots and Francis Marion Roots founded The Roots Blower Company in Connersville, Indiana.

Roots superchargers operate by pumping air with a pair of meshing lobes resembling a set of stretched gears. The incoming air is trapped in pockets surrounding the lobes and carried from the intake side to the exhaust of the blower.

TWIN-SCREW SUPERCHARGERS In 1935, Swedish engineer Alf Lysholm patented a new air pump design as well as a method for its manufacture that improved upon the limitations of Roots blowers. Lysholm had replaced the lobes with screws, creating the rotary-screw compressor.

CENTRIFUGAL SUPERCHARGERS...

INTERCOOLERS

Forcing more air into a cylinder with boost easily creates more power in an engine by increasing the air mass of the intake charge beyond what is possible with natural aspiration. This also inherently pushes volumetric efficiency well beyond 100%

Because forced induction occurs outside of the engine the properties of the air mass can be further enhanced by cooling, by passing the compressed air through a heat-exchange device known as an intercooler.

TURBOCHARGERS

In some extreme cases, it can take as much as ⅓ of the base engine's power to drive the supercharger to produce a net gain in power.

The first turbocharger design was patented in 1905 by Swiss Engineer Alfred Büchi. He had conceptualized a compound radial engine with an exhaust-driven axial flow turbine and compressor mounted on a common shaft.

Turbochargers work by converting the heat and kinetic energy contained within engine exhaust gases, as they leave a cylinder. Radial inflow turbines work on a perpendicular gas flow stream, similar to a water wheel.

This shaft is housed within the center section of a turbocharger known as the center hub rotating assembly. Not only must it contain a bearing system to suspend the shaft spinning at 100,000s of RPMs, but it must also contend with the high temperatures created by exhaust gases.

In automotive applications, the bearing system found in most turbochargers are typically journal bearings or ball bearings. Of the two, journal bearings are more common due to its lower costs and effectiveness. It consists of two types of plain bearings; cylindrical bearings to contain radial loads and a flat thrust bearing to manage thrust loads.

Turbine aspect ratio - This is the ratio of the area of the turbine inlet relative to the distance between the centroid of the inlet and the center of the turbine wheel.

Compressors Trim -This is the relationship between the compressor wheels’ inducer and exducer diameter.

WASTEGATES

In order to prevent safe pressures and speeds from being exceeded, a mechanism called a wastegate is employed. Wastgates work by opening a valve at a predetermined compressor pressure that diverts exhaust gases away from the turbine, limiting its rpm. In its most common form, wastegates are integrated directly into the turbine housing, employing a poppet type valve. The valve is opened by boost pressure pushing a diaphragm against a spring of a predetermined force rating, diverting exhaust gases away from the turbine.

BLOW OFF VALVES

On engines with throttles, such as gasoline engines, a sudden closing of the throttle plate with the turbine spinning at high speed causes a rapid reduction in airflow beyond the surge line of the compressor. A blow-off valve is used to prevent this.

MULTI-CHARGING

Twincharging started to appear in commercial automotive use during the 1980s, with Volkswagen being a major adopter of the technology. In its most common configuration, a supercharger would feed directly into a larger turbocharger.

TWIN-SCROLL TURBOCHARGER

Twin-scroll turbochargers have two exhaust gas inlets that feed two gas nozzles. One directs exhaust gases to the outer edge of the turbine blades, helping the turbocharger to spin faster, reducing lag, while the other directs gases to the inner surfaces of the turbine blades, improving the response of the turbocharger during higher flow conditions.

VARIABLE GEOMETRY Variable-geometry turbochargers are another example of turbocharger development. They generally work by allowing the effective aspect ratio of the turbocharger’s turbine to be altered as conditions change.

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