Posted tagged ‘race cars’

Super Charger

September 28, 2011

 

Engines combust (burn) fuel and use the energy of that combustion to do work. The more fuel that is combusted in any given time then the more energy is available to carry out the engines task. Fuel requires air (or the oxygen contained within air) to burn so if there isn’t enough air mixed with the fuel it will not burn. This also means that the amount of air entering an engine determines how much fuel can be burnt and consequently how much energy (or power) an engine can produce. Superchargers are essentially an air pump designed to cram extra air into an engine allowing it to combust more fuel than would otherwise be possible.

2011-Mustang-Supercharger

Mercedes pioneered automotive superchargers on their race cars during the 1920’s. These were simple reciprocating compressors attached to the engine by an electrically operated clutch. A switch activated by the accelerator pedal turned the pump on when extra power (full throttle) was required. A flurry of engineering endeavor ensued in order to reign in Mercedes advantage on the racetrack. Within a few short years most of the basic designs for modern superchargers had appeared.

mmfp-0609-02z-2b1999-ford-f150-lightning-2bwhipple-supercharger

During the 1930’s superchargers were largely the preserve of marine engines, aircraft and race vehicles but gradually found their way into commercial diesel engines by the 1950’s. It has been common for truck engines to be turbo supercharged (a.k.a. turbocharged) for decades but car engines originally had difficulty in effectively employing this technology.

01-supercharger-layout

Superchargers mostly fall into one of two categories, mechanically driven superchargers and turbo superchargers driven by exhaust gasses. A third category is starting to make an appearance and that is electrically powered superchargers.

01-super-charger-work

Turbo superchargers (a.k.a. turbochargers or turbo’s) are relatively compact, lightweight and efficient but suffer from turbo lag and heat stress. By turbo lag we mean the amount of time it takes for the turbo’s rotor to speed up to full efficiency. Some of the earliest turbo charged vehicles took so long for the turbo to produce a usable amount of boost that they were all but useless. Modern turbo chargers are much better in this regard but turbo lag is still a problem. Heat is another bane of turbo chargers. Exhaust gasses are extremely hot and can cause so much heat to build up in the turbo that oil will burn and congeal within its galleries leading to a bearing failure. This is why many turbo chargers have a turbo timer. The timer will cause an engine to continue idling for a few minutes after it is switched off allowing excess heat to be dissipated.

01-super charger schematic diagram

Mechanically driven superchargers usually don’t suffer from turbo lag and can often produce more boost than an exhaust driven charger (turbo). On the negative side they are generally bulky, heavy, and have a cumbersome drive mechanism (usually belt drive). Furthermore most chargers of this type have to supply air at all engine speeds and loads making them difficult to match various engine conditions precisely.

As our supercharger is electrically driven we have devoted an entire article to the advantages and disadvantages of this type.

Heat exchangers (intercoolers) are frequently used in conjunction with superchargers. Compressing air increases its temperature thus making it less dense. By re-cooling the compressed volume of air before it enters, density is increased allowing even more air to be forced into the engine. Intercoolers are more important for turbo superchargers as there are two heating sources present, the act of compression and heat from exhaust gasses both increase air temperature.

Variable Turbo Chargers Geometry (VTG)

September 25, 2011

Variable geometry turbochargers (VGTs) are a family of turbochargers, usually designed to allow the effective aspect ratio (sometimes called A/R Ratio) of the turbo to be altered as conditions change. This is done because optimum aspect ratio at low engine speeds is very different from that at high engine speeds. If the aspect ratio is too large, the turbo will fail to create boost at low speeds; if the aspect ratio is too small, the turbo will choke the engine at high speeds, leading to high exhaust manifold pressures, high pumping losses, and ultimately lower power output. By altering the geometry of the turbine housing as the engine accelerates, the turbo’s aspect ratio can be maintained at its optimum. Because of this, VGTs have a minimal amount of lag, have a low boost threshold, and are very efficient at higher engine speeds. VGTs do not require a waste gate.

01-variable turbine geometry-turbocharger-vtg-sequence

Most common designs
The two most common implementations include a ring of aerodynamically-shaped vanes in the turbine housing at the turbine inlet. Generally for light duty engines (passenger cars, race cars, and light commercial vehicles) the vanes rotate in unison to vary the gas swirl angle and the cross sectional area. Generally for heavy duty engines the vanes do not rotate, but instead the axial width of the inlet is selectively blocked by an axially sliding wall (either the vanes are selectively covered by a moving slotted shroud, or the vanes selectively move vs a stationary slotted shroud). Either way the area between the tips of the vanes changes, leading to a variable aspect ratio.

01-normal_turbo charger-vtg turbo-turbine section-compressor section

Actuation
Often the vanes are controlled by a membrane actuator identical to that of a waste gate, however increasingly electric servo actuation is used. Hydraulic actuators have also been used in some applications.

01-Twincharger_theory-turbocharger layout diagram

Main suppliers

Several companies supply the rotating vane type of variable geometry turbocharger, including Garrett (Honeywell), Borg Warner and MHI (Mitsubishi Heavy Industries). The rotating vane design is mostly limited to small engines and/or to light duty applications (passenger cars, race cars and light commercial vehicles). The only supplier of the sliding vane type of variable geometry turbocharger is Cummins Turbo Technologies (Holset), who are effectively the sole supplier of variable geometry turbochargers for applications involving large engines and heavy duty use (i.e. trucks and off highway applications).

01-turbo-parts-turbocharger section-compressor air discharge

Other common uses
In trucks, VG turbochargers are also used to control the ratio of exhaust re-circulated back to the engine inlet (they can be controlled to selectively increase the exhaust manifold pressure exceeds the inlet manifold pressure, which promotes exhaust gas recirculation (EGR)). Although excessive engine back pressure is detrimental to overall fuel economy, ensuring a sufficient EGR rate even during transient events (e.g. gear changes) can be sufficient to reduce nitrogen oxide emissions down to that required by emissions legislation (e.g. Euro 5 for Europe and EPA 10 for the USA).

01-turbocharger-Vtg-cross sectional diagram-control system

Another use for the sliding vane type of turbocharger is as downstream engine exhaust brake (non-decompression type), so that an extra exhaust throttle valve isn’t needed. Also the mechanism can be deliberately modified to reduce the turbine efficiency in a predefined position. This mode can be selected to sustain a raised exhaust temperature to promote “light-off” and “regeneration” of a diesel particulate filter (this involves heating the carbon particles stuck in the filter until they oxidize away in a semi-self sustaining reaction – rather like the self-cleaning process some ovens offer). Actuation of a VG turbocharger for EGR flow control or to implement braking or regeneration modes generally requires hydraulic or electric servo actuation.