Comprex™ Technology
The supercharger consists of a cylindric housing tube in which a cell wheel rotates. Furthermore, there are cases on each front end of the housing tube which have each one or more inlets and outlets for hot emissions (exhaust casing) and fresh air (fresh air casing). These three basic casing components are screwed together to one unit.
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The cell wheel is driven mechanically by a V-belt, toothed belt or an electromotor and runs at a higher speed than the crankshaft of the combustion engine (typically 3:1). There are also variants with an electrically driven rotor available. This is an advantage, because when the crankshaft rotates at low speed, the optimum speed for the cell wheel is closer to 5:1. Therefore, the electric drive, unlike the firmly coupled belt drive, always ensures an optimum rotor speed. There is also a greater flexibility considering the mounting location when compared with a belt drive. The supercharger can also be positioned vertically or obliquely. The rotor bearing is done with permanently lubricated roller bearings. Among the older generations, they are both integrated in the intake casing (rotor is overhung-mounted).
The latest variant of the Comprex™ has a water-cooled gas housing that makes it possible to implant a bearing there, which subsequently enables constantly very tight clearances between the rotor and the housing. In this variant, the rotor consists of two halves which enable the heat expansion of the components by a small gap in between. As for the efficiency, it is extremely important to keep the clearances small without rubbing the rotor. This former problem is thus reliably solved. Furthermore, especially the cold start problem with petrol engines with a Comprex™ could also be solved. The problem was as follows: such engines had caused large thermal expansions of the rotor due to hotter exhaust gases, therefore, they had to be equipped with a larger rotor clearance. This is no longer necessary. Moreover, water cooling prevents the ignition of hot exhaust gases in the exhaust tract of petrol engines that are enriched with fuel (Lambda = 0,75) for the cooling of exhaust gas. However, this point is no longer relevant as the new emission legislation consistently foresees a stoichiometric Lambda = 1-mode. Thanks to water-cooling, the high temperatures of exhaust gases due to the stoichiometric mode of the engine do not affect the new generation of chargers. Also, the mechanical durability is high with Comprex™ chargers as the circumferential speed of the cell wheel at 80 – 100 m/s is far below the one of a turbo charger where for instance 500 m/s are quite common. In addition, measurements have shown that the presence of a Comprex™ charger have led to lower temperatures of exhaust gases up to 80⁰ C compared to a turbo engine identical in construction.
The boost pressure is caused by the energy of the incoming exhaust gases. The exhaust gases are passed to one or several cells of the cell wheel and compress the fresh air in the cells (principle of a pressure exchanger). The proper speed-timing of the cell wheel causes the exhaust pressure impulse to compress fresh air in the respective actuated cell. The continued rotation of the cell wheel holds the fresh air pressure in the cell, and shortly afterwards the air thus compressed is conducted into the induction tract. It is very often assumed that the Comprex™ receives its pressure waves from the engine. However, the Comprex™ generates its own pressure waves by accumulating the exhaust gas in front of the fresh air chamber. It will be released and causes a pressure wave. The cell which is provided with fresh air by the continued rotation of the low-pressure part is led in front the hot gas duct which is under high pressure. It is true, however, that the engine causes pressure impulses, but they are unnecessary for the smooth operation of the supercharger or are, at the worst, even obstructive. The supercharger can also be driven on a hot gas test bench with constantly streaming hot gas.
The drive of the cell wheel does not, in contrast to a compressor or a Roots blower, transfer energy for the pressure build-up. Furthermore, it does not have to overcome any forces apart from the bearing friction. The drive of the Comprex™ cell wheel is a speed synchronisation in order to time the gas-dynamic processes in the supercharger in the best possible way. The position of the pressure waves must always be accurate to its position in the supercharger during operation to create a suction wave for the fresh air. This suction wave conveys the exhaust emissions out of the rotor cell and gets fresh air into the respective cell. This cell is then ready for the next compression. Normally, a rotor in the automotive field can, for reasons of space, serve two gas-dynamic cycles per rotation. In the case of larger superchargers, for example for high-speed Diesel engines with 1000 kW and more, there can be three or four cycles. Typical rotor diameters measure from 70 mm up to 200 mm. A rule of thumb for a rotor is that its diameter and length are equal.
The Comprex™ charger receives the energy for the pressure build-up from the exhaust gases. It causes flow resistance in the exhaust gas system, similar to a turbocharger, and thus generates a so-called nozzle area from where a pressure increases between engine and charger results. Analogous to the turbocharger a large nozzle area generates little pressure and vice versa. For the regulation of the boost pressure, the Comprex™ charger of BBC/ABB had a waste gate outlet by which excess pressure could be deflated, analogous to the turbocharger. Newer chargers have so-called variable gas pockets; these are recesses in the exhaust casings by which the waste gas is normally deflected in the direction of the air housing. Among the new generations these pockets are also related with the exhaust gas system of the engine and can be opened to a greater or smaller degree, by the means of an outlet. The result of this, analogous to a variable turbine geometry (VGT) of the turbocharger, is a smaller or larger nozzle area with which the resulting boost pressure can be regulated very fast and precisely. This kind of charger enables the regulation of the performance of petrol engines over long distances with a throttle valve fully open.