Comprex™ technology

The Comprex™ pressure wave supercharger uses exhaust energy to compress fresh air, enabling immediate power delivery without delay. It reduces fuel consumption, lowers emissions, and improves engine performance. The latest generation, featuring a water-cooled housing, offers enhanced efficiency and reliability.

Principle of the Comprex™ Charger for an Internal Combustion Engine

The charger consists of a cylindrical housing (casing) in which a cell wheel rotates, along with housings on each end of the casing. These end housings contain one or more inlets and outlets for hot exhaust gases (exhaust housing) and fresh air (fresh air housing). These three fundamental housing components are bolted together as a single unit.

The cell wheel is mechanically driven by a V-belt or timing belt and runs at a higher rotational speed than the engine’s crankshaft (typically a 3:1 ratio). Variants with electrically driven rotors are also now available. Since the optimal speed for the cell wheel with a slowly rotating crankshaft is closer to 5:1, an electric drive offers an advantage. It can always guarantee an optimal rotor speed compared to a fixed belt drive. Additionally, the flexibility regarding installation location is significantly greater than with a belt drive. The charger can be positioned at an angle or vertically. The rotor is supported by permanently lubricated roller bearings, which in older generations were both housed in the air housing (floating rotor bearing).

The latest version of the Comprex™ features a water-cooled gas housing, enabling the use of a bearing that allows consistently very tight clearances between rotor and housing. In this variant, the rotor consists of two halves, which accommodate thermal expansion of the components through a small gap in the middle.

All previous problems related to maintaining the very small clearances critical for efficiency without rotor contact have been reliably solved. This also addresses the cold start problem for gasoline engines equipped with Comprex™, which due to hotter exhaust gases caused greater thermal expansion of the rotor and thus required larger rotor clearances.

Moreover, the issue of after burning of hot exhaust gases in the exhaust tract of gasoline engines running rich for exhaust cooling (lambda = 0.75) is now a thing of the past thanks to water cooling. However, the new exhaust regulations consistently mandate stoichiometric lambda = 1 operation, so this point has practically become obsolete.

The high exhaust temperatures resulting from stoichiometric operation pose no problem for the new charger generation due to the water cooling. Mechanical durability is also high, as the peripheral speeds of the rotor range from 80–100 m/s, far below those of turbochargers, where speeds of 550 m/s are not uncommon today.

Furthermore, measurements have shown that the mere presence of a Comprex™ charger caused exhaust temperatures to drop by up to 80 °C compared to an identical turbocharged engine.

Boost Pressure Build-Up Principle

The boost pressure is generated using the energy of the incoming exhaust gases. The exhaust gases are directed into one or multiple cells of the rotor wheel, compressing the fresh air contained within those cells (principle of a pressure exchanger). Through precise rotational speed timing of the rotor wheel, the exhaust pressure pulse compresses a portion of fresh air in the currently activated cell. As the rotor continues to turn, the fresh air pressure in the cell is maintained and shortly thereafter the compressed air is supplied to the intake manifold.

To clarify a common misconception that the Comprex™ obtains its pressure waves directly from the engine: the Comprex™ actually generates its pressure waves internally at the moment a cell, freshly filled with low-pressure fresh air, rotates in front of the high-pressure exhaust channel.

Although pressure pulses do come from the engine, they are neither necessary nor beneficial for the clean operation of the charger and can at worst be obstructive. The charger can be operated on a turbocharger test bench with a constant flow of hot gas. However, this requires a ROOTS blower supplying charging air to the combustion chamber to simulate operation similar to an engine. Without a Roots blower, testing is possible but the consequences of such operation must be understood.

Unlike a compressor or Roots blower, the drive of the rotor wheel does not transfer mechanical energy for pressure build-up and only needs to overcome bearing friction. The drive’s purpose is purely rotational speed synchronization to optimally time the gas-dynamic processes inside the charger. Pressure waves must always arrive at precise moments and locations inside the charger so that a suction wave is generated, expelling exhaust gases from the rotor cell and drawing fresh air into that cell, which is then available for the next compression phase.

Typically, due to space constraints related to connection lines, an automotive rotor can handle two gas-dynamic cycles per revolution. Larger chargers, for example on high-speed diesel engines above 1000 kW, can have three or four cycles. Typical rotor diameters start at 70 mm and can exceed 200 mm. A practical rule of thumb is that rotor diameter and length are approximately equal.

The Comprex™ charger obtains the energy to build boost pressure from the exhaust gas. It represents a flow resistance in the exhaust tract, similar to a turbocharger, creating a so-called backpressure area that results in a pressure rise between engine and charger. Similar to a turbocharger, a large backpressure area produces low pressure, and vice versa. To regulate boost pressure, the Comprex™ chargers from BBC/ABB were equipped with a wastegate valve to vent excess pressure, functioning analogously to turbochargers. Newer chargers feature variable gas pockets — indentations in the exhaust housing that normally direct exhaust flow towards the air housing. In modern generations, these pockets are connected to the engine’s exhaust tract and can be more or less opened via a roller valve. This results in a variable back pressure area similar to a Variable Turbine Geometry (VTG) in turbochargers, enabling very precise and rapid control of boost pressure.

With this type of charger, it is also possible to regulate the power output of gasoline engines over wide operating ranges at fully open throttle.