Comprex loader

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Double row cell wheel of a pressure wave supercharger

The Comprex charger (also called pressure wave charger ) is a type of engine supercharging of gasoline and diesel engines .

Structure and principle of operation

Principle of Comprex charging a diesel engine

The charger consists of a cylindrical housing (casing) in which a cell wheel rotates, and casings with one or more inlets and outlets for hot exhaust gases (exhaust casing) and fresh air (fresh air casing) on ​​each face of the casing. These three basic housing parts are screwed together to form a unit.

The cellular wheel is mechanically driven by a V-belt or toothed belt and runs at a higher speed than the crankshaft of the internal combustion engine (typically 3: 1). Variants with an electrically driven rotor are now also available. Since the optimal speed for the cellular wheel with a slowly rotating crankshaft is closer to 5: 1, this is an advantage. Thus, the electric drive can adapt the rotor speed better than a belt drive with a fixed gear ratio. The installation location can also be selected more flexibly than with a belt drive. The loader can also be positioned at an angle or vertically. The rotor is supported by permanently lubricated roller bearings, which in the older generations are both housed in the air housing (rotor overhung). The latest version of the Comprex has a water-cooled gas housing, which makes it possible to use a bearing, which allows constant small clearances between the rotor and the housing. In this variant, the rotor consists of two halves, which allow the components to expand through a small gap in the middle. All previous problems of keeping these clearances, which are extremely important for efficiency, small without touching the rotor are thus reliably solved. In particular, this also applies to cold start problems in gasoline engines with Comprex chargers, which due to the hotter exhaust gases caused greater thermal expansion of the rotor and therefore had to be equipped with greater rotor clearance. The post-ignition of hot exhaust gases in gasoline engines that are enriched for exhaust gas cooling (lambda = 0.75) is a thing of the past thanks to the water cooling. However, since the new exhaust gas legislation provides for stoichiometric lambda = 1 operation throughout, this point has practically been resolved. But the high exhaust gas temperatures resulting from the stoichiometric operation of the engine do not affect the new generation of turbochargers thanks to the water cooling, and the mechanical durability of Comprex turbochargers is also high, as the circumferential speeds of the rotor of 80–100 m / s are far below those of a turbocharger which meanwhile reach peripheral speeds of 550 m / s. Measurements have also shown that the presence of a Comprex charger alone has resulted in the exhaust gas temperatures falling by up to 80 ° C compared to a turbo engine of the same design.

The boost pressure builds up with the help of the energy of the inflowing exhaust gases. The exhaust gases are directed into a single cell or several cells of the cell wheel and compress the fresh air contained therein (principle of a pressure exchanger). A suitable position of the inlet and outlet openings on the housing of the rotary valve ensures that the exhaust pressure surge compresses a fresh air section in a cell that is currently being activated. Then the fresh air pressure is maintained in the cell by further turning the cell wheel, and shortly afterwards the air compressed in this way is fed to the intake tract. In order to anticipate a widespread mistake that the Comprex gets its pressure waves from the engine, it should be noted here that the Comprex generates its pressure waves itself at the moment when a cell is equipped with fresh air by further turning the low-pressure part in front of the high-pressure exhaust duct comes. It is true that pressure surges come from the engine, but these are not necessary for the clean operation of the charger or, in the worst case, they are even a hindrance. The charger can also be operated on a turbocharger test bench with a constant flow of hot gas. However, this must be equipped with a Roots blower that feeds the charge air to the cylinder in order to guarantee operation similar to engine operation. If this is not possible, you can also measure without a Roots blower, but you have to know the consequences of this operation.

In contrast to a compressor or Roots blower, the drive of the cell wheel does not transmit any mechanical energy to build up pressure and apart from bearing friction, it does not have to overcome any forces; When driving the Comprex cellular wheel, it is only a matter of speed synchronization in order to synchronize the gas dynamic processes in the charger. The pressure waves always have to arrive at a certain place in the charger at the right moment so that a suction wave is created which transports the exhaust gases out of the rotor cell and draws fresh air into the relevant cell. This is then available for the next compression. Usually, a rotor in the automotive sector can operate two gas-dynamic cycles per revolution for reasons of space (the connection lines are a hindrance); with larger chargers for high-speed diesel engines with 1000 kW and more, for example, it can also be three or four cycles. Typical rotor diameters start at 70 mm and can reach over 200 mm. As a rule of thumb, the rotor diameter and length are the same.

The Comprex charger receives the energy to build up the pressure of the charge air from the exhaust gas. It is a flow resistance in the exhaust system similar to a turbocharger and thus generates a so-called accumulation area, which results in a pressure increase between the engine and the charger. Similar to the turbocharger, a large accumulation area generates little pressure and vice versa. To regulate the boost pressure, the Comprex loaders from BBC / ABB, like the turbocharger, had a wastegate valve through which excess exhaust gas could be blown off. Newer loaders have so-called variable gas pockets; these are depressions in the exhaust housing through which the exhaust gas is normally diverted towards the air housing. In the newer generations, these pockets are also connected to the exhaust system from the engine and can be opened more or less by means of a roller valve. Analogous to a variable turbine geometry (VTG) with the turbocharger, this also results in a more or less large accumulation area with which the resulting boost pressure can be regulated very precisely and quickly. With this type of charger, it is also possible to regulate the power of gasoline engines over long distances with the throttle valve fully open.

history

The Comprex loader was developed in the 1970s and 1980s by Brown, Boveri & Cie. in Baden (Canton Aargau in Switzerland).

Comprex loaders were briefly installed in the last Senator diesel models by the manufacturer Opel from 1985 onwards . At that time, other manufacturers already had turbo-diesel engines in their sales program, so that Opel was no longer competitive with its 2.3-liter naturally aspirated diesel, which produced only 71 hp (52 kW). The range of diesel engines was further expanded: In the 1984 model year, the 86 hp (63 kW) turbo variant was added, which was also available in the Senator. From 1985 the 2.3-liter Comprex diesel with 95 hp (70 kW) was available as an option as a special technical feature. The Senator Comprex diesel was only available with a 5-speed manual transmission. According to the nameplate, the manufacturer was not Opel, but Irmscher . Only small numbers were produced.

The torque curve shows the advantage of the Comprex charger compared to the turbocharger: unlike the turbocharger , the boost pressure starts at the lowest engine speeds and, above all, without delay, as there is no need to accelerate the exhaust gas turbine. An energy-saving driving style at low speeds is made possible.

The peculiarity of the first Comprex loader was that the cells of the same size produced a singing tone, which some customers found unpleasant, the pitch of which followed the speed. This was remedied by a new cellular wheel, which no longer consisted of cells but of a package of tubes with different diameters. Since the charger was made of thin-walled ceramic, this made production more expensive.

BBC did not get the desired response from vehicle manufacturers in Europe with the Comprex loader; there was also no new money for this project after the merger of BBC with ASEA to form ABB. The system was then sold to Mazda, where a Comprex-charged 2.0-liter diesel engine was installed in the model 626 from 1988 to 1997, which had an output of 55 kW (75 hp). In comparison, the 2.0-liter naturally aspirated diesel developed 44 kW (60 hp).

In 1996 Greenpeace presented the Twingo Smile to the public. This prototype based on the Renault Twingo had a 358 cm³ gasoline engine with a Comprex charger. The engine developed 40 kW (55 hp), which results in a specific output of 154 hp per liter of displacement. The Twingo Smile used less than 3.3 liters of fuel per 100 km, which is about half as much as the original vehicle with its outdated bumper motor .

The company swissauto Wenko AG in Burgdorf (Switzerland) was in charge of this development and in 2008 also built a demo vehicle based on a VW Golf V with the self-developed Hyprex pressure wave charger. The engine (an EA111) had 1000 cc and made 110 kW (150 hp); but above all the immediately available torque of 210 Nm at 1250 rpm impressed every test driver. A larger version of the charger was also to be installed in the AMG Mercedes A45 in 2012, but the existing teething problems of the Hyprex led to the project being canceled. The name Hyprex is a mix of Hybrid and Comprex as the main characteristic is an electric drive. In addition, this charger has a rotatable setting of the control edges in the air housing and the aforementioned variable gas pockets.

The latest expansion stage of the Comprex now has a water-cooled exhaust housing, a new stable bearing, very tight rotor clearances regardless of the rotor temperature and instead of the backlash-affected rotatable backdrop, a cycle shutdown, which offers further advantages.

Advantages and disadvantages

The Comprex loader increases efficiency because it takes useful energy from the exhaust gas that would otherwise be lost. The exhaust gases transfer the pressure to the fresh air without delay, which means that there is no “turbo lag” with the Comprex charger, but the charging effect starts immediately. In contrast to the turbocharger compressor, the supercharger has no surge limit and can provide comparatively high boost pressures even with low throughputs. Since the charger does not have to be brought to a higher speed in order to generate a higher boost pressure, it is well suited for high-altitude charging. With regard to CO 2 and pollutant emissions , the following advantages can be claimed: The engine usually sees a lower back pressure than with a turbo engine. This means lower fuel consumption due to the reduced piston extension work. Because of the high pulling power at low speeds, the engine can be operated very lazily at higher loads, i.e. with gasoline engines with the throttle valve wide open, which also saves fuel. High boost pressures also go well with gas engines , as the methane gas used usually has a very high octane rating (up to 130). In the case of a petrol-powered engine, the 3-way catalytic converter and the particle filter can be installed between the engine and the charger. This is an advantage when you consider that the vast majority of harmful emissions arise during a cold start, until the catalytic converter has reached its working temperature. With an arrangement immediately after the engine, the catalytic converter warms up more quickly and emissions are reduced accordingly. This makes the Comprex very suitable for downsizing gasoline engines and complements the turbocharger, especially for small engines under 1.5 liters. Turbochargers have a hard time with these small displacements, as the small turbochargers tend to have poor efficiencies and this is particularly important due to the lack of torque of the small base engine (weak start-up). Another advantage is the permanent lubrication of the bearings, which eliminates the usual problems with the engine oil of turbo engines. In addition, a Comprex engine does not need a complex exhaust system with large silencers, the loader already absorbs sound. The advantage here is a lower space requirement for the exhaust system, which is associated with lower weight and costs.

One of the disadvantages is that the Comprex loader is relatively difficult to adapt to an engine, which makes development very expensive and complex, but only if one is only dependent on test bench results. All common engine cycle programs now have a pressure wave charger module so that the charger can be designed very well on the basis of such simulations. Another interesting fact is that these are completely physical models and that, unlike a turbocharger, any charger can be simulated without measured maps.

Furthermore, the Comprex loader is sensitive to both the exhaust gas and the fresh air side, so that even small changes in the intake (dirty air filter) or exhaust system (snow in the exhaust) can have effects. Such errors often led to the problem of unwanted exhaust gas recirculation , which the engine responded to with severe stuttering and choking. With the new generation of chargers, however, such effects can largely be prevented by taking certain measures. Because the exhaust gas and fresh air sides are not separated in the cell rotor, it used to be necessary to ensure that no exhaust gases could get into the intake duct for the starting process when the cell rotor was rotating extremely slowly, otherwise starting would be impossible. The measures required, such as an electromagnetically operated flap, made the system more complicated, more fragile and more expensive. However, this problem is solved with an electrically powered charger, since a much higher rotor speed can be set while the engine is starting, which avoids the starting problem. The electric motor and control unit required for this are nowadays inexpensive to obtain and very reliable.

literature

  • Dr. Andreas Meyer, Motorentechnische Zeitschrift (MTZ), Comprex, missed opportunities, 2017
  • Lukas Flückiger, Motorentechnische Zeitschrift (MTZ), Supercharging with pressure wave superchargers for gasoline engines, 2006
  • Gert Hack, Iris Langkabel: Turbo and compressor motors . 1st edition, Motorbuch Verlag, Stuttgart, 1999, ISBN 3-613-01950-7
  • Hans-Hermann Braess, Ulrich Seiffert: Vieweg manual automotive technology. 2nd edition, Friedrich Vieweg & Sohn Verlagsgesellschaft mbH, Braunschweig / Wiesbaden, 2001, ISBN 3-528-13114-4

See also

Web links