The engine charge is a method in which the power or the efficiency of combustion engines by supplying air with an increased pressure is increased. Due to the higher pressure of the filling ratio improved so that more air for the combustion of fuel is available, what the per cycle done work increased. The suction work can thus be converted into useful work.
The pressure of the air sucked in by the engine can be increased by flow vibrations, fans, turbochargers and compressors. Due to the higher thermal and mechanical loads on the engine, this process is subject to material and design limits. Modern turbocharged engines usually have a lower specific consumption than naturally aspirated engines of the same power.
Increased performance through charging
Formula for the indicated engine power.
P i = internal engine power
p i = indicated mean effective pressure
V h = displacement per cylinder
z = number of cylinders
n = speed
i = work cycles per revolution (i.e. two-stroke engine 1 and four-stroke engine 0.5)
The other variables , z, n and i also contribute to the increase in performance and are implemented according to the application and the special limits. An increase in performance by increasing the speed n is practically only feasible up to a certain limit.
The absolute boost pressure is approx. 2 bar (car, single-stage, gasoline engine) to 4 bar (large diesel engine).
According to areas of application
Typical applications of loaders:
- Marine diesel engines ( four-stroke & two-stroke diesel engine ), also in combined heat and power plants
- Truck - and car - diesel engines : Turbo Diesel
- Aircraft engines
- Generating sets: small generators + large systems for (emergency) power supply
- Passenger car gasoline engines (also motorsport )
- Opposite piston engines
- Motors in stationary systems and special vehicles where a high power reserve is required.
According to the method of compaction
Various methods are known for compressing the fresh air for an engine:
- Resonance charging (optimized for a certain speed): ideal with constant generator speed for power units , combined heat and power plants, etc.
- Dynamic gas compressors: turbo compressors , especially exhaust gas turbochargers
- Blower: The air is sucked in and transported to the side with higher pressure without internal compression: Roots blower (efficiency?)
- Displacement machines: Within the compressor, a decrease in volume leads to the compression of the gas, as is the case with reciprocating compressors, vane-type chargers, scroll and screw compressors and the Wankel ro-charger. The advantages of these machines are lower noise generation and high isentropic efficiency .
Charge air cooling is always important for high efficiency.
Loaders are differentiated according to their operating principles. The most commonly used are exhaust gas turbochargers, mechanical superchargers and pressure wave superchargers. Mechanical loaders are mostly driven directly by the charged engine through gearboxes or belts, in some cases the loaders have their own drive, such as an electric motor (external charging).
In a broader sense, this also includes so-called dynamic charging (resonance charging ), in which specially designed intake and exhaust pipes amplify the gas oscillations at certain speeds through resonance, which improves the gas exchange in the cylinder. The resonance principle can be combined with other types of chargers.
Exhaust gas turbocharger (ATL)
The most important type of charger today is the exhaust gas turbocharger , in which an exhaust gas turbine drives the turbo compressor, which is firmly coupled via a shaft , at high speed . The exhaust gas turbine is as close as possible to the exhaust gas outlet of the engine, directly in the exhaust gas flow. Usually right next to it is the reversed turbo compressor, whose compressor wheel presses the charge air into the intake tract via a charge air cooler (" intercooler ") .
In addition to the desired supercharging, exhaust gas turbochargers have the advantage that they use part of the residual pressure of the exhaust gases that is otherwise unused (typically 3–5 bar) and couple it to the crankshaft as an additional gain in performance when the piston is driven with the excess pressure of the charge air during the intake cycle instead of having to work against negative pressure as with the naturally aspirated engine .
A well-known disadvantage, the so-called turbo lag , occurred especially in early models with turbocharging. The additional torque was only available from a certain speed of the supercharger, the exhaust gas flow was only large enough to drive the supercharger sufficiently from a certain load level, which then enabled the engine to generate more exhaust gas for the turbine due to the increasing boost pressure . This delayed the increase in torque. This effect is largely compensated for in modern machines with electronic boost pressure control and relief valves.
History and development of the exhaust gas turbocharger
Exhaust gas turbochargers have been around since the early 20th century, initially being used for diesel engines and becoming the standard in particular for large two-stroke diesel engines such as marine diesel engines . Since the 1990s, turbochargers have also been used increasingly for passenger car diesel engines ( turbodiesel ), and only recently also for gasoline engines , whose exhaust gas temperatures are significantly higher and whose throttle valve also impairs response behavior.
Exhaust gas turbocharger for gasoline engines
The turbine in modern gasoline engines is exposed to an exhaust gas flow of up to 1000 ° C during operation and runs at extremely high speeds of up to 400,000 revolutions per minute. As little heat as possible should be transferred to the compressor. In order to keep the inertia and centrifugal forces low at these speeds, the materials of the running gear must be very light, at the same time dimensionally stable and high-strength over a rapidly changing temperature range of up to 1000 ° C, and the bearings must be almost play-free, but also smooth. This could only be achieved with the development of modern materials and techniques in the late 20th century.
Electrically powered compressors
In addition to the classic mechanically driven compressors, electrically driven compressors are also used in various series vehicles or exhaust gas turbochargers are supported by an electric motor in order to overcome the “ turbo lag ”. The electric drive decouples them from the engine speed and from the exhaust gas mass flow. These are used in a range of up to 2000 rpm.
Mechanically driven compressors
"Mechanical loader," mostly " compressor " mentioned (English supercharger ) can be directly from the engine ( crankshaft driven) via a chain, belt or gear transmission, possibly with intermediate clutch. Alternatively, it can also be driven by its own electric motor (see separate section below).
Rotary piston loader ( Roots loader )
These superchargers, named after the inventors of the design principle, Roots blowers , have two rotors rotating in opposite directions, the two or three club-shaped “wings” of which alternately interlock. Similar to a gear pump , the air is sucked in on one side, pushed along the inner wall of the oval housing by the “wings” and pushed out on the opposite side. The shafts of the two rotary pistons are connected outside of the housing via gear wheels. The pistons run to each other and to the housing without contact.
Roots loaders work without internal compression. Due to their mode of action, they only work effectively when there is a larger amount of air and are therefore relatively large and heavy. They are mostly used in medium and large diesel engines. Their big advantage over the turbocharger is that they work immediately and there is no turbo lag . In addition, because of the lower speed, the lower thermal pressure load and the non-contact operation, they are much more durable and economical to manufacture.
Roots loaders were often installed in heavy trucks ( MAN ) and Mercedes racing cars in the past . At the beginning of the 1920s, the Mercedes supercharger was a guarantee for victory in car races. In the 1924 edition of the car magazine Der Herrenfahrer - Das Blatt vom Auto und other conveniences in life wrote a “Dr. Ritter "a hymn to this drive technology:
“The over-employed industrial captain or bank tycoon of our day would probably regard it as an ideal situation at certain times if he did not need to eat, drink or sleep. Food in a highly concentrated form, freed from all waste, possibly fed to him during work without any manual action on his part, for all cases of special mental or physical high performance, a can of power reserve in the vest pocket, a nerve regenerator that welcomes sleep-refreshed youth into a spraying shower Poured over him any number of times, would work together to make a giant of success out of this man such as the world has never seen him. However, the rhythm of his life would also be destroyed. It is precisely that the wave of life rises and falls, that the crest of waves of energy and performance alternates with the trough of calm and enjoyment, constitutes the content and charm of our life. [...] But what doesn’t exist for humans and shouldn’t exist either, has actually existed for the engine for several years. It's the compressor . "
The Roots scavenger fans used in the two-stroke engines from Krupp and Commer are not real superchargers. Lancia was the first manufacturer after the Second World War to install Roots loaders in production cars, 1983 to 1985 in the Volumex models. In the 1950s and 1960s, Hanomag installed Roots blowers, both as flushing blowers in their two-stroke diesel engines (e.g. D611 in the R12) and to increase performance in four-stroke diesel engines (e.g. D28 LAS in the Hanomag L 28 ). It was not until the mid-1990s that Mercedes-Benz started building compressor models again. The rotary piston charger with one wing and three inner runners is a further development .
Piston charge pumps
DKW developed in the 1930s for racing motorcycles a two-stroke - double piston engine with crankcase scavenging and charging by another piston in the crankcase to the system of Arnold Zoller . The engine was continuously developed (DKW experimented with four different positions of the charge pump); In particular the DKW ULD 250 was superior to other machines at the end of the 1930s due to its high performance in motorcycle races.
They work on the principle of the liquid pumps of the same name or vice versa like the so-called air motors in many compressed air tools . In a housing with a circular cross-section, an eccentrically mounted rotor runs in which several blades made of plastic or hard tissue are arranged radially. The blades are guided in grooves in the rotor and can be moved in the radial direction. In the operating state, they are pressed with their outer edges against the inner housing wall by centrifugal force, more rarely by spring force, and slide on its surface. This creates enclosed spaces, also called cells , between neighboring wings , in which the air is transported. The housing has an inlet and an outlet opening. Due to the eccentric mounting, the cells are initially enlarged during the rotation on the suction side, which creates a slight negative pressure. In the direction of the pressure side, they continue to decrease in size up to the outlet opening. This means that the air is pre-compressed and fed into the engine's intake tract. The eccentricity can be changed and the charge can be adjusted without any problems using adjustment devices that are easy to implement. Vane cell loaders achieve a lower compression performance than turbo and roots loaders. The possible speed is higher than with the Rootslader, but limited by the centrifugal forces and friction. They are small, light and relatively cheap to manufacture. Since they are subject to increased wear due to the friction of the cell wings, their service life is quite limited. Because of these properties, they are particularly suitable for small gasoline engines in sports cars.
From the 1950s to the early 1960s, it was also installed in the VW Beetle to improve performance (Judson compressor).
See also: rotary vane pump
The principle of this group of positive displacement chargers was invented by Léon Creux at the beginning of the 20th century and patented in France and the USA . However, the practical application failed due to the complicated production and the material requirements. It was not until the 1970s that Volkswagen took up the idea again and, after tests with a few hundred copies and many detailed changes, it was used in larger numbers in the mid-1980s. The company VW named their model G-Lader , which was produced in two different sizes and was used in the VW Polo G40 as well as in the Golf G60 , Passat G60 and Corrado . The name refers to the construction. The housing, which is round in cross section, consists of two halves, each of which has two spiral-shaped webs cast into it, which, like the spiral-shaped displacer, are reminiscent of the capital letter G. The numbers 40 and 60 indicate the height / width of the case in millimeters. The displacer is driven by the crankshaft via a belt drive with a main shaft and is guided by a secondary shaft coupled with a toothed belt. Eccentrics sit on both shafts so that the displacer does not rotate, but revolves around a circular path. The movement of the runner causes constantly decreasing volumes between the webs in which the air is conveyed. The air enters the housing tangentially, is trapped there between the spiral webs of the housing and the displacer and transported towards the center of the housing, from where it flows axially into the intake tract.
Because of the considerable friction of the complex sealing elements and springs, which are arranged between the end faces of the displacer and the housing, G-Superchargers have a short service life if the wear parts concerned are not checked and replaced at regular intervals. Due to the difficult production and the high repair and replacement costs, they could not prevail. VW stopped manufacturing in the early 1990s.
In 2007 Handtmann took up this principle again and developed the Handtmann Spiral Loader (HSL) . The main difference to the G-Lader lies in the single-spiral version whereas the G-Lader was double-spiral.
Centrifugal loader or centrifugal compressor
A centrifugal supercharger (also radial compressor or centrifugal compressor called) compresses the air with a rapidly rotating blade wheels, in which the air is in accelerated by the centrifugal force radially outwardly, which results in the desired compression. Other designs also had tubes arranged in a star shape. The centrifugal loader is driven by the motor via V-belts, toothed belts, gears or shafts. There were centrifugal loaders in aircraft engines and marine diesel engines. In some cases, this principle can still be found today in retrofit parts for tuning cars. Thanks to the direct drive, no contact with hot exhaust gas is necessary. The piping and the attachment of a centrifugal loader are simplified accordingly. In engines whose speed varies greatly, one of the disadvantages of this form of supercharging comes into play, since the amount of air delivered does not increase proportionally with the speed of the supercharger, which is proportional to the engine speed. Some aero engines with radial compressors had switchable gears in order to be able to better adapt the air volume and boost pressure for the operating range of the engine.
Comprex loader or pressure wave loader (DWL)
The construction, also known as the Comprex charger (from Compression / Expansion) or DWL ( pressure wave charger ), uses the kinetic energy of the hot exhaust gases (expansion), in contrast to the turbocharger, directly to compress the intake air. The rotor is designed as a cellular wheel (similar to the drum of a drum turret) and is enclosed by the air and gas housing with a common jacket. On the opposite end faces, there are two air or exhaust gas openings in the form of circular segments. When the air-filled cells are rotated in front of the exhaust gas inlet opening (coming from the engine), the air is pushed by the hot, pressurized gas towards the opposite side. As the rotor continues to rotate, the cells reach the opening of the charge air line, and the compressed air flows into the engine. Before the exhaust gas also reaches the opening, the cells have already passed the charge air line and are closed. After further rotation, the pressurized exhaust gas escapes into the exhaust line released shortly thereafter. This creates a negative pressure that draws in fresh air from the intake line that has now been reached. Although air and exhaust gas come into direct contact with each other, they only mix in a narrow zone of the cells. Since, in contrast to the turbocharger, there is a gas exchange in the cells, the synchronization of the cell wheel with the engine speed is necessary. This is done either directly from the crankshaft via toothed or V-belts or from a speed-controlled electric motor. The short pressure peaks caused by the gas dynamics can be compensated for by spherical extensions of the air inlet and gas outlet lines, the so-called sensors .
Advantages of pressure wave charging
- High boost pressure at low speeds (also available in the lower speed range; no "turbo lag" and therefore high elasticity)
- Very fast response behavior (no turbine acceleration required)
- With a reduced displacement, it enables a high proportion of full suction and thus good efficiency.
- Can also be used with very small gasoline engines (less than 1 liter displacement).
Since, in contrast to the turbocharger, a cell tube is used instead of a turbine, the boost pressure is available almost immediately and is also available at low speeds with lower exhaust gas speeds. The cooling and pre-expansion of the exhaust gases by the fresh intake air has a positive effect on the pollutant content.
Disadvantages of pressure wave charging
- The current exhaust gas regulations can only be met with great effort (mixing of exhaust gas with fresh gas and slight resonance problems of the exhaust gas, therefore soot).
- High costs, large space requirements
- Sensitive to differential pressure between the intake and exhaust sides (air filter, diesel catalytic converter, soot particle filter).
- Slight differential pressure sensitivity between ambient and exhaust gas temperature.
- Boost pressure drops at high engine speeds.
- Only available during the warm-up phase (risk of exhaust gas entering the intake system).
The material of the rotor is subjected to high thermal and mechanical loads (temperature and pressure changes with high frequency). Sealing and storage are complex. If the synchronization with the engine speed is poor, the performance drops sharply.
Application of pressure wave charging in vehicle engines
The Twingo Smile from Greenpeace 1996 had a pressure wave charger, as did the Mazda 626 Diesel. Of the Opel Senator A 2, Irmscher Automobilbau built 1,709 units with a 2.3 liter Comprex diesel engine.
Exhaust gas charging pump
In 2014, a new type of exhaust gas charging system called the exhaust gas charging pump was published. The exhaust gas charging pump is a diaphragm pump that is driven by exhaust gas pressure pulsations that arise when the exhaust valves or exhaust ports are opened in the exhaust gas tract and which uses this energy to pre-compress fresh gas. Physically, the operation of the exhaust gas charging pump is based on shock wave charging. The exhaust gas charging pump clocks with its pumping frequency proportional to the ignition frequency of the engine. The maximum amount of air required per work cycle is determined by the geometrically specified pump volume. Thus, the exhaust gas charging pump works as a passively driven pump in a self-regulating manner and no active pressure control mechanisms such as wastegates are required. Depending on the system, the exhaust gas charging pump is also particularly suitable for single-cylinder engines and small individual cubic capacities. The exhaust gas charging pump does not require any mechanical coupling with the engine, but is connected to it exclusively via an exhaust branch line and a pressure storage line for the pre-compressed fresh gas. The exhaust branch pipe, in which the exhaust gas is merely pushed back and forth, ensures extensive thermal insulation of the exhaust gas charging pump from the hot engine exhaust so that the temperatures in the exhaust gas charging pump are below 100 ° C. With charging by the exhaust gas charging pump, torque or power increases of 30–45% compared to naturally aspirated engines can be achieved.
With this method, the charging is not achieved by an independent charger, but by using the kinetic energy of the air flowing in the intake tract and the vibrations of the gas column, which are excited by the discontinuous flow. The maximum achievable increase in torque is significantly lower than with supercharging by means of a compressor and a special construction of the intake manifold is necessary. Compared with fan charging, however, an increase in performance can be achieved with relatively little additional design effort.
Simple constructions suck in the air through a pipe, the length of which is dimensioned such that the air flowing into the pipe begins to accumulate at the inlet valve as soon as the valve opens. The kinetic energy of the air is used to improve the degree of filling. However, the charging effect only works in a relatively narrow speed range, and the maximum output is limited by the throttle effect of the longer intake pipe.
By changing the length of the intake pipe, the supercharging can be achieved in a broader speed range, whereby the throttling of the engine at high speeds is also reduced. The length is changed either continuously or by flaps (variable intake manifold ). In the lower speed range, the air flows through the long intake path. The short suction paths are closed by the flaps or rotary valves. At high speeds, the flaps are opened electro-pneumatically or electrically. The length of the intake tract is thus adapted to the higher gas exchange frequency, and the shorter intake paths also enable a higher gas throughput.
When the intake timing coincides with the frequency of the gas oscillation, resonance occurs . This causes an additional increase in pressure, stimulated by the rhythm of the intake strokes of the cylinder group. At medium speeds, long suction pipes in conjunction with a resonance tank cause long oscillating gas columns with high pressure in front of the inlet valve. In this speed range, the resonance oscillation causes a charge and thus a better filling. The formation of cylinder groups avoids an overlapping of the flow processes through the next cylinder in the firing order. Each resonance container is therefore connected to a resonance suction pipe.
Such systems were used by Lancia , Ford , Audi and BMW, among others . BMW used this principle in motorcycle engines as early as the 1950s. On the R25 / 3 , the intake pipe was led through the tank due to the length required. There was an increase in performance of 1 HP compared to the previous model R25 / 2 , which did not have this oscillating tube. This principle was particularly efficient with the Wankel engine : due to the lack of intake valves, the NSU Spider with a suitable design of the intake tract was able to achieve delivery rates of over one as early as the early 1960s (this is the ratio of the amount of fresh mixture drawn in to the amount possible due to the displacement).
In two-stroke engines , resonance charging takes place through the exhaust gas oscillations in the exhaust tract, a so-called resonance exhaust . This is designed in such a way that more fresh gas is sucked out of the crankcase in the area of maximum torque than could flow into the cylinder through the displacement of the piston alone. The excess fresh gas initially flows into the exhaust tract and would thus be lost as a scavenging loss, but is pushed back into the cylinder in the area of resonance by the pressure wave reflected on the mating cone of the exhaust. This supercharging effect results in a very narrow usable speed range that can only be used in motorsport with maximum power output . This type of resonance charging is usually limited to small individual cubic capacities, since each combustion chamber requires its own, relatively voluminous exhaust.
There are a number of other designs, which, however, exist more as technical concepts - almost exclusively as mechanical superchargers - and have hardly achieved any importance in practice. Also worth mentioning is the screw loader , which is currently being developed further. Otherwise, technical detail improvements and various combinations of the mentioned loaders are used to further increase performance, such as B. the variable inlet control, arrangements of several chargers in parallel or in series (register, cascade charging) and others. In racing vehicles and some series-produced motorcycles, performance is increased in some cases by means of ram-air systems with special intake openings that use the dynamic pressure of the air to increase the air supply at high speeds .
Charge air cooling
The power of an engine is proportional to the air flow that the engine draws in. This in turn is proportional to the air density. Through the above Methods of charging therefore increase throughput. Since the compression of the air increases its temperature and its density decreases, the effect of the charging would be reduced. This is counteracted by charge air coolers ("intercoolers"). The increase in density associated with falling temperature is converted into higher output, efficiency is increased and the lower process temperature reduces the thermal load on the motor. The NO x content is also reduced by the lower gas temperature and the permissible compression of the engine is increased at the same distance from the knock limit .
Gasoline and diesel engines differ fundamentally in the working process. Gasoline engines with a regulated catalytic converter require a precisely defined ratio of fuel and air volume, which must deviate as little as possible from the theoretical stoichiometric ratio (see lambda control ). If the amount of air increases, the amount of fuel required increases proportionally. The compression of the gasoline engine is limited because of the undesirable self-ignition. For this reason, the compression ratio must be reduced in turbocharged gasoline engines.
The diesel engine always works with excess air . An increase in the amount of air without a simultaneous increase in the amount of fuel has no negative effects on the exhaust gas values, unlike in a gasoline engine with a regulated catalytic converter. Furthermore, the newer emissions standards can only be met with a high excess of air, which in turn can only be achieved by using a supercharger without significant loss of torque. The compression of diesel engines is higher; they can be operated with the most favorable compression for efficiency, which is around 16: 1 for turbocharged engines with direct injection . Almost all newly registered diesel cars worldwide have turbocharged engines, as the lower fuel consumption resulting from the better thermodynamic efficiency saves costs and leads to lower CO 2 emissions. For the same reason, all diesel trucks and marine diesel have long been supplied with loaders.
It was not until the 1990s, with the development of high-performance digital engine controls and smaller, lighter chargers with improved efficiency and service life, that engine charging (along with other measures) has been used to a greater extent in series-production cars.
- Gert Hack, Iris Langkabel: Turbo and compressor motors . 1st edition, Motorbuch Verlag, Stuttgart, 1999, ISBN 3-613-01950-7
- John D. Humphries: Automotive Technical Manual for Turbochargers and Supercharged Engines. 1st edition, Schrader Verlag GmbH, Suderburg-Hösseringen, 1993, ISBN 3-921796-05-9
- Michael Graf Wolff Metternich: The compressor. From the Zoller compressor to the loader of tomorrow. Schmidt Verlag, Munich (approx. 1982)
- Thomas Heiduk, Ulrich Weiß, Andreas Fröhlich, Jan Helbig: The new V8 TDI engine from Audi Part 1: Aggregate architecture and charging concept with electric compressor , in: MTZ 77 (2016), No. 6, pp. 24–31, doi : 10.1007 / s35146-016-0042-3 .
- Gersdorf / Grasmann / Schubert, aircraft engines and jet engines, Bernard & Graefe Verlag Bonn 1995, ISBN 3-7637-6107-1