Variable compression
Variable compression is the name of a technology that can help reduce fuel consumption in internal combustion engines . In the professional world, the term VCR (variable compression ratio) has established itself. In the case of an engine in which VCR technology is implemented, the compression ratio can be selectively changed during operation. A distinction is made between VCR engines with a continuously adjustable compression ratio and VCR engines with a two-stage adjustable compression ratio. Although VCR technology has been researched and developed intensively for years, it has not found its way into series production for a long time. In April 2017 , the Japanese automobile manufacturer Infiniti presented the world's first mass-produced engine with variable compression in Vienna . This is the first to be used in the second generation of the Infiniti QX50 , which was first presented at the LA Auto Show in November 2017. The sixth generation of the Nissan Altima followed in autumn 2018.
background
In conventional internal combustion engines, the compression ratio is determined by the design of the engine components and thus represents an unchangeable parameter. This parameter is of central importance for the engine work process. The designer should always choose the compression ratio as high as possible to achieve high levels of efficiency .
Depending on the work process, the fuel used and the operating point, however, phenomena occur that must be taken into account when dimensioning the compression ratio. When Otto engine , the phenomenon can in high load operation knocking combustion can occur. Various measures can be taken to suppress this engine-damaging phenomenon. It is common practice to shift the ignition point to “retarded” starting from the most efficient setting. The later release of heat reduces the engine efficiency. However , this measure alone is often not sufficient, particularly in the case of turbocharged gasoline engines. To represent high torques, the compression ratio must therefore also be selected to be correspondingly low. This means that the efficiency of a supercharged gasoline engine at partial load is usually lower than that of a naturally aspirated engine with the same displacement and a correspondingly higher compression. In the case of a VCR engine, the compression ratio can be adapted to the respective operating point. At operating points at which knocking occurs, a correspondingly low compression ratio is set, while at all knock-free operating points a correspondingly higher compression ratio is set.
Constructions
The idea of varying the compression volume through additional volumes in the cylinder head , which initially appears to be very obvious , is practically ineffective when taking into account the geometric conditions of modern four-stroke engines with four-valve technology. Against this background, all newer concepts are based on a variation of the compression volume by varying the piston position at top dead center (TDC).
Variable compression in internal combustion engines was already known at the end of the 19th century. In his work The Origin of the Diesel Engine, the German engineer Rudolf Diesel describes that in the fall of 1898, as part of a repair work on a demonstration diesel engine , "a device was made that allowed the compression chamber to be changed by adjusting the piston rod."
Unconventional crank drives
Unconventional crank drives are used in the VCR systems of this first type. There are various types of transmission technology, mostly using multi-link connecting rods . Common to all systems is a support that can be displaced relative to the crankcase . By shifting this support, the lifting kinematics of the crank drive is influenced, whereby the compression ratio can usually be adjusted continuously.
Multi-link
With the Multi-Link system, the power is transmitted from the piston to the crank pin via a two-part connecting rod. This connecting rod, also referred to as an articulated connecting rod, is connected to the crankcase in an articulated manner via a further connecting rod. This pivot point on the crankcase side is shifted by means of an eccentric shaft. Both Daimler and Nissan have developed and tested test engines with a multi-link crank drive.
MCE-5
The company MCE-5 Development SA has developed a VCR system which, like the Multi-Link system, is based on an unconventional crank drive and also enables the compression ratio to be varied continuously. The piston force is transmitted via a toothed rack to a toothed wheel, which rolls on a second toothed rack fixed to the motor housing. The force acting on the axis of rotation of the gear is transmitted to a crankshaft via a connecting rod. The vertical displacement of the crankcase-side rack affects the lifting kinematics and with it the TDC position of the piston.
Variation of the distance between cylinder head and crankshaft
The VCR systems of this second type have conventional crank drives. The TDC position of the piston is varied by changing the distance between the cylinder head and the crankshaft. The compression ratio is usually varied continuously.
Swiveling cylinder head
The VCR system introduced by Saab in 2000 is based on a swiveling cylinder head and cylinder unit. This assembly, also known as “monohead”, is rotatably mounted in relation to the crankcase and can be swiveled by 4 °, whereby the distance between the crankshaft and cylinder head is varied and consequently the compression ratio.
Displaceable crankshaft
The engineering company FEV has developed a VCR system based on an eccentrically mounted crankshaft. The eccentrics rotatably mounted in the crankcase are connected to form a torsionally rigid unit. The distance between the crankshaft and the cylinder head can be varied by turning this eccentric unit. The resulting axial offset between the crankshaft and the transmission input shaft is compensated for by a parallel crank clutch .
Variable crank drive components
The VCR systems of this third type are based on crank drives, the components of which are variable in their geometric dimensions. These variable crank drive components are installed instead of the conventional components. The necessary adjustments to existing engine architectures are comparatively small, so that such systems can be more easily integrated into existing engine architectures.
Connecting rod with eccentric piston pin bearings
The engineering company FEV has developed a two-stage switchable VCR connecting rod that uses the gas and inertia forces acting on the piston pin for adjustment. The length variation takes place via an eccentric bearing of the piston pin in the small connecting rod eye. The torque generated at the eccentric is supported hydraulically by means of two oil-filled pressure chambers. The two pressure chambers can be opened and closed alternately so that the eccentric can only rotate in one desired direction. The free-running direction of the eccentric and thus the direction of adjustment of the compression can be switched via a mechanically or hydraulically operated switching valve. In 2005, the function of this VCR system was verified for the first time by means of test bench tests. In the following years, the system was applied to various engines and continuously developed and also tested in a test vehicle.
Telescopic connecting rods
The engineering company AVL is working in cooperation with the chain manufacturer Iwis on the development of a two-stage switchable telescopic connecting rod. The connecting rod shaft is designed as a double-acting hydraulic cylinder . In the extended position, the gas force resulting from the combustion pressure is fully supported on a volume of oil. The forces acting on the piston pin are used for adjustment. The switching valve is operated hydraulically by varying the supply oil pressure applied to the connecting rod bearing. The function of the system was proven on the engine test bench.
Variable compression height piston
The principle of varying the compression height was already used in the USA in the 1960s in an air-cooled 12-cylinder diesel engine. The executed VCR pistons consist of two parts, an outer piston which is guided in the cylinder tube and an inner part which is connected to the piston pin. On the upper and lower end faces of the inner piston there are pressure spaces on which the gas or the upward inertia force of the piston outer part can be supported. The hydraulic interconnection of the pressure chambers and thus the adjustment of the compression level is carried out by hydraulic valves that work depending on the combustion pressure. The compression level is thus set automatically and cannot be selected independently of the engine operating point.
In the 1980s, research at Daimler-Benz AG and the piston manufacturer Mahle developed a VCR piston for passenger car gasoline engines in a joint research project.
Crankshaft with variable crank radius
The Dutch company Gomecsys develops crankshafts with a variable crank radius. In the latest design of the Gomecsys crankshaft, eccentrics rotate around each crank pin at exactly half the crankshaft speed. The eccentric rotations are synchronized with the help of gears. An electric servomotor attached to the free end of the crankshaft allows the phase position of the eccentric rotations to be varied continuously compared to the crankshaft rotation. This changes the effective crank radius in relation to the dead center positions of the piston, which results in a continuously variable compression ratio. Another effect of this principle is that the compression stroke and the expansion stroke are of different lengths, whereby additional efficiency advantages can be opened up. In cooperation with Peugeot, Gomecsys operates test vehicles equipped with this VCR technology.
swell
- ↑ Uli Baumann: Variable compression is ready for series production. auto-motor-und-sport.de, April 28, 2017, accessed on December 17, 2017 .
- ↑ Uli Baumann: Mid-range SUV with a super engine. auto-motor-und-sport.de, November 29, 2017, accessed on December 17, 2017 .
- ↑ Uli Baumann: New Nissan Altima (2018): Mid-size sedan gets a new design. In: auto-motor-und-sport.de. March 29, 2018, accessed December 18, 2018 .
- ↑ a b c Rolf Weinowski, Karsten Wittek, Bernd Haake, Carsten Dieterich, Jörg Seibel, Markus Schwaderlapp: CO 2 potential of a two-stage VCR system in combination with future gasoline engine drive concepts. In: Proceedings of the 33rd Vienna Motor Symposium, 2012.
- ↑ Rudolf Diesel: The Origin of the Diesel Engine . Springer, Berlin 1913. ISBN 978-3-642-64940-0 . P. 92f.
- ↑ Shunichi Aoyama, Katsuya Moteki, Naoki Takahashi, Ryousuke Hiyoshi: Investigation of a variable compression ratio system with a multiple link mechanism and its effect on engine performance. In: Proceedings of the 15th Aachen Colloquium Vehicle and Engine Technology, 2006.
- ^ Erhard Rau, Herbert Kohler, Günter Karl, Klaus Fieweger, Jochen Betsch, Bernd Krutzsch: Future potentials through variability in the gasoline engine. In: Proceedings of the 35th Vienna Motor Symposium, 2014.
- ↑ Vianney Rabhi, Frédéric Dionnet, Jacques Beroff: The MCE-5 technology: a new technical possibility of the variable compression ratio in SI engines. In: Proceedings of the 13th Aachen Colloquium Vehicle and Engine Technology, 2004.
- ↑ Hans Drangel, Lars Bergsten: The new Saab SVC engine - an interplay to reduce consumption of variable compression, supercharging and downsizing. In: Proceedings of the 9th Aachen Colloquium Vehicle and Engine Technology, 2000.
- ↑ Kurt Imren Yapici: Variable compression ratio in the internal combustion engine through eccentric crankshaft displacement . Dissertation. RWTH Aachen, Aachen 2001.
- ↑ Christoph Bollig, Knut Habermann, Markus Schwaderlapp, Kurt Imren Yapici: Variable compression ratio in the internal combustion engine through eccentric crankshaft displacement . In: Motortechnische Zeitschrift . Issue 12, 2001.
- ↑ Karsten Wittek: Variable compression ratio in the internal combustion engine by utilizing the forces acting in the engine . Dissertation. RWTH Aachen, Aachen 2006.
- ↑ G. Fraidl, P. Kapus, H. Melde, S. Lösch, W. Schöffmann, H. Sorger, M. Weißbäck, J. Wolkerstorfer: Variable compression - in technology competition? In: Proceedings of the 37th Vienna Motor Symposium, 2016.
- ↑ John Basiletti, Edward Blackburne: Recent Developments in variable compression ratio engines. SAE Technical Paper 660344, 1966.
- ↑ Klaus Binder, Viktor Pfeffer, Friedrich Wirbeleit: Development work on pistons with variable compression heights to increase the efficiency and power density of internal combustion engines. In: Motortechnische Zeitschrift. MTZ 47, 1986.
- ↑ Berger, L .; Nowak, M .; De Gooijer, B .: PSA Gasoline Engine Next Generation: Gomecsys VCR Concept as a Solution? In: Proceedings of the 25th Aachen Colloquium Vehicle and Engine Technology, 2016.