VCR connecting rods

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The VCR connecting rod is a special design that, in contrast to the normal connecting rod, allows the hole spacing - also known as the connecting rod length - to be changed while the engine is running, thereby varying the engine's compression ratio . VCR stands for variable compression ratio .

VCR connecting rods are part of variable compression technology and, like all other VCR systems, are still in the research and development stage. In principle, any reciprocating internal combustion engine can be equipped with a VCR connecting rod. Because it is relatively easy to integrate it into existing engine designs, the VCR connecting rod is an attractive approach to implementing variable compression. There are different design configurations of VCR connecting rods, the majority of the known designs using the forces acting on the connecting rod bearing points for adjustment. It is shortened or lengthened by these forces. The adjustment times from one end position to the other are dependent on the force available and thus dependent on the current engine operating point. Most of the time, VCR connecting rods can be switched in two stages.

FEV VCR connecting rods

The engineer Karsten Wittek invented the system known as the FEV-VCR connecting rod as part of his dissertation at the Chair of Internal Combustion Engines at RWTH Aachen University . The engineering company FEV applied for a patent for Wittek's invention in 2005. Since then, FEV engineers have applied this system to various engines and continuously developed it further.

Compared to other VCR systems, the VCR connecting rod is less complex, easier to adapt to any reciprocating piston engine and to manufacture significantly more cheaply. The fuel consumption of a modern downsizing -Ottomotors can with a two-stage switchable VCR system to about 5-7%, depending on the mode of operation can be reduced.

Construction and principle of operation

First prototype for a car engine

The length variation is achieved by an eccentric bearing of the piston pin in the small connecting rod eye. The eccentric rotation range is typically around 60 °, so that, depending on the selected eccentricity, connecting rod length variations of 2 mm to 5 mm can be represented. The eccentric is connected to a hydraulic support device that holds the eccentric in one of the two end positions by supporting the moments that arise on the eccentric. In addition, the support device ensures that the eccentric is transferred from one end position to the other at a defined speed. The support device thus takes on the function of a switchable freewheel, similar to that of a switch ratchet for tightening or loosening screws. The support device comprises two oil-filled support chambers and a support rod that converts the rotary movement of the eccentric into stroke movements of the support piston. If the eccentric is in its upper end position, a corresponding support pressure builds up in the left support chamber due to the combustion pressure acting on the piston. If the eccentric is in its lower end position, the right support chamber is loaded as a result of the inertial force acting on the piston pin and directed upwards during the gas exchange. Both support chambers have outflow lines that open into the crankcase or the connecting rod bearing. These lines can be opened and closed alternately by means of a switching valve so that one of the two support pistons can immerse in each case. In the cylinder, which is then just expanding, oil flows through a check valve. Calibrated orifices in the outflow lines generate a defined hydraulic resistance. This prevents the support piston from moving in at too high a speed and from being damaged when it reaches its mechanical end stop. The switching times in the respective adjustment directions depend on the forces prevailing on the piston pin, which results in a dependence on the engine operating point. The switching valve is operated mechanically via a mechanism in the cylinder crankcase.

Test vehicle

At the Vienna Motor Symposium 2012, FEV presented a test vehicle equipped with VCR connecting rods, the test engine of which is based on a turbocharged 1.8-liter five-valve engine from the EA113 engine series from Volkswagen . The displacement was reduced to 1.65 liters; the compression can be switched between 8.8: 1 and 12.0: 1. The compression changes are automatically triggered by the engine control , depending on the engine load and the speed. The current compression of each individual cylinder is displayed in the cockpit. Installed in the mid-engined Lotus Elise Series 1 sports car , the functional behavior of the VCR system can also be tested during very dynamic driving maneuvers.

  • Second prototype for a car engine

  • Test engine with VCR connecting rods

  • Lotus Elise test vehicle

Current developments

One of the newer connecting rod prototypes was adapted to the 1.0 liter three-cylinder engine of the Ford EcoBoost engine family as part of a research project . The selected length variation range of the connecting rod is 2.2 mm, with which the compression levels 12.11: 1 and 9.56: 1 were realized. The switching valve, the orifices and the non-return valves are integrated in this construction in one assembly, which is screwed to the connecting rod bearing cap. This has the advantage that the hydraulic parameters can be changed quickly during the test phase. In addition, this considerably simplifies the machining of the connecting rod body. The mass of the ready-to-install VCR connecting rod is 650 g, which is 231 g more than that of the standard connecting rod. The specially made piston has almost the same mass as the series piston.

Hydraulic actuation of the switching valves can significantly reduce the integration effort of the VCR connecting rod, since the mechanical actuation mechanism can be omitted. Such a hydraulically operated switching valve can be installed instead of the mechanically operated valve. In the simplest case, the control pressure connection of this valve is connected directly to the supply system in the connecting rod. For actuation, the oil pressure level on the engine side must then be varied. With the help of a separate control pressure line system, the switching valve can also be operated independently of the connecting rod bearing supply oil pressure.

The function of the connecting rod equipped with different actuation systems was characterized on the basis of engine test bench tests. The times for a switchover from high to low compression are 0.5 to 1 second in the relevant operating range in the design. The influence of the oil temperature on the switching time is negligibly small in the examined temperature range.

  • Prototype for a 1.0 liter three-cylinder gasoline engine

  • Hydraulically operated switching valve

  • Functional testing on the engine test bench

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  1. ^ A b c David Crolla: Encyclopedia of Automotive Engineering . John Wiley & Sons. Hoboken. 2015. ISBN 978-0470974025 . P. 584 (in English) [1]
  2. ↑ a b Johannes Liebl, Christian Beidl (ed.): International Motor Congress 2016: With Conference Commercial Vehicle Engine Technology. Springer publishing house. 2016. ISBN 978-3-658-12918-7 . Pp. 248, 249
  3. ↑ a b c d Karsten Wittek: Variable compression ratio in the internal combustion engine by utilizing the forces acting in the engine . Dissertation. RWTH Aachen, Aachen 2006.
  4. ↑ Patent DE102005055199 : Reciprocating internal combustion engine with adjustable compression ratio. Registered on November 19, 2005 , applicant: FEV GmbH, inventor: Karsten Wittek. ‌
  5. ↑ a b 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.
  6. ↑ a b c Karsten Wittek, Frank Geiger, Jakob Andert: The VCR connecting rod with eccentrically piston pin suspension - design evolutions and current status . In: FEV Conference - Variable Compression Ratio . February 8, 2017.

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