Camshaft adjustment

from Wikipedia, the free encyclopedia

Camshaft adjustment (also known as variable camshaft control or variable valve control ) describes a method for changing the valve timing of the valve control of four-stroke engines during operation. This must be distinguished from the one-off adjustment of the control times when the engine is installed, which is possible, for example, with special disc springs with an offset when the drive gear is joined to the camshaft.

Adjusting the valve opening times allows the engine to be more efficient , depending on the respective load behavior. This increase can come into play as a gain in power and torque and as a fuel saving .

A further possibility of use is achieved in that by changing the valve timing, the direction of rotation of the crankshaft can theoretically also be reversed. Due to the sequence in which the valves of a four-stroke engine are actuated, it is not possible to start the engine contrary to its usual direction of rotation of the crankshaft without changing the valve timing. This method of camshaft adjustment is used for slow-running inland waterway engines and the like. a. from Deutz AG , MAN and Maschinenbau Kiel as well as the high-speed engines Mercedes-Benz MB 518 , a further development of the Mercedes airship engines of the type LOF 6.

In the pre-chamber diesel engine XII Jv 170/240 from Ganz & Co., the timing of the intake camshaft is changed during the starting process, so that the intake valves only open very late. This creates a negative pressure in the combustion chamber, which ensures that the incoming intake air experiences a temperature increase due to the sudden increase in pressure; in this way, the ignition temperature in the engine can be achieved without glow plugs.

The term “variable camshaft”, which is sometimes used, is misleading, as the camshaft itself is not changed, only its angle of rotation or the valve lift.

background

Four-stroke cycle of an ideal-typical slow-running gasoline engine : 1: "Prime" ➝ 2: "compacting" ➝ 3: "Expand" ➝ 4: "pushing" real remains running fast on the cycle 1 inlet on the UT also opened to more time to allow the fresh charge to
flow in while the piston is already starting to compress; the exhaust valve opens at the end of cycle 3 and between cycle 4 and 1 the opening times of the exhaust and intake valve overlap

In the case of camshaft drives without adjustment, the camshaft is driven by the crankshaft at half the speed via a fixed connection (such as a toothed belt , chain , toothed wheel or vertical shaft ) . The valve opening times of the engine, given as to the position of the crankshaft related rotation angle 0-720 ° CA (degree  crank angle ), are defined constructively. They will also control times called because it the charge cycle control of the motor.

The time that the exhaust and intake valves are open at the same time is called valve overlap . By extending the overlap, the maximum power at high speeds can be optimized, but at the expense of the torque at low speeds.

The reason for this is the flow processes in the engine during the intake stroke at different speeds:

  • At high speeds, due to the inertia of the fresh gas column, further gas can flow in even after bottom dead center (BDC) if the valve closes late while the piston is already starting to compress. This is called reloading and leads to an increase in performance with a higher degree of delivery .
  • On the other hand, at low engine speeds, an inlet that closes too late causes fresh gas that has already been drawn in to flow out of the cylinder again (reflection on the piston crown) or is pushed out by the piston.
  • Basically, a long valve overlap causes the exhaust gas flowing out to entrain the fresh gas flowing in via the inlet valve and thus the cylinder is thoroughly flushed, whereby more fresh charge reaches the cylinder, which causes a higher torque and thus a higher power during combustion. At high speeds, however, there are losses due to fresh gas flowing into the exhaust, and at low speeds, a large overlap causes exhaust gas that has already been exhausted to be sucked back into the combustion chamber, which acts like an internal exhaust gas recirculation , but dilutes the fresh gas and thus less Torque up to combustion misfires when idling.

With the camshaft adjustment, the intake and exhaust control times are changed as a function of engine speed and throttle valve position (load) in order to achieve the most efficient cylinder filling possible in all engine speed ranges. Variable control times of the inlet valves also allow the quantity to be controlled solely for the degree of delivery , in order to avoid the considerable flow losses at the throttle valve. (A throttle valve is usually also installed in vehicle engines in order to close for the function of the engine brake when you take your foot off the accelerator.)

It is also possible to control the exhaust gas flow through individual turbocharger stages by opening different exhaust valves. Cylinder shutdown can also be implemented by means of camshaft adjustment .

technology

Phase adjustment

The most widespread today is the hydraulic phase adjuster: A swivel motor known from hydraulics , which is equipped with several blades to increase the transferable torque, which limits the swivel angle to 11 ° to 35 °. This swivel motor phase adjuster (SMV) is operated with engine oil pressure and can only be operated in conjunction with a check valve due to the high dynamics of the changing moments of the cams. The SMV is usually placed on the camshaft ends in the power transmission (rotary motion).

In engines with two camshafts, the desired effect can be achieved with just one SMV on the intake camshaft, and an overlap (intake and exhaust valves are open simultaneously for a short time) of the valve opening times is possible. If, on the other hand, another SMV is used on the exhaust camshaft, the designer can work with a larger overlap and has more freedom in optimizing the gas flows. This allows z. B. a targeted re-suction of exhaust gases can be achieved, which replaces the complex and costly external exhaust gas recirculation or allows a small design of this.

Stroke adjustment

The effect of the timing adjustment can be further increased by changing the valve lift. This allows air to flow after BDC even in lower speed ranges, which increases the torque and thus the power at these speeds. For example, at BMW, the throttle valve is only used in special operating modes (e.g. emergency operation). BMW calls the technology for camshaft adjustment VANOS (see below) and that for lift adjustment Valvetronic .

Others

Racing engines in Formula 1 have for some time pneumatic valve springs : Here compressed air replaces the steel spring that otherwise pushes the valve into the valve seat back and at high speeds (up to 18,000 min -1 caused) problems. The valves are still opened by the cams on the camshafts.

In the planning and testing stage, there are still methods that no longer control the valve via a fixed mechanical coupling with the crankshaft, but rather move the valves directly via hydraulics , pneumatics or electrical actuators . The mechanically complex camshaft drive, which is afflicted with friction losses and wear , could then be dispensed with, any control curves of the valves would be possible, and a cylinder-specific control would also be possible. The challenge lies in the coordination of the parameters costs, power requirements, accuracy and reliability. Such a hydraulically controlled system was already placed on the market in the USA towards the end of the 1980s, but it did not prove itself and resulted in extremely high warranty costs for the manufacturer.

Implementations

For the cam adjustment, the automobile manufacturers use different technical solutions under their own technical abbreviations, whereby a basic distinction must be made between systems that adjust the timing of the intake camshaft (and possibly also the exhaust camshaft) and systems with stroke adjustment or fully variable systems.

Systems for adjusting the timing (phase adjustment)

Neo VVL (Nissan)

N Nissan E cology O riented V ariable V alve L ifting & timing of Nissan , originally in response to Japan was resolved in law for the " L ow E mission V ehicle" to reach the local emission standards. The engines are only available in Japan, the best-known representative is the SR20VE, a 2.0-l DOHC engine from the Japanese Primera 2.0Te-V from 1997 to 2000 with 140 kW (190 PS) output and the successor Primera 20V from 2000 to 2003 as a version with 150 kW (204 PS). Nissan also has a turbo engine with camshaft adjustment in its range with the SR20VET (206 kW / 280 PS) from the Nissan X-Trail GT. With this system, the valve lift is also changed with the help of different camshaft profiles for the intake and exhaust sides.

VCT (Ford)

V ariable C am T iming from Ford . This system was first used in the Ford Puma with a 1.7 liter Zetec engine and later and in the Ford Focus ST170 . With this system only one camshaft is adjusted, in this case on the intake side.

Ti-VCT (Ford)

T win i ndependent V ariable C on T IMing by Ford . Ti-VCT is a further development of VCT and was presented by Ford with the 1.6-liter Duratec engine, which was used in the Focus and Focus C-Max from 2005. Here, both camshafts are adjusted independently of one another using hydraulic actuators , similar to VANOS actuators. Depending on the engine load, the position is continuously adjusted with an electronic map control.

VANOS (BMW)

VANOS system with vane cells in the BMW N52 engine

VANOS is the va riable No camshafts, s price increases for BMW .

VarioCam (Porsche)

The system used in the Porsche models 968 , 911 (996) , 911 (997) , 911 (991) , Cayenne , Cayman , Boxster and Panamera to adjust the phase of the intake camshafts is known as VarioCam .

VFD (Fiat)

V ariatore di F ase D inamico is the term used by the Fiat Group , also known as “phase adjuster” in German: hydraulic adjustment of the inlet camshaft and return to the initial position by means of a spring. This means that 90% of the maximum torque is already available at 2000 rpm.

Fiat was the first automobile manufacturer to apply for a patent for a system for the variable adjustment of valve timing, including valve lift (US patent 3,641,988). The basis for this was the hydraulic camshaft adjustment developed by Giovanni Torazza in the late 1960s. The range of variation of the valve opening times achieved in this way was already 37%. Fiat had recognized for the first time that more flexible valve timing could improve the potential and efficiency of an engine. It shouldn't be the last time (see Multiair).

The Alfa Spider 1750 cc (model 1980) produced for the American market was the first car to have a purely mechanical variable valve control (US Patent 4,231,330), which was necessary at the time to comply with the strict American emission regulations. Thus, for the first time, a phase adjustment for the intake valves was possible. In 1983 Alfa Romeo was the first to mass-produce electronically controlled variable valve timing for the European market (in a two-liter, four-cylinder engine).

VVT-i and Dual VVT-i (Toyota)

A VVT-i engine

V ariable V alve T IMing - i is ntelligent a camshaft with intelligent variable valve timing that of Toyota was developed. To improve the degree of filling of the cylinders, the intelligent variable valve control VVT-i regulates the opening and closing angles of the intake valves and the like. a. based on engine speed and load condition. The regulation takes place continuously; the system works with a control valve on the camshaft, which is connected to the engine's lubricating oil circuit.

In contrast to the simple VVT-i, with the Dual VVT-i from Toyota , the opening and closing angles of the exhaust valves are also controlled depending on the load and speed.

The VVT-i system replaces the VVT ​​offered since 1991.

VVT-iE (Toyota)

VVT-iE ( V ariable V alve T iming i ntelligent by E lectric motor) is a further development of the Dual VVT-i which uses an electrically operated actuator to set and maintain the intake camshaft timing. The exhaust camshaft control is also controlled by a hydraulic actuator. This form of variable valve control technology was initially developed for Lexus vehicles.

Systems for stroke adjustment (stroke switching and fully variable systems)

AVS (Audi)

The A Audi V alvelift S is ystem since 2008 in almost all FSI and TFSI engines of the brand Audi in use. The system is also used in identical engines from the Volkswagen Group. For each cylinder, this system consists of a cam piece that can be axially displaced on the main shaft and two electromagnetic actuators (actuators). The cam pieces have two profiles arranged next to one another with different cam contours for realizing small and large valve lifts. Depending on the speed and engine load, the actuators engage in spiral grooves on the flanks of the rotating cam pieces and move them to one of the end positions (large or small cam). The actuators are reset mechanically via a throw-back ramp on the cam piece. The entire switching process is completed after one revolution of the camshaft. If the engine is idling or running at low speed, the small cam profile is in action. At full load, however, the large cam profile is active. According to published reports, the system promises five percent more power delivery while saving up to seven percent in fuel. The system is expanded to include the option to permanently close individual valves in a targeted manner. This is used both for cylinder deactivation and for BiTurbos. When the cylinder is switched off, the inlet valves are no longer opened. If there are several turbos, the exhaust valves are only opened if the corresponding turbo should or can also be used.

Camtronic (Mercedes-Benz)

Mercedes-Benz calls CAMTRONIC a two-stage system for valve lift switching. In order to reduce the throttling losses, it is possible to switch to a smaller cam and thus a lower valve lift in the partial load. The system was first used in the M 270 / M 274 engine with 1.6 L displacement.

MIVEC (Mitsubishi)

M itsubishi I nnovative V alve timing E lectronic C ontrol system, from Mitsubishi . There are different variants from switching to fully variable. In the former, the inlet time and the height of the valve are influenced and two different camshaft profiles are used. Mivec is also used for turbo engines (since Lancer Evolution IX) and as the first manufacturer for diesel engines.

MultiAir (Fiat)

MultiAir is the name of a fully variable hydraulic valve control developed jointly by Fiat and the Schaeffler Group , which until then had never been used in a car engine. It was initially available for the Alfa Romeo MiTo with 1.4-liter petrol and turbo engines from September 2009. It is also used in the Fiat Twin-Air two-cylinder engine.

The cam operates the intake valve via a hydraulic system that allows the valve opening times and lifts to be varied. An electronically controlled hydraulic valve allows oil to flow out of the system during the cam lift so that the inlet valve no longer has to follow the profile of the camshaft, but can open later and less widely and close earlier. The hydraulic system is repeatedly refilled by the engine oil circuit and also acts as a hydraulic valve clearance compensation.

The advantage of this technology is the reduction of pumping and flow losses, since the air supply no longer has to be controlled via the throttle valve: The options for varying the opening time and stroke of the valves are also used to regulate the amount of air required. As a result, not only about 10% more power and 15% more torque can be gained at low speeds, but fuel consumption can also be reduced by up to 25% at the same time.

This system is still limited to the control of the inlet valves. There are already plans for a 0.9-liter two-cylinder engine in which all valves will be electro-hydraulically fully variably controlled in the near future. According to Fiat, this technology can also be applied to diesel engines with similarly good results.

Valvematic (Toyota)

Valvematic is a fully variable valve train from Toyota . It is the further development of VVTL-i. The valve lift of the inlet valves is continuously adjusted, which means that torque and speed control via a throttle valve is no longer necessary. The system also has timing adjustment (as with the Dual VVT-i), but the timing is not completely variable.

Valvetronic (BMW)

The fully variable valve control, which BMW calls Valvetronic , is a supplement to the VANOS system . The valves are operated here via intermediate levers that can be adjusted by an electric motor, so that the maximum valve lift can be varied continuously. By combining VANOS and Valvetronic, the opening time and the stroke of the valves can be controlled independently of each other. The system was initially controlled by a separate control unit and is now integrated into the engine control.

VarioCam Plus (Porsche)

In addition to the phase adjustment ( VarioCam ), the VarioCam Plus also has a valve lift adjustment of the inlet valves, which is implemented using switchable bucket tappets . The system was developed together with the Schaeffler Group .

VTEC (Honda)

Cylinder head with iVTEC technology

The V ariable Valve T IMing and Lift E lectronic C ontrol VTEC system comes from Honda . It exists in different versions (DOHC-VTEC, SOHC-VTEC, SOHC-VTEC-E, 3-stage VTEC and i-VTEC).

Basically, VTEC provides the option of switching between different cam profiles while the engine is running. The first engine equipped with VTEC appeared in 1983 in the CBR400F Super Four HYPER VTEC motorcycle. The current (2010) highest liter output of 88 kW / l was achieved with the VTEC in the S2000 . The advantages of this technology lie in a variable cylinder filling level in the respective speed range, which can be implemented cost-effectively. By this characteristic, the speed range can be up to over 9000 rpm at certain engine models -1 are raised. VTEC is electronically regulated by the control unit and includes values ​​such as cooling water temperature, oil pressure, load, speed, etc. One motivation for the VTEC system was the Japanese tax policy, which made larger cubic capacities financially unattractive for the general population.

DOHC-VTEC was first available in Germany in 1989 in the Honda Civic and CRX with 16-valve four-cylinder (1.6i with 110 kW / 150 PS, engine type B16A1) and worked in two stages. Between the two (inlet / outlet) cam profiles of the first stage there is another cam, the profile of which stands for earlier opening, greater lift and later closing. In normal operation, the rocker arm controlled by this cam presses “empty” on a set of springs that is attached between the two valves or springs. There are cross holes and locking pins in the rocker arms. If oil pressure is now released from the VTEC valve to the rocker arms at higher speeds, the locking pins within the rocker arm slide to the side and connect all three rocker arms to form a fixed group. Because the middle rocker arm opens earlier and further because of the larger cam profile and closes later, the two other rocker arms now also follow this profile and at the same time no longer have any contact with the cams of the first stage. To switch back to the first stage, the oil pressure is removed from the VTEC valve and the locking pins slide back into their rest position by means of a spring.

When SOHC VTEC uses the same principle as the DOHC VTEC, but only on the inlet side, since the space for the VTEC rocker arm on the exhaust side occupied by a spark plug.

In the SOHC-VTEC-E , an inlet valve is "switched off" in the lower speed range in order to save fuel. It opens only minimally to avoid sooting. Both inlet valves are only fully opened when there is a higher load requirement or higher speeds. The speed range for these engine models is relatively short, as the cam profile corresponds to that of a normal engine when the VTEC is activated. VTEC-E was first available in 1991 in the Civic VEi . With a standard consumption of 4.5 l per 100 km, this 66 kW (90 PS) car was one of the most economical petrol engines at the time.

Engines with 3-stage VTEC (from 1995) are based on the VTEC-E concept. They combine the economy of the VTEC-E engine in the partial load range with the performance of the SOHC-VTEC engine in the full load range. They also have the usual third cam of the normal VTEC principle. This is realized by double locking pins.

  • 1st stage: All locking pins are in their rest position and both rocker arms follow their own cam profile (one opens weakly, the other normal).
  • 2nd stage: The 1st set of locking pins is activated. Both valves now follow the cam profile for normal opening
  • 3rd stage: The 2nd set of locking pins is also activated. Both valves now follow the middle cam profile

The iVTEC system is used in newer vehicles, which combines the properties of the DOHC-VTEC and the VTEC-E with an additional phase shift of the camshaft, known as VTC (Variable Timing Control). The inlet camshaft can be continuously adjusted by up to 50 °.

VVTL-i (Toyota)

V ariable V alve T iming and L ift - i ntelligent from Toyota . It is a further development of the VVT-i (see above). This system is similar to Honda's VTEC system . It has a "normal" cam profile and a sporty one. In the control unit, the engine switches to the sporty (“sharp”) cam profile from a specified speed. The control times are extended, that is, the valves open earlier and close later and the valve lift is increased. As a result, the maximum torque is greater at higher engine speeds, since the cylinder's degree of filling is higher due to the longer control times. The maximum power is correspondingly greater, but without the disadvantages of a sporty design, since the "tame" cam profile is used at low speeds and results in a better filling level and higher torque.

Like the VVT-i, the system includes phase adjustment of the camshafts relative to the crankshaft.

swell

  1. Peter Kleinheins: The large zeppelins: The history of airship construction . 3. Edition. Springer, Berlin 2005, ISBN 3-540-21170-5 , pp. 94 ( limited preview in Google Book search).
  2. H. Kremser: The structure of high-speed internal combustion engines for motor vehicles and railcars . In: Hans List (Ed.): The internal combustion engine . tape 11 . Springer, Vienna 1942, ISBN 978-3-7091-5016-0 , p. 190 , doi : 10.1007 / 978-3-7091-5016-0 ( limited preview in Google book search).
  3. ^ Grohe, Heinz: Otto and diesel engines, Vogel-Verlag Würzburg, ISBN 3-8023-1559-6
  4. Grohe, Heinz: Otto and diesel engines, Vogel-Verlag Würzburg, ISBN 3-8023-1559-6 , page 132, "Determination of the timing "
  5. 'Tagung Variable Valve Control' ( Memento of the original from September 28, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , hdt-essen.de @1@ 2Template: Webachiv / IABot / www.hdt-essen.de
  6. US patent
  7. http://history.nissan.co.jp/PRIMERA/9809PRIMERA/index.html
  8. Archived copy ( memento of the original from October 29, 2006 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.nissan.co.jp
  9. http://history.nissan.co.jp/X-TRAIL/T30/0010/CATALOG/MECHA/index.html
  10. Harald Kaufeld, Ulrich Kölsch, Manfred Rechs, Helmut Ruhland, Klaus Moritz Springer: The New Ford Duratec 1.6 l Ti-VCT Engine , MTZ 3/2005, 66th year.
  11. Archived copy ( memento of the original from October 13, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.bmw.de
  12. Archived copy ( Memento of the original from August 17, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.audi.de
  13. Audi Technology Portal - cylinder on demand. In: Audi Technology Portal. Retrieved May 7, 2016 .
  14. Audi SQ7 TDI - animation EAV and 48-volt sub-electrical system - Audi MediaTV. In: www.audi-mediacenter.com. Retrieved May 7, 2016 .
  15. CAMTRONIC valve lift switchover - accelerating with the camshaft in the new A-Class
  16. http://www.auto.de/magazin/showArticle/article/34482/Genf-2010-Mitsubishi-ASX-erster-Diesel-mit-variabler-Ventilsteuerung
  17. http://www.autobild.de/artikel/erste-fahrt-mit-fiat-multiair_923741.html

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