01-07-2005, 11:12 AM
(Modification du message : 01-07-2005, 11:25 AM par PetruSeven.)
Il y aurait un mécanisme identique pour les cylindres 1 et 2.
Et voici les explications trouvées ici:
http://www.mgcars.org.uk/news/news174.html
Variable Valve Control:
The concept which Rover uses was first designed and patented by the UK firm AE Ltd (Associated Engineering Group) in the early 1970's. However, its execution in mechanical terms is pure Rover and was designed and patented in the early 1990's as a means of expanding the range of cars capable of using the K Series engine. It was therefore designed before the MGF was developed as part of Rover's shelf engineering activity.
Several other manufacturers including Alfa-Romeo, BMW, Porsche and Honda have developed systems giving more control than a fixed camshaft. None give the range of control patented by Rover.
The Holy Grail of engine design, is to make a small engine feel and perform like a larger one. Which is exactly the process started by Henry 86 years ago.
The Mechanics of How it Works:
In its simplest form, what the VVC Mechanism does, is to speed up and slow down the camshaft so that the length of time the inlet valves are open can be varied without the profile of camshaft changing. Although the drive to the VVC Mechanism is at a constant half engine (or crankshaft) speed, the camshaft velocity varies within each revolution in such a way as to maintain an average of half engine speed.
The heart of the MGF VVC lies just underneath the cam cover.
The first obvious thing one sees is that the inlet camshaft is not a single casting as in most engines, but is in four sections. Inlet valves for cylinders 1 and 2 are controlled by lobes on a half cam-shaft driven by a VVC Mechanism (the most important components of which are two drive rings) at the front of the engine. Inlet valves for cylinders 3 and 4 are controlled by lobes on a half cam-shaft driven by a second VVC Mechanism (incorporating its own drive rings) at the rear of the engine, the drive for the latter is taken from the exhaust camshaft.
Each half camshaft is in two parts, an inner independent shaft and an external shaft on which the cam lobes are located. Each VVC Mechanism housing is machined to swiss watch standards and contains a pair of needle roller bearings.
The other thing one notices is a cast hydraulic control unit and next to it, two 1 square plastic coated solenoids. These are driven from the Engine Management System (EMS) and, through a spool valve, control shaft connected to the toothed control sleeve on the outside of each driving ring assembly. One of these half-inlet camshafts, that for cylinders 3 and 4 is shown in illustration 4.
The really ingenious bit is the design of the VVC Mechanism. As the cross-sectional diagrams in illustrations 5 show, the outer control sleeve is machined such that it is much thinner on one side than the other, (i.e. it's bore is eccentric to it's outside diameter).
The sleeve can be adjusted through about a quarter-turn which causes the pair of drive rings within the sleeve to move outside the camshaft's centre of rotation.
As the drive ring rotates within the sleeve, the camshaft drive pin moves from the inside of the radia slot to the outside, depending on whether the drive ring is at the thin side or the thick side.
A pin at the end of the external shaft slots into a hole in the face of the drive ring thus transferring the desired amount of movement to the external shaft and hence to the cam lobes.
This picture shows exactly what happens within each revolution of the camshaft. If you examine the centre row of the diagrams, you can see that at the 260 degree cam period, the centre of the radial slot always revolves around the central axis. This ensures the drive pin (which transfers the movement to the external shaft on which the cam lobes are cast) stays in the centre of the pin clearance hole. In other words it performs as a normal fixed profile camshaft would.
The top and bottom rows shows the extremes of movement available. The drive pin is no longer always in the centre but can move clockwise or anti-clockwise to the edge of the pin clearance hole. The direction and amount of movement is determined by the extent to which the drive ring speeds up or slows down, relative to the input speed.
The top row of diagrams show the pin and radial slot in positions which allow the camshaft to open and close the valves faster thus allowing less fuel/air mixture in because less power is needed. In the bottom row, the valves are being opened and closed more slowly, thus allowing more time for more fuel/air mixture to be burnt, giving more power.
Having understood how it works, the next question is, how does the VVC mechanism know by how much to alter things as you drive along?
Information from the EMS MAP:
In order to translate the mechanical system into the measurable performance effect described below, the EMS receives information on a continuous basis from non-contact sensors in several parts of the engine.
These tell it the current status of:
1 Inlet cam position.
2 Crankshaft rotation speed.
3 Oil temperature in the hydraulic control unit.
4 Inlet manifold air temperature and pressure.
5 Coolant temperature at the top hose.
The difference between what the current status of each is and what the software inside the EMS has been programmed to decide the valve opening period needs to be at any particular fraction of a second, is then transmitted through the hydraulically controlled mechanical linkage of the VVC Mechanism to the camshaft lobes.
What is the effect?:
The software in the EMS ensures that the VVC Mechanism allows the length of time during which each inlet camshaft lobe permits each inlet valve to stay open, to be varied as the camshaft rotates. The amount of variation is huge - 75 degrees between 220 to 295 degrees, or about 40 degrees either side of the basic camshaft profile of 260 degrees if you prefer.
The amount of precision control is incredible. The EMS can map and if necessary alter, the degree of valve opening required within each single camshaft revolution.
To quote Laurence Pomeroy again: Increased valve overlap (either side of top-dead-centre) and the extension of the total period of inlet valve opening contribute to increased effectiveness for a given valve area
Et voici les explications trouvées ici:
http://www.mgcars.org.uk/news/news174.html
Variable Valve Control:
The concept which Rover uses was first designed and patented by the UK firm AE Ltd (Associated Engineering Group) in the early 1970's. However, its execution in mechanical terms is pure Rover and was designed and patented in the early 1990's as a means of expanding the range of cars capable of using the K Series engine. It was therefore designed before the MGF was developed as part of Rover's shelf engineering activity.
Several other manufacturers including Alfa-Romeo, BMW, Porsche and Honda have developed systems giving more control than a fixed camshaft. None give the range of control patented by Rover.
The Holy Grail of engine design, is to make a small engine feel and perform like a larger one. Which is exactly the process started by Henry 86 years ago.
The Mechanics of How it Works:
In its simplest form, what the VVC Mechanism does, is to speed up and slow down the camshaft so that the length of time the inlet valves are open can be varied without the profile of camshaft changing. Although the drive to the VVC Mechanism is at a constant half engine (or crankshaft) speed, the camshaft velocity varies within each revolution in such a way as to maintain an average of half engine speed.
The heart of the MGF VVC lies just underneath the cam cover.
The first obvious thing one sees is that the inlet camshaft is not a single casting as in most engines, but is in four sections. Inlet valves for cylinders 1 and 2 are controlled by lobes on a half cam-shaft driven by a VVC Mechanism (the most important components of which are two drive rings) at the front of the engine. Inlet valves for cylinders 3 and 4 are controlled by lobes on a half cam-shaft driven by a second VVC Mechanism (incorporating its own drive rings) at the rear of the engine, the drive for the latter is taken from the exhaust camshaft.
Each half camshaft is in two parts, an inner independent shaft and an external shaft on which the cam lobes are located. Each VVC Mechanism housing is machined to swiss watch standards and contains a pair of needle roller bearings.
The other thing one notices is a cast hydraulic control unit and next to it, two 1 square plastic coated solenoids. These are driven from the Engine Management System (EMS) and, through a spool valve, control shaft connected to the toothed control sleeve on the outside of each driving ring assembly. One of these half-inlet camshafts, that for cylinders 3 and 4 is shown in illustration 4.
The really ingenious bit is the design of the VVC Mechanism. As the cross-sectional diagrams in illustrations 5 show, the outer control sleeve is machined such that it is much thinner on one side than the other, (i.e. it's bore is eccentric to it's outside diameter).
The sleeve can be adjusted through about a quarter-turn which causes the pair of drive rings within the sleeve to move outside the camshaft's centre of rotation.
As the drive ring rotates within the sleeve, the camshaft drive pin moves from the inside of the radia slot to the outside, depending on whether the drive ring is at the thin side or the thick side.
A pin at the end of the external shaft slots into a hole in the face of the drive ring thus transferring the desired amount of movement to the external shaft and hence to the cam lobes.
This picture shows exactly what happens within each revolution of the camshaft. If you examine the centre row of the diagrams, you can see that at the 260 degree cam period, the centre of the radial slot always revolves around the central axis. This ensures the drive pin (which transfers the movement to the external shaft on which the cam lobes are cast) stays in the centre of the pin clearance hole. In other words it performs as a normal fixed profile camshaft would.
The top and bottom rows shows the extremes of movement available. The drive pin is no longer always in the centre but can move clockwise or anti-clockwise to the edge of the pin clearance hole. The direction and amount of movement is determined by the extent to which the drive ring speeds up or slows down, relative to the input speed.
The top row of diagrams show the pin and radial slot in positions which allow the camshaft to open and close the valves faster thus allowing less fuel/air mixture in because less power is needed. In the bottom row, the valves are being opened and closed more slowly, thus allowing more time for more fuel/air mixture to be burnt, giving more power.
Having understood how it works, the next question is, how does the VVC mechanism know by how much to alter things as you drive along?
Information from the EMS MAP:
In order to translate the mechanical system into the measurable performance effect described below, the EMS receives information on a continuous basis from non-contact sensors in several parts of the engine.
These tell it the current status of:
1 Inlet cam position.
2 Crankshaft rotation speed.
3 Oil temperature in the hydraulic control unit.
4 Inlet manifold air temperature and pressure.
5 Coolant temperature at the top hose.
The difference between what the current status of each is and what the software inside the EMS has been programmed to decide the valve opening period needs to be at any particular fraction of a second, is then transmitted through the hydraulically controlled mechanical linkage of the VVC Mechanism to the camshaft lobes.
What is the effect?:
The software in the EMS ensures that the VVC Mechanism allows the length of time during which each inlet camshaft lobe permits each inlet valve to stay open, to be varied as the camshaft rotates. The amount of variation is huge - 75 degrees between 220 to 295 degrees, or about 40 degrees either side of the basic camshaft profile of 260 degrees if you prefer.
The amount of precision control is incredible. The EMS can map and if necessary alter, the degree of valve opening required within each single camshaft revolution.
To quote Laurence Pomeroy again: Increased valve overlap (either side of top-dead-centre) and the extension of the total period of inlet valve opening contribute to increased effectiveness for a given valve area
![[Image: icnecatapetrusrm3.gif]](http://img147.imageshack.us/img147/1927/icnecatapetrusrm3.gif)
