It is a Hydraulic Desmodromic valve train: the hydraulic liquid (typically the lubricating oil of the engine) opens the valve positively and closes the valve positively without needing restoring valve springs. The HyDesmo provides infinite valve lift profiles starting from zero lift.
The mass production of the MultiAir / TwinAir / UniAir of Fiat / INA proves the reliability of the hydraulic actuation of the valves of the modern internal combustion engines. The infinite valve lift profiles make the engine greener: the Fiat 500 TwinAir engine has been voted as the greenest gasoline engine, and has been awarded as "by far" the best engine of the year 2011.
Removing the restoring valve springs from the valves, the hydraulic system has easier work to do; the valve train consumes less power, the reciprocating mass of the valve assembly reduces, the rev limit is higher, the height and the cost of the engine decrease. The HyDesmo can operate according all the MultiAir strategies and more (Outgoing Air Control explained at PatAir ). Besides, the HyDesmo fits with the actuation of the exhaust valves, too.
The control by high speed solenoid valves, like those of the MultiAir, is not mandatory for the HyDesmo. The angular displacement of the oil piston can vary the valve lift and the valve duration, just like the rotation of the piston of a jerk pump (Bosch high pressure fuel pump) varies the quantity of fuel injected. Slow, cheap, low power servomotors can micro-align the angular displacement of different oil pistons to balance the load between different cylinders.
The cam 6, rotating in synchronization to the crankshaft of the engine, forces the oil piston 7 to perform an actuating motion wherein the oil piston displaces oil out from the oil chamber 8; the oil is directed either to the opening oil chamber 4 to displace the valve 1 to open positively, or to the closing oil chamber 5 to displace the valve to close positively.
With a "refresh" the three animations get in phase.
In the beginning of the valve opening, the oil displaces the check valve 16 allowing the closing oil chamber 5 to communicate with the oil reservoir 11, while oil passing through the one way valve 17 enters into the opening oil chamber 4 displacing the valve to open. With the valve piston uncovering the "braking holes" around the opening oil chamber 4, the oil starts entering to the opening oil chamber through both, the one way valve 17 and the "braking holes".
At a crankshaft angle, depending on the angular displacement of the oil piston 7 relative to the oil chamber 8, the opening oil chamber 4 stops communicating with the oil chamber 8 and starts communicating with the oil reservoir 11, while the closing oil chamber 5 stops communicating with the oil reservoir 11 and starts communicating with the oil chamber 8.
Now the oil exiting from the oil chamber 8 is directed to the closing oil chamber 5, displacing the valve 1 to close; the excess oil from the opening oil chamber 4 is directed to the oil reservoir 11.
When the valve 1 is about to land on the valve seat, the "holes" around the opening oil chamber 4 are progressively covered by the valve piston 2, decelerating / braking the valve motion and enabling smooth valve landing.
With the valve closed, the oil chamber 8 starts communicating with the oil reservoir 11; the check valve 16 does not allow the oil to exit from the closing oil chamber 5, and the valve 1 remains firmly closed.
During the restoring (upwardly) motion of the oil piston 7, oil entering through the one way valve 23 fills the oil chamber 8.
The auxiliary / safety spring 18 holds the valve from "dropping" towards the combustion chamber during engine's stall.
The hydraulic lash adjuster (spring 19, one way valve 20) controls the valve lash.
The auxiliary piston 21 is slidably fitted to, and seals, one side of the closing oil chamber 5; together with its supporting spring 22, it serves as an oil pressure surge compensation means: during the pressure surging, the piston 21 moves slightly outwards of the closing oil chamber 5 dumping the pressure surge; when the pressure drops, the piston 21 moves, by the supporting spring 22, slightly inwards the closing oil chamber 5 to compensate any oil leakage and to keep, this way, the valve firmly closed until the next valve opening.
The niches / galleries of the oil piston 7 and the ports of the oil chamber 8 are (no matter what the angular displacement of the oil piston 7 relative to the oil chamber 8 is) such that:
the oil reservoir 11 communicates with the closing oil chamber 5 for as long as oil from the oil chamber 8 goes to the opening oil chamber 4,
the oil reservoir 11 communicates with the opening oil chamber 4 for as long as oil from the oil chamber 8 goes to the closing oil chamber 5,
the oil reservoir 11 communicates with the oil chamber 8 after the valve closing.
The mission of the oil piston 7 is multiple:
- to displace, in synchronization to the engine crankshaft, the oil actuating the opening and the closing of the valve,
- to control, by its angular displacement, the valve lift and the valve duration,
- to manage the communication between the participating oil chambers.
With an adequately long part of the actuating (downwardly) motion of the oil piston dedicated, for all the available modes of operation, to the valve closing, the hydraulic-mechanical system guarantees that at the end of the actuating motion of the oil piston 7, the valve 1 will always be closed.
Having a single opening ramp of a rotating cam to cause both, the opening and the closing of the valve, and having a single part, the oil piston, to manage the communication between the various oil chambers, the system becomes as accurate, as simple and as reliable as it gets.
The roller of the rocker arm rolls initially along the "valve opening or closing" section of the cam lobe; depending on the angular displacement of the oil piston 7 relative to the oil chamber 8, the valve can either open or close.
Near the middle stroke of the oil piston 7, i.e. near the middle of the opening ramp of the cam 6, the roller of the rocker arm starts rolling along the "valve closing" section of the cam; if the valve is still open, it closes; if the valve is closed, it remains closed. The "valve opening or closing" section and the "valve closing" section comprise the opening ramp of the cam.
After the "valve closing" section, the roller of the rocker arm rolls along the "oil chamber refill" section of the cam, which is the closing ramp of the cam; the valve remains closed; the restoring spring 24 pushes the oil piston upwards to follow the "oil chamber refill" section of the cam; oil enters and fills the oil chamber 8, through the one way valve 23. Then the roller of the rocker arm rolls along the "basic circle" section of the cam during which the valve remains closed.
The duration of the opening ramp of the cam lobe, i.e. the duration of the "valve opening or closing" section plus the duration of the "valve closing" section equals to the maximum valve duration because the opening ramp of the cam controls the valve motion not only during the valve opening but also during the valve closing. For instance, for 270 crank degrees valve duration (in the conventional engine this means nearly 135 crank degrees opening ramp duration), the duration of the opening ramp of the HyDesmo cam lobe needs to be of at least 270 crank degrees. Given the desirable full lift profile and given the geometry of the system, the opening ramp of the cam lobe is calculated. Until the maximum valve lift, the valve lift profile and the oil piston lift profile "match", i.e. they have similar shape. After the maximum valve lift, the oil piston lift profile "matches" with the negative valve lift profile.
If the opening oil chamber stops communicating with the oil chamber at 40 crankshaft degrees, the valve moves according the A valve lift profile:
the valve starts opening at -30 crankshaft degrees and follows, for 70 crankshaft degrees, the full lift curve B; then the valve starts closing.
Until the angle the curve B is maximized, the closing curve A is the symmetrical of the full lift curve B about the horizontal line from the intersection point of the B and A curves; then the curve A keeps constant distance from the curve B.
By angularly displacing the oil piston relative to the oil chamber, the duration and the lift of the valve vary. The system can offer a continuous range of valve lifts starting from zero, enabling the throttling by the intake valves, i.e. enabling the throttle-less operation.
What is shown above, is the oil piston into the oil chamber, the valve seat and the lower part of the valve.
In each of the four triads, at left is the full lift / full duration mode, at middle is a medium-lift / short-duration mode, at right is the valve deactivation mode.
The top left triad is for a cam angle wherein the oil piston starts moving downwards.
The top right triad is for a later cam angle; the valve at left continuous to open; the valve at the middle is starting to close.
In the bottom left triad the cam is further rotated; the valve at left is at the maximum lift, while the valve at middle is almost closed.
In the bottom right triad, the cam is further rotated; here the valve is closed in all modes.
The pure mechanical control is an option: in a HyDesmo motorcycle, for instance, the driver can rotate, by the grip and the gas-cable, the oil piston relative to the oil chamber, just like he controls the throttle valve in a conventional engine. The fuel system can align the quantity of the fuel with the air entering the cylinder (oil piston angular displacement sensor, rev sensor, lambda sensor, loop control etc).
The electronic control is another option. With servomotors controlled by the ECU, the angles of the oil pistons of different oil chambers are micro-aligned (fine tuning) in order to balance the operation of different cylinders based on lambda sensors, on the feedback control etc. This fine-tuning needs not the high-power instant-response solenoid electromagnetic valves of the MultiAir system. The ECU can complete the micro alignment in a few rotations of the crankshaft. In case the ECU fails to control the servomotors, the engine continues to operate based on the mechanical control.
Applied on a Vee engine (like the current V-6 with the four camshafts, or like the older Vee with a single camshaft) the reprofiled cam (the duration of the opening ramp doubles) displaces directly the oil pistons;
the oil displaced by the oil pistons out from the oil chambers goes, through proper piping, to the valves wherein the opening oil chambers and the closing oil chambers reside, and controls positively both, the valve opening and the valve closing, eliminating many parts and making the engine cheaper, shorter and simpler; besides, with the infinite valve lift profiles of the HyDesmo, the engine becomes greener and more fuel efficient.
A desmodromic "MultiAir clone".
Below is a HyDesmo version having an electronic control similar to that of the MultiAir. A high speed electromagnetic valve 10 controls the communication of the oil chamber 8 with the opening oil chamber 4 and the closing oil chamber 5. The oil piston 7 is connected to an eccentric pin 6 (this is the cam) via a connecting rod. No restoring spring for the oil piston is required.
The offset of the crankshaft provides a different oil piston motion profile, enabling longer "opening ramp" duration (for the "Outgoing Air Control" mode, for instance).
The larger area (by the surface of the valve stem section) of the valve piston at the "opening oil chamber" side, enables a stronger force - for a given pressure into the oil chamber - for the opening of the valve (significant for the actuation of the exhaust valves of supercharged engines); it also enables more degrees of the "cam" to be dedicated to the "valve opening or closing" section of the "cam" relative to the degrees for the "valve closing" section.
The following plot shows some, of the infinite available, valve lift profiles of the above arrangement (case of zero offset of the actuating crank - or cam - relative to the oil chamber axis):
Instead of the eccentric crank-pin 6, a cam and a rocker arm and an oil piston restoring spring 24, like those of the HyDesmo version with the angularly displaceable oil piston, can be used.
The following plot shows the available strategies of the HyDesmo with the high speed electromagnetic solenoid valves for the case the cam lobe provides pure sinusoidal full lift profile:
The valve opens and closes positively. The closing of the valve is controlled by the cam opening ramp, being independent on the restoring force of a valve spring and on the characteristics (viscosity etc) of the hydraulic liquid.
The Hydraulic-Mechanical system guarantees that at the end of the downward motion of the oil piston the valve will always be closed, no matter what the state, or the functionality, of the control system is.
The moment just before the opening of an exhaust valve, the MultiAir has to overcome the force applied to the valve by the pressure into the combustion chamber, and the force necessary to accelerate the valve, and the force the restoring valve spring applies to the valve. Rid of restoring valve springs, the HyDesmo fits better to the actuation of exhaust valves.
By controlling the moment the exhaust valve closes, the actual overlap is controlled without phasers.
HyDesmo animations
Click on the image above to open the HyDesmoTwin controllable windows exe animation.
A single camshaft is actuating four oil pistons, each of the oil piston is actuating two intake or two exhaust valves.
Click on the image above to run the controllable windows exe animation (then press the A key for random mode change, or the SpaceBar key to select the mode of operation, or the , . and / keys to select - deselect and control the "step by step mode", or move the mouse along the animation to change the rhythm, or press ESC - or double click - to quit the animation).
