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Home , Inlet manifold, Valve timing

Valve timing IVO : 10°-40°before TDC IVC : 40°-60°after BDC EVO : 40°-60°before BDC EVC : 10°-30°after TDC

In the ideal cycles, valves open and close instantaneously at dead centre. In practice, they open and close in finite time (to maintain acceptable accelerations and velocities) and often quite far from the piston dead centre, for fluid and/or thermo-dynamic reasons. Usually the opening time periods for the inlet and exhaust valve are reported on two circular diagrams or in a linear diagram.

EVO (Exhaust valve opening) Exhaust valves open 40°-60°before BDC to lower the pressure inside the , before the beginning of the exhaust , but avoiding, on the other hand, a heavy reduction of the expansion work. When piston approaches BDC, the pressure inside the cylinder is quite low (0.3-0.6 MPa), the valve flow area increases slowly and the arm is smaller and smaller. For this reason, this early opening does not represent a significant loss in terms of expansion work, but it produces a spontaneous discharge (blow-down) of part of the gases into the , due to the pressure head across the seat valve. In this way, the work done by the piston to expel the gases during the exhaust stroke is reduced.

By advancing the EVO, the work lost in expansion increases, but the negative expulsion works decreases. The optimum EVO value, for a given speed, is the one which offers the best compromise between these two contrasting effects. This value can be initially estimated by a suitable computer model, and then refined by experimental tests.

IVC (Inlet valve closing) valve closes (IVC) quite late (40°-80°after BDC), to provide more time for fresh charge to enter the cylinder, when the pressure level in the inlet pipe is higher than inside the cylinder. After BDC, the piston is no more able to suck charge. However, in the inlet manifolds there are always pressure oscillations due to unsteady gas flow and pressure wave propagation. If there is a pressure peak just around BDC, for a given engine speed, the fresh charge continues to enter the cylinder until IVC, increasing λv (ram effect). At lower engine speed, the closing lag is usually too long and part of the fresh charge, already inside the cylinder, may return back to the (backflow). At higher engine speeds, the closing lag is generally too short and the pressure peak cannot be fully exploited.

Effects of different valve closing lags on . The shape of the curve reflects the one previously illustrated. Once the IVC is fixed, there is an optimum engine speed for which the volumetric efficiency is maximum. The higher the closing lag, the higher the optimum engine speed. For fixed angular valve timings, it is possible to choose the IVC to optimize the engine operation at low, medium or high engine speeds.

Valve overlap period Usually, the exhaust valve closing is delayed, while the intake valve is opened before TDC. There is a period, called valve overlap period, during which both intake and exhaust valves are opened.

Effects at high engine speeds and loads If the engine is running at high-speed (and high loads in SI ), then, when the piston approaches TDC towards the end of the exhaust stroke, burned gases are leaving the cylinder at high velocity. Therefore the gas keeps its high momentum, directed towards the exhaust port, even if the intake valve opens. Indeed, the pressure head between the cylinder and the induction manifold is usually low and unable to change the flow direction. The inertia of the outgoing gases is then used to draw fresh charge through the inlet valve (partially opened), the and increasing the cylinder volumetric efficiency. Moreover, the dynamic effects, in the intake and exhaust systems can greatly help this process, if during the overlap period there is a positive pressure pulse in the intake manifold and a depression in the exhaust pipe.

Effects at low engine speeds and loads With fixed timings, when the engine velocity is lower than the optimum value for the fixed overlap, the time of contemporary opening of the two valves becomes too long and part of the new charge may follow the burned gases out of the exhaust port. In direct-injection engines, only air is lost. In PFI engines, air and fuel are lost through the exhaust port and this creates pollution problems (unburned hydrocarbons).

Effects at partial loads In throttled SI engines, as the valve is progressively closed, the depression in the induction pipe increases. On the other hand, the gas mass per cycle is now small, due to the partial load operation, and therefore the momentum of burned gases, flow towards the exhaust port, is low. Then, some burned gases may be drawn into the intake manifold. Here they dilute the new charge, that is later aspirated into the cylinder, when the piston begins its induction stroke.

A sort of internal recycling of burned gas occurs, that may help in reducing the NOx emissions. However, with fixed valve timings, the overlap period cannot be controlled as a function of engine speed and load. Therefore, at very low load, the amount of recycled gas may be excessive and may make difficult and incomplete the combustion of the fresh charge (with consequent fuel consumption and pollution problems).

Variable Conventional engines have a fixed angular valve timing for reasons of design simplicity, cost and reliability. This means that the crank-angles of valve opening and closing do not vary with the engine speeds and loads. Therefore the length of the time available for the exchange process (being t=θ/ω) decreases when the engine speed increases. Hence fixed timings can be optimized only for a narrow range of speed and loads. However, advantages in terms of performance, fuel consumption and pollution are so high that many engine builders are trying to use the most advanced technologies with the objective of driving all the valve movements (lifts as well as opening and closing times) with optimum values, in the engine operating range.

Variable valve timing - first generation •They can vary (in a limited and discontinuous way) the valve timings. •They are usually based on the action of a union sleeve with external helical splines and internal straight splines, that is axially moved by lubricating oil under pressure, rotating the (generally of the intake valve) of a fixed crank-angle (Δθ= 20°-25°).

•If the engine has two different , it becomes possible to optimize the valve overlap for different engine speeds and loads: - Low speeds and low loads: no valve overlap, to avoid fresh charge escaping through the exhaust port. - High speeds and high loads: maximum valve overlap to increase the cylinder filling. - second generation •The axis has three adjacent . The central one has a low profile, acting on a follower of small diameter located at the top of the valve stem. •The two lateral cams (with the same profile), have a maximum lift three times the one of the central profile) and acts on a circular crown follower with the same axis of the central one. •When the motion of the last follower is not bound to the valve stem, only the central cam acts on its follower, imposing a low lift to the valve. •When the two followers are blocked together (by means of a hydraulic or electromagnetic driving), the valve is moved by the two lateral cams, which impose to it the maximum lift.

•Possibility to control the valve lift and timing. •Such devices allow the power control of SI engines based on the variation of the intake valve lift and no longer only on the throttling of the induction duct, so reducing the negative effect of pumping losses and fuel consumption at partial loads.

Variable valve timing - third generation Third generation systems fully exploit the innovation potential of variable valve timing, with a full and flexible control of valve movements as a function of loads and speeds. The different solutions available can be classified on the basis of the driving type, which can be mainly of mechanical, hydraulic or electrical nature.

BMW-VALVETRONIC The cam does not act directly on the of the valve, but through a special follower. An electric motor, changes the position of the shaped follower with respect to the cam, so that the maximum lift transmitted to the stem of the valve is reduced, with continuity, from its full load value, up to approximately one tenth. Two additional rollers are also necessary to avoid the scraping of touching surfaces, reducing their wear and the noise emitted. A hydraulic group is also used to recover all the mechanical clearances. Fiat - Multiair, hydraulic The camshaft acts on a rocker arm which is then connected with a hydraulic chamber, where an electro-valve controls the amount of oil that is then transmitted to the valve tapped by means of hydraulic actuators. This system, which is rather complex, allow to control the valve lift and timing for a wide range of speed and loads, achieving significant reduction in fuel consumption at partial load.

EIVC: Early intake valve closing LIVC: Late intake valve closing

These systems allow a full and flexible control of valve motion for each engine load and speeds, and their advantages can be summarized as follows:

1) Possibility to optimize the valve opening and closing timings, maximizing the filling of all cylinders in the whole engine operating field, thus improving its performance and its response during transient conditions. 2) In SI engines, the load can be controlled by varying the mass of air trapped in each cylinder changing the valve lifts, so removing the pumping losses and improving the engine fuel consumption, especially at low-loads 3) The combustion process can be controlled by generating turbulent flows inside the cylinder (using different operating laws for the two intake valves) and by optimizing the internal recycling of burned gases.