When the solenoid valve is
energized, it opens and allows the oil to flow into the hydraulic
chamber. As a result, no oil will flow to the valve actuator, thus the
intake valve will close under the force of its rebound spring. In this
way, Multiair can shut down the intake valves at any desired instant. (Fig 3 and Fig 4)
Suppose the intake valve has closed for a while, then the solenoid valve of hydraulic chamber is closed again. What will happen ? In this case, oil will flow directly to valve actuator again, thus the intake valve will follow the cam profile and open again. However, as some time and oil volume has already "lost" (at the hydraulic chamber) during the solenoid valve opening, the valve lift will be reduced. The degree of reduction depends on the instant of solenoid valve closure. The later the solenoid valve close, the lower valve lift will be obtained. In this way, Multiair can vary the lift and opening duration of intake valves. (Fig 2)
Now let us see the above valve lift graphs again:
Fig 1 is suitable for high rpm running.
Fig 2 is sutiable for low-load operation. Its late valve opening leads to a partial vacuum in the combustion chamber. In addition to the low valve lift, the intake air stream is greatly speeded up, generating turbulence thus improve air and fuel mixture. This benefits fuel economy and emission.
Fig 3 is suitable for a wide range of part-load operation. Depending on the requirement of power, the amount of air can be controlled by the early closing of intake valves. This eliminates the need of throttle butterfly (like BMW Valvetronic) and reduce pumping loss by up to 10%.
Fig 4 is designed for enhanced low-rpm acceleration. While it enables more intake air volume compare with Fig 2 & 3, its early valve closure ensures no air flow back into the intake manifolds near the end of the intake stroke. (Remark: the combination of fast cam timing and low rpm operation could lead to backflow, that's why Multiair needs to close the valves earlier. Other engines do not have this issue because they either use variable cam phasing or compromised cam timing)
Fig 5 is so-called "Multilift" mode and designed for very low rpm operation. It combines the strategy of Fig 2 & 3 and their benefits - regulated consumption and improved quality of air-fuel mixture.
Combining these modes, FIAT claims Multiair improves maximum power by 10%, low-rpm torque by 15% and fuel economy by 10%. Moreover, cold-start emission of HC/CO and NOx are reduced by 40% and 60% respectively due to its ability of exhaust gas recirculation. This technology is also compatitble with diesel engines, which means substantial cost reduction.
However, I can see a few weaknesses of Multiair: Firstly, at the moment it is compatible with SOHC engines only because of the bulky mechanism. This mean while it enables variable timing and lift for intake valves, it offers neither for exhaust valves. The addition of variable exhaust cam phasing may require a complex cam-in-cam mechanism like that used by Dodge Viper 8.4. Secondly, the SOHC design and the complicated electrohydraulic mechanism could generate extra friction, thus it is not suitable to high-revving high performance engines, which is a common problem shared with BMW Valvetronic. It is more suitable to mass production engines and low-revving turbocharged engines. Lastly, the electrohydraulic mechanism might complicate servicing and raise reliability issues.
Suppose the intake valve has closed for a while, then the solenoid valve of hydraulic chamber is closed again. What will happen ? In this case, oil will flow directly to valve actuator again, thus the intake valve will follow the cam profile and open again. However, as some time and oil volume has already "lost" (at the hydraulic chamber) during the solenoid valve opening, the valve lift will be reduced. The degree of reduction depends on the instant of solenoid valve closure. The later the solenoid valve close, the lower valve lift will be obtained. In this way, Multiair can vary the lift and opening duration of intake valves. (Fig 2)
Now let us see the above valve lift graphs again:
Fig 1 is suitable for high rpm running.
Fig 2 is sutiable for low-load operation. Its late valve opening leads to a partial vacuum in the combustion chamber. In addition to the low valve lift, the intake air stream is greatly speeded up, generating turbulence thus improve air and fuel mixture. This benefits fuel economy and emission.
Fig 3 is suitable for a wide range of part-load operation. Depending on the requirement of power, the amount of air can be controlled by the early closing of intake valves. This eliminates the need of throttle butterfly (like BMW Valvetronic) and reduce pumping loss by up to 10%.
Fig 4 is designed for enhanced low-rpm acceleration. While it enables more intake air volume compare with Fig 2 & 3, its early valve closure ensures no air flow back into the intake manifolds near the end of the intake stroke. (Remark: the combination of fast cam timing and low rpm operation could lead to backflow, that's why Multiair needs to close the valves earlier. Other engines do not have this issue because they either use variable cam phasing or compromised cam timing)
Fig 5 is so-called "Multilift" mode and designed for very low rpm operation. It combines the strategy of Fig 2 & 3 and their benefits - regulated consumption and improved quality of air-fuel mixture.
Combining these modes, FIAT claims Multiair improves maximum power by 10%, low-rpm torque by 15% and fuel economy by 10%. Moreover, cold-start emission of HC/CO and NOx are reduced by 40% and 60% respectively due to its ability of exhaust gas recirculation. This technology is also compatitble with diesel engines, which means substantial cost reduction.
However, I can see a few weaknesses of Multiair: Firstly, at the moment it is compatible with SOHC engines only because of the bulky mechanism. This mean while it enables variable timing and lift for intake valves, it offers neither for exhaust valves. The addition of variable exhaust cam phasing may require a complex cam-in-cam mechanism like that used by Dodge Viper 8.4. Secondly, the SOHC design and the complicated electrohydraulic mechanism could generate extra friction, thus it is not suitable to high-revving high performance engines, which is a common problem shared with BMW Valvetronic. It is more suitable to mass production engines and low-revving turbocharged engines. Lastly, the electrohydraulic mechanism might complicate servicing and raise reliability issues.