04.03.2009

The Fiat Multiair Technology: how it works


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The operating principle of the system, applied to intake valves, is the following: a piston, moved by a mechanical intake cam, is connected to the intake valve through a hydraulic chamber, which is controlled by a normally open on/off Solenoid Valve.
When the Solenoid Valve is closed, the oil in the hydraulic chamber behaves like a solid body and transmits to the intake valves the lift schedule imposed by the mechanical intake cam. When the solenoid valve is open, the hydraulic chamber and the intake valves are de-coupled; the intake valves do not follow the intake cam anymore and close under the valve spring action. The final part of the valve closing stroke is controlled by a dedicated hydraulic brake, to ensure a soft and regular landing phase in any engine operating conditions.
Through Solenoid Valve opening and closing time control, a wide range of optimum intake valve opening schedules can be easily obtained.
For maximum power, the Solenoid Valve is always closed and full valve opening is achieved following completely the mechanical cam, which was specifically designed to maximize power at high engine speed (long opening time).
For low-rpm Torque, the Solenoid Valve is opened near the end of the cam profile, leading to early intake valve closing. This eliminates unwanted backflow into the manifold and maximizes the air mass trapped in the cylinders.
In engine part load, the Solenoid Valve is opened earlier causing partial valve openings to control the trapped air mass as a function of the required torque. Alternatively the intake valves can be partially opened by closing the Solenoid Valve once the mechanical cam action has already started. In this case the air stream into the cylinder is faster and results in higher in-cylinder turbulence.
The last two actuation modes can be combined in the same intake stroke, generating a so-called “Multilift” mode, that enhances turbulence and combustion rate at very low loads.


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