Fiat Group and Fiat Powertrain Technology revealed its new air control technology 'Multiair' at the Geneva Motor Show. Multiair is an electro-hydraulic method of engine valves for dynamic and direct control of air and combustion, cylinder by cylinder and stroke by stroke. Multiair supports reducing fuel consumption, and pollutant emissions are furthermore reduced by combustion control.
The Fiat Multiair Technology, the evolution of the Common Rail technology for Diesel engines noted a discovery in the passenger car market. To be competitive in the field of gasoline engines, Fiat Group chose to follow the same approach and focus on finding new technologies.
The fundamental parameter to control Diesel engine combustion and consequently performance, emissions and fuel consumption is the amount and characteristics of the fuel injected into cylinders. That is the reason why the Common Rail electronic Diesel fuel injection system was such a fundamental breakthrough in Direct Injection Diesel engine technology.
In the beginning, world-wide analysis efforts were concentrated on the electromagnetic actuation concept, following which valve opening and closing are obtained by alternatively exciting upper and lower magnets with an armature connected to the valve. This actuating system had the intrinsic appeal of maximum flexibility and dynamic response in valve control, but despite a decade of significant development efforts the main drawbacks of the concept - its being intrinsically not fail-safe and its high energy absorption - could not be fully overcome.
The Fiat Multiair Technology: how it works
The working principle of the system, applied to intake valves, is the following: a piston, moved by a mechanical intake cam, is correlated 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 works 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.
The Multiair Technology Benefits
The Multiair Technology potential benefits for gasoline engines exploited so far can be summarized as follows:
- Maximum Power is increased by up to 10% thanks to the adoption of a power-oriented mechanical cam profile
- Low-rpm Torque is improved by up to 15% through early intake valve closing strategies that maximize the air mass trapped in the cylinders.
- Elimination of pumping losses brings a 10% reduction of Fuel Consumption and CO2 emissions, both in Naturally Aspirated and Turbocharged engines with the same displacement
- Multiair Turbocharged and downsized engines can achieve up to 25% Fuel Economy improvement over conventional Naturally Aspirated engines with the same level of performance
- Optimum valve control strategies during engine warm-up and internal Exhaust Gas Recirculation, realized by reopening the intake valves during the exhaust stroke, result in emissions reduction ranging from 40% for HC / CO to 60% for NOx
- Constant upstream air pressure, atmospheric for Naturally Aspirated and higher for Turbocharged engines, together with the extremely fast air mass control, cylinder-by-cylinder and stroke-by-stroke, result in a superior dynamic engine response
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