|Multiair system components. Click to enlarge.|
Fiat Group and Fiat Powertrain Technology introduced their new air management technology Multiair at the Geneva Motor Show. Multiair is an electro-hydraulic valve-timing system that provides dynamic and direct control of air and combustion, cylinder by cylinder and stroke by stroke.
Fiat expects that Multiair, with direct control of the air through the intake engine valves without using the throttle, can help reduce fuel consumption by up to 10% for turbocharged or naturally aspirated gasoline engines. Pollutant emissions are likewise reduced through combustion control. Applied to a turbocharged, downsized engine, MultiAir can contribute to up to a 25% reduction in fuel consumption over conventional naturally aspirated engines with the same performance, according to Fiat.
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.
Potential benefits for gasoline engines include:
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; and
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.
The first world-wide application of the Multiair technology will be the Fire 1400 cc 16V naturally aspirated and turbocharged engines. The second application will be a new small gasoline engine (SGE - 900 cc twin-cylinder) where the cylinder head design has been specifically optimized for the Multiair actuator integration.
Here again, there will be both a naturally aspirated and a turbocharged version. A specific turbocharged engine version will be bi-fuel (gasoline-CNG). Thanks to radical downsizing, the Turbocharged Small Gasoline Engine achieves diesel-like CO2 emission levels, which are further reduced in its natural gas version with CO2 emissions lower than 80 g/km in many vehicle applications.
Fiat says that Multiair will lead to further innovations such as:
Integration of the Multiair Direct air mass control with Direct gasoline Injection to further improve transient response and fuel economy.
Introduction of more advanced multiple valve opening strategies to further reduce emissions.
Innovative engine-Turbocharger matching to control trapped air mass through combination of optimum boost pressure and valve opening strategies.
Fiat began working on electro-hydraulic actuation in the mid-1990s, leveraging on the know-how gained during the Common Rail development. The goal was to reach the desired flexibility of valve opening schedule air mass control on a cylinder-by-cylinder and stroke-by-stroke basis. The electro-hydraulic variable valve actuation technology developed by Fiat was selected for its relative simplicity, low power requirements, intrinsic fail safe nature and low cost potential.