MULTIAIR: THE ULTIMATE AIR MANAGEMENT STRATEGY
Multiair is a versatile technology, easily applicable to all gasoline engines and with future potential developments also for Diesel engines
FUEL CONSUMPTION AND EMISSIONS REDUCTION
INCREASE OF BOTH MAX POWER AND TORQUE
ENHANCED PERFORMANCE AND FUN TO DRIVE
The Fiat Multiair Technology: some history
The key parameter to control Diesel engine combustion and therefore performance, emissions and fuel consumption is the quantity 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.
The key parameter to control gasoline engine combustion, and therefore performance, emissions and fuel consumption, is the quantity and characteristics of the fresh air charge in the cylinders. In conventional gasoline engines the air mass trapped in the cylinders is controlled by keeping the intake valves opening constant and adjusting upstream pressure through a throttle valve. One of the drawbacks of this simple conventional mechanical control is that the engine wastes about 10% of the input energy in pumping the air charge from a lower intake pressure to the atmospheric exhaust pressure.
A fundamental breakthrough in air mass control, and therefore in gasoline engine technology, is based on direct air charge metering at the cylinder inlet ports by means of an advanced electronic actuation and control of the intake valves, while maintaining a constant natural upstream pressure.
Research on this key technology started in the 80’s, when engine electronic control technologies reached the stage of mature technologies.
At this point most automotive companies fell back on the development of the simpler, robust and well-known electromechanical concepts, based on the valve lift variation through dedicated mechanisms, usually combined with cam phasers to allow control of both valve lift and phase.
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.
The Fiat Multiair Technology: how it works
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.
Inside Fiat Multiair
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.
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.
Application of the Multiair Technology to FPT Engines
The second application is a new Small Gasoline Engine (SGE - 900cc Twin-cylinder) where 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.
Further Potential of the Multiair Technology
All breakthrough technologies open a new world of further potential benefits, which are usually not fully exploited in the first generation, in order to minimize industrial risk.
The Common Rail technology, a Fiat worldwide premiere in 1997, paved the way to more than a decade of further technological evolutions such as "Multijet" for multiple injections, Small Diesel Engines and the very recent Modular Injection technology, soon to be launched on the market.
Similarly, the Multiair technology, a Fiat worldwide premiere in 2009, will pave the way to a wave of further technological evolutions for gasoline engines:
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.
While electronic gasoline fuel injection developed in the 70’s and Common Rail developed in the 90’s were fuel specific breakthrough technologies, the Multiair Electronic Valve Control technology can be applied to all internal combustion engines whatever fuel they burn.
Multiair, initially developed for Spark Ignition engines burning light fuel ranging from gasoline to Natural Gas and hydrogen, has wide potential also for Diesel engine emissions reduction.
Intrinsic NOx reduction of up to 60% can be obtained by internal Exhaust Gas Recirculation (iEGR) realized with intake valves reopening during the exhaust stroke, while optimal valve control strategies during cold start and warm-up bring up to 40% HC and CO reduction of emissions. Further substantial reduction comes from the more efficient management and regeneration of the Diesel Particulate Filter and NOx Storage Catalyst, thanks to the highly dynamic air mass flow control during transient engine operation.
Diesel engine performance improvement is similar to that of the gasoline engine and is based on the same physical principles. Instead, fuel consumption benefits are limited to few percentage points because of the low pumping losses of Diesel engines, one of the reasons of their superior fuel economy.
In the future, powertrain technical evolution might benefit from a progressive unification of gasoline and Diesel engines architectures.
A Multiair engine cylinder head can be therefore conceived and developed, where both combustion systems can be fully optimized without compromises. Moreover the Multiair electro-hydraulic actuator is physically the same, with minor machining differences, while internal subcomponents are all carry over from the Fire and SGE applications.