Multi Air system full report
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1.INTRODUCTION

Fiat Group was one of the first manufacturers to adopt what has become the increasingly common practice of improving official fuel economy and CO2 emissions by creating a small forced-induction engine which uses fuel at a modest rate when the turbocharger isn't operating but produces similar power to a much larger unit when it is. In 2010, it has taken the idea a stage further by introducing various versions the 1.4-litre MultiAir petrol engine to the Punto Evo and Alfa Romeo MiTo ranges.
In the Geneva Auto Show to launch a new engine technology which could ultimately be as important as the common rail diesel technology it invented 15 years ago. Dubbed MultiAir, the hydraulically-actuated variable valve timing (VVT) technology was first announced as a concept two years ago, and offers a more controllable flow of air during the combustion cycle in comparison with mechanical VVT systems. Vastly reduced fuel consumption and emmissions plus significantly more power are claimed, and the technology is even more effective when used with a supercharger or a diesel engine.
Fiat claims Multiair is a fundamental breakthrough in petrol engine design that will dramatically cut fuel consumption, as well as significantly boosting power and torque, cutting carbon dioxide emissions by between 10 and 25 percent, and up to a 60 percent reduction in other engine pollutants.
This higher output will allow Fiat to replace larger engines with smaller, more efficient ones, and the company's 1.0 liter and 1.4 liter engines will be the first to get the new technology, along with a new 900cc twin cylinder engine.
Unlike the common rail diesel technology, which it sold to Bosch during a financial crisis, and has regreted ever since, FIAT will not be relinquishing ownership of the new Multiair system, having announced it will license it to other manufacturers or provide entire engines.







History
The Multiair technology took over a decade to get finished. The vice president of Fiat Powertrain Research & Development Rinaldo Rinolfi led the team who developed the technology. Development costs were over $100 million. There was also delay in development, in the time (2000-2005) when Fiat was in partnership with General Motors.


Other systems

Currently ready alternatives to industrialization do not exist, but there are under development also totally camless systems. The Valvetronic system used by BMW allows the valve timing and lift to be varied but not the cam profile. The ability to vary the latter is characteristic of camless and the Multiair systems.




Development of the Fiat MultiAir system

In the last decade, the development of Common Rail technology for diesel engines marked a breakthrough in the passenger car market. To be equally competitive in the field of petrol engines, Fiat Group decided to follow the same approach and focus on breakthrough technologies.
The aim was to provide customers with substantial benefits in terms of fuel economy and driving pleasure, while maintaining the engine’s intrinsic refinement, based on a smooth combustion process and on light structures and components.
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 the 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.
However, the key to controlling petrol engine combustion, and therefore performance, emissions and fuel consumption is the quantity and characteristics of the fresh air charge in the cylinders. In conventional petrol engines the air mass trapped in the cylinders is controlled by keeping the intake valve 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 per cent 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 petrol engine technology, is based on direct air charge metering at the cylinder inlet ports by means of advanced electronic actuation and control of the intake valves, while maintaining a constant natural upstream pressure.
Research on this key technology started in the ’80s, when engine electronic control reached the stage of a mature technology.



At the outset, world-wide research efforts were focused on the electromagnetic actuation concept, by which valve opening and closing is obtained by alternatively energising upper and lower magnets with an armature connected to the valve. This actuating principle 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 – it being intrinsically not fail-safe and its high energy absorption – could not be fully overcome.
At this point most automotive companies fell back on the development of the simpler, robust and well-known electromechanical concepts, based on valve lift variation through dedicated mechanisms, usually combined with camshaft phasers to allow control of both valve lift and phase.
The main limitation of these systems is low flexibility in valve opening schedules and a much lower dynamic response; for example, all the cylinders of an engine bank are actuated simultaneously, thereby excluding any cylinder selective actions. Many similar electromechanical valve control systems were subsequently introduced over the past decade.
In the mid ’90s, Fiat Group research efforts switched to electro-hydraulic actuation, leveraging on the know-how gained during its 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.






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 camshaft, 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 camshaft.
When the solenoid valve is open, the hydraulic chamber and the intake valves are de-coupled; the intake valves do not follow the intake camshaft 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 camshaft, which is specifically designed to maximise power at high engine speed (long opening time).
For low-rpm torque, the solenoid valve is opened near the end of the camshaft profile, leading to early intake valve closing. This eliminates unwanted backflow into the manifold and maximises 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 camshaft 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.



Further Potential of MultiAir Technology

All breakthrough technologies open a new world of further potential benefits, which are usually not fully exploited in the first generation.
Common Rail technology, a Fiat Group 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 recent Modular Injection technology, soon to be launched on the market.
Similarly, MultiAir technology will pave the way to further technological evolutions for petrol engines:
Integration of the MultiAir Direct air mass control with direct petrol 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 a combination of optimum boost pressure and valve opening strategies.
While electronic petrol injection developed in the ’70s and Common Rail developed in the ’90s were fuel-specific breakthrough technologies, 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 petrol to natural gas and hydrogen, also has wide potential for diesel engine emissions reduction.

Intrinsic NOx reduction of up to 60 per cent can be obtained by internal exhaust gas recirculation (iEGR) realised with intake valves reopening during the exhaust stroke, while optimal valve control strategies during cold start and warm-up bring up to 40 per cent HC and CO reduction of emissions. Further substantial reductions come 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 petrol 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 for their superior fuel economy.
In the future, powertrain technical evolution might benefit from a progressive unification of petrol and diesel engine designs.
A MultiAir engine cylinder head can therefore be conceived and developed, where both combustion systems can be fully optimised without compromise. The MultiAir electro-hydraulic actuator is physically the same, with minor machining differences, while internal sub-components are all carried over from Fiat’s FIRE and SGE applications.



Difference between MultiAir and existing variable valve timing (VVT) systems


Current VVT systems rely on mechanical systems to open and close the valves. Engineers have long understood the benefits of changing valve opening and closing times to tweak an engine's power and emissions performance, depending on the need for power or parsimony.

Valves are an engine's nose and mouth – it inhales through inlet valves and exhales through exhaust valves. Sounds simple enough, but actually engines are a lot like people. Depending on what they're doing, they need to breathe more or less air and the timing and rate of their breathing needs to vary. Like competitive swimmers who time their breathing to match the stroke, an engine wants to take long deep breaths when it's working hard and short shallow ones when it isn't.

Trouble is, it can't. The ancient method of opening and closing valve, the camshaft, is still in use today because it's simple to make, robust and very effective. Each valve is opened by a rotating cam on the camshaft whose shape and size controls how the valve opens and shuts and when it does so. The valve is closed by a simple spring because, in 100 years, no-one's found a better tool for the job. But what's right for developing high power at high rpm isn't right for that torquey, low-speed slog around town and greater variability of valve opening and closing helps reduce consumption and CO2 emissions too.

A lot of modern engines try to overcome the inadequacies of the traditional valvetrain with phasers to vary the timing of when valves open and shut. They may also have cam profile switching (like the Honda VTEC system), which switches to a hotter cam profile at higher revs. But the effect is limited. If the engine were a swimmer, it would still be gagging to get the right amount of air at exactly the right time, like when its face was under water.





The MultiAir system replaces the twin camshafts of a four-valves-per-cylinder engine. It's so cleverly designed, not only can it be incorporated in new engines, it fits exsiting motors too – so potentially all sorts of engines (not just Fiat's) could use it. The single camshaft opens up all four valves. Exhaust valves are not variable and are opened in the usual way by mechanical cam lobes. But between the inlet cam lobes and inlet valves are hydraulic chambers from which oil can be released by electronic solenoid valves.





MultiAir Technology benefits

Maximum power is increased by up to 10 per cent thanks to the adoption of a power-orientated mechanical camshaft profile. Low RPM torque is improved by up to 15 per cent through early intake valve closing strategies that maximise the air mass trapped in the cylinders. Elimination of pumping losses brings a 10 per cent reduction in 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 per cent 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, realised by reopening the intake valves during the exhaust stroke, result in emissions reductions ranging from 40 per cent for unburnt hydrocarbons and carbon monoxide (HC/CO), and up to a 60 per cent cut in oxides of nitrogen, (NOx). Constant upstream air pressure, atmospheric for naturally aspirated and higher for turbocharged engines, together with extremely fast air mass control, cylinder-by-cylinder and stroke-by-stroke,

result in a superior dynamic engine response, and enhanced driving pleasure. MultiAir is applicable to all internal combustion engines, regardless of the fuel used. It can be adapted for diesel engines to reduce their NOx emissions and make particulate filters significantly more effective.
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