Published on Apr 02, 2024
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.
MultiJet for multiple injections, small diesel engines, and the recent Modular Injection technology, soon to be
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
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
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.
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