How does Fiat's MultiAir vary from 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.How do other variable valve timing systems work?
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.How MultiAir works
When the electronic solenoid valve is closed, the chamber becomes a solid body and the valve follows the profile of the cam exactly. But if the electronic solenoid is opened to release some of the oil, the chamber shrinks to absorb the cam movement and the valve opens less. Choosing when to bleed the oil from the chamber, as well as by how much, makes it possible to control timing and lift as well as the duration of opening on an individual, valve-by-valve basis.
The mechanical inlet cam profile is 'hot', that is to say designed for high power at high rpm. For maximum power, the chambers remain full so the valves are opened to their maximum for longer. At slow speed, the solenoid is opened near the end of the cam's 'ramp' to close the valve early, maximising the amount of air trapped in the cylinder and improving torque. At part throttle, different strategies are used, partially opening the valves to trap just the right amount of air and speed up air flow.
Claimed results are impressive. Maximum power is up 10%, consumption and CO2 down by around 15% and downsized Multiair turbocharged engines can achieve 25 percent better fuel economy than larger naturally aspirated engines of the same power. And because the inlet valves can be opened at the same time as the exhaust valves to promote internal exhaust gas recirculation, emissions of HC and NOx drop by 40% and 60%.
MuliAir is much more than just another gizmo, then; it's an important step forward in engine design. Bravo, Fiat.>> This article originally appeared in the June 2009 issue of CAR Magazine
Fiat this year launches what it claims is a big step forward in variable valve timing: MultiAir. It's the first time that a manufacturer has made a properly variable valve system a production reality, and will slowly be rolled out across most of Fiat's and Alfa's car ranges.