How MultiAir Works
by David Finlay (11 March 2010)
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.
To understand MultiAir you have to know how a conventional engine works. The rotary motion which leaves the engine and eventually reaches the wheels is the result of an initial up-and-down motion in which pistons (connected by conrods to a crankshaft) move up and down cylinders hidden within the engine block. In the universally-adopted four-stroke cycle, a downward movement of the piston draws air - and generally fuel, though in direct-injection engines this is a separate process - into the cylinder, then an upward movement compresses the fuel/air mix; that mix is ignited, and the energy released in burning it pushes the piston back down, and as it returns to the top again it pushes exhaust gas out of the cylinder.
Air is allowed into the cylinder, and exhaust allowed out of it, by means of valves which open and close at appropriate times. The valves are operated by eccentic lobes on a camshaft, and the design of those lobes determines both the extent to which the valves open and how long they stay open.
If this all sounds reasonably simple, be assured that a huge amount has been left out because it's not immediately relevant. One thing that is relevant, though, is that the valve timing - the behaviour of the valves as determined by the cam profiles - does not work equally well at all engine speeds. Honda attended to that twenty years ago by using lobes with two different characteristics on the same cam and developing a system which allowed the engine to swap from one to the other as required. Other methods of varying valve timing have been used in the intervening years.
Fiat's MultiAir system does that too, though with an unusually high level of control. The exhaust valves are operated normally, but there is no direct mechanical link between the camshaft and the inlet valves. Instead, a link is created by oil compressed so that it reaches pressures of around 120 pounds per square inch - enough to make it effectively solid, or at least solid enough to be able to push open the valves against the springs that normally hold them shut.
However, the oil is only allowed to do this if it is directed into the appropriate channel. If it isn't, it is instead sent elsewhere, and the valves do not open because there is nothing to open them with. The oil is either directed towards the valve or not according to whether a nearby solenoid is operated, and the decision is made by the engine management system.
So, the valves can either behave as directed by the profile on the cam lobes, or they can just sit there doing nothing. The cunning part of MultiAir is the control of the in-between states, in which the valves may be opened late, or shut early, or in one situation slightly opened twice during the same intake stroke. By these means the MultiAir engine effectively has different cam profiles for different situations, resulting in more power at low engine speeds and better turbulence of the fuel/air mix (leading to a more efficient burn and in turn to better economy and CO2 emissions) during gentle driving. The "real" cam profile is used only when maximum power is required, which in normal motoring isn't very often.
MultiAir has come to market long before it has reached the end of its development, so although it already works very well in the Punto Evo and the MiTo there is certainly more to come. Fiat engineers won't go into details, of course, but we do know that a direct petrol injection version of the engine is being worked on.