Fiat's Multiair Engine

Don Sherman
#Fiat, #Jeep

The truly cool thing about internal combustion engines is there's always another creative way to skin the power/efficiency/emissions cat. Fiat's Multiair technology is a prime example. Even though variable valve timing (VVT) was first used here thirty years ago (by Alfa Romeo), Fiat engineers invented a clever twist on the idea which is already in production for Europe and will surely be used future Chrysler engines.

The standard VVT approach is to use electronic controls to adjust the mechanical actuation between the engine's cam lobes and the valve stems. Altering both the timing and sometimes the lift of the valves can increase the power produced while diminishing both emissions and fuel consumption.

Multiair changes an important piece of the puzzle. In place of the adjustable mechanical link between cam and valve, Multiair imposes a small hydraulic connection to convey the forces that open the valve. An electronic solenoid valve plumbed into this hydraulic cam-to-valve connection quickly alters valve opening and lift in response to commands from the engine computer. So far Multiair works only on the intake valves.

What Multiair achieves that's not possible with alternative VVT approaches is cylinder-to-cylinder and cycle-to-cycle adjustability. It's so quick on its feet that two distinct intake valve opening events can be achieved in the same combustion cycle. That, according to Fiat, is handy for inducing turbulence (swirl and or tumble) in the combustion chamber, promoting quick flame travel and more complete burning of the fuel-air charge.

High-lift, long-duration intake cam lobes provide maximum power throughout the rpm range. Multiair essentially plays dead under these conditions to allow maximum intake valve lift and (open) duration. But when Multiair is programmed to lift the intake valve less than the full amount and to close it earlier than normal, torque is improved in the lower half of the rpm range.

During cruising, intake valve lift is lessened so that the throttle can be opened wide to greatly diminish pumping losses. With Multiair, it's possible to coordinate intake valve lift and timing throughout the entire rpm band and at all load settings to provide optimum performance.

Fiat claims a 10-percent gain in peak power, 15-percent more torque, and 10-percent better fuel economy. Combining Multiair with turbocharging and reduced displacement raises the efficiency benefit to 25-percent. There may also be cost benefits over conventional VVT since the hardware required here is quite modest in scope. One possible drawback is the congestion that results at the top of the cylinder head. This may be why Fiat has not yet combined Multiair with direct fuel injection.

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