Automakers have long sought cost-effective solutions for improving the breathing of gasoline engines in all operating conditions. Witness the many innovative electromechanical variable valve timing (VVT) systems in the marketplace, including Honda’s VTEC, BMW’s Valvetronic, Toyota’s VVTi, Nissan’s Variable Valve Event and Lift, as well as other phaser and cam-switching approaches.
Four decades ago Fiat pioneered a VVT system based on hydraulic actuation. Recently the company introduced its novel MultiAir system, a versatile but relatively inexpensive VVT unit that uses electrohydraulic technology to control the intake valves.
Installing MultiAir in naturally aspirated and turbocharged engines can boost power by 10%, low-end torque by 15%, and fuel efficiency by as much as 10%, according to Lucio Bernard, diesel engine director at Fiat Powertrain Technologies (FPT)—Research and Technology. FPT is located amid Fiat’s Central Research Laboratories in Orbassano, Italy, near Turin.
The fuel-economy gains can jump to 25%, Bernard continued, if a planned engine is downsized from, for example, a V6 to a turbocharged four, or a four-cylinder to a turbocharged twin-cylinder.
Other claimed benefits of MultiAir include substantial potential reductions in NOx (by 60%), carbon monoxide (CO), and hydrocarbons (HC) (by 40%). The reduced emissions result from optimized valve control during engine warm-up and internal exhaust gas recirculation (EGR), achieved by reopening the intake valves during the exhaust stroke.
Significantly, the straightforward, modular design helped minimize MultiAir's system cost. In volume production, a MultiAir unit costs on the order of 20 to 40 euros ($25 to $50). That is approximately 1% of the engine’s unit cost, according to Bernard. He said additional costs associated with using the MultiAir are reasonable, partly because the new technology eliminates one of the camshafts and other components.
Many industry observers believe that MultiAir, which is applicable to a range of engine sizes and types including hybrid-electric drivetrains, will strengthen the Italian automaker’s bottom line while helping its American subsidiary, Chrysler, retrieve lagging domestic market share. Fiat launched its MultiAir technology in Europe in late 2009 with the introduction of the Alfa Romeo MiTo—Italy’s answer to the Mini.
Fiat reported plans to install as many as 1 million MultiAir units on European engines annually during the next three years.
The Fiat 500, which will be built in Mexico for U.S. sale beginning in 2011, will feature the system on a 1.4L FIRE engine that is being built in Dundee, MI. Chrysler plans to fit the MultiAir system to its new Pentastar V6 family as well.
How MultiAir Works
The MultiAir system is elegantly simple. An electrohydraulic actuator—a high-response, electronically activated solenoid—controls the pressure applied to hydraulic fluid (engine oil drawn from the sump) that fills a thin passageway that connects the intake valves and the camshaft. The solenoid valve regulates the amount of oil that is pumped by the cam action either to the valve or a bypass reservoir.
When pressurized, the hydraulic line behaves like a solid body and transmits the lift schedule imparted by intake cam directly to the intake valve. When the solenoid is disengaged a spring takes over valve actuation duties.
This electrohydraulic link allows independent operation of the two components, which enables near real-time control over the valve lift profiles, said Bernard. Whereas a closed solenoid transmits the pressure generated by the camshaft’s intake profile to the valve in the normal fashion, an open solenoid breaks the hydraulic link between cam and valve, decoupling their operations.
FPT engineers have optimized the response of the electronic controls to make the system work smoothly on the microsecond time-scale. MultiAir can thus vary the timing of the valve openings constantly, based on pedal inputs and driving conditions.
With an essentially unlimited range of air-management strategies to implement, each cylinder can operate independently of the next, which enables an engine to react much more quickly to changes in driver inputs.
The electrohydraulic VVT mechanism delivers greater precision and versatility than the electromechanical systems, claimed Bernard.
“For example, MultiAir allows us to implement narrower cam profiles multiple times, which permits the system to flexibly manage the quality of the air charge and the turbulence it causes in the cylinder,” he told AEI.
MultiAir Running Modes
MultiAir can provide five principal operational modes, each tailored to a particular type of driving.
When the engine starts up and is running at idle, the valve is opened partially by closing the solenoid valve after the mechanical cam action has begun. As a result air rushes into the chamber, causing greater in-cylinder turbulence and thus better combustion.
To provide more low-rpm torque, the solenoid valve engages near the end of the cam profile, closing the intake valve early, halting undesirable backflow into the intake manifold and trapping the maximum amount of air in the cylinder, increasing volumetric efficiency.
Under partial load conditions, the solenoid opens early to produce partial valve lifts that optimize the volume of the trapped air mass to deliver the required torque.
In the multilift mode, which is intended mostly for saving fuel in stop-and-go urban driving, the system opens the intake valve twice during each intake stroke, enhancing air turbulence and boosting combustion rates at low loads. This strategy can also be used for cylinder deactivation.
The full-lift mode provides maximum power for highway speeds by keeping the solenoid valve closed, whereby the camshaft produces maximum (late) intake valve lift.
“Each of these five strategies,” Bernard said, “incorporate an essentially infinite number of continuously variable valve-lift profiles.”
Road to Electrohydraulic VVT
Such a neat package did not come about easily, however. “It took us about 15 years to develop MultiAir,” said Bernard, a physicist by education. In the mid-1990s, he recalled, “we knew that we needed variable valve technology because the throttle-based air control loses about 10% of the input energy, as the piston tries to pump air through a small orifice.”
Bernard said the engineers wanted to attain unparalleled levels of valve control to allow direct air charge metering at the inlet ports. The goal was to be able to precisely and flexibly manage in real time the quantity and turbulent behavior of the fresh air charge that enters the cylinder according to the driving mode.
Experience had shown existing electromechanical VVT systems provided insufficiently flexible air intake control, slow dynamic response, and no possibility of “cylinder-selective actions.” In addition, electromechanical technology could not be installed as a module nor would it work with diesel engines. In the meantime, the FPT specialists deemed electromagnetic actuation, a potential alternative technique, to be unreliable and power-hungry.
The Fiat team decided to base its VVT system on electrohydraulic actuation, which had formed the foundations of Fiat’s earlier, ground-breaking work on the now-ubiquitous common-rail fuel delivery technology for diesels (which the company had unfortunately failed to market due to financial problems).
“In our common-rail program,” Bernard recalled, “we had found that electrohydraulic technology was simple, robust, and inexpensive and requires minimal power to operate.” By 2001, the team had proved the feasibility of the MultiAir concept and has worked since to perfect it for commercial introduction.
“Second-generation MultiAir systems will add even more flexibility—more sophisticated valve-control modes and strategies, including new multivalve opening schedules to further cut fuel consumption and emissions,” Bernard said. Other targets are direct control of the intake air mass in turbocharged engines using fully integrated approaches that combine boost pressure and valve lift strategies. The system also will be more integrated with gasoline direct-injection systems to improve their transient response and fuel economy.
MultiAir also works with diesel engines. According to Bernard, it could help reduce intrinsic diesel NOx emissions by up to 30% using internal exhaust gas recirculation (iEGR) techniques. Optimized valve control strategies during cold starting and warm-up could cut HC and CO emissions levels by up to 40%.
“We think that MultiAir is as significant for spark-ignition engines as the common-rail was for diesel,” Bernard concluded.