MultiAir engine tech from Fiat

New fuel charge management technology promises higher performance and efficiency for gasoline direct injection engines

Fiat Powertrain Technologies has been awarded the 2009 edition of the Technobest prize for its brand new Multiair Technology. Described in a nutshell as the ultimate air management strategy, the Multiair engine technology is the new electro-hydraulic system of engine valves for dynamic and direct control of air and combustion, cylinder by cylinder and stroke by stroke. In the last decade, the development of the common rail diesel engine technology has marked a breakthrough in the passenger car market. Attempts to apply a technology similar to common-rail on petrol started in the 80s, when engine electronic control technologies reached the stage of mature technologies. A renewed thrust on gasoline direct injection technology from most automakers including Fiat comes at a time when emission norms dictate near miraculous power and torque output from downsized engines by making maximum use of their efficiency and also by complementing their performance with the addition of turbochargers, etc. Interestingly, the key parameter to control diesel engine combustion and therefore performance, emissions and fuel consumption is the quantity and characteristics of the fuel injected into cylinders. That is the reason why the common rail electronic diesel fuel injection system was such a fundamental breakthrough in direct injection diesel engine technology.

The key parameter to control gasoline engine combustion, and therefore performance, emissions and fuel consumption, is the quantity and characteristics of the fresh air charge in the cylinders. In conventional gasoline engines the air mass trapped in the cylinders is controlled by keeping the intake valves opening constant and adjusting upstream pressure through a throttle valve. One of the drawbacks of this simple conventional mechanical control is that the engine wastes about 10 per cent of the input energy in pumping the air charge from a lower intake pressure to the atmospheric exhaust pressure. A fundamental breakthrough in air mass control, and therefore in gasoline engine technology, is based on direct air charge metering at the cylinder inlet ports by means of an advanced electronic actuation and control of the intake valves, while maintaining a constant natural upstream pressure.

At the beginning, worldwide research efforts were focused on the electromagnetic actuation concept, following which valve opening and closing is obtained by alternatively energising upper and lower magnets with an armature connected to the valve. This actuating principle had the intrinsic appeal of maximum flexibility and dynamic response in valve control, but despite a decade of significant development efforts the main drawbacks of the concept - it being intrinsically not fail-safe and its high energy absorption - could not be fully overcome. At this point, most automotive companies fell back on the development of the simpler, robust and well-known electromechanical concepts, based on the valve lift variation through dedicated mechanisms, usually combined with cam phasers to allow control of both valve lift and phase. The main limitation of these systems is low flexibility in valve opening schedules and a much lower dynamic response; for example all the cylinders of an engine bank are actuated simultaneously thereby excluding any cylinder selective actions. Many similar electromechanical valve control systems were then introduced over the past decade.

In the mid 90’s, engineers at Fiat concentrated on electro-hydraulic actuation, leveraging on the know-how gained during the common rail development. The goal was to reach the desired flexibility of valve opening schedule air mass control on a cylinder-by-cylinder and stroke-by-stroke basis. The electro-hydraulic variable valve actuation technology developed by Fiat was selected for its relative simplicity, low power requirements, intrinsic fail safe nature and low cost potential. The Multiair technology, applied to the intake valves, thus work on a principle where a piston moved by a mechanical intake cam lobe, 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 cam. When the solenoid valve is open, the hydraulic chamber and the intake valves are decoupled; the intake valves do not follow the intake cam 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 cam, which was 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 cam 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 cam 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.

Potential benefits of the Multiair technology include maximum power increase of up to 10 per cent with the adoption of a power-oriented mechanical cam profile. Low-rpm torque is improved by up to 15 per cent through early intake valve closing strategies that maximise the air mass trapped in the cylinders. Elimination of pumping losses brings a 10 per cent reduction of fuel consumption and CO2 emissions, both in naturally aspirated and turbocharged engines with the same displacement. Multiair turbocharged and downsized engines can in fact achieve up to 25 per cent fuel economy improvement over conventional naturally aspirated engines with the same level of performance. Furthermore, optimum valve control strategies during engine warm-up and internal exhaust gas recirculation, realised by reopening the intake valves during the exhaust stroke, result in emissions reduction ranging from 40per cent for HC/CO to 60 per cent for NOx While the constant upstream air pressure, atmospheric for naturally aspirated and higher for turbocharged engines, together with the extremely fast air mass control, cylinder-by-cylinder and stroke-by-stroke, result in a superior dynamic engine response, the first world-wide application of the Multiair technology will be the FIRE 1400cc 16V naturally aspirated and turbocharged engines.

The second application is a new naturally aspirated as well as turbocharged small gasoline engine (SGE - 900cc Twin-cylinder) where cylinder head design has been specifically optimised for the Multiair actuator integration. What's best is that one of the two variants of the two cylinder engine employing Multiair technology could be powering the small car Fiat is developing for India! So far, the Alfa Romeo MiTo has become the first car in the Fiat Group to get a 105hp and 130 hp, 1.4-litre naturally aspirated Multiair engine.