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 PuntoEvo and AlfaRomeoMiTo ranges.
In the Geneva Auto Show to launch a new engine technology which could ultimately be as important as the common rail diesel technology it invented 15 years ago. Dubbed MultiAir, the hydraulically-actuated variable valve timing (VVT) technology was first announced as a concept two years ago, and offers a more controllable flow of air during the combustion cycle in comparison with mechanical VVT systems. Vastly reduced fuel consumption and emmissions plus significantly more power are claimed, and the technology is even more effective when used with a supercharger or a diesel engine.
Fiat claims Multiair is a fundamental breakthrough in petrol engine design that will dramatically cut fuel consumption, as well as significantly boosting power and torque, cutting carbon dioxide emissions by between 10 and 25 percent, and up to a 60 percent reduction in other engine pollutants.
This higher output will allow Fiat to replace larger engines with smaller, more efficient ones, and the company’s 1.0 liter and 1.4 liter engines will be the first to get the new technology, along with a new 900cc twin cylinder engine.
Unlike the common rail diesel technology, which it sold to Bosch during a financial crisis, and has regreted ever since, FIAT will not be relinquishing ownership of the new Multiair system, having announced it will license it to other manufacturers or provide entire engines.
MultiAir Technology: how it works??
The operating principle of the system, applied to intake valves, is the following: a piston, moved by a mechanical intake camshaft, 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 camshaft.
When the solenoid valve is open, the hydraulic chamber and the intake valves are de-coupled; the intake valves do not follow the intake camshaft 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 camshaft, which is 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 camshaft 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 camshaft 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.