Fiat’s Multi-Air is an electro-hydraulic system for controlling intake valve lift and, in a ‘virtual’ sense, camshaft profile. This control over the behaviour of the intake valves is used to gain a direct control over the air entering the engine, independently of the throttle valve. In a Multi-Air engine, the intake manifold pressure does not drop below atmospheric.
The system’s control over intake valve lift and timing is dynamic, and can be adjusted from one intake stroke to the next; each cylinder is controlled independently.
Independent valve actuation has been under development since the 1980s. Early research efforts were focused on electromagnetic control, in which the valve was opened and closed by alternately energising upper and lower magnets with an armature connected to the valve. This approach offered very flexible and responsive valve control, but it was not fail-safe and required a great deal of (electrical) energy to operate.
At this point, most manufacturers fell back on the development of the simpler, robust and well-known electromechanical systems for controlling the lift and timing of the valves. The main limitation of these systems is that they offer relatively limited flexibility in their control régimes. Also, because all of the intake valves are controlled together, the facility to control each cylinder independently is lost, sacrificing an extra degree of precision in controlling the combustion process.
In the mid-90s, the Fiat Group’s research efforts switched to electro-hydraulic actuation of the valves. The final design was selected for its relative simplicity, low power requirements, intrinsic fail-safe nature and its potential to be built at low cost.
Currently, all of Fiat’s Multi-Air power-units are indirectly fuelled, though the Company says that the system is easily applicable to all petrol engines. There is also the potential to develop the technology for diesels, though as diesel engines aren’t throttled, the usefulness of this application is limited to enhancing intake air speeds.
Gas-flow is fundamental to efficient combustion in any engine. Additionally, a petrol engine needs to regulate the quantity of intake air as well as its flow dynamics. Conventionally, the control of intake air mass is achieved using the throttle valve: at light loads, when when the throttle opening is very restricted, there is a substantial pumping loss.
The operating principle of the Multi-Air system is this. A piston, moved by an intake cam lobe, acts on the oil inside a hydraulic chamber. A separate hydraulic oil gallery leads to the chamber: this oil gallery is controlled by a solenoid relief valve that is normally open. In this condition, the cam-follower is completely decoupled from the intake valve.
When the solenoid valve is closed, the oil in the hydraulic chamber transmits the cam-follower’s movement to the intake valve directly. Here, the intake valve follows the profile of the cam.
If the relief valve to the hydraulic chamber is opened while the valve is open, the cam-follower and the intake valve are immediately de-coupled; the intake valve closes under the force of the valve-spring. 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.
So by controlling the solenoid relief valve that opens and closes the hydraulic chamber, coupling and decoupling the cam-follower and the valves, not only can valve-timing be controlled — within the outer limits defined by the physical cam-lobe — but a virtual cam profile can be created.
For maximum power, the solenoid valve remains shut. Because the valve is following the cam profile directly, the maximum opening and duration are achieved.
For low-end torque, the solenoid valve opens near the end of the cam profile, leading (obviously) to the intake valve closing early. This eliminates unwanted back-flow into the manifold and maximises the air mass trapped in the cylinders.
If the engine is at part load, the solenoid valve is opened earlier still, using 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 slightly after the mechanical cam action has started. In this case, the air stream into the cylinder is faster, producing higher gas speeds and turbulence in the cylinder.
The schemes described in the last two paragraphs can be combined in the same intake stroke, generating a so-called ‘Multi-lift’ mode. This enhances turbulence (and thus combustion) at very low loads.
Fiat claims that Multi-Air’s potential benefits for petrol engines include:
Maximum power is increased by up to 10 per cent., because the mechanical cam profile, if followed faithfully by the system, is biased towards power.
Low-end torque is improved by up to 15 per cent. using 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 atmospheric and turbocharged engines.
Multi-Air turbocharged and downsized engines can achieve up to 25 per cent. fuel economy improvement over conventional atmospheric engines with the same level of performance.
Valve control strategies during engine warm-up, along with internal exhaust gas recirculation effected by reopening the intake valves during the exhaust stroke, result in an emissions reduction ranging from 40 per cent. for HCs and CO to 60 per cent. for NOx.
Fiat also boasts that the lack of an intake manifold vacuum, together with fast air mass control, cylinder-by-cylinder and stroke-by-stroke, result in a superior dynamic engine response.
Applications of Multi-Air
The first power unit to be fitted with a Multi-Air head and control system was the four-valve FIRE 1.4l engine, which recently made its début in the Alfa Romeo Giulietta. Atmospheric and turbocharged forms are available. (The blown version won an Engine of the Year award in June.) The second application is the little Twin-Air vertical twin, which we have described elsewhere: this is an 875cc unit due to appear in the Fiat 500 towards the end of 2010. Again, atmospheric and turbocharged versions will be offered, including a turbocharged petrol-CNG bi-fuel derivative promising CO2 returns around 80g/km.
Fiat Multi-Air valve-control régimes.
Fiat anticipates a number of developments of Multi-Air in the relatively near future. These include:
Integration with directly-fuelled petrol engines.
Introduction of more advanced multiple valve-opening strategies.
New applications of the turbocharger to control trapped air mass through a combination of optimum boost pressure and valve-opening strategies.
A reduction of up to 60 per cent. in the NOx produced by a diesel engine is apparently possible by using internal exhaust gas recirculation. This is achieved, according to Fiat, by reopening the intake valves during the exhaust stroke. This clearly relies on the exhaust gas pressure overcoming the supercharged intake manifold pressure from the turbocharger.
Another potential benefit for diesels comes from a claimed improvement in the management and regeneration of the diesel particulate filter and NOx catalyst.
Potentially, Multi-Air’s highly dynamic air mass flow control can also make a significant contribution to improving emissions during transitional load phases. This means that during transitional (or transient) load conditions — a transition from low load to high load — the intake valve can be made to open late and sharply, increasing gas-flow appreciably to counter transitional load emissions spikes. These spikes affect all engines, though they are visible only with diesels, and are caused by a combination of gas-flow — actually the lesser issue — and the cooling of the combustion chamber walls when the engine is run at light loads. Hence transitional load emissions spikes can and do occur at relatively high crankshaft speeds, because cool combustion chambers cause premature quenching of the combustion process. Improving gas-flow alone cannot cure the problem, but it can help to minimise it, particularly at low crankshaft speeds.
The improvements in the performance of a diesel are apparently similar to those enjoyed by an Otto-cycle engine, though benefits to fuel consumption are limited to a few percentage points, because the diesel starts with low pumping losses.