Fiat MultiAir in detail, incl BMW Valvetronic comparison...

[martinbo]

There has been fair mention of FIAT introducing a new valve actuation method over a two-year period, just about. Now, FIAT have released details of this new engine and its application in the facelifted Grande Punto, so I thought to provide the forum with a basic explanation.

If you’re familiar with petrol engine induction then you’ll know that the throttle butterfly valve is used to control the combustion process by restricting the amount of air being supplied to the engine inlet manifold. Hence the term throttle because, effectively, this “strangles” the inlet system of the maximum air flow when this valve is closed or partially open. As a result, at partial throttle openings, this restriction requires the engine to work harder in order to ingest the air needed for combustion and this in turn leads to a drop in efficiency. Clearly, a removal of the restriction caused by the throttle body would result in a more efficient, freer-breathing engine.


Throttle Butterfly at Idle – Throttle Butterfly at Partial Load – Throttle Butterfly at Full Load

The first manufacturer to apply the concept of throttle-less air intake in mass-produced engines was BMW with its innovative Valvetronic concept which is an electro-mechanical system. Now, FIAT has shown in detail its new MultiAir electro-hydraulic system which replaces the inlet camshaft with hydraulic fluid-actuated valves and is also designed to obviate the need for a throttle butterfly. Both systems do this by using valve-lift (the depth of the inlet valve opening into the combustion chamber) to control the volume of air ingested into the cylinder. The smaller the lift opening, the less air introduced and vice versa. In this way the engine gets precisely the right amount of air relative to what’s needed of it without having to work against the throttle effect.

The new 1400cc, 16-valve, inline 4 cylinder engine makes use of a single conventional camshaft to actuate the exhaust valves in the normal mechanical manner and to mechanically initiate the actuation of the inlet valves that are coupled with a set of electro-hydraulically controlled valves. These valves not only regulate the timing of the valve opening and closing but also the amount of valve-lift needed relative to the accelerator pedal position. The system is intended for use in both naturally aspirated and forced induction applications.

Before covering the functional aspects of the system, mention needs to be made as to why FIAT adopted this approach as opposed to the much speculated and theorised electro-magnetic control of valves which was deemed to be the next step in future valvetrain activation and timing management. According to FIAT, such electro-magnetic systems that control the movement of each valve had to be discarded after years of tests as a result of reliability problems and the amount of energy needed. FIAT says instead that, the electro-hydraulic "MultiAir” system is relatively simple, low cost, reliable and consumes little power.



As can be seen in the image above, the inlet camshaft – usually aside the visible exhaust cam – has been replaced by a series of spring-loaded pistons, levers and fluid valves. This is in turn connected to the traditional inlet valves for the combustion chamber as seen in the image below:



The camshaft actuates a piston. This is hydraulically connected to a fluid reservoir controlled by an electromagnetically regulated valve. This fluid is then connected to the intake valve of the cylinder. A solenoid controls the opening and closing of the valve.
When the electromagnetic valve is closed, the fluid from the reservoir acts as a solid, transmitting the movement of the camshaft thereby opening the inlet valve. When the electromagnetic valve is opened, there is no rigid connection between the inlet valve and the cam, so that the inlet valve is kept closed by spring force. The fundamental concept regarding the connection of the camshaft to the inlet valves is that the system defaults to full throttle position. So when maximum power is called for the electro-hydraulic valve is closed and the inlet valves follow the rotational profile of the camshaft as shown in the image below. This is thus optimised for high speed efficiency and performance.



Now the system can be near-infinitely applied to control the opening lift, timing and duration under various throttle loads:
At low speed and full engine load, the electromagnetic valve opens to anticipate closing the intake valve, eliminating flow-back of unwanted combustion gases and maximising induction air mass. At partial engine load the electromagnetic valve is opened in advance, partially opening the valves in order to control the air mass introduced by the required torque. As can be seen from the following diagram, the precise control of intake valve lift, timing and duration is greatly enhanced by micro-processor control of the inlet valve’s actuation.



~~~

In contrast, BMW’s Valvetronic system uses a more conventional approach whereby use is still made of a normal inlet camshaft. Instead, the BMW system uses the rotational output of an electric motor to control the extent of the valve lift. The motor drives a gear which is in turn connected to a set of levers or rockers that extend and retract, thereby adjusting the extent of the valve lift as seen in the diagrams below:




The extent of the valve lift may vary from 0.0 to 9.7 mm, depending on the amount of air needed for combustion. Interestingly, a conventional throttle butterfly is still fitted as a redundant system, in the fully open position, for in the event of engine malfunction or for diagnostics purposes.



This picture clearly shows the location of the valve lift componentry relative to the inlet and exhaust camshafts. It’s also a nice illustration as to why, initially, Direct Injection wasn’t available with Valvetronic due to cylinder head space constraints. This has now been worked around by using a more compact, space-efficient version of Valvetronic that allows for the central location of the injector units.

[dr Dunkel]

Hrmm... the first thing that spring to mind is "what will that look like in 50,000 kms"...

[Giannis]

Fantastic post as always Martin. There is noone like you when it comes to engine technology topics!

Thank you!

[Wunderkind]

Originally Posted by dr Dunkel

Hrmm... the first thing that spring to mind is "what will that look like in 50,000 kms"...

That was always the problem. Electrification of the valve train has its advantages and drawbacks. The main advantages (compared to hydraulic wedge pin systems) are lighter componentry, less energy consumption, and quicker adjustments/movements and the scope to vary each cylinder individually (this can also be accomplished by individual spark control in each cylinder). But the drawback may well be medium-long term reliability.

[martinbo]

^ It's interesting that your refer to one of the advantages of electromagnetic valvetrains as having less energy consumption. This is true when we consider the energy losses that arise from the traditional, mechanical valve train being driven off the crankshaft. Certainly this does draw energy from the engines total power output.

In theory, an electromagnetic system would negate this mechanical loss of efficiency, leaving the engine unburdened by having to turn the camshafts and push valves against spring forces.

Unfortunately, the power needed to drive such electromagnetic systems is high, necessitating upwards of 36 Volts if I recall correctly and so the energy requirement and subsequent loss of effiency across the entire system renders the use of electromagnetic valve trains unfeasible for now.

Which, is a shame really because, in theory, electromagnetic valve trains could dramatically increase the combustion efficiency of spark ignition engines by providing micro-precise control of valve timing and opening with amazing speed.

[dr Dunkel]

Would this be fixed by the electromagnets being smaller (more efficient)?

[Wunderkind]

Originally Posted by dr Dunkel

Would this be fixed by the electromagnets being smaller (more efficient)?

smaller size will no doubt help. but the electromagnets will still need to generate the required force to operate the valves and their timing.

so it will be a question of using the smallest magnet modules that could generate the necessary force.

[dr Dunkel]

Originally Posted by Wunderkind

smaller size will no doubt help. but the electromagnets will still need to generate the required force to operate the valves and their timing.

so it will be a question of using the smallest magnet modules that could generate the necessary force.

I was thinking that a lighter train would require a smaller magnet -> less power.

I did not think that through, of course one would need more power to make the electromagnets smaller. Silly me.