There is a large amount of research and technology applied to valve control, especially on the intake side. For as long as 4-stroke internal combustion engines have been built with camshafts to operate intake and exhaust valves, engineers have known that the camshaft is always a compromise. No other component has as much influence on the power characteristics of a given engine as the camshaft.
The crucial events the camshaft controls are intake valve opening (IVO), intake valve closing (IVC), exhaust valve opening, (EVO) and exhaust valve closing (EVC). The timing of these four events determines the power and torque curve, idle quality and where the power is produced such as a low end torquey engine, an engine with good mid-rpm power or a high- rpm screamer. The compromise comes from the fact that changing the cam lobe profile is not possible once a conventional cam is ground. Having a cam profile with early intake valve closing will produce good low-rpm torque but this will limit higher rpm power and vice versa.
Fiat powertrain engineers realize this and have come up with a unique solution to the problem of a fixed cam lobe design. By utilizing modern, high speed computer and hydraulic technology they have built a high output 4-cylinder engine that incorporates fully variable intake valve lift and timing control through the use of hydraulics to control valve operation. The Fiat Multi-Air engine is a 4-cylinder engine with four valves per cylinder and a single overhead camshaft that operates the exhaust valves directly through inverted bucket tappets and a unique hydraulic actuator assembly that sits above the intake valves and controls their operation.
Multi-Air technology was patented in 2002 and introduced in Europe in 2009 on the Alpha Romeo MiTo. The first U.S. application was the 1.4-liter Multi-Air engine in the 2010 Fiat 500 along with a turbocharged version for the 500 Abarth. A 2.4 liter Tigershark engine with Multi-Air is available in the Dodge Dart and Jeep Cherokee. Current Multi-Air engines are port fuel injected and do not use EGR valves or cam phasing systems.
Multi-Air technology can be adapted to many different engine designs and allows Fiat the opportunity to license the technology to other manufacturers. Multi-Air benefits include increased power and torque, reduced fuel consumption, faster throttle response, reduced CO2 emissions and lower pumping losses. All of the benefits are due to instantaneous, fully variable intake valve lift and timing control. Intake valve actuation is no longer directly controlled by the intake cam lobe profile but rather by electrical control of the hydraulic actuator. Let’s take a closer look at exactly how the intake valve really works.
Inside The “Brick”
Sitting directly above the intake valves on the cylinder head is the Multi-Air actuator, often called the “brick” (Figure 1). A look at the camshaft reveals there are three lobes per cylinder, two identical exhaust lobes and a single intake lobe that operates a follower which moves a piston in and out of a bore in the Multi-Air actuator - this is the high pressure oil pump (Figure 2).
The stroking movement of this piston pressurizes engine oil that will be directed to a hydraulic piston, which will open the intake valve. A close look at the camshaft lobes reveals that the lobe lift appears much greater on the exhaust lobes than the single intake lobe. Service manual information lists the exhaust valve lobe lift at .295 ich or 7.5 mm and intake valve lobe lift at .145 inch or 3.81 mm. In actuality, due to rocker arm ratio and hydraulic system multiplication the actual intake valve lift is .370 inch or 9.3 mm. The Multi-Air actuator contains a high speed oil control solenoid and two hydraulic brake pumping elements per cylinder, each operating a single intake valve.
In Figure 3, the camshaft driven hydraulic pump can be seen on the right with a return spring attached, the oil control solenoid in red on the left and the hydraulic brake\pumping element in the center directly above the intake valve. The hydraulic brake\pumping element receives high pressure engine oil from the solenoid and pushes the valve open. This actuator has several functions, it limits the maximum travel of the valve, acts as a hydraulic lash adjuster and also as a brake as the valve closes to slow the valve down and prevent hammering of the valve seat and face.
Also located inside the Multi-Air assembly is an oil accumulator for each cylinder to absorb pressure pulsations when the solenoid valve is opened and to maintain pressure in the low pressure circuit. The oil control solenoid is the component that allows the Powertrain Control Module to control when the intake valve opens, how long it stays open or whether it opens at all. Keep in mind that this is a lost motion system, meaning that if the valve opening point is delayed or the closing point is advanced, some of the cam lobe lift is lost and the valve lift and duration will be less than the lobe profile.
The oil control solenoid is a normally open solenoid and in this condition high pressure oil provided by the camshaft driven pumping element will be vented to the accumulator chamber in the Multi-Air brick and no valve opening will occur. When the PCM energizes the solenoid the vent chamber is blocked and high pressure engine oil is directed to the hydraulic brake/pumping elements and the valve is opened. This means the PCM has to energize the Multi-Air solenoid to open the intake valves.
Any fault the prevents the circuit from working such as a failed PCM fuse or relay may cause a cranking no-start that mimics an engine with no compression, like a broken timing belt. This computer control of intake valve actuation allows for a number of unique Multi-Air control modes which are, Full Lift, EIVC (early intake valve closing), LIVO (late intake valve opening), Multi Lift (multiple valve opening events) and No Lift (no valve opening). Full Lift mode is used when maximum engine power is requested or when there is a stored engine fault such as a misfire code. In full lift mode the solenoid is commanded on before the intake cam lobe contacts the hydraulic pump roller follower and remains energized during the entire cam lobe duration providing maximum valve lift and duration. This mode is seldom in use as was seen during extensive test driving of Multi-Air equipped vehicles.
EIVC mode is used extensively while driving and can be used as a primary load control mode that allows for less throttle control by the throttle body. The PCM determines the required engine torque and will close the intake valve once the necessary air mass has been inducted into the cylinder, thus you can think of this as throttling via the intake valve. It can be seen while driving that during light engine load there is very little vacuum in the intake manifold thus lowering intake pumping losses and making the engine more efficient.
The two scan data captures (Figures 4 and 5) are from a Fiat 500 and show how as the engine is warming up it operates in LIVO mode and the intake pressure closely follows the throttle angle as in all throttled engines. After frame 220 as the engine accelerates, it changes to EIVC mode and the intake manifold pressure remains high at about 12.5 PSI (3.5 in.hg.) and no longer mirrors the throttle angle. Similar to how BMW targets a small vacuum level in the intake manifold on valvetronic equipped engines, the approximately 3.5 inches of mercury vacuum seen in Multi-Air engines appears to be a target level and allows for crankcase ventilation and charcoal canister purging. Due to this relatively high manifold pressure, or lack of intake vacuum, the engine is equipped with a camshaft driven vacuum pump to assist the vacuum brake booster.
The LIVO mode is used primarily at idle and low engine speeds. By delaying activation of the oil control solenoid some or most of the valve lift profile is lost depending on how late activation occurs. This allows for no valve overlap and very smooth idle and low speed operation. Any chance of charge dilution with exhaust gases from valve overlap are not possible and no EGR effect occurs with LIVO so it can only be used at idle or very light load.
Multi-Lift mode is a combination of EIVC and LIVO and can be used to increase valve open duration with small valve opening levels. Multi-Lift may allow for good charge motion that may prove helpful if GDI is adapted to these engines. No-Lift mode would allow complete cylinder de-activation and would be helpful on larger V6 or V8 engines. As of this writing Multi-Lift and No Lift modes are not in use on current engines but may be implemented in future applications.
Oil Control Solenoid
The oil control solenoid is precisely controlled and closely monitored by the PCM and produces a unique current signature. By scope testing the oil control solenoid a better understanding of system operation can be gained. It must be understood that regardless of when the solenoid is turned on or off the valve movement is still dependent on the intake cam profile stroking the oil pump and producing oil pressure that can be applied to the hydraulic brake/pumping actuator. You cannot confuse solenoid activation current with intake valve movement as sometimes the solenoid is activated ahead of when the cam actually moves the follower.
When scope testing the oil control solenoid, the solenoid on-time may be longer in engine degrees of rotation than the listed intake cam lobe duration but the cam lobe moving the pump is what causes valve movement. The oil control solenoid is a low resistance solenoid and uses a peak and hold current control strategy. The first waveform seen (Figure 6) was taken from a known good rental vehicle and shows a peak current of about 10 amps with a hold current at 5 amps. The solenoid is similar to a GDI injector in that both wires are controlled by the PCM with no shared connection between all 4 cylinders. The solenoid feed wire is biased at 8 volts and pulsed to 12 volts for current control while the ground wire is also biased at 8 volts then held to ground while the solenoid is energized.
Figure 6: Scope test with Multi-Air solenoid low voltage circuit on bottom, solenoid feed voltage circuit above that and solenoid current above that. Cylinder #1 ignition firing is the top pattern.
By adding rotation rulers to the waveform framing the cylinder ignition firing event you can see the current waveform hold section shows the solenoid was energized for 241 degrees of crank rotation (Figure 7). The engine was running in LIVO mode at the time. The turn-off event of the current waveform happens much sooner in EIVC mode and can be seen in the third waveform capture (Figure 8).
Due to close monitoring of the Multi-Air system, diagnosis of the system is highly DTC driven. There are extensive circuit codes for the solenoids that allow not only electrical fault identification but also hydraulic system problem diagnosis. There is a P1523 code for low oil pressure in the Multi-Air brick. By monitoring overall manifold vacuum, vacuum pulsations and RPM fluctuation during cranking the PCM can determine if the intake valves are operating.
The engine must be cranked for 10 seconds to set this code and no theft codes must be present as active SKIM codes may cause the PCM to disable the oil control solenoids yet the parameters for the P1523 code are still monitored. No MAP sensor codes can be present for this code test to run. To prove out the reliability of the self- diagnostics some experiments were performed. Using small diameter 18- gauge jumper wires to add some resistance to the solenoid circuit, the engine was started and the oil control solenoid current tested. Figure 9 shows the setup and the engine had a constant misfire with the jumper wires in place. The current waveform with jumper wires installed can be seen in the following scope capture (Figure 10).
Figure 9: Picture of the setup with jumper wires installed for the circuit resistance experiment.
The AC coupled MAP sensor voltage at the top of the waveform confirms there is no intake pull even though there is solenoid current present, but the current level is low. This experiment set 2 codes, a P1041-00 Implausible data from cylinder #1 oil supply solenoid valve received and P1061-00, Cylinder No. 1 oil supply solenoid valve stuck. The system can identify issues but the installed resistance was well below the threshold allowed using an ohmmeter to test the circuit as mentioned in the code chart so scope testing is the best way to identify problems.
There are only four serviceable items in the Multi-Air system, the entire Multi-Air “brick,” the roller followers, a screw in oil temperature sensor and the oil supply O-ring between the brick and the cylinder head. While the oil pumping elements and hydraulic brake/pumping elements can be removed from the brick they cannot be purchased separately at this time. Speaking of service, there are a couple of special tools required to perform timing belt service on the engine as there are no timing marks.
There is also a special spring compressor tool to collapse the pumping elements for easier Multi-Air brick removal and installation. The cam timing tools can be seen installed on the engine, the crankshaft locking tool is Miller No. 10276 (Figure 11) and the camshaft locking tool is Miller No. 10277. The spring compressor is Miller No. 10259B. The camshaft locking tool is affixed to the back of the cylinder head once the camshaft driven vacuum pump is removed (Figure 12).
Fiat Multi-Air technology is expected to spread to further applications and will be showing up in your service bays sooner or later. Getting familiar with this innovative technology will keep you ready to service and repair these powertrains when trouble crops up.