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.