Fiat may not be well known in North America right now, but they do have a big presence in other parts of the world. Now, with their investment in Chrysler, we are going to see some of their technologies come to our shores, and one of the interesting ones is called MultiAir.
MultiAir is an innovative method of engine control that can regulate the air entering a cylinder with the engine valve opening to control everything from engine idle to exhaust emissions. Other companies have tried this concept in the past, but using different designs. One of the more successful is BMW’s Valvetronic system, which uses electric motors to move eccentric lobes that will vary valve lift. This is a complex system that works well, but Fiat’s MultiAir takes the concept further and does it in a simpler manner.
We’re seeing the MultiAir system first on the 1.4-litre engine in the Fiat 500. The technology is currently only applied to the intake valves, but the mechanisms could be incorporated on exhaust valves in the future to meet tighter fuel economy and emission standards. It starts with the camshaft – a long shaft with eccentric bumps or lobes on it that moves the valves.
A conventional camshaft must open the valves for a short time that will enable the engine to run smoothly at idle yet open long enough to allow adequate airflow at high engine speeds to develop maximum power. This is an impossible task, so a conventional camshaft is a compromise. Different methods have been used to overcome these limitations. For example, Honda’s VTEC uses camshafts with two different lobe shapes. One shape opens the valves at lower rpm while the other shape opens the valves more at higher rpm. The engine computer commands the switching between the two lobe designs.
On Fiat’s MultiAir, only one camshaft lobe design is used and it is a very aggressive design. By aggressive, I mean it would open the valve for a long time and lift it high to allow maximum airflow. This is great for high rpm power, but the engine wouldn’t idle unless something was done to modify the valve opening and lift. The MultiAir system does this.
When the camshaft lobe rotates, it moves a rocker arm or follower. This follower pushes on a small piston inside a cylinder. Trapped inside the cylinder is engine oil, which pushes on another small piston to open the valve. Oil can’t be compressed, so any movement of the cam lobe is transferred to the valve to open it. However, there is also an electric solenoid at each cylinder that can control the oil inside the MultiAir chamber.
When the cam lobe starts to move the follower and piston, the solenoid can allow oil to escape the chamber and the valve doesn’t open. When the solenoid blocks the release of oil, the valve then starts to open, but for a shorter time and lift. By operating the solenoid on and off rapidly during each intake valve opening cycle, the intake valve rate of opening and lift can be tailored to each driving condition. For example, the valve could lift rapidly initially but then slow its lift rate momentarily, only to lift faster again near the end of the cycle.
Ford has used a similar concept of solenoid control to regulate oil flow in the fuel injectors of its earlier PowerStroke diesels, but now the MultiAir design uses oil regulation to control the operation of each intake valve independently.
Better fuel economy, increased engine performance at any engine speed and lower emissions are all advantages of the MultiAir system. Fiat claims a 10 per cent reduction in CO2 emissions, 10 per cent engine power increase and a 15 per cent increase in torque, along with improved fuel economy. Now that’s performance!
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發表於 2011-3-1 11:59 AM
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