Background
After developing in the 1980s a conical cams system with variable valve timings and lifts, the Fiat Research Centre (CRF) developed an electrohydraulic concept dubbed MultiAir. The outcome of lengthy development, this technology is today used in Fire 1400 cc engines with 16 aspirated and turbocharged valves. Forthcoming applications will concern the future 900 cc twin cylinder petrol engine and applications will also follow in diesel engines from 2011 on.
Operation
MultiAir technology consists in replacing the intake camshaft with an electrohydraulic system controlling the opening and closure of the intake valves whereby the time can be chosen when the valve is to open and close in terms of many parameters such as load, intake temperature and engine speed.
The system is applied exclusively to intake valves because the advantages of an exhaust valves application would not justify the additional cost.
Advantages
MultiAir technology offers many advantages:
- Load control without a throttle valve reduces pumping losses and therefore fuel consumption.
- Maximum power increased by up to 10% and low RPM torque improved by 15%.
- MultiAir turbocharged and downsized engines can achieve up to 25% fuel economy improvement over conventionally aspirated engines with the same level of performance.
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Optimum valve control strategies during engine warm-up and internal exhaust gas recirculation result in emissions reductions of up to 40% for HC and CO and up to 60% for Nox.
Lubricant impact
In a 4-cylinder, 2-litre engine, the system is now reliable up to more than 7000 RPM, 8000 RPM being possible. With a 5W-40 grade oil, an engine can be started at -30°C and the system remains operational up to oil temperatures of 150°C. Outside these extremes, too low a viscosity increases the speed at which valves touch their seat leading to knocking, whereas too high a viscosity delays closure excessively.
THE SOLENOID
Of the linear type, it is composed of a spool comprising a magnet in its centre. When the solenoid (C) is open the valve is disconnected from the camshaft and closes under the pressure of the valve spring (D).
THE VARIOUS INTAKE LAWS
To obtain maximum power, the solenoid is closed and the valves follow the movement of the cam as in a traditional engine.
This operating mode provides the greatest possible opening of the valves. For low speeds and low torques, the solenoid valve opens before the end of the maximum cam lift (A) which closes the valve in advance. Filling and torque are improved.
THE OIL IS VITALLY IMPORTANT
The camshaft drives a tappet (A) which activates a piston (B) which pushes the oil into the valves system composed of a piston with a return spring and an hydraulic brake.
This system uses a single overhead camshaft (SOHC) and can be used to replace a double overhead camshaft (DOHC).
