Driving engine efficiency

A collaboration between the Schaeffler Group and Fiat Powertrain has resulted in the development of what is claimed to be the world’s first fully variable hydraulic valve-control system for car engines.

The Uniair system was originally developed by engineers at the Centro Ricerche Fiat (CRF). In 2001 Fiat granted a licence to Schaeffler to continue to develop the technology and in 2003 the first functioning Schaeffler prototype car was available, with further prototype vehicles manufactured two years later. In 2004 the development of the Fiat Fire Multiair engine was started.

The final specification of the system was completed in 2007 and manufacturing facilities for series production started at various Schaeffler Group locations in the same year. Since 2009, the company has been working with Fiat and Alfa Romeo to develop specific bespoke versions of the technology for their new lines of cars.

In conventional cam-actuated valve systems, a mixture of fuel and air is forced into the cylinder of an engine through an intake valve. The valve then closes and the fuel-air mixture is compressed and ignited – after which the products of combustion exit through an exhaust valve.

The opening and closing of the valves in such engines is operated by a camshaft with a series of cams along its length, each designed to open a valve during the appropriate part of an intake or exhaust stroke. As such, it is impossible to individually control the opening and closing of the fuel-air intake valve of each cylinder, or to control how much they open or close during a combustion cycle.

The air charge that enters the cylinders, and hence the power output of the engine, is simply controlled through the use of the throttle. As such, it is impossible to optimise the performance of the engine under different driving conditions. The result is an inefficient engine.

That is where the Uniair system claims to have an advantage over the traditional approach. Instead of providing a mechanical linkage from the camshaft to the valves, the connection is performed hydraulically. This allows not only variations in the valve stroke, but also in the opening and closing of valves several times during one cycle – at different points in time.

’Specifically,’ said Michael Haas, director of advanced engineering and business development at Schaeffler Group Automotive’s Engine Systems unit in Germany, ’the variable hydraulic valve-control technology is based on the use of a high-pressure oil chamber that sits between the cam and the intake valve, and is managed by a solenoid valve connected to an electronic control unit.’

At one end of the hydraulic chamber is a piston pump that is driven by the camshaft of the engine. At the other is a brake piston that is mechanically coupled to the valve itself. The fluid volume in the high-pressure oil chamber is controlled by the solenoid valve.

If the high-pressure chamber is kept closed by the solenoid, when the pump piston creates a high pressure in the chamber, all the pressure will be transferred to the brake piston, producing a full valve lift. However, by using the solenoid valve to modify the volume of hydraulic fluid in the chamber, it is possible to modify the lift and timing of each of the valves in the engine.

’The Uniair system can provide full coverage of every conceivable airflow possibility, from zero lift to maximum lift, dictated by the camshaft lobe for each individual cylinder or valve. The system can provide early intake-valve closing or late intake-valve opening to maximise engine breathing relative to certain engine speed and load conditions. It offers unique valve-lift events for every valve – individually and cycle to cycle. In contrast, an electromechanically actuated, fully variable valve train requires several cam revolutions to achieve the same lift differential,’ said Haas.

At engine startup and idling, for example, late intake-valve opening (LIVO) can be deployed, while at medium to low RPM torques and engine partial load, early intake-valve closing (EIVC) can be used. LIVO has an influence on the opening and closing point of the valve lift. EIVC keeps the intake-valve opening point in a fixed position while moving the closing point.

The system is said to reduce carbon-dioxide emissions immensely. During the engine warm-up phase, for example, Fiat claims that hydrocarbon (HC) emissions are up to 40 per cent less and nitrogen oxide (NOx) is reduced by up to 60 per cent.

In developing the system, Haas explained that one of the most difficult engineering challenges was ensuring the reliability of the electromechanical solenoid valve. The Schaeffler design team had to ensure that it would work very precisely over the whole engine lifetime, which is over 330 million cycles.

Aside from the mechanical issues, the team had to develop bespoke valve-control software to drive the solenoid-valve actuation and, hence, the timing of the valves in the system. Once developed for an engine, the valve-control software module is then incorporated into the electronic control unit (ECU) of a car. But, Haas explained, since the characteristics of each car model may well be required to be unique, that software must be customised and calibrated on a model-by-model basis, as it will determine how the car performs on the road.

Aside from its benefits of reducing fuel consumption and lowering CO2 emissions, the system will also allow engineers to downsize the engines in vehicles while maintaining their performance. Fiat announced that its fully variable valve train will reduce fuel consumption and CO2 emissions – as well as downsizing – by up to 25 per cent.

For its part, Fiat claims that the system allows an engine to produce 15 per cent higher torque at lower engine speeds than engines using a traditional mechanical cam drive. That means that the engine can be downsized compared to the larger, existing one.

The system can also be used on diesel engines, but Haas said that some additional testing will need to be performed before the system will be deployed in such engines. Because the hydraulic system uses the existing oil in the engine, wear due to particulates in the oil becomes an important design parameter. ’We have to show that the durability in the diesel engine will be as good [as in a Port Fuel Injection petrol engine] because the higher soot content in the oil will undoubtedly adversely affect the wear of the components – particularly the solenoid valve. We also have to prove that the solenoid valve’s durability can withstand 330m cycles under these somewhat harsher conditions,’ he said.

Nevertheless, the Uniair system already made its debut in two petrol-powered vehicles at last September’s German Automotive Fair in Frankfurt – on Fiat’s Punto Evolution and Alfa Romeo’s Mito. Haas has also said that his Automotive Engine Systems unit is receiving a lot of serious interest for the system from automotive companies all over the world.