BMW takes a fresh look at car fuel consumption

With emissions of carbon monoxide and other pollutants from car exhausts drastically reduced, attention is turning to improved fuel economy to cut carbon dioxide levels.

New EU regulations call for car makers to achieve an average of 140mg of CO2/km across their model ranges by 2008.

Most experts are betting on direct injection petrol engines to achieve this. BMW, though, has taken a different approach in its new Valvetronic engines, to be built at its Hams Hall plant and introduced in the 316ti in June.

Only 22% of the available energy in the fuel burnt in a petrol engine ends up driving the wheels. There are many small losses, but it has been known for some time that at least 10% is due to the throttle butterfly valve which regulates engine air flow.

Direct injection of the fuel into the cylinder is one way to remove the need for the butterfly. But BMW argues that petrol direct injection is insufficiently developed. In-line engine project manager Theodor Melcher says: ‘The theoretical efficiencies are not achieved. But emission performance is the biggest problem.’

Instead, BMW looked at ways of controlling the amount the inlet valves open.

In the Valvetronic, the camshaft does not operate directly on the valve rocker arms. An intermediate lever, one per valve, is interposed. A second eccentric shaft is added at the top of the engine. This can be rotated through small arcs by an electric motor.

As it rotates it adjusts the position of the levers relative to the camshaft and changes the amount the valves open. Valve lift is continuously variable between 0 and 9.7mm.

Advantages are smoother running, improved cold start, more direct response to throttle inputs, but most importantly a 10% improvement in fuel economy.

The system has been made possible by greater computer power and high precision manufacturing. It has its own 40MHz 32-bit computer networked to the separate engine management unit, giving a total capacity of 1.6MB. Neural networks were needed to control all other variables, such as camshaft timing and fuel injection. ‘Normal algorithms can’t cope,’ said Melcher.

The Valvetronic unit is produced as a self-contained module at Hams Hall. Valve lift at idle is 0.25mm. The profile of the cam levers is finished to an accuracy of 8microns. ‘These are levels of accuracy not even thought about a few years ago,’ said Melcher. The technology will progressively be added to BMW’s 8, 12 and six cylinder engines.

On the web