Bye bye butterfly

Offering fuel savings and reduced emissions, BMW’s Valvetronic engine has been described as an advance comparable to the changeover from the carburettor to fuel injection. So how was it designed?

The throttle butterfly, positioned inside the inlet manifold, has restricted the free ventilation of the internal combustion engine ever since its invention. The purpose of the throttle butterfly is to control engine output. Without it, the engine would run constantly at full throttle. However, whenever it’s not fully open, the throttle butterfly causes a loss in power and wastes petrol.

To illustrate, at light throttle, the throttle butterfly partially closes. The pistons, still running, take air from the partially closed intake manifold. The intake manifold between the throttle and the combustion chamber has a partial vacuum, resisting the sucking and pumping action of the pistons, wasting energy. The slower the engine runs, the more the throttle butterfly closes and the more energy is lost.

Theoder Melcher, BMW product manager for four cylinder engines, draws a comparison with the human body. ‘When required to make a great effort, human beings breathe in a deep and long process of ventilation,’ he says. ‘When we need less air, we don’t throttle the supply of air by closing our nose or mouth, but simply breathe in a shorter, flatter process of ventilation.’

Valvetronic is able to breathe in the same way as nature – always in line with current requirements, without any kind of throttling effect and therefore with maximum efficiency. Within the Valvetronic engine, BMW has replaced the conventional throttle butterfly function with infinitely variable intake valve lift.

Its most important feature is that it is able to save at least 10% fuel, with a corresponding reduction in exhaust emissions, regardless of fuel quality. Also, the efficient operation of Valvetronic doesn’t require any unusual types and grades of oil. Valvetronic is an important element in BMW’s aim of meeting the 2008 carbon dioxide fleet requirements of 140 gm/km.

The system is based on BMW’s double-VANOS (Variable Adjustment Of the Camshafts) with infinite camshaft adjustment to meet specific requirements. The additional, variable valve lift adjusts the effective cam action and, accordingly, the opening cross-section of the valves.

This is done by a lever positioned between the camshaft and the intake valves, its distance from the camshaft being adjusted infinitely by an additional eccentric shaft operated by an electric motor. Depending on the position of the Valvetronic control system, the lever converts the cam contour into a larger or smaller valve lift. Thus, Valvetronic has the ability to get deep, long ventilation (large valve lift) and flat, short ventilation (short valve lift), depending on the demands placed on the engine.

Based on signals formerly taken mechanically from the accelerator pedal, a stepper motor changes the phase of the eccentric cam, modifying the action of the intake valves.

Valvetronic was introduced on the all-alloy 1.8-litre, 4-cylinder engine for the E46 316ti Compact, and will subsequently be applied to most eight and 12-cylinder engines within a few years.

Valvetronic has its own computer housed in a separate unit away from the engine management system, networked with the digital engine management system incorporating a 40MHz, 32-bit computer.

The entire system is pre-assembled and inserted as a module into its position in the cylinder head. Valvetronic engines are built at BMW’s new engine plant at Hams Hall near Coventry in England.

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