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How to repair BMW Valvetronic systems

Friday, September 1, 2017 - 06:00
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The Otto-cycle 4-stroke internal combustion engine has been around for well over a century and continues to be the main source of propulsion for the automotive and light truck industries. While the basic engine concept has served the needs of the industry so far, there is constant pressure to improve efficiency, emissions and performance of the engines that are used to power vehicles.  This article will discuss the refinements that BMW has engineered to the intake system of the gasoline internal combustion engine, or ICE for short.

The need
If you have studied basic engine design you are aware that a camshaft opens and closes the intake and exhaust valves and that a throttle is used to control engine load, or airflow, through the engine. A typical intake camshaft lobe will open the intake valve around 3/8 of an inch or .375” (9-10mm). This valve lift amount is actually only required for wide open throttle engine operation or full-load. During any other load situation the throttle controls airflow and the maximum valve lift is unnecessary. BMW has demonstrated that 80% of an engine load map can be accomplished with 3mm or less of valve lift.

Additionally, as a camshaft turns, its rotation is hindered by the force needed to compress the valve spring. This restriction creates a need for engine power to overcome this restriction. Engineers consider this a “mechanical” loss. Another problem when operating a throttle controlled engine in the low or partial load range is called “pumping” loss, which can be defined as the pressure difference above and below the piston on the intake stroke. The crankcase is under only a very slight vacuum controlled by the crankcase ventilation system, say 13.5 to 14 psi. When the intake valve opens at idle or low load, the top of the piston is exposed to intake manifold pressure which may be only 4-5 psi. The difference in pressure would want to move the piston up if it were not connected to the crankshaft. But in a running engine, the crankshaft will pull the piston down against this pressure differential and uses some of the engine’s power to do so.

Background and operation

BMW has developed a fully mechanical system using an electronically driven eccentric shaft and an intermediate lever to control intake valve lift and duration, thereby greatly reducing these losses during partial load conditions. This system is called Valvetronic and to date there are 3 generations of the system.

BMW first introduced Valvetronic in 2002 on the N62 V\8 engine and the N73 V\12 engine. In 2006 the newly redesigned in-line six cylinder dubbed the N52 was fitted with the second generation Valvetronic system. In 2010 the system saw an extensive re-design for the in-line six cylinder N55 engine and the new 4-cylinder N-20 engine, which is considered the third generation Valvetronic design.

Valvetronic has been adapted to 4-, 6-, 8- and 12-cylinder engines and can be found on most current production BMW cars and SUV’s. The system operates on the “lost motion” principle, meaning that there is a conventional intake camshaft but through the use of the added eccentric shaft and intermediate lever only a portion of the effective cam lobe profile may be used during any given intake stoke. If only a partial portion of the cam lobe is utilized, the intake valve lift and duration will be reduced. The intake valve will open later, close sooner and the valve lift will be less. This allows the engine airflow and hence load to be controlled at the intake port by the amount of valve lift rather than by the amount of throttle opening. BMW calls this throttle-free load control.

The Valvetronic servo-motors are circled and clearly visible in this picture of a 550i, N62 4.8 Liter V/8 engine.

The rotation of the eccentric shaft pushes the intermediate lever closer to the camshaft. This changes the pivot point of the intermediate lever that acts upon the roller follower which opens and closes the valve. The special tapered profile of the bottom of the intermediate lever allows the valve lift to vary between 0.3 millimeters and 9.85 millimeters. The reversible, high speed direct current servo-motor can vary from minimum lift to maximum lift in 300 milliseconds and operates at a frequency of 16khz. Because the cylinder charge airflow is being controlled by how far the valve lifts from its seat and not by the position of the throttle plate, valve lift differences between cylinders is critical and must be held to a very close tolerance to avoid a rough idle or misfire at idle.

One of the main problems with the first generation Valvetronic system was the contact point between the intermediate lever and eccentric shaft. This point is a sliding contact pad and was susceptible to wear if oil maintenance was neglected. This contact point is subjected to the pressure applied by the closing pressure of the valve spring pushing up on the roller follower and intermediate lever. As this contact point wears, the actual valve lift will decrease and cylinder-to-cylinder air intake quantities will vary and cause idle roughness.

Gen 2 intermediate lever with roller interface at top, against the retainer called a gate block.

Because of this wear issue there was a BMW scan tool function that would allow the technician to change the minimum lift setting of the Valvetronic system from .3mm to .8mm to see if the engine ran smoother. This change effectively made the engine a throttle controlled engine at idle. If the engine ran better, a thorough inspection of the Valvetronic system was necessary. In order to eliminate this problem, the intermediate lever was re-designed and a roller was added at this interface point. This is the primary difference between 1st and 2nd generation systems.

The other primary difference is a lowering of the minimum valve lift setting to .18mm or about 7 thousandths of an inch. With the engine running at minimum lift during idle operation the actual intake valve movement is almost imperceptible.

The Gen 2 valvetronic system, the intermediate lever roller is shown. The lever is seen in min and max lift positions.
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