Operation times diagram and setpoint values

Idle speed: 

At idle speed, the camshafts are adjusted in such a way that there is a small valve overlap or none at all. The low residual gas content leads to stable combustion and thus to stable idling. The smallest valve overlap is achieved with great to the greatest possible intake spread and the greatest possible exhaust spread. The VANOS solenoid valves are de-energised here. This camshaft position is also assumed on stopping the engine.

Power output: 

To achieve good power output at high engine speeds, the exhaust valves are opened late. This enables the expansion of combustion to move the piston for longer. The VANOS solenoid valve opens after TDC and closes late after BDC. This exploits the dynamic ram effects of the incoming air to increase performance.

Torque and power output: 

To achieve high torque, a high volumetric efficiency must be achieved. Depending on the phase position of the intake pipe pressure peaks and valleys, the intake or exhaust valves must be opened or closed earlier or later. Depending on the intake pipe length, an engine without VANOS has precisely one engine speed with optimized cylinder charge. An engine with VANOS lies within a broad engine speed range with optimized cylinder charge. Reason: Both ejection of the fresh gases back into the intake pipe and a flow of residual gas back into the cylinder can be avoided.

Increasing torque with turbocharging 

On the turbocharged engine, 'over-scavenging' - and thus significantly more torque - can be achieved at low engine speeds in the charged range with a positive scavenging divide by means of a large valve overlap. The effect: More air than is necessary for combustion flows through the engine. This means the compressor is not in the pump limit. Second effect: There is virtually no more residual gas in the cylinder.

Internal exhaust-gas recirculation with partial load 

In contrast to the torque-optimized and power-optimized VANOS position, high exhaust-gas recirculation can also be forced with adjustment of the intake and exhaust camshaft. Decisive for the amount of internal exhaust-gas recirculation is: The size of the valve overlap as well as the pressure difference between the exhaust manifold and intake pipe. Internal exhaust-gas recirculation has the following characteristics:

• Fast response times compared to external exhaust-gas recirculation (with internal exhaust-gas recirculation, there is no residual gas in the intake plenum)
• Fast exhaust-gas heat recirculation into the cylinder (with a cold engine, the additional heat improves the mixture preparation and leads to lower hydrocarbon emissions)
• Reduction in the peak temperatures of the combustion and thus a reduction in the nitrogen oxide emission

Fig 1: Intake/Exhaust Camshaft Spread Graph

Courtesy of BMW OF NORTH AMERICA, INC.

Observe the following setpoint values for the VANOS solenoid valve: