VANOS solenoid actuator, intake and exhaust

The variable camshaft timing control improves the torque in the lower and middle engine speed range. At the same time, the most favorable valve operation times for idle speed and maximum power output are adjusted. A greater valve overlap results in lower amounts of residual gas at idle speed. The exhaust-gas recirculation in the partial load range reduces the volume of nitrogen oxide.

The following is also achieved:

• faster heating of the catalytic converters
• lower pollutant emissions after a cold start
• reduction in the fuel consumption.
  Fig 1: Identifying Intake And Exhaust VANOS Solenoid Actuator Components

  Courtesy of BMW OF NORTH AMERICA, INC.

A VANOS solenoid actuator activates the VANOS unit. The required positions of the intake and exhaust camshafts are calculated from the engine speed and the load signal (depending on the intake air temperature and coolant temperature). The Digital Engine Electronics (DME) activates the VANOS unit via the VANOS solenoid actuator.

The VANOS central valve handles the mounting of the VANOS unit with the camshaft. At the same time, the oil flow rate into the VANOS unit is also controlled with this VANOS central valve.

A VANOS solenoid actuator moves the VANOS central valve. The piston thereby presses the VANOS solenoid actuators on the piston of the VANOS central valve.

Design of VANOS central valve 

Fig 2: Identifying Pistons, Holding Sleeve, Filter And Oil Duct

Courtesy of BMW OF NORTH AMERICA, INC.

Idle speed: 

At idle speed, the camshafts are adjusted in such a way that there is a slight valve overlap to optimize consumption and operational smoothness. 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. In this state, the exhaust camshaft adjusters lock so that with a subsequent engine start there is a stable camshaft adjustment. This stable camshaft adjustment is also achieved when the oil pump has not yet built up adequate oil pressure to adjust the camshaft. With the first adjustment request, the oil flowing in unlocks the exhaust camshaft adjusters again.

Power output: 

To achieve high torque at low engine speeds, the exhaust valves are opened late. This enables the expansion of combustion to move the piston for longer. At high engine speeds, the greater valve overlap (exhaust valve opening is advanced and exhaust valve opening is late) achieves height power output.

To achieve high torque, a high volumetric efficiency must be achieved. Depending on the intake pipe vacuum (charging pressure) and exhaust back pressure, the intake or exhaust valves must or opened or closed earlier or later. An engine with VANOS lies within a broad engine speed range with optimized cylinder charge. An engine with VANOS requires less charging pressure than an engine with a rigid camshaft position for the same filling (corresponds to torque).

Reason: Both ejection of the fresh gases back into the intake pipe and a flow of residual gas 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 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 twin-scroll exhaust turbocharger is not in the pumping range.

Second effect: There is virtually no residual gas present in the cylinder.

Internal exhaust-gas recirculation with partial load 

In contrast to the torque-optimized and power-optimized position of the intake and exhaust camshafts, high exhaust-gas recirculation can also be forced with adjustment of the intake and exhaust camshafts. 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 emission of hydrocarbons)
• Reduction in the temperatures of the combustion and thus a reduction in the nitrogen oxide emission.

The following graphic relates to engine N55:

Fig 3: Exhaust Camshaft Spread Graph

Courtesy of BMW OF NORTH AMERICA, INC.