Engine Lubrication: Description and Operation

Oil Supply

Oil is supplied according to requirements which means that only the amount of engine oil actually used is supplied. The following illustration shows the components for the oil supply on the N55 engine.







Brief component description
Descriptions of the following components of the oil supply are provided:
- Oil pressure sensor
- Volume-flow-controlled oil pump
- Oil pressure regulating valve
- Oil condition sensor
- Engine oil cooler.

Oil pressure sensor
The oil pressure sensor monitors the lubricating system. The oil pressure indicator light lights up if the oil pressure does not exceed a defined value. This value is approx. 0.2 to 0.5 bar. The oil pressure sensor is connected across a signal line with the of the Digital Engine Electronics (DME). If the switch is not operated, there is a voltage of 12 Volts on this signal line; if the switch is operated, there is a voltage of 0 Volts. The data interchange is digital.











Volumetric flow-controlled oil pump
The engine has a volume-flow-regulated oil pump. This volume-flow-controlled oil pump only delivers exactly as much oil as each operating range of the engine requires. Depending on the driving situation, for example, the following oil quantity is required:
- High engine speed: little oil
- High load: a lot of oil.

The volume-flow-controlled oil pump is an external gear pump that is driven via a drive chain by the crankshaft. On this volumetric-flow-controlled oil pump, the non-driven gear (oil pump drive gear) can be shifted axially, which varies the delivery volume. The operation of the volumetric-flow-controlled oil pump means that only the amount of oil required is delivered. This requirement-based control means that an average of up to 160 Watts less power is required than for a conventional oil pump. At approx. 6000 rpm, up to 1.25 kW of power is saved. The average fuel consumption is lowered by approximately 1%.







Oil pressure regulating valve
For the volume-flow-controlled oil pump, the oil pressure control valve has been integrated.
The following graphic shows the oil pressure control valve and the volume-flow-controlled oil pump on the N55 engine.







The characteristic map control influences the oil pressure in the control oil chamber. Two valves are involved in the characteristic map control:
- Oil pressure regulating valve
- Pressure limiting valve as fallback level.

The oil pressure control valve can influence the oil pressure in the control oil chamber steplessly. The further the oil pressure is reduced, the more volume the volume-flow-controlled oil pump delivers. The main spring in the volumetric-flow-controlled oil pump that affects the slider is softer than in conventional systems. The slider is easier to move to a centred position, which means that the volume-flow-controlled oil pump adjusts to minimum delivery even with slight oil pressure in the control oil chamber. This reduces the pressure ratio in the oil circuit, which in turn means less energy to drive the volumetric-flow-controlled oil pump.
The use of characteristic map control for the volumetric-flow-controlled oil pump enables oil supply in line with requirements and a reduction in the average pressure level in the oil circuit. The oil pressure control valve enables the Digital Engine Electronics (DME) to control the fuel delivery rate individually.
As a second stage of the characteristic map control, there is a pressure limiting valve located in the housing of the volumetric-flow-controlled oil pump between the oil pressure control valve and the control oil chamber. The pressure limiting valve is a 3/2-way valve that can switch the main oil pressure into the control oil chamber of the volumetric-flow-controlled oil pump. Oil from the main oil duct presses a piston back against a spring until the duct to the volumetric-flow-controlled oil pump is released. The oil pressure from the oil pressure control valve affects the other side of the piston.







The oil pressure control valve is closed when de-energised . This means the oil circuit is automatically in the fallback level in the event of failure of the oil pressure control valve and the pressure limiting valve ensures an oil pressure of approx. 5.5 bar. During characteristic map operation, the oil pressure lies between 1.5 and 4.5 bar.

Oil condition sensor
The oil condition sensor increases the function range of the thermal oil level sensor. The oil condition sensor measures the following variables:
- Engine oil temperature
- Oil level
- Dielectric conductivity.

The Digital Engine Electronics (DME) evaluate these measured variables. With the oil condition sensor, the electrical properties of the engine oil are also determined. The characteristics change with wear of the engine as well as changes to the engine oil (for example, ageing, deposits).
The oil condition sensor is attached to the oil sump and is accessible from below. There is no longer an oil dipstick. Electronic oil-level monitoring is now a feature of all engines.
The oil condition sensor consists of 2 cylindrical condensers. The condensers are mounted above one another. 2 metal tubes are inserted one into the other to serve as electrodes. The engine oil is used as a dielectric medium between the electrodes.







The temperature sensor is seated on the housing of the oil condition sensor. The housing of the oil condition sensor contains an electronic evaluation unit. The electronic evaluation unit has self-diagnosis. A fault in the oil condition sensor is entered in the fault memory of the Digital Engine Electronics (DME).
The electrical material properties of the engine oil change as the engine oil wears and ages. The changed electrical properties of the engine oil change the capacitance of the capacitor.
The oil level is measured for the electronic oil level check. The 2nd capacitor in the upper part of the oil condition sensor registers the oil level with the engine running. The capacitor is at the same level as the oil level in the oil sump. As the oil level changes, the capacitance of the capacitor changes. The evaluation electronics unit converts the measured capacity into a digital signal. The Digital Engine Electronics (DME) uses this to calculate the oil level. The electronic oil level check is displayed on the Central Information Display (CID) as well as in the instrument panel (KOMBI). On vehicles without a Central Information Display (CID), the oil level is displayed exclusively in the instrument panel (KOMBI).

Engine oil cooler
Engine oil cooling is implemented by means of an oil-to-water heat exchanger. In the variant for hot countries, an additional engine oil cooler is deployed.







System overview







System functions
The following system functions are described:
- Pressure circulation lubrication.

Pressure circulation lubrication
The engine is equipped with pressure circulation lubrication. The oil drawn in by the oil pump via an intake pipe from the oil pan flows through the full-flow oil filter. The oil then enters the main oil gallery, which usually runs in the engine block parallel to the crankshaft. Ducts lead to the main bearings of the crankshaft. The crankshaft and conrod are supplied with oil by the main bearings, as the crankshaft has corresponding bore holes. Part of the oil is branched off the main oil gallery and fed to the cylinder head at the corresponding positions.
We can assume no liability for printing errors or inaccuracies in this document and reserve the right to introduce technical modifications at any time.