Engine Lubrication

Lubrication is the lifeblood of an engine. We often don't think about it, but the only way all those incredibly fast moving parts keep moving is because there is a constant supply of oil pumped and sprayed at them. As with everything in the M5, the lubrication system needs to be beefed up to keep up with the power output.

The M5 uses low viscosity oils 0W-30 or 0W-40. "Viscosity" is a measure of how "thick" a liquid is. Molasses is viscous, water is not. It is actually a measure of a liquid's resistance to being deformed when under stress.

In "0W-30" oil, the "30" part means that is the viscosity of the oil when tested at 100 C according to SAE (Society for Automobile Engineering) standards. Higher numbers indicate thicker oil. The "0W" means that at low starting temperatures, the oil viscosity is like that for oil that tests at viscosity 0 at high temperature (100 C).

At a minimum, the oil should be 5W-30 or 5W-40. The approved oil type is ACEA A3/B4 which is described by the European Automobile Manufacturers Association (ACEA) to be a "stable, stay-in-grade oil intended for use in high performance gasoline and direct injection diesel engines". Up to 1L of A3/B3 can be mixed in in a pinch if A3/B4 is not available. Castrol Edge with Syntec 0W-30 Synthetic Oil satisfies these conditions.

The purpose of the oil is to mainly lubricate but also cool fast moving engine parts and in certain cases to also act as a hydraulic fluid (for the variable valve timing VANOS in the case of the M5). Lubrication is specifically directed at the crankshaft, timing chains, camshafts, VANOS wheels (particularly oil hungry), valves, turbochargers, and piston crowns. Other parts of the engine, such as the Valvetronic worm gear, and the pistons are lubricated via carefully designed oil splash from the camshafts and crankshaft respectively.

The illustration below shows the oil pan and the oil cooler.

The oil collects in the oil pan at the bottom of the engine. A lot of the oil is pumped through holes in the engine block onto the crankshaft at the bearing and simply falls down into oil pan. It is cooled using an oil-air heat exchanger (3) located under the front bumper (see photo below) under control of a thermostat (1) that allows the oil to flow to the radiator through connection (2) only after the oil hits 100 C.

The oil pump is driven by a roller chain off the rear of the engine near the flywheel.

The oil pump is responsible for a considerable amount of leeching of engine power. Moreover, the VANOS system uses hydraulic oil pressure to advance and retard timing, and is quite oil hungry as a result (see my blog post Valvetrain for an explanation of how it works). However, VANOS only needs the oil when it is adjusting, and not in cruising situations, therefore the oil pump is of a new design that adapts to the oil requirements of the engine. There is also a second oil pump in the front of the oil pan that sucks fluid back to the sump when braking and cornering hard.

The oil pump is a volumetric flow-controlled sliding-vane positive displacement pump, and only supplies as much oil as is required by the engine at any given time.

The rotor (8) pivots around the pump shaft (2) on pivot pin (10) against the tension of the compression spring (3). When demand for oil pressure is high, this creates lack of oil and hence lack of pressure in the oil chamber (7) which causes the rotor (8) to drop off centre  When off-centre, the difference in volume between the chambers formed by the lower vanes and the upper ones force more oil to pump. As demand drops the rotor returns to a more central position and less oil is pumped.

The oil pump in the oil pan is shown below.

The intake pipe (9) is positioned deep in the rear area of the oil pan sump to ensure a good oil supply under even "dynamic driving conditions" (i.e., driving like a maniac). The oil return line (1,2,3) returns oil into the rotor pump (4) where it drains into the sump. Pressurized oil flows out through the outlet (6) into the oil filter, the radiator if above 100 C, and then onto the engine. When less oil is required by the engine, it accumulates in the sump.

The oil filter has a valve that bypasses the filter if the pressure differential on either side of the filter is more than 2.5 bar. This is for cold start situations when the oil is too viscous to flow properly through the filter.

There are spray nozzles placed in strategic locations near cylinder heads, chain drives, and bearings, to direct oil where it is needed.

In addition, there are passageways in the engine block where oil flows and is directed to where it needs to go (the below is not for the M5, but gives the general idea).