BMW 1989-1995 525i Service Manual page 44

100-2
ENGINE-GENERAL
Cylinder Head and Valvetrain
On 6-cylinder engines. the cylinder head is an aluminum al-
loy casting that bolts to the cylinder block . V-8 engines use
two 4-cylinder aluminum alloy cylinder heads. The cylinder
head casting(s) employs a crossflow design for greater power
and efficiency. Intake air enters the combustion chamber from
one side while exhaust gasses exit from the other. Oilways in
the head provide lubrication for the camshaft(s) and val-
vetra in.
On M50 and M60 engines, chain -driven twin camshafts (in-
take and exhaust) operate four valves per cylinder through hy-
draulic lifters. See Fig. 1.
When the engine is running , the piston housing is supplied
with pressurized engine oil via the soleno id-actuated spool
valve . Depending on the position of the spool valve. oil is di-
rected to either the front or back side of the gear cup piston.
When the solenoid is in the off position. engine oil is direct-
ed to the back side of the piston . This holds the gear cup for-
ward and valve timing is maintained at the normal "late"
position. When the solenoid is energized, the spool valve is
moved forward and oil pressure is directed to the front side of
the piston . This in turn moves the gear cup further into the
camshaft secondary drive, causing the cams haft to "advance"
12.5 0
•
The helical gears are cut so that forward motion of the
gear cup is translated into rotational motion of the camshaft.
See 117 Camshaft Timing Chain for test ing and repair infor-
mation on the VANOS system .
811 000
Piston
housing
Solenoid
Camshaft
B1 1oo1
Fig . 1. M50 twin-cam. four-valve-per-cylinder engine.
VANOS (Variable Valve Timing)
1993 and later M50TU (Technical Update) engines are
equipped with a variable intake valve timing system, known as
VANOS (from the German words Variable Nockenwellen
Steurung). The VANOS system electro -hydraulically adjusts
intake valve timing for enhanced mid-range performance. The
VANOS system is controlled by the DME 3.3.1 Motronic con-
trol module , using engine speed , engine load and engine tem-
perature as the primary inputs.
At low speeds , the intake valves open late to ensure smooth
engine operation. At mid-range speeds . the valves open early
(valve timing advanced. VANOS actuated) for increased
torque , improved driveability, and reduced emissions. And at
high speeds , the valves again open late for optimum power
and performance.
The main components of the VANOS system are the piston
housing with integral spool valve and soleno id, and the modi-
fied intake camshaft and sprocket assembly. See Fig. 2.
GENERAL
Fig. 2. VANOS (variable intake valve timing) system used on 1993
and later 525i models. When solenoid is actuated. oil pres-
sure is directed to front side of gear cup piston. This forces
gear cup into camshaft to advance intake valve timing.
Lubrication System
The lubrication system is pressurized whenever the engine
is running. The oil pump draws oil through a strainer in the bot-
tom of the oil pan, then forces it through a replaceable oil filter
and into the engine oil passages. A pressure relief valve limits
the maximum system pressure . A bypass valve prevents the
oil filter from burst ing and insures engine lubrication should
the filter become plugged.
BMW engines are equipped with a dynamic oil pressure
warning system and an oil level warn ing system. See 119 Lu-
brication System for additional information.
Fuel Supply System
The fuel supply system stores fuel and provides the injec-
tion system with pressurized fuel. The fuel tank is installed un-
der the rear of the car. The fuel gauge sending unit and the
electric fuel pump are mounted in the tank. An in-line fuel filter
cleans the fuel before it reaches the engine. See 160 Fuel
Tank and Fuel Pump for additional information .