Depending on the model, the following power units are installed:
- Model 518i: 4-cylinder M40 engine with a displacement of 1.8 liters.
- Models 520i/525i (with a working volume of 2.0 l and 2.5 l): until April 1990 engine M20, from May 1990 (more powerful 4-valve engine M50.
- Models 530i/535i: M30 engine with a displacement of 3.0 liters and 3.5 liters.
- Model 524td: M21 turbodiesel engine with a displacement of 2.4 liters.
- Models 525td/525tds Turbodiesel engine M51 with a working volume of 2.5 liters.
The engine block made of grey cast iron has cylinder bores. If there is significant wear or scratches on the cylinder walls, the cylinders can be honed, i.e. ground, in a special workshop.
After this, only pistons of increased dimensions should be installed. In the lower part of the engine block, there is a crankshaft, mounted on its bearings. The connecting rods that connect with the pistons are mounted on the crankshaft plain bearings. The lower part of the engine is completed by an oil sump, in which the oil necessary for lubrication and cooling of the engine is collected. The cylinder head is bolted on top of the engine block. It is aluminum, since this metal has greater thermal conductivity and lower specific gravity than cast iron.
The cylinder head is constructed according to the so-called cross-flow principle. The camshaft is located in the upper part of the cylinder head. The camshaft is driven on M40, M20 and M21 engines from the crankshaft via a toothed belt, and on M50 and M51 engines via a single-row roller chain. The camshaft on M20 and M30 engines controls the operation of the V-shaped inlet and outlet valves. The 4-valve M50 engine has a second camshaft; each of the shafts controls either the intake or exhaust valves directly by maintenance-free hydraulic tappets. The M51 diesel engine also has hydraulic tappets. On the M21 diesel engine and on the 4-cylinder M40 engine, the camshaft controls the valves via rocker arms resting on the side opposite the valve on ball joints (on the M40 engine self-compensating) fingers. On M20, M21 and M30 engines, regular checking and adjustment of valve clearances is required as part of routine maintenance and after repair work.
Since September 1992, the 6-cylinder petrol engines have been further developed, including the variable camshaft control system, abbreviated as VANOS. In this case, the intake camshaft is rotated by a certain angle relative to the chain using an adjusting element, depending on the engine speed, thereby maintaining the optimum ignition timing. The adjusting element is controlled by the engine control unit. Other improvements have been made to ensure the smoothest possible engine operation, torque characteristics, exhaust gas composition and fuel consumption: an increase in the compression ratio in combination with selective detonation control, the installation of simple valve springs with low spring forces, lightweight pistons with extended connecting rods.
Engine lubrication is provided by an oil pump, which is mounted on the 6-cylinder petrol engine and on the diesel engine with a working volume of 2.4 liters in the front part of the engine crankcase and is driven, depending on the engine type, either directly from the crankshaft or via an intermediate shaft. On the 4-cylinder and on the diesel engine with a working volume of 2.5 liters, the oil pump is located in the cover of the timing case and the end of the crankshaft and is connected to the latter by a gear transmission. Oil from the oil sump gets through holes and channels to the bearings of the crankshaft and camshaft, as well as to the working surfaces of the cylinders.
The water pump is located at the front of the engine block; its shaft at the appropriate temperature is connected by a viscous coupling to the radiator fan. The water pump is driven by a V-belt, which also serves as the generator drive, and on the M40 engine by a toothed belt of the camshaft. It should be borne in mind that the cooling system should be filled with a mixture of antifreeze and anticorrosive concentrate with lime-free water throughout the year.
An electronic ignition and fuel injection device is used to prepare the flammable air-fuel mixture, which guarantees a stable composition of exhaust gases.
In the front part of the cylinder head, instead of the usual ignition distributor, a so-called high-voltage distributor is installed, which is driven directly from the camshaft. The ignition system of the M50 engine does not have a distributor at all, i.e. on these engines there are no moving parts in the ignition system. On diesel engines, fuel dosing is also controlled electronically using the DDE system (Digital Diesel Engine Management System).
[The original entry can be found on the portal «bmwman.ru»]