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 M20 engine, from May 1990 (more powerful 4-valve M50 engine.
- Models 530i/535i: 3.0L and 3.5L M30 engine.
- Model 524td: M21 turbo diesel engine with a displacement of 2.4 liters.
- Models 525td/525tds 2.5L M51 turbodiesel engine.
The gray cast iron engine block has cylinder bores. With significant wear or scratches on the cylinder walls, the cylinders can be honed, that is, ground in a special workshop.
After that, only oversized pistons should be installed. At the bottom of the engine block is a crankshaft mounted on its bearings. The connecting rods that communicate with the pistons are planted on crankshaft boulder plain bearings. The lower part of the engine is completed by the oil sump. which collects the oil needed to lubricate and cool the engine. The cylinder head is bolted to the top of the engine block. It is aluminum, since this metal, in comparison with cast iron, has a higher thermal conductivity and a lower specific gravity.
The cylinder head is built according to the so-called cross-flow principle. At the top of the cylinder head is the camshaft. The camshaft is driven on the M40, M20 and M21 engines from the crankshaft through a toothed belt, and on the M50 and M51 engines through a single-row roller chain. The camshaft on the M20 and M30 engines controls the V-shaped intake and exhaust valves. The 4-valve M50 engine has a second camshaft; each shaft drives either intake or exhaust valves directly with maintenance-free hydraulic tappets. The M51 diesel engine also has hydraulic pushers. On the M21 diesel engine and on the M40 4-cylinder engine, the camshaft controls the valves through the rocker arms, based on the side opposite the valve, on the ball (on the M40 engine self-compensating) fingers. On M20, M21 and M30 engines, regular checks and valve clearances are required as part of routine maintenance and after repair work.
Since September 1992, the 6-cylinder petrol engines have been further developed, among other things, they are equipped with a variable camshaft control system, abbreviated as VANOS. In this case, with the help of an adjusting element, depending on the engine speed, the intake camshaft is rotated by a certain angle relative to the chain, as a result of which the optimum ignition moment is maintained. The setting element is controlled by the engine control unit. Other improvements have been to ensure the smoothest possible operation of the engine, torque characteristics, exhaust gas composition and fuel consumption: an increase in the compression ratio in conjunction with selective knock 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 mounted on the 6-cylinder petrol engine and on the 2.4 liter diesel engine at the front of the engine crankcase and, depending on the type of engine, either directly from the crankshaft or via an intermediate shaft. On the 4-cylinder and diesel engine with a displacement of 2.5 liters, the oil pump is located in the timing case cover and the end of the crankshaft and is connected to the last gear train. Oil from the oil sump enters through drillings 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 through a V-belt, which also serves as a generator drive, and on the M40 engine through a camshaft toothed belt. It should be borne in mind that the cooling system must be filled with a mixture of antifreeze and anti-corrosion concentrate with lime-free water throughout the year.
To prepare a flammable air-fuel mixture, an electronic ignition and fuel injection device is used, which guarantees a stable exhaust gas composition.
In front of the cylinder head, instead of a conventional 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, that is, on these engines there are no moving parts in the ignition system. On diesel engines, fuel metering is also controlled electronically using the DDE system (Digital Diesel Engine Management System).