The power unit is located longitudinally in the engine compartment and can only be pulled upwards using a suitable crane or lifting device.
6-cylinder gasoline engines are designated MZO or M52. The block head and cylinder block are made of aluminum alloy. This alloy provides better thermal conductivity and lighter weight than conventional cast iron. This improves cooling and fuel efficiency. Cylinder head with 4 valves per cylinder has an adjustable camshaft for intake valves (VANOS designation according to BMW specification).
The 725tds model is equipped with a 6-cylinder M51 turbodiesel with a displacement of 2.5 liters. The cylinder block is made of gray cast iron, the cylinder head is made of light alloy.
The cylinder bores are located in the cylinder block. In case of severe wear or scratches on the cylinder walls, the cylinders can be honed in the workshop or reground. After that, you will need to install oversized pistons.
The cylinder head on all engines is made according to the cross-flow principle. This means that fresh air/fuel mixture enters one side of the head and exhaust gases exit the other side of the head. By using this principle, faster gas exchange is ensured. At the top of the cylinder head is the camshaft/shafts. The diesel engine has only one camshaft. Gasoline engines with 4 valves per cylinder have 2 camshafts, one of which drives only the intake valves and the other the exhaust valves. Also, like a diesel engine, the camshafts are driven using a roller chain. The intake and exhaust valves of modern engines are equipped with maintenance-free hydraulic tappets. Valve clearances are not adjusted on these engines.
6-cylinder engines are equipped with a variable valve timing system on the camshaft, which is abbreviated as VANOS. During its operation, the intake camshaft, depending on the engine speed, is rotated by an actuator relative to the drive sprocket, which ensures optimal valve timing. The actuator is controlled by the engine control unit, which opens and closes the oil pressure valve.
The engine is lubricated by an oil pump, which, on 6-cylinder petrol engines, is located in the front of the oil sump and is driven by a chain from the crankshaft. On a diesel engine, the oil pump is located in the cover of the timing gear housing (timing) from the side of the crankshaft and is connected to it by a gear train. The engine oil in the sump flows through the holes and pipelines to the crankshaft and camshaft bearings and to the cylinder bores.
The water pump is located in front of the cylinder block and its shaft, at the appropriate temperature, is connected via a viscous coupling to the radiator fan. The water pump is driven through a V-ribbed belt, which also drives the generator. It should be borne in mind that the cooling system is filled with a special coolant all year round (coolant), which is an aqueous solution of antifreeze containing special anti-corrosion additives.
An electronic ignition and injection system is used to prepare the flammable petrol-air mixture, which ensures long-term low values of exhaust toxicity. The ignition system with separate ignition coils has no moving parts, so it works without wear. On a diesel engine, the fuel dosage is controlled by the DDE electronic device (diesel digital electronics).
Gasoline engines use 95 octane fuel, but other grades of gasoline can be used with cylinder knock control. When using gasoline with an octane rating of 98, fuel consumption during normal operation is reduced and power is increased. When using gasoline with an octane rating of 91, power drops and fuel consumption increases.
Main technical parameters of engines
M30 engine
1 - inlet valve; 2 - ignition distributor; 3 - camshaft; 4 - rocker; 5 - intake manifold; 6 - connecting rod; 7 - crankshaft; 8 - piston; 9 - oil level sensor; 10 - generator; 11 - camshaft drive chain.
M70 engine (twelve-cylinder engine)
M52 engine (four valves per cylinder)
M51 engine (725tds)
1 - camshaft; 2 - hydraulic pusher; 3 - valve; 4 - turbocharger (turbocharger); 5 - thermostat; 6 - water pump; 7 - oil pump; 8 - viscous coupling; 9 - V-ribbed belt; 10 - poly V-belt tensioner; 11 - timing chain; 12 - vacuum pump; 13 - intake manifold.