Reproduced from the BMW TIS.
BMW has developed a new six-cylinder, four valve engine which will be found in the 525i model. Known internally as the M50 engine, it uses the same technological concept as the four-cylinder, four valve engine (M42) found in the 318i and 318is models.
The main development objectives were to maximize power output and torque and to maintain high quality standards and long life, with even easier maintenance and servicing. And, above all, to combine these objectives with the silky smooth running of an inline six-cylinder engine.
Continuing the tradition of the other BMW multi-valve engines, the new M50 provides a high specific power output - 141 KW/188 HP @ 5900 rpm, a generous torque curve - 245Nm @ 4700 rpm (= maximum torque) and low emission and fuel consumption levels.
Additional advanced design features include: direct solid state ignition with separate coils mounted above spark plugs, a new generation of Digital Motor Electronics - DME 3.1, maintenance-free ribbed belt drive to the auxiliaries, self-adjusting hydraulic valve tappets, vertical oil filter configuration, a plastic intake manifold assembly and asbestos-free gaskets and seals.
Despite its use of four valves per cylinder and its higher performance, the M50 weighs only slightly more than the M20.
The installed angle of the M50 is 30° versus 20° for the M20.Four Valve Technology
Using four valves per cylinder has various advantages:
- Less energy is lost during gas exchange, so that power output and torque are higher at all engine speeds. This, in effect, raises the maximum torque value from 222Nm/168 lb. ft. of the M20 to 245Nm/181 lb. ft. for the M50, while also ensuring higher torque values throughout the entire speed range.
- Ideal spark plug position. Flame paths inside the combustion chamber are all equally short, to raise the knock limit. A higher compression ratio can be utilized to increase thermal efficiency and make better use of the energy potential in the fuel.
- Reduced moving mass at each valve. Lower (closing) force valve springs can be used to reduce frictional losses while still ensuring that the valves follow the cam contours even at high speeds.
The grey cast iron crankcase possesses exceptional torsional rigidity, yet is approximately 5 percent lighter than the M20 crankcase. The cylinder bores are siamesed (cast together in pairs) using the same design principles as on the M20 engine.
The crankshaft is a spheroidal graphite casting supported by seven main bearings with 12 counterweights. Precise design measures have reduced the crankshaft weight of the M50 by approximately 3 kg (6.5 lbs) compared to the M20.
The piston crowns incorporate a symmetrical combustion bowl and are notched for valve head clearance. Oil jets mounted in the main bearing oil supply spray oil into the underside of the piston crown for piston cooling.
The forged connecting rod has been redesigned to reduce weight by 10 grams compared to the M20 which promotes a smooth running engine and ensures reliable operation even at higher speeds.
The most important design feature is the crossflow cylinder head with four valves per cylinder, double overhead camshafts (DOHC) and bucket-type tappets with hydraulic lash adjusters.
Compares with the M20, the intake and exhaust port areas, exposed when the valves are open, have gone up by 40 percent. Mixture flow through the intake valves is 30 percent higher. Cylinder filling is therefore more complete and the charge-exhaust cycle more efficient.
The combustion chambers are ideally shaped for short flame paths and uniform flame propagation. This makes for efficient combustion and a high level of thermal efficiency, and therefore high specific power output with good fuel economy and low exhaust emissions.
The intake and exhaust valves are arranged in a V pattern, and use dual valve springs to ensure rapid, bounce-free closure.
The self-adjusting hydraulic tappets simplify maintenance, eliminating routine valve adjustments. Fluctuations in performance or exhaust emission levels due to valve clearances changing are a thing of the past. Valve train noise and thermal load on the valves are also reduced due to permanent contact between the camshafts and tappets.
The double overhead camshafts are hollow castings (M20: solid castings) and rotate in seven bearing journals. The camshaft housings are separate components which permit removal of the camshaft and self-adjusting hydraulic tappets without cylinder head removal.
In view of the higher loads imposed by the new valve train, the camshafts are driven by single roller chains which are lubricated and cooled by oil spray jets.
The main drive (primary chain) connects the crankshaft to the exhaust camshaft, with a guide rail on the tension run of the chain and a hydraulically dampened tensioning rail.
The secondary chain runs from the exhaust to the intake camshaft and also has a guide rail and a hydraulically dampened tensioning rail.
The cylinder head cover has been reduced in weight. New rubber elements and components limit noise radiation.
The ignition leads for the direct solid-state distribution system with its six individual coils are integrated into the cylinder head cover.
An oil trap is also integrated into the cylinder head cover. A labyrinth formed by cast webs is sealed at the bottom by a sheet-metal cover with moulded rubber seal. Return openings in the webs allow the trapped oil to flow back into the cylinder head. The almost oil-free fumes that remain are conducted to the intake manifold for combustion, in order to comply with pollutant emissions legislation.
An oil shield on the intake camshaft reduces the oil splash accumulation in the crankcase breather.
The single-piece plastic intake manifold with its optimized intake pipe lengths and cross-sections provides a torque curve which ensures immediate response and good flexibility in all engine-speed ranges.
With a weight of approx. 3 kg (6.5 lbs), the plastic air intake system is about 50% lighter than a comparable aluminium system. Its very smooth inside walls make additional treatment unnecessary. The resulting drop in flow resistance ensures optimum operating efficiency.
The air collector is extremely rigid and therefore emits less noise.
The oil pump is a "Duocentric" also known as an Eaton Rotary type of tandem design (similar to M70 oil pump).
The pump is bolted to the engine block in the oil sump, and driven directly from the crankshaft by a single roller chain with a tensioner.
The single-piece pressure-cast aluminium oil pan incorporates an oil deflector and a baffle plate which maintains the oil supply reliably in extreme conditions and prevents engine foaming.
A single-piece gearbox support is integrated into the oil sump so that a separate stiffener is not needed. This rigid link between engine and gearbox reduces vibration loads on the entire driveline, and therefore keeps noise transmission low.
The oil filter is mounted vertically on the intake side of the engine. The paper filter element is accessed from above when replacing it.Lubrication oil routing - engine cross section
Oil supply to the valves and crankshaft, with spray cooling for underside of pistons
The cooling system incorporates a new style water pump which is integrated into the timing case cover (similar to M70). The pump features a ceramic axial seal, a plastic impeller wheel, and an aluminium pump housing which promotes a longer working life, reduced weight, and high efficiency.
The thermostat housing located at the front of the cylinder head contains a thermostat that is designed to open at 88°C (190°F). Coolant flows from the front of the engine block to the rear. At the rear, the coolant rises through connecting passages into the cylinder head and flows from the rear back to the front.
All accessory component rotary drive is accomplished with two poly-ribbed V-type drive belts. Once correctly installed the belts are self-adjusting with automatic hydraulic tensioners and thus are maintenance free.
Drive belt #1 (main V-belt):
Crankshaft to water pump (fan), alternator and power steering pump.
Drive belt #2 (secondary V-belt):
Crankshaft to air conditioning compressor.
1 BELT 1
2 BELT 2
The M50 is the first engine to use the latest Digital Motor Electronics management system designated DME 3.1. The control unit size has been reduced to occupy 40% less space than previous DME 1.3 control units.
The M50 control unit contains two computers instead of one:
- an 8 bit computer (for fuel injection only); and
- a 16 bit computer (main computer).
DME 3.1 can process more information considerably faster in the same period of time as DME 1.3.
The extended functions of DME 3.1 control unit, i.e., six individual output stages for the fuel injection and six for the ignition system, calls for a control unit with 88 pins.DME 3.1 New Features
|DME 3.1||DME 1.1-1.3|
|Ignition||Direct, solid-state||Rotary distributor|
|Cylinder identification||Camshaft transmitter||Sensor on ignition lead 6 for semi-sequential fuel injection|
|Load sensing||Hot wire air mass metering||Volumetricairflow metering|
|Type of fuel injection||Fully sequential||Parallel or semisequential|
|Throttleposition sensing||Throttle butterfly potentiometer, adaptive idle setting; part- and full load by way of resistance value||Throttle butterfly switch, idle and fully load by mechanical switch contact|
|Starting||Advance fuel pump run and injection; double spark (max. 250 ms); fully sequential injection||Parallel injection|
|Full load ignition timing||Altitude compensation; dependent on engine speed and cylinder charge||Dependent on engine speed|
|Idling control||Adaptive value without/with air conditoning||Single adaptive value|
|Model application identification||By mapping characteristic||By variant coding|
|Memory board||40 kbyte||32 kbyte|
|Electromagnetic compatibility||4x multilayer technique||Additional supression|
In comparison to DME 1.3 the number of items in the diagnostic system have been greatly increased. Up to 30 malfunctions can be memorized, with an automatic scale of priorities according to the importance of the fault. Workshop personnel can obtain a readout of the stored data with the Service Tester or MODIC.
All data values needed to ensure correct and reliable engine operation are accompanied by substitute values, so that in the event a fault should develop, i.e., a missing input to the control unit, an emergency run program allows the vehicle to be driven to the nearest BMW Dealership for repairs.
Fully sequential injection is used for the first time on the M50 engine. All six injectors are controlled separately through six individual output stages in the Motronic Control Unit in contrast to semi-sequential fuel injection (M42, M30, M70) which meters fuel to pairs or groups of cylinders only.
Fully-sequential fuel injection means that conditions at every cylinder are ideal for the formation of precisely controlled and correct fuel-air mixture.
- Injection timing is varied to suit the engine's momentary operating condition.
- The length of the injection is matched to the needs of each individual cylinder.
- The injectors are included in the diagnosis system.
- Fuel is delivered to all six cylinders at once during cold starts.
- Primary ignition circuit monitoring.
The M50 utilizes a new version of fuel injector nozzle with a double-spray pattern instead of a single-spray pattern. The new fuel metering principle also optimizes injector sealing when they are de-energized.
An inductive transmitter on the camshaft drive gear identifies the fuel injection and ignition sequence. This ensures the correct injection order immediately after the engine is started (cylinder 1 is identified).
1. DME M3.1 control unit
2. 3-way catalytic converter
4. Ignition switch
5. Main relay
6. Fuel pump relay
7. Lambda probe
8. Idle speed adjuster
10. Hot-wire air mass meter
11. Intake air temperature sensor (intake system)
12. Fuel injection rail
14. Camshaft position sensor
15. Spark plug with triangular ground electrode
16. Coolant temperature sensor
17. Crankshaft positions sensor
18. Lambda probe
19. Individual coils
20. Fuel tank vent valve
21. Activated carbon fuel vent filter
22. Fuel pump
23. Fuel catch tank
24. Fuel filter
The solid state ignition system of the M50 is controlled through six individual output stages in the DME 3.1 control unit.
Each cylinder has a separate ignition coil rated at 30KV which is bolted to the cylinder head directly above the spark plug. The new direct ignition system eliminates the following causes of engine misfiring:
- mechanical wear (carbon brushes) as on a rotary ignition distributor system;
- leakage voltage (arcing) as a result of moisture, road dirt or salt;
- risk of damage to ignition wires by rodents
thus increasing overall ignition system reliability.
Spark plugs with the triangular ground electrode concept, introduced on the M42 are also used on the M50 engine. In order to cope with the extremely rapid combustion process of a four valve engine the tri-electrode spark plug was developed for BMW. The electrode gap is 0.8mm ± 0.1 mm and requires no adjustment. The resistance value of the spark plug is > 1 KOhm.
2 BOSCHM50/M20 COMPARISON
|at engine speed||5900 rpm|
|Max. Torque||245 Nm|
|at engine speed||4700 rpm|
|Max. Permissable engine speed||6500 rpm ± 40|
|Compression pressure||10 bar - 11 bar (minimum)|
|Intake valve diameter||2x33mm|
|Exhaust valve diameter||2 x 30.5 mm|
|Valve clearance||Self-adjusting, hydraulic|
|Oil pressure||4 bar|
|Oil capacity||4 ltr (4.2 qts) + 0.25 ltr (0.3 qts) for oil filter|
|Ignition||Direct solid state ignition|
|Distributor||One ignition coil per cylinder|
|Spark plus||Bosch F03DAR NGK BCPR7ER (with triangular electrode)|
|Electrode gap||0.9 mm ± 0.1 mm (non-adjustable)|
|CO level||0.7 + 0.5% (non-adjustable)|
|Idle speed||800 ± 30/min. (non-adjustable)|
|Fuel injection||DME 3.1 with hot wire air flow sensor|
|Fuel grade||90 AKI/95 RON Premium, unleaded|