BMW M50 Straight-6 Engine

The BMW M50 is a straight-6 DOHC piston engine was produced from 1990-1996. It was released in the E34 520i and 525i, to replace the M20 engine. It began to be phased out following the introduction of the M52 engine in 1994.

It is a straight-six engine - gasoline engine of the car manufacturer BMW and was in late 1989 as the successor to the BMW M20  and heritage of the BMW M30 presented first in the "five" (model E34 ) and then in the "triple" (model E36 ) used. It was used worldwide with a displacement of 2.0 or 2.5 liters (with 150 or 192 hp) in the BMW E36 and BMW E34 .

With the BMW M50, a new generation of engines replaced the BMW M20 after a good twelve-year construction period. Constructive innovations compared to the M20 were four-valve technology, timing chain and hydraulic tappets , which make the regular checking / adjusting the valve clearance unnecessary.

In February 1990 series production started in the BMW engine plant Steyr . The press launch of the new generation of engines made at the end February 1990 equipped the BMW plant in Landshut, from May 1990 were the BMW 520i and BMW 525i with M50 engines. The engines were to 1995 the BMW E36 and until mid-1996 in the five-Touring BMW E34 used. A total of 943,795 units were produced by the BMW M50.

Based on the design principle of the M50, the BMW M-GmbH developed the S50 sports engine series with 3 and 3.2 liter cubic capacity. These engines came u. a. in the BMW M3 E36 and BMW Z3 M used.

Compared with its M20 predecessor, the M50 features DOHC, 4 valves per cylinder, coil-on-plug ignition, a knock sensor and a lightweight plastic intake manifold. As per the M20, the M50 uses an iron block with an alloy head.

In September 1992, the M50 was upgraded to the M50TU ("technical update"), which added variable valve timing to the intake camshaft (called single VANOS by BMW).

The E36 M3 was powered by the S50 engine series, which is a high output version of the M50.

As with the predecessor BMW M20 , a smaller cylinder clearance than the BMW M30 (100 mm) was selected with 91 mm , whereby the crankcase of the BMW M50 has the same external dimensions as the predecessor component. The main dimensions of the cylinder crankcase and the overall engine are thus relatively compact and allowed the installation of the engine in all former BMW vehicle series.

The crankcase is made of pearlitic gray cast iron , which offers advantages in terms of strength , damping behavior and corrosion. The 2.0-liter engine has a 80-mm bore with freestanding bushes , and the 2.5-liter, 74-mm bore engine with tapped bushes. Lowering the oil pan flange by 60 mm below the center of the crankshaft made the lower engine section particularly stiffened. Lightweight casting allowed a crankcase weight of 48 kg to be achieved.

The engine weight according to DIN 70 020-A is only 194 kg for both displacement variants - despite the more complex design of the BMW M20 with four-valve technology, swept flywheel and V-ribbed belt drive of the ancillaries. The only 12 kg extra weight compared to the predecessor model could be achieved by constructive lightweight construction using FEM and CAD.

The oil sump is in one piece and consists of an aluminum alloy , which is processed by die-casting technology . With a shell integrated into the oil sump, the lower half of the gearbox is additionally bolted to the engine / gearbox to improve overall stiffness. Arranged in the sump is a Duocentric oil pump, d. H. a regulated gear pumpdriven by a single row chain from the crankshaft. The lubrication system contains 5.8 l of oil, with the oil pressure in the system being regulated to 4 bar.

The crankshafts with a stroke of 66 mm (2.0 l) and 75 mm (2.5 l) were manufactured in nodular cast iron . The main bearing diameter is 60 mm, the connecting rod bearing diameter 45 mm. These dimensions resulted in both crankshafts to a very large overlap between the main and connecting rod journal and thus to a high rigidity of the crankshafts.

The forged connecting from C45 were carried out uniformly with 135 mm length, allowing the use of existing production facilities. By waisting the connecting rod shaft a weight reduction could be achieved while the durability increased.

The lightweight pistons with 9 mm fire bridges have a bolt diameter of 22 mm. Among other things due to the different compression ratio are the different designs of the piston: The 2.0-l engine (ε = 10.5) has flat piston without trough, the 2.5-l engine (ε = 10.0) has a Centric ball trough with about 4 mm depth. There are four valve pockets in the pistons, two each for intake and exhaust valves. The cooling of the piston plates is done with spray oil nozzles. These are arranged in the crankcase in the region of the crankshaft bearing blocks.

Data of piston rings:

• Upper compression ring: rectangular ring, chrome-plated, 1.5 mm high
• Lower compression ring: nose minute ring, 1.75 mm high
• Oil scraper ring: so-called oil slit ring with hose spring, 3 mm high

Cylinder head 

For the BMW M50, four-valve technology was used for the six-cylinder mass production for the first time. For the BMW M50, a completely new DOHC crossflow cylinder head with 4 valves per cylinder has been developed.

The valves are operated by means of two camshafts with bucket tappets with hydraulic valve clearance compensation (HVA). The two overhead, hollow cast camshafts made of die-cast magnesium are mounted seven times, which ensures high rigidity between two cams. With built-in camshafts, accessibility to the cylinder head bolts is guaranteed. The camshafts are driven by two single roller chains:

• Main drive (primary chain):

From the crankshaft to the exhaust camshaft with guide rail in the drawn chain center; hydraulically damped tensioning rail.

• PTO (secondary chain):

From exhaust to intake camshaft; Guide rail and hydraulically damped tensioner.

The use of the 4-valve technology allowed a reduction of the valve dimensions compared to the BMW M20, whereby the valve disks are housed in the bore size of the cylinders. Smaller valves provide better heat dissipation and thus durability and result in reduced moving masses, which in turn leads to reduced closing forces of the valve springs. The smaller valve masses allow precise valve control - even in high speed ranges.

The cylinder head cover made of die - cast magnesium is acoustically decoupled from the cylinder head by means of a large volume rubber gasket and rubber elements on the mounting screws. The electrical coupling is done by means of earth strap. The individual ignition coils are protected by a plastic cover against dirt and spray. The cover of the chain drive is made of die-cast aluminum; For engine bleeding, it ensures pressure equalization between the crankcase and the oil chamber in the cylinder head.

The 4-valve technology allows by the overall larger cross-section of the inlet and outlet openings particularly favorable flow conditions of the sucked air-fuel mixture and the combustion gases. By optimizing the lengths and cross sections of the entire air exchange tract on the intake and exhaust side, a high degree of filling has been achieved - the essential requirement for high power and torque values ​​over a wide engine speed range.

Very small valve angles (inlet side 20 ° 15 ', outlet side 19 ° 15') allow a flat combustion chamber with the concentration of the focal volume around the centrally located spark plug - symmetrically arranged between the valves. A compact combustion chamber with a small surface-to-volume ratio results in good thermal efficiency and balanced emissions due to favorable combustion conditions due to short combustion paths and low wall heat losses.

The evenly long flame paths allow a faster and less knock-prone combustion of the mixture. The low knock tendency of the 4-valve engine allows an increase in the compression ratio. The resulting benefits are

• Increase of the thermal efficiency
• Increased torque and improved torque curve
• Reduction of specific fuel consumption and
• optimized emissions.

In summary, the main advantages of the 4-valve technology are:

• Lower gas exchange work,
• ideal spark plug and
• smaller moving masses per valve.

Taking into account the combustion chamber, the plastic suction system was designed with short tubes of the same length so that a high dynamic range in the speed range between 4000 and 6000 min ^-1 is achieved. The streamlined inlets into the suction pipes and the smooth surface reduce the losses. For mixture formation and charge exchange, it proved to be advantageous to separate the inlet channels shortly before the cylinder inlet, wherein the channels are made so large that there is no channel narrowing even at maximum valve opening. The one-piece intake system is produced as a plastic injection-molded part using the core remelting process (joint component development by BMW, BASF and Mann + Hummel), this method was used for the first time in a mass production. The suction system made of glass-fiber reinforced, heat-stabilized polyamide (trade name: Ultramid ) has the necessary mechanical strength, rigidity and heat resistance, even over 130 ° C.

Low back pressure and favorable dynamic behavior were the main design criteria for the exhaust system . By appropriate dimensioning of pipe and catalyst cross sections and the muffler volumes , the first specification was achieved. A favorable dynamic behavior and thus good torque output in the medium speed range was achieved by as long as possible, separate front exhaust pipes to Durchmischungsstrecke before the catalyst .

The optimized design of the intake and exhaust side allowed the definition of relatively short control times (opening angle: inlet side 240 °, exhaust side 228 °). The short intake timing with early intake closure results in a high filling in the low and medium speed range, the short exhaust timing supports the high torque yield in the medium speed range.

Data Cylinder Head 

Models

M50B20

The 1,991 cc (121 cu in) M50B20 was introduced with the 1990 520i. It has an 80 mm (3.1 in) bore and 66 mm (2.6 in) stroke and produces 110 kW (150 hp). Compression Ratio 10.5:1.

Applications:

• 1990-1992 E34 520i
• 1991-1992 E36 320i

M50B20TU

The M50B20 was updated with single VANOS in 1992. Compression Ratio raised to 11:1. Peak torque became available at 4200 rpm.

Applications:

• 1992-1994 E36 320i
• 1992-1996 E34 520i

M50B24

This is a 2.4 L engine used in the Thailand and Oceania markets. It is based on the 2.5 L M50B25 with a reduced stroke.

M50B25

The 2,494 cc (152 cu in) M50B25 was introduced with the 1990 525i and 525ix. It has an 84 mm (3.3 in) bore and 75 mm (3.0 in) stroke and produces 141 kW (189 hp) at 6000 rpm and 245 N·m (181 lb·ft) at 4700 rpm.

Applications:

• 1990-1992 E34 525i, 525ix
• 1991-1992 E36 325i, 325is

M50B25TU

The M50B25 was updated with single VANOS in 1992. Peak torque became available at 4200 rpm.

Applications:

• 1993-1995 E36 325i, 325is
• 1992-1996 E34 525i, 525ix

S50

The S50 is the high performance version of the M50 which was used in the M3. Like the M50, it has an iron block and aluminum head with four valves per cylinder.

In the USA, 1994-1995 M3 models are powered by the "S50B30US" (which was replaced by the 3.2 S52 in 1996).

S50B30

The S50B30 is a 2,990 cc (182 cu in) higher output version of the M50 which powered the E36 M3, except in the USA. Canada had a limited production run of 45 cars with the S50B30 engine.

Engine management is provided by a Bosch Motronic M3.3 ECU with a separate "VNC" Vanos control unit providing fully variable single camshaft adjustment on the intake side. It produces 213.5 kW (286 hp), has a bore of 86 millimetres (3.4 in) and a stroke of 86 millimetres (3.4 in), and a compression ratio of 10.8:1. The limited edition "M3 GT" model from 1995 had different camshafts and a redesigned sump and oil pump, and produced 220 kW (295 hp).

Upgrades over the M50 include:

• Individual throttle plates for each cylinder
• Continuously variable "Vanos" valve timing.
• Lightweight pistons
• Graphite-coated connecting rods
• Larger inlet valves
• Redesigned equal length exhaust manifolds

Applications:

• 1992-1995 E36 M3 (except for USA)

S50B30US

USA versions of the 1994-1995 E36 M3 are powered by a 3.0L tuned version of the M50 which produces 179 kW (240 hp). This engine was based on the M50B25TU and uses the same compression ratio,but uses a different crankshaft, connecting rods, and pistons. At the 1996 facelift, this engine was replaced by the 3.2 L S52.

Applications:

• 1994-1995 E36 M3 (USA only)

S50B32

In 1996, the 3,201 cc (195 cu in) S50B32 replaced the S50B30 (except in Canada and USA, where the M52-based S52 engine was used instead). Engine management is provided by a Siemens MSS50 engine control unit featuring integrated Vanos control for both the intake and exhaust camshafts. The MSS50 ECU also encompassed an advanced knock control system which can monitor each of the s50b32 6 cylinders via 3 knock sensors allowing the engine run a more refined calibration on standard high octane pump gas. The oil sump is a smaller capacity unit first seen on the M3GT and features a dual pick-up system to lower the risk of oil being forced away from a single pick-up point under cornering. This engine produces 239 kW (321 hp). The compression ratio is 11.3:1, the bore is 86.4 mm (3.4 in) and the stroke is 91 mm (3.6 in).

Applications:

• 1996-1999 E36 M3 (except Canada and USA)
• 1996-1999 Z3 M coupe and M roadster (except Canada and USA)

Technical revision 

After about 500,000 units produced, the BMW M50 underwent a major overhaul. Goals of further development were

• Reduction of fuel consumption and emissions
• Improvement of elasticity in the lower and middle speed range
• Comfort optimization (acoustics)
• Optimization of the idling quality and
• Compatibility with fuels ROZ 91/95/98.

The technically modified engines are called BMW M50 "TU" (technically revised, English technical update ). They were equipped with the variable camshaft control VANOS and went into production in September 1992. 

Measures 

In order to improve fuel consumption, emissions, idling quality and acoustics, an important objective in the revision of the basic engine was the reduction of friction in the piston group and in the valve train as well as the adaptation of intake camshaft control. The technical innovations led to a reduction of the friction torque - depending on displacement and speed - by 10 to 18%. At low speeds (800 min ^-1 ) the Reibmomentabnahme in the valve train is most pronounced. At 2000 min ^-1 a Reibmomentabnahme in the piston group and valve train is effective. At 6000 min ^-1 , the influence of the piston group on the total friction prevails.

Technical revision of the basic engine 

Since the dimensions of the crankcase, bearings, and crankshaft lift should be maintained, friction could be substantially reduced by reducing the connecting rod ratio, the oscillating masses, and the supporting piston and piston ring treads. The 135 mm standard rivet length had to be abandoned to reduce the bearing area of ​​the piston. The 2.0-liter engine was 145 mm long and the connecting rods of the 2.5-liter engine were extended to 140 mm. Together with the increase in compression, the compression height of the piston is shortened accordingly. The 2.0 l engine reduces the piston skirt length (as a function of the bearing piston area) by 11.6 mm and the 2.5 l engine by 9.8 mm. With the reduction in compression heights, the masses of the piston were reduced by 100 and 50 g, respectively. Despite lengthening of the connecting rods, this resulted in a reduction of the oscillating masses by 12 and 6%, respectively. Sufficient strength of the pistons could be achieved despite these changes by using the box, X and half slipper type.

The piston cooling has also been improved. For this purpose, on the one hand, the throughput of the spray oil cooling was increased by one spray nozzle per cylinder in the bearing block by 100% and on the other hand could be kept in an acceptable frame by optimizing the piston inner contour and an extension of the injection duration of the piston piston.

The weight- and shape-optimized connecting rods are now made of micro-alloyed carbon-manganese forged steel C40 mod BY. The six-cylinder connecting rods were thus adapted to the connecting rods of the four-cylinder engines.

Also new is the axial vibration damper, which replaces the radial vibration damper. At approximately the same moment of inertia , this has a lower weight and lower acoustic properties.

The piston rings have also been revised:

• Upper compression ring: rectangular ring, 1.5 mm high - now convex, with sharp lower edge. For the 2.0 l engine, the tread is chromed as before, and the 2.5 l engine was plasma coated because of the higher thermal load.
• Lower compression ring: nose minute ring, 1.5 mm high (previously 1.75 mm)
• Oil scraper ring: 2 mm high three-piece steel disk ring or a 2 mm high two-piece oil slit ring with hose spring (formerly 3 mm high)

Technical revision of the cylinder head 

A reduction in the valve train friction could be achieved by reducing the valve spring forces and the oscillating valve train masses, which led to a reduction in the contact forces between the friction partners cam and bucket tappets. The diameter of bucket tappet and HVA element has not changed. The mass of the bucket tappets could be reduced by optimizing the wall thicknesses of the bucket tappet housing and altering the interior sheet metal parts that serve to supply oil to the HVA element and its mount.

Valve disc diameter and valve material have remained unchanged, however, the valve stem diameter has been reduced from 7 mm to 6 mm, which reduces the valve masses by 20% on average. The upper spring plate, which is still made of steel, has been structurally adapted to the single valve spring, reducing its mass by 21%.

By changing the cam lift and the optimized acceleration curve along with the mass reduction of the oscillating valve train parts was a reduction of the maximum valve spring force by 30% possible, which allowed the replacement of the existing double valve springs by single valve springs. Thanks to an optimized material compared to the double valve spring, the dynamic safety of the single valve spring could even be increased while maintaining the same thrust and stroke tension.

Further revisions, VANOS

Other modifications in the course of the revision were the use of hot-film air mass meters , modified crankshaft vibration dampers and a new idler actuator ZWD-5 (two-winding turntable) in the 2.5-liter engine. The use of a knock control allowed a slight increase in the compression ratio.

Performance and exhaust emissions as well as the running characteristics of a 4-stroke gasoline engine can be improved by an adjustable during operation camshaft spreading. By Va riable No ckenwellen- S preizung ( VANOS ) d can be realized the spread of the variable intake camshaft of the BMW M50TU. H. be adjusted from late to early or vice versa depending on the load and operating conditions. After extensive engine tests, a maximum adjustment angle of 25 ° CA (crankshaft angle) was determined for both engine variants.

Technical revision of the engine control 

The technical revision was accompanied by an adaptation of the engine control. In the 2.5-l engines, the digital engine electronics DME 3.3.1 with knock control was now used. All BMW E34 and E36 vehicles with M50B20TU engines received the SIEMENS MS 40.1 engine control.