Ignition order of a 6-cylinder in-line diesel engine. The order of operation of the cylinders in different engines

System Components

System overview

Mechanical components and parts of the diesel engine First described the following engine is divided into three large parts.

• Crankcase
• crank mechanism
• Gas distribution mechanism

These three parts are in constant interaction. relationships that have a significant impact on the properties of the engine:
• interval between ignitions;
• the order of operation of the cylinders;
• mass balancing.

Ignition Interval
The mechanical elements of the engine are mainly divided into three groups: the engine crankcase, the crank mechanism and the valve actuator. These three groups are closely interconnected and must be mutually agreed upon. The ignition interval is the angle of rotation of the crankshaft between two successive ignitions.
During one working cycle, the fuel-air mixture ignites once in each cylinder. The duty cycle (suction, compression, stroke, exhaust) of a four-stroke engine takes two complete revolutions of the crankshaft, i.e. the angle of rotation is 720 °.
The same interval between ignitions ensures uniform operation of the engine at all speeds. This interval between ignitions is obtained as follows:
ignition interval = 720°: number of cylinders

Examples:

• four-cylinder engine: 180° crankshaft (KB)
• six-cylinder engine: 120° KB
• eight-cylinder engine: 90° SV.

The greater the number of cylinders, the shorter the interval between ignitions. The shorter the interval between ignitions, the more evenly the engine runs.
At least theoretically, because mass balancing is added to this, which depends on the design of the engine and the order of operation of the cylinders. In order for a cylinder to ignite, the corresponding piston must be at "TDC at the end of the compression stroke", i.e. the corresponding intake and exhaust valves must be closed. This can only take place when the crankshaft and camshaft are correctly positioned relative to each other. The ignition interval is determined by the relative position of the connecting rod journals (the angular distance between the knees) of the crankshaft, i.e. the angle between the journals of successive cylinders (cylinder firing order).In V-engines, the camber angle must be equal to the ignition interval to achieve uniform performance.
Therefore, eight-cylinder BMW engines have an angle between the cylinder banks of 90°.

The order of operation of the cylinders
The firing order of the cylinders is the sequence in which ignition occurs in the engine's cylinders.
The order of the cylinders is directly responsible for the smooth operation of the engine. It is determined depending on the design of the engine, the number of cylinders and the interval between ignitions.
The firing order of the cylinders is always indicated starting with the first cylinder.

Mass balancing
As previously described, engine smoothness is dependent on engine design, number of cylinders, cylinder firing order, and firing interval.
Their influence can be illustrated by the example of the six-cylinder engine that BMW manufactures as an in-line engine, although it takes up more space and is more labor-intensive to manufacture. The difference can be understood by comparing the mass balancing of inline and V-shaped six-cylinder engines.
The following figure shows the moment of inertia curves for a BMW inline six engine, a 60° V6 engine and a 90° V6 engine.
The difference is obvious. In the case of an inline six-cylinder engine, the mass movements are balanced to such an extent that the entire engine is practically stationary. V-shaped six-cylinder engines, on the contrary, have a clear tendency to move, which manifests itself in uneven operation.

Body parts
The body parts of the engine take on isolation from the environment and perceive various forces, that occur during engine operation.

The engine body parts consist of the main parts shown in the following figure. Gaskets and bolts are also needed to perform the tasks of the crankcase.

Main goals:

• perception of the forces arising during the operation of the engine;
• sealing of combustion chambers, oil pan and cooling jacket;
• placement of the crank mechanism and valve drive, as well as other components.

Crank mechanism
The crank mechanism is responsible for converting the pressure that occurs during the combustion of the fuel-air mixture into useful movement. In this case, the piston receives a rectilinear acceleration. The connecting rod transmits this movement to the crankshaft, which turns it into rotational movement.

The crank mechanism is a functional group that converts the pressure in the combustion chamber into kinetic energy. In this case, the reciprocating motion of the piston turns into the rotational motion of the crankshaft. The crank mechanism is the optimal solution in terms of work output, efficiency and technical feasibility.

Of course, there are the following technical limitations and design requirements:

• speed limitation due to inertia forces;
• inconstancy of forces during the working cycle;
• the occurrence of torsional vibrations that create loads on the transmission and on the crankshaft;
• interaction of various friction surfaces.

Valve drive
The valve actuator controls the charge change. In modern BMW diesel engines, only the done valve train with four valves per cylinder is used. The transmission of movement to the valve is carried out through the pusher lever.

The engine must be periodically supplied with outside air while the exhaust gas it produces must be vented. In the case of a four-stroke engine, the intake of outside air and the exhaust of exhaust gas is called charge change or gas exchange. During the charge change process, the inlet and outlet ports are periodically opened and closed by means of the inlet and outlet valves.
Lift valves are used as intake and exhaust valves. The duration and sequence of valve movements are provided by the camshaft.

Design
The valve actuator consists of the following parts:

• camshafts;
• transmission elements (roller levers of pushers);
• valves (whole group);
• hydraulic valve clearance compensation (HVA) if equipped;
• valve guides with valve springs.

The following figure shows the design of a four-valve cylinder head (M47 engine) with roller rocker arms and hydraulic valve clearance compensation.

Constructions
The valve actuator is available in various designs. They are distinguished by the following features:

• number and arrangement of valves;
• number and location of camshafts;
• method of transmitting movement to the valves;
• valve clearance adjustment method.

BMW diesel engines today exclusively have four valves per cylinder and two overhead camshafts for each cylinder bank (dohc). BMW M21 / M41 / M51 engines had only two valves per cylinder and one camshaft for each cylinder bank (ohc).
The transmission of the movement of the camshaft cams to the valves in BMW diesel engines is carried out by roller tappet levers. In this case, the required clearance between the camshaft cam and the so-called cam follower (for example, the roller lever of the pusher) is ensured by a mechanical or hydraulic valve clearance compensation (HVA).
The following figure shows the valve actuator parts of the M57 engine.

crankcase

The block crankcase, also called the cylinder block, includes the cylinders, the cooling jacket and the crankcase of the drive mechanism. The demands and challenges placed on the crankcase are high due to the complexity of today's "Highttech" engines. However, crankcase improvement is proceeding at the same pace, especially since many new or improved systems interact with the crankcase.

Below are the main tasks.

• Perception of forces and moments
• Placement of the crank mechanism
• Placement and connection of cylinders
• Location of the crankshaft bearings
• Placement of coolant channels and lubrication system
• Ventilation system integration
• Fastening of various auxiliary and attached equipment
• Sealing the crankcase cavity

These tasks give rise to different and overlapping requirements for tensile and compressive strength, bending and twisting. In particular:

• the forces of the impact of gases that are perceived by the threaded connections of the cylinder head and the crankshaft bearings;
• internal forces of inertia (bending forces) resulting from inertia forces during rotation and oscillation;
• internal torsional forces (twisting forces) between the individual cylinders;
• torque of the crankshaft and, as a result, the reaction forces of the engine mounts;
• free forces and moments of inertia, as a result of the inertia forces during vibrations, which are perceived by the engine mounts.

Design
The basic shape of the block crankcase hasn't changed much since the beginning of the motorstory. Changes in the design touched on particulars, for example, how many parts the crankcase is made of or how its individual parts are made. Designs can be classified depending on the execution:

• top plate;
• area of ​​the bed of the main bearing;
• cylinders.

Top plate
The top plate can be made in two different designs: closed and open. The design affects both the casting process and the rigidity of the crankcase.
In the closed version, the top plate of the crankcase is completely closed around the cylinder.
There are holes and channels for supplying oil under pressure, draining oil, coolant, crankcase ventilation and threaded connections of the cylinder head.
The coolant holes connect the water jacket that surrounds the cylinder to the water jacket in the cylinder head.
This design has disadvantages in terms of cylinder cooling in the TDC zone. The advantage of the closed version compared to the open version is a higher rigidity of the top plate and thus less deformation of the plate, less displacement of the cylinders and better acoustics.
In the open design, the water jacket surrounding the cylinder is open at the top. This improves the cooling of the cylinders at the top. Less rigidity is currently offset by the use of a metal head gasket.

Fig.2 - Closed version of the upper plate of the M57TU2 engine The crankcases of BMW diesel engines are made of gray cast iron. Starting with engines M57TU2 and U67TU, the crankcase is made of high strength aluminum alloy.

BMW diesel engines use a closed plate design. Main bearing bed area
The design of the area of ​​the main bearing bed is of particular importance, since in this place the forces acting on the crankshaft bearing are perceived.
The versions differ in the plane of the crankcase and oil pan separation and in the design of the main bearing caps.
Parting plane versions:

• oil pan flange in the center of the crankshaft;
• oil pan flange below the center of the crankshaft.

Main bearing cap designs:
• individual main bearing caps;
• integration into one frame structure.

Main bearing bed
The bearing bed is the upper part of the crankshaft support in the crankcase. The bearing beds are always integrated into the casting of the crankcase.
The number of bearing beds depends on the design of the engine, primarily on the number of cylinders and their location. Today, for reasons of vibration reduction, the maximum number of crankshaft main bearings is used. The maximum number means that there is a main bearing next to each crankshaft elbow.
When the engine is running, the gas in the crankcase cavity is constantly in motion. The movements of the pistons act on the gas like pumps. To reduce losses for this work, many engines today have holes in the bearing beds. This facilitates pressure equalization throughout the crankcase.

Crankcase split plane

The separation plane of the crankcase and oil pan forms the oil pan flange. There are two designs. In the first case, the parting plane lies in the center of the crankshaft. Because this design is economical to manufacture, but has significant disadvantages in terms of stiffness and acoustics, it is not used in BMW diesel engines.
With the second design (IN) the oil pan flange is located below the center of the crankshaft. At the same time, a crankcase with lowered walls and a crankcase are distinguished
with top and bottom, the latter is called bedplate construction (WITH). BMW diesel engines have a crankcase with lowered walls.

The M67 engine also uses a lowered wall design. This ensures high dynamic rigidity and good acoustics. The steel bridge reduces stress on the bearing cap bolts and further reinforces the main bearing bed area.

Fig.6 - Support beam concept

Support beam concept
In order to achieve high dynamic rigidity, the crankcases of BMW diesel engines are designed according to the support beam principle. With this design, horizontal and vertical box-section elements are cast in the walls of the crankcase. In addition, the crankcase has lowered walls that extend up to 60 mm below the center of the crankshaft and end with a plane for mounting the oil pan.

main bearing cap
The main bearing caps are the underside of the crankshaft bearings. In the manufacture of the crankcase, the beds and main bearing caps are machined together. Therefore, their fixed position relative to each other is necessary. This is usually done using centering sleeves or surfaces made on the sides in the beds. If the crankcase and main bearing caps are made from the same material, the caps can be made using the split method.
When separating the main bearing cap by the breaking method, a precise breaking surface is formed. This surface structure accurately centers the main bearing cap when installed on the bed. Additional surface treatment is not required.

Another possibility for precise positioning is punching the surfaces of the bed and the main bearing cap.
This fixation ensures an absolutely smooth transition between the bed and the cap in the main bearing hole after reassembly.

Fig. 8 - Stamping the surface of the main bearing cap of the M67TU engine
1 main bearing cap
2 Punching the surface of the main bearing cap
3 The mating shape of the surface of the main bearing bed
4 Main bearing bed

When the surface is stamped, the main bearing cap receives a certain profile. When the main bearing cap bolts are first tightened, this profile is imprinted on the surface of the bed and ensures that there is no movement in the transverse and longitudinal directions.
Main bearing caps are almost always made from gray cast iron. General machining with an aluminum crankcase, although demanding, is common today for high-volume production. The combination of an aluminum crankcase with gray cast iron main bearing caps offers certain advantages. The low coefficient of thermal expansion of gray cast iron limits the operating clearances of the crankshaft. Together with the high rigidity of gray cast iron, this leads to a reduction in noise in the area of ​​the main bearing bed.

The cylinder and piston form the combustion chamber. The piston is inserted into the cylinder liner. The smooth machined surface of the cylinder liner together with the piston rings ensures an effective seal. In addition, the cylinder gives off heat to the crankcase or directly to the coolant. Cylinder designs differ in the material used:

• monometallic construction (cylinder liner and crankcase are made of the same material);
• insertion technology (cylinder liner and crankcase are made of different materials physically connected);
• connection technology (cylinder liner and crankcase are made of different materials, connected metal).

Monometallic construction
With a mono-metal design, the cylinder is made of the same material as the crankcase. First of all, the gray cast iron crankcase and the AISi crankcase are manufactured according to the principle of monometallic construction. The required surface quality is achieved by repeated processing. BMW diesel engines only have crankcases in mono-metal construction made of gray cast iron, as the maximum ignition pressure is up to 180 bar.

Insert technology
The crankcase material does not always meet the requirements for the cylinder. Therefore, often the cylinder is made of another material, usually in combination with an aluminum crankcase. Cylinder liners are distinguished:

1. according to the method of connecting the crankcase with the sleeve

• integrated into the casting
• pressed
• crimped
• plug-in.

2. according to the principle of operation in the crankcase

• wet and
• dry

3. by material

• gray cast iron or
• aluminum

Wet cylinder liners are in direct contact with the water jacket, i.e. the cylinder liners and the cast crankcase form a water jacket. The water jacket with dry cylinder liners is completely in the cast crankcase - similar to a monometallic design. The cylinder liner has no direct contact with the water jacket.

Fig.9 - Dry and wet cylinder liners
A Cylinder with dry sleeve
IN Wet liner cylinder
1 crankcase
2 Cylinder liner
3 Water jacket

Wet cylinder liners have an advantage in terms of heat transfer, while dry liners have an advantage in production and processing capability. As a rule, the production cost of cylinder liners is reduced with a large quantity. Gray cast iron liners for both M57TU2 and M67TU engines are heat treated.

Connection technology
Another possibility of manufacturing a cylinder mirror, with an aluminum crankcase, is the connection technology. In this case, too, the cylinder liners are inserted during casting. Of course, this is carried out using a special process (for example, under high pressure), the so-called intermetallic bonding to the crankcase. Thus, the cylinder mirror and crankcase are inseparable. This technology limits the use of casting processes and thus the design of the crankcase. BMW diesel engines do not currently use this technology.

Machining cylinder mirrors
The cylinder bore is the sliding and sealing surface for the piston and piston rings. The surface quality of the cylinder mirror is decisive for the formation and distribution of the oil film between the contacting parts. Therefore, the roughness of the cylinder wall is largely responsible for oil consumption and engine wear. The final processing of the cylinder mirror is carried out by honing. Honing - polishing the surface with the help of combined rotational and reciprocating movements of the cutting tool. In this way, an extremely small deviation in the shape of the cylinder and a uniformly low surface roughness are obtained. Machining must be gentle on the material to avoid chipping, uneven transitions and burrs.

materials

Even now, the crankcase is one of the heaviest parts of the entire car. And it occupies the most critical place for driving dynamics: the place above the front axle. Therefore, it is here that attempts are made to fully exploit the potential for mass reduction. Gray cast iron, which has been used as a crankcase material for decades, is increasingly being replaced in BMW diesel engines by aluminum alloys. This allows a significant weight reduction to be obtained. In the M57TU engine, it is 22 kg.
But, the advantage in mass is not the only difference that occurs when processing and using a different material. Acoustics, anti-corrosion properties, production processing requirements and service volumes are also changing.

Gray cast iron
Cast iron is an alloy of iron with more than 2% carbon and more than 1.5% silicon. Gray cast iron contains excess carbon in the form of graphite.
For crankcases of BMW diesel engines, cast iron with lamellar graphite has been and is used, which got its name from the location of the graphite in it. Other constituents of the alloy are manganese, sulfur and phosphorus in very small amounts.
From the very beginning, cast iron was proposed as a material for block crankcases of serial engines, since this material is not expensive, it is simply processed and has the necessary properties. Light alloys could not meet these requirements for a long time. BMW uses lamellar graphite cast iron for its engines due to its particularly favorable properties.
Namely:

• good thermal conductivity;
• good strength properties;
• simple machining;
• good casting properties;
• very good damping.

Outstanding damping is one of the hallmarks of flake graphite cast iron. It means the ability to perceive vibrations and damp them due to internal friction. Due to this, the vibration and acoustic characteristics of the engine are significantly improved.
Good properties, strength and easy processing make the gray cast iron crankcase competitive today. Due to their high strength, M petrol engines and diesel engines are still made with gray cast iron crankcases today. Increasing requirements for the mass of the engine in a passenger car in the future will only be able to meet light alloys.

Aluminum alloys
Aluminum alloy crankcases are still relatively new for BMW diesel engines. The first representatives of the new generation are the M57TU2 and M67TU engines.
The density of aluminum alloys is about a third compared to gray cast iron. However, this does not mean that the advantage in mass has the same ratio, because due to the lower strength, such a block crankcase has to be made more massive.

Other properties of aluminum alloys:

• good thermal conductivity;
• good chemical resistance;
• good strength properties;
• simple machining.

Pure aluminum is not suitable for casting a crankcase, because it does not have good enough strength properties. In contrast to gray cast iron, the main alloying components are added here in relatively large quantities.

Alloys are divided into four groups, depending on the predominant alloying additive.
These additives:

• silicon (Si);
• copper (Ci);
• magnesium (Md);
• zinc (Zn).

AlSi alloys are used exclusively for the aluminum crankcases of BMW diesel engines. They are improved with small additions of copper or magnesium.
Silicon has a positive effect on the strength of the alloy. If the component is more than 12%, then a very high surface hardness can be obtained by special processing, although cutting will be complicated. In the region of 12%, outstanding casting properties take place.
The addition of copper (2-4%) can improve the casting properties of the alloy if the silicon content is less than 12%.
A small addition of magnesium (0.2-0.5%) significantly increases the strength values.
Both BMW diesel engines use AISi7MgCuO.5 aluminum alloy. The material has already been used by BMW for diesel engine cylinder heads.
As can be seen from the designation AISL7MgCuO.5, this alloy contains 7% silicon and 0.5% copper.
It has high dynamic strength. Other positive properties are good casting properties and ductility. True, it does not allow to achieve a sufficiently wear-resistant surface, which is necessary for the cylinder mirror. Therefore crankcases made of AISI7MgCuO,5 have to be made with cylinder liners (see chapter "Cylinders").

Tabular overview

general information
The assembled cylinder head determines performance characteristics such as power output, torque and emissions, fuel consumption and acoustics like no other engine functional group. Almost the entire gas distribution mechanism is located in the cylinder head.
Accordingly, the tasks that the cylinder head must solve are also extensive:

• perception of forces;
• placement of the valve drive;
• placement of channels for changing the charge;
• placement of glow plugs;
• placement of nozzles;
• placement of coolant channels and lubrication systems;
• limiting the cylinder from above;
• heat dissipation to the coolant;
• fastening of auxiliary and attached equipment and sensors.

The following loads follow from the tasks:
• the forces of the effects of gases that are perceived by the threaded connections of the cylinder head;
• torque of camshafts;
• forces generated in the camshaft bearings.

Injection processes
In diesel engines, depending on the design and layout of the combustion chamber, direct and indirect injection are distinguished. Moreover, in the case of indirect injection, in turn, a vortex-chamber and an ancestral mixture formation are distinguished.

Pre-chamber mixing

The prechamber is located in the center relative to the main combustion chamber. The pre-combustion fuel is injected into this prechamber. The main combustion occurs with a known self-ignition delay in the main chamber. The prechamber is connected to the main chamber by several holes.
The fuel is injected by means of an injector providing a staged injection of fuel at a pressure of about 300 bar. The reflective surface in the center of the chamber breaks the jet of fuel and mixes with air. The reflective surface thus contributes to rapid mixture formation and streamlining of air movement.

The disadvantage of this technology is the large prechamber cooling surface. Compressed air cools relatively quickly. Therefore, such engines are started without the help of glow plugs, as a rule, only at a coolant temperature of at least 50 ° C.
Thanks to the two-stage combustion (first in the prechamber and then in the main chamber), combustion occurs gently and almost completely with relatively smooth engine operation. Such an engine provides reduced emissions of harmful substances, but at the same time develops less power compared to a direct injection engine.

Vortex chamber mixing
Vortex chamber injection, like the ancestor-dimensional one, is a variant of indirect injection.
The swirl chamber is designed in the form of a ball and is located separately on the edge of the main combustion chamber. The main combustion chamber and the vortex chamber are connected by a straight tangential channel. The tangentially directed straight channel, when compressed, creates a strong air turbulence. Diesel fuel is supplied through a nozzle that provides staged injection. The opening pressure of the nozzle, which provides a staged fuel injection, is 100-150 bar. When a finely atomized cloud of fuel is injected, the mixture is partially ignited and develops its full power in the main combustion chamber. The design of the swirl chamber, as well as the location of the nozzle and glow plug, are factors that determine the quality of combustion.
This means that combustion starts in the spherical vortex chamber and ends in the main combustion chamber. Glow plugs are necessary to start the engine, as there is a large surface between the combustion chamber and the swirl chamber, which contributes to the rapid cooling of the intake air.
The first mass-produced BMW M21D24 diesel engine operates on the principle of vortex chamber mixing.

direct injection
This technology eliminates the separation of the combustion chamber. This means that with direct injection there is no preparation of the working mixture in the adjacent chamber. Fuel is injected by a nozzle directly into the combustion chamber above the piston.
In contrast to indirect injection, multi-jet nozzles are used. Their jets must be optimized and adapted to the design of the combustion chamber. Due to the high pressure of the injected jets, instantaneous combustion occurs, which on earlier models led to loud engine operation. However, such combustion releases more energy, which can then be used more efficiently. This reduces fuel consumption. Direct injection requires a higher injection pressure and therefore a more complex injection system.
At temperatures below 0 °C, as a rule, preheating is not required, since heat losses through the walls due to a single combustion chamber are noticeably less than in engines with adjacent combustion chambers.

Design
The design of cylinder heads has changed a lot in the process of improving engines. The shape of the cylinder head is highly dependent on the parts it includes.

Basically, the following factors influence the shape of the cylinder head:

• number and arrangement of valves;
• number and arrangement of camshafts;
• the position of the glow plugs;
• nozzle position;
• the shape of the channels for changing the charge.

Another requirement for the cylinder head is to be as compact as possible.
The shape of the cylinder head is primarily determined by the concept of the valve drive. To ensure high engine power, low emissions and low fuel consumption, it is necessary, if possible, to have an efficient and flexible charge change and a high degree of filling of the cylinders. In the past, the following has been done to optimize these properties:

• the top arrangement of valves;
• overhead camshaft;
• 4 valves per cylinder.

The special shape of the inlet and outlet ports also improves charge exchange. Basically, cylinder heads are distinguished according to the following criteria:

• number of parts;
• number of valves;
• cooling concept.

At this point, it should be mentioned once again that only the cylinder head is considered here as a separate part. Due to its complexity and strong dependence on named parts, it is often described as a single functional group. Further topics can be found in the respective chapters.

Number of parts
A cylinder head is called a single-piece when it consists of only one single large casting. Small parts such as camshaft bearing caps are not covered here. Multi-part cylinder heads are assembled from several individual parts. A common example of this is cylinder heads with screwed-on camshaft retainers. However, only single-piece cylinder heads are currently used in BMW diesel engines.

Fig.15 - Comparison of heads with two and four valves
A Cylinder head with two valves
IN Cylinder head with four valves
1- Combustion chamber cover
2- valves
3- Direct channel (vortex-chamber mixing with two valves)
4- Glow plug position (4 valves)
5- Injector position (direct injection with four valves)

Number of valves
Early four-stroke diesel engines had two valves per cylinder. One exhaust and one intake valve. Thanks to the installation of an exhaust gas turbocharger, a good filling of the cylinders was obtained even with 2 valves. But for several years now, all diesel engines have four valves per cylinder. Compared to two valves, this results in a larger total valve area and thus a better flow area. Four valves per cylinder also allow the nozzle to be centrally located. This combination is necessary in order to provide high power with low exhaust emissions.
Fig. 16 - Vortex channel and filling channel of the M57 engine
1- outlet channel
2- exhaust valves
3- vortex channel
4- Nozzle
5- intake valves
6- Filling channel
7- swirl valve
8- glow plug

In the swirl channel, the incoming air is rotated for good mixture formation at low engine speeds.
Through the tangential channel, air can flow freely in a straight line into the combustion chamber. This improves the filling of the cylinders, especially at high speeds. A swirl valve is sometimes installed to control the filling of the cylinders. It closes the tangential channel at low speeds (strong swirl) and opens it smoothly at higher speeds (good filling).
The cylinder head in modern BMW diesel engines includes a swirl and filling channel, as well as a centrally located nozzle.

• Combination of both types

In crossflow cooling, the coolant flows from the hot side of the outlet to the cold side of the inlet. This has the advantage that an even distribution of heat takes place throughout the cylinder head. In contrast, with longitudinal flow cooling, the coolant flows along the axis of the cylinder head, i.e. from the front side to the power take-off side or vice versa. The coolant heats up more and more as it moves from cylinder to cylinder, which means a very uneven distribution of heat. In addition, this means a pressure drop in the cooling circuit.
The combination of both types cannot eliminate the disadvantages of longitudinal flow cooling. Therefore, BMW diesel engines use cross-flow cooling exclusively.

• seals the cylinder head from above;
• reduces the noise of the engine;
• removes crankcase gases from the crankcase;
• location of the oil separation system

• placement of crankcase ventilation pressure control valve;
• placement of sensors;
• placement of pipeline outlets.

Cylinder head gasket
The cylinder head gasket (ZKD) in any internal combustion engine, be it gasoline or diesel, is a very important part. It is subjected to extreme thermal and mechanical stress.

ZKD functions include isolating four substances from each other:

• burning fuel in the combustion chamber
• atmospheric air
• oil in oil channels
• coolant

Sealing gaskets are mainly divided into soft and metal.

Soft seals
This type of seals are made from soft materials but have a metal frame or carrier plate. On this plate, soft pads are held on both sides. Soft grips often have a plastic coating applied to them. This design allows it to withstand the stresses that cylinder head gaskets are normally subjected to. The holes in the ZKD leading into the combustion chamber have a metal edging due to loads. Elastomeric coatings are often used to stabilize coolant and oil passages.

Metal seals
Metal seals are used in engines operating under heavy loads. Such gaskets include several steel plates. The main feature of metal gaskets is that the seal is carried out mainly due to the corrugated plates and stoppers located between the spring steel plates. The deformation properties of ZKD allow it, firstly, to optimally lie in the region of the cylinder head and, secondly, to compensate the deformation to a large extent due to elastic recovery. Such elastic recovery takes place due to thermal and mechanical loads.

The thickness of the required ZKD is determined by the protrusion of the piston crown relative to the cylinder. Decisive is the highest value measured on all cylinders. Three thicknesses of the cylinder head gasket are available.
The difference in shim thickness is determined by the thickness of the intermediate shim. See TIS for details on piston crown projection.

oil pan

The oil pan serves as a reservoir for engine oil. It is made by aluminum die casting or double steel sheet.

General remarks
The oil pan closes the engine crankcase from below. For BMW diesel engines, the oil pan flange is always below the center of the crankshaft. The oil pan performs the following tasks:

• serves as a reservoir for engine oil and
• collects draining engine oil;
• closes the crankcase from below;
• is an element of strengthening the engine and sometimes the gearbox;
• serves as a place for installing sensors and
• guide tube for oil dipstick;
• here is the oil drain plug;
• reduces engine noise.

A steel seal is installed as a seal. Plug seals that have been installed in the past have shrunk, which can lead to loose threads.
To ensure the operation of the steel gasket, oil must not get on the rubber surfaces during its installation. Under certain circumstances, the seal may slip off the sealing surface. Therefore, the flange surfaces must be cleaned immediately before installation. In addition, it must be ensured that oil does not drip from the engine and does not get on the flange surfaces and gasket.

crankcase ventilation

During operation, parterre gases are formed in the crankcase cavity. They must be removed to prevent oil seepage in places of sealed surfaces under the action of excess pressure. Connecting to a clean air piping, which has a lower pressure, cuts off ventilation. In modern engines, the ventilation system is controlled by a pressure control valve. The oil separator cleans the crankcase gases from oil, and it returns through the outlet pipe to the oil pan.

General remarks
When the engine is running, crankcase gases enter the crankcase from the cylinder due to the pressure difference.
Blow-by gases contain unburned fuel and all components of the exhaust gases. In the crankcase cavity, they mix with engine oil, which is present there as an oil mist.
The amount of crankcase gases depends on the load. An excess pressure arises in the crankcase cavity, which depends on the movement of the piston and on the crankshaft speed. This overpressure builds up in all cavities associated with the crankcase cavity (eg oil drain line, timing case, etc.) and can lead to oil leakage at the seals.
To prevent this, a crankcase ventilation system was developed. At first, crankcase gases mixed with engine oil were simply thrown into the atmosphere. For environmental reasons, crankcase ventilation systems have been used for a long time.
The crankcase ventilation system diverts the crankcase gases separated from the engine oil into the intake manifold, and drops of engine oil through the oil drain pipe into the oil pan. In addition, the crankcase ventilation system ensures that no excess pressure builds up in the crankcase.

Unregulated crankcase ventilation
In the case of unregulated crankcase ventilation, crankcase gases mixed with oil are vented by vacuum at the highest engine speeds. This vacuum is created when connected to the inlet. From there, the mixture enters the oil separator. There is a separation of crankcase gases and engine oil.
In BMW diesel engines with non-adjustable crankcase ventilation, the separation is carried out using a wire mesh. The “cleaned” crankcase gases are vented to the engine intake manifold, while the engine oil returns to the oil pan. (crankshaft oil seals, oil sump flange gasket, etc.) Unfiltered air enters the engine, resulting in oil aging and sludge formation.

Adjustable crankcase ventilation
The M51TU engine was the first BMW diesel engine to feature a variable crankcase ventilation system.
BMW diesel engines with variable crankcase ventilation for oil separation can be equipped with a cyclonic, labyrinth or mesh oil separator.
In the case of controlled crankcase ventilation, the crankcase cavity is connected to the clean air pipeline after the air filter through the following components:

• ventilation duct;
• calming chamber;
• crankcase gas channel;
• oil separator;
• pressure control valve.

Fig. 23 - oil separator of the M47 engine
1- Raw crankcase gases
2- Cyclone oil separator
3- Net oil separator
4- Pressure control valve
5- Air filter
6- Channel to clean air pipeline
7- Hose to clean air duct
8- Clean air pipeline

There is a vacuum in the clean air pipeline due to the operation of the exhaust gas turbocharger.
Under the influence of the pressure difference relative to the crankcase, the crankcase gases enter the cylinder head and first reach the stilling chamber there.
The damping chamber is used to ensure that the sprayed oil, for example, through the camshafts, enters the crankcase ventilation system. If oil separation is carried out using a labyrinth, the task of the stilling chamber is to eliminate fluctuations in crankcase gases. This will prevent excitation of the membrane in the pressure control valve. For engines with a cyclone oil separator, these fluctuations are quite acceptable, since this increases the efficiency of oil separation. The gas is then settled in a cyclone oil separator. Therefore, here the stilling chamber has a different design than in the case of labyrinth oil separation.
The crankcase gases enter the oil separator through the supply line, where the engine oil is separated. The separated engine oil flows back into the oil pan. Cleaned crankcase gases are continuously fed through a pressure control valve into the clean air line upstream of the exhaust gas turbocharger. Modern BMW diesel engines are equipped with 2-component oil separators. First, preliminary oil separation is carried out using a cyclone oil separator, and then the final oil separation is carried out in the next grid oil separator. Almost all modern BMW diesel engines have both oil separators in the same housing. The exception is the M67 engine. Here, oil separation is also carried out by cyclone and grid oil separators, but they are not combined into one unit. The preliminary oil separation takes place in the cylinder head (aluminum), and the final oil separation by means of a mesh oil separator takes place in a separate plastic housing.

Adjustment process
When the engine is not running, the pressure control valve is open (state A). Ambient pressure acts on both sides of the diaphragm, i.e. the diaphragm is fully open under the action of a spring.
When the engine is started, the vacuum in the intake manifold builds up and the pressure control valve closes (state IN). This state is always maintained at idle or when coasting, since there are no crankcase gases in this case. The inner side of the membrane is thus subjected to a large relative vacuum (relative to the ambient pressure). In this case, the ambient pressure, which acts on the outer side of the diaphragm, closes the valve against the force of the spring. When the crankshaft is loaded and rotated, crankcase gases appear. crankcase gases ( 8 ) reduce the relative vacuum that acts on the membrane. As a result, the spring can open the valve, and crankcase gases escape. The valve remains open until an equilibrium is established between the ambient pressure and the vacuum in the crankcase plus the spring force (state WITH). The more crankcase gases are released, the smaller the relative vacuum acting on the inside of the membrane becomes, and the more the pressure control valve opens. This maintains a certain vacuum in the crankcase (approx. 15 mbar).

Oil separation

To release crankcase gases from engine oil, various oil separators are used depending on the type of engine.

• Cyclone oil separator
• Labyrinth oil separator
• Net oil separator

When cyclone oil separator crankcase gases are directed into a cylindrical chamber in such a way that they rotate there. Under the influence of centrifugal force, heavy oil is squeezed out of the gas outward to the walls of the cylinder. From there, it can drain through the oil drain pipe into the oil pan. The cyclone oil separator is very efficient. But it requires a lot of space.
IN labyrinth oil separator crankcase gases are passed through a labyrinth of plastic partitions. Such an oil separator is located in the housing in the cylinder head cover. Oil remains on the baffles and can drain into the cylinder head through special holes and from there back into the oil pan.
Net oil separator able to filter even the smallest droplets. The core of the mesh filter is a fibrous material. However, thin non-woven fibers with a high carbon black content are prone to rapid fouling of the pores. Therefore, the oil separator screen has a limited service life and must be replaced as part of maintenance.

Crankshaft with bearings

The crankshaft converts the linear motion of the piston into rotational motion. The loads that act on the crankshaft are very large and extremely complex. Crankshafts are cast off or forged for operation under increased loads. The crankshafts are mounted with plain bearings, into which oil is supplied. while one bearing is guiding in the axial direction.

general information
The crankshaft converts the linear (reciprocating) motion of the pistons into rotational motion. Forces are transmitted through the connecting rods to the crankshaft and converted into torque. In this case, the crankshaft rests on the main bearings.

Additionally, the crankshaft takes on the following tasks:

• drive of auxiliary and attached equipment by means of belts;
• valve drive;
• often an oil pump drive;
• in some cases, the drive of balance shafts.

Under the action of forces changing in time and direction, torques and bending moments, as well as excited vibrations, a load arises. Such complex loads place very high demands on the crankshaft.
The service life of the crankshaft depends on the following factors:

• bending strength (weak points are the transitions between the bearing seats and the shaft cheeks);
• torsional strength (it is usually reduced by lubrication holes);
• resistance to torsional vibrations (this affects not only stiffness, but also noisiness);
• wear resistance (at the supports);
• wear of oil seals (loss of engine oil due to leaks).

Design
The crankshaft consists of a single piece, cast or forged, which is divided into a large number of different sections. The main bearing journals fit into the bearings in the crankcase.
Through the so-called cheeks (or sometimes earrings), the connecting rod journals are connected to the crankshaft. This part with the connecting rod neck and cheeks is called the knee. BMW diesel engines have a crankshaft main bearing next to each crankpin. In in-line engines, one connecting rod is connected to each crankpin through a bearing, and two in V-engines. This means that the crankshaft of a 6-cylinder in-line engine has seven main bearing journals. Main bearings are numbered consecutively from front to back.
The distance between the connecting rod journal and the axis of the crankshaft determines the stroke of the piston. The angle between the connecting rod journals determines the interval between ignitions in individual cylinders. For two complete revolutions of the crankshaft or 720 ° in each cylinder, one ignition occurs.
This angle, which is called the distance between the crankpins or the angle between the knees, is calculated depending on the number of cylinders, the design (V-shaped or in-line engine) and the order of operation of the cylinders. The goal here is to run the engine smoothly and evenly. For example, in the case of a 6-cylinder engine, we get the following calculation. An angle of 720° divided by 6 cylinders results in a crankpin spacing or firing interval of 120° of the crankshaft.
There are oil holes in the crankshaft. They supply the connecting rod bearings with oil. They run from the journals of the main bearings to the connecting rod journals and are connected through the bearing beds to the engine oil circuit.
The counterweights form a mass symmetrical about the axis of the crankshaft and thus contribute to the uniform operation of the engine. They are made in such a way that, along with the forces of inertia of rotation, they also compensate for part of the forces of inertia of the reciprocating motion.
Without counterweights, the crankshaft would be severely deformed, which would lead to imbalance and uneven running, as well as high stresses in dangerous sections of the crankshaft.
The number of counterweights is different. Historically, most crankshafts had two counterweights, symmetrically to the left and right of the crankpin. V-shaped eight-cylinder engines, such as the M67, have six identical counterweights.
To reduce weight, the crankshafts can be made hollow in the area of ​​​​the middle main bearings. In the case of forged crankshafts, this is achieved by drilling.

Manufacturing and properties
Crankshafts are either cast or forged. High torque engines are fitted with forged crankshafts.

Advantages of cast crankshafts over forged ones:

• cast crankshafts are significantly cheaper;
• casting materials lend themselves very well to surface treatment to increase vibration strength;
• cast crankshafts in the same version have a weight of less than approx. on 10 %;
• cast crankshafts are better machined;
• the cheeks of the crankshaft can usually not be processed.

Advantages of forged crankshafts over cast ones:

• forged crankshafts are stiffer and have better vibration resistance;
• in combination with an aluminum crankcase, the transmission should be as rigid as possible, since the crankcase itself has a low rigidity;
• forged crankshafts have low bearing journal wear.

The advantages of forged crankshafts can be offset by cast shafts with:

• larger diameter in the bearing area;
• expensive vibration damping systems;
• very rigid crankcase design.

Bearings

As already mentioned, the crankshaft in a BMW diesel engine is mounted in bearings on both sides of the crankpin. These main bearings hold the crankshaft in the crankcase. The loaded side is in the bearing cap. Here, the force generated during the combustion process is perceived.
Low-wear main bearings are required for reliable engine operation. Therefore, bearing shells are used, the sliding surface of which is coated with special bearing materials. The sliding surface is inside, i.e. the bearing shells do not rotate with the shaft, but are fixed in the crankcase.
Low wear is ensured if the sliding surfaces are separated by a thin oil film. This means that sufficient oil supply must be ensured. This is ideally carried out from the unloaded side, i.e. in this case from the side of the main bearing bed. Lubrication with engine oil occurs through the lubrication hole. Circular groove (in radial direction) improves oil distribution. However, it reduces the sliding surface and thus increases the effective pressure. More precisely, the bearing is divided into two halves with a lower bearing capacity. Therefore, the oil grooves are usually found only in the unloaded area. Engine oil also cools the bearing.

Bearings with a three-layer insert
Crankshaft main bearings, which are subject to high demands, are often designed as bearings with a three-layer insert. A layer of babbitt is additionally galvanically applied to the metal bearing coating (for example, lead or aluminum bronze) on the steel bushing. This gives an improvement in dynamic properties. The strength of such a layer is higher, the thinner the layer. The thickness of the babbitt is approx. 0.02 mm, the thickness of the metal base of the bearing is between 0.4 and 1 mm.

Coated bearings
Another type of crankshaft bearing is the coated bearing. This is a bearing with a three-layer shell with a layer sprayed on the sliding surface that can withstand very high loads. Such bearings are used in highly loaded engines.
Coated bearings are very hard due to their material properties. Therefore, such bearings are usually used in places where the heaviest loads take place. This means that coated bearings are mounted on one side only (pressure side). On the opposite side, a softer bearing is always installed, namely a bearing with a three-layer liner. The softer material of such a bearing is able to absorb dirt particles from the part. This is extremely important to prevent damage to it.
Vacuuming separates the smallest particles. With the help of electromagnetic fields, these particles are applied to the sliding surface of a bearing with a three-layer insert. This process is called sputtering. The sprayed sliding layer is characterized by an optimal distribution of the individual components.
Coated bearings in the area of ​​the crankshaft are installed in BMW diesel engines with maximum power and in TOP versions.

Careful handling of bearing shells is essential, as the very thin metal layer of the bearing cannot compensate for plastic deformation.
Coated bearings can be identified by the embossed "S" on the back of the bearing cover.
Thrust bearing
The crankshaft has only one thrust bearing, which is often referred to as a centering or thrust bearing. The bearing holds the crankshaft in the axial direction and must absorb forces acting in the longitudinal direction. These forces are generated by:

• gears with oblique teeth to drive the oil pump;
• clutch control drive;
• car acceleration.

The thrust bearing may be in the form of a shoulder bearing or a compound bearing with thrust washers.
The shouldered thrust bearing has 2 ground bearing surfaces for the crankshaft and rests on the main bearing bed in the crankcase. A flanged bearing is a one-piece bearing half, with a flat surface perpendicular or parallel to the axis. On earlier engines, only one half of the flanged bearing was installed. The crankshaft had an axial bearing of only 180°.
Composite bearings are made up of several parts. With this technology, one thrust half-ring is installed on both sides. They provide a stable, free connection to the crankshaft. Due to this, the thrust half rings are movable and fit evenly, which reduces wear. In modern diesel engines, two halves of a composite bearing are installed to guide the crankshaft. As a result, the crankshaft is supported by 360°, which ensures very good resistance to axial movement.
It is important that engine oil lubrication is ensured. Thrust bearing failure is usually caused by overheating.
A worn thrust bearing starts to make noise, primarily in the area of ​​the torsional vibration damper. Another symptom may be a malfunctioning crankshaft sensor, which in cars with automatic transmission manifests itself through hard shocks when shifting gears.

Connecting rods with bearings General information
The connecting rod in the crank mechanism connects the piston to the crankshaft. It converts the linear motion of the piston into the rotational motion of the crankshaft. In addition, it transfers the forces arising from the combustion of fuel and acting on the piston from the piston to the crankshaft. Since it is a part that experiences very large accelerations, its mass has a direct impact on the power and smoothness of the engine. Therefore, when creating the most comfortable running engines, great importance is attached to optimizing the mass of the connecting rods. The connecting rod experiences loads of forces from gases in the combustion chamber and inertial masses (including its own). The connecting rod is subjected to variable compression and tension loads. In high-speed gasoline engines, tensile loads are decisive. In addition, due to the lateral deviations of the connecting rod, a centrifugal force is generated, which causes bending.

The features of the connecting rods are:

• M47 / M57 / M67 engines: parts of the bearings on the connecting rod rod are made in the form of coated bearings;
• M57 engine: the connecting rod is the same as that of the M47 engine, material C45 V85;
• M67 engine: trapezoidal connecting rod with a bottom head made by breaking, material C70;
• M67TU: Conrod bearing shell thickness increased to 2mm. Connecting rod bolts are installed with sealant for the first time.

The connecting rod transmits the force and pressure from the piston to the crankshaft. Connecting rods today are made of forged steel, and the connector on the large head is made by breaking. The break, among other things, has the advantages that the break planes do not require additional processing and both parts are precisely positioned relative to each other.

Design
The connecting rod has two heads. Through the small head, the connecting rod is connected to the piston using a piston pin. Due to the lateral deflection of the connecting rod during rotation of the crankshaft, it must be able to rotate in the piston. This is done using a plain bearing. To do this, a sleeve is pressed into the small head of the connecting rod.
Through the hole in this end of the connecting rod (on the piston side), oil is supplied to the bearing. On the side of the crankshaft is a large split connecting rod head. The large connecting rod head splits so that the connecting rod can be connected to the crankshaft. The operation of this assembly is provided by a plain bearing. The plain bearing consists of two bushings. An oil hole in the crankshaft supplies the bearing with engine oil.
The following illustrations show the stem geometry of straight and oblique split connecting rods. Connecting rods with an oblique connector are used mainly in V-shaped engines.
V-shaped engines, due to high loads, have a large diameter of the connecting rod journals. The oblique connector allows you to make the crankcase more compact, because when the crankshaft rotates, it describes a smaller curve at the bottom.

Trapezoidal connecting rod
In the case of a trapezoidal connecting rod, the small head in cross section has the shape of a trapezoid. This means that the connecting rod becomes thinner from the base, adjacent to the rod of the connecting rod, to the end at the small head of the connecting rod. This allows for further weight savings as material is saved on the “unloaded” side while the full width of the bearing is maintained on the loaded side. It also reduces the distance between bosses, which in turn reduces piston pin deflection. .Another advantage is the absence of an oil hole in the small end of the connecting rod, since the oil enters through the beveled sidewall of the plain bearing.Due to the lack of a hole, its negative effect on strength is eliminated, which allows the connecting rod to be made even thinner in this place.Thus not only saves weight , but it also results in a gain in the space of the piston.

Manufacturing and properties
The rod blank can be made in various ways.

hot stamping
The starting material for the manufacture of the connecting rod blank is a steel rod, which is heated to approx. up to 1250-1300 "C. Rolling redistributes the masses towards the connecting rod heads. When the main form is formed during stamping, a flash is formed due to excess material, which is then removed. stamping properties are improved by heat treatment.

Casting
When casting connecting rods, a plastic or metal model is used. This model consists of two halves that together form a connecting rod. Each half is molded in the sand so that the reverse halves are produced accordingly. If you now connect them, you get a mold for casting a connecting rod. For greater efficiency, many connecting rods are cast next to each other in one mold. The mold is filled with liquid iron, which then slowly cools.

Treatment
Regardless of how the blanks were made, they are machined to their final dimensions.
To ensure smooth operation of the engine, the connecting rods must have a specified mass within a narrow tolerance range. Previously, additional processing dimensions were set for this, which were then milled if necessary. With modern manufacturing methods, technological parameters are controlled so precisely that it allows the production of connecting rods within acceptable weight limits.
Only the end surfaces of the large and small heads and the connecting rod heads themselves are processed. If the connection of the connecting rod head is performed by cutting, then the surfaces of the connection must be processed additionally. The inner surface of the large connecting rod head is then drilled and honed.

Fracture connector
In this case, the large head is divided as a result of a break. In this case, the specified fault location is marked by punching, broach or with the help of a laser. Then the connecting rod head is clamped on a special two-piece mandrel and separated by pressing a wedge.
This requires a material that breaks without stretching too much beforehand (deformation When the connecting rod cap breaks, both in the case of a steel connecting rod and in the case of a powder materials connecting rod, a fracture surface is formed. This surface structure accurately centers the main bearing cap during installation on the connecting rod.
Fracture has the advantage that no further processing of the parting surface is required. Both halves match exactly. Positioning with centering sleeves or screws is not required. If the connecting rod cap is reversed or installed on the other rod of the connecting rod, the fracture structure of both parts is destroyed and the cap is not centered. In this case, it is necessary to replace the entire connecting rod with a new one.

Threaded fastening

The threaded fastening of the connecting rod requires a special approach, since it is subjected to very high loads.
The threaded fasteners of the connecting rods are subjected to very rapidly changing loads during the rotation of the crankshaft. Since the connecting rod and its fastening bolts are moving parts of the engine, their weight should be minimal. In addition, limited space requires a compact threaded fastener. This results in a very high load on the threaded fastening of the connecting rod, which requires particularly careful handling.
See TIS and ETC for details on connecting rod threads such as threads, tightening order, etc.
When installing new set of connecting rods:
connecting rod bolts can only be tightened once when installing the connecting rod to check the bearing clearance and then at the final installation. Since the connecting rod bolts have already been tightened three times during the processing of the connecting rod, they have already reached their maximum tensile strength.
If the connecting rods are reused and only the connecting rod bolts are replaced: the connecting rod bolts must be tightened again after checking the bearing clearances, loosened again and tightened a third time until they reach maximum tensile strength.
If the connecting rod bolts have been tightened at least three times or more than five times, engine damage will result.

Piston with rings and piston pin

The pistons convert the gas pressure generated during combustion into motion. The shape of the piston head is decisive for mixture formation. Piston rings ensure a thorough sealing of the combustion chamber and regulate the thickness of the oil film on the cylinder wall.
general information
The piston is the first link in the chain of parts that transmit engine power. The task of the piston is to absorb the pressure forces generated during combustion and transfer them through the piston pin and connecting rod to the crankshaft. That is, it converts the thermal energy of combustion into mechanical energy. In addition, the piston must drive the upper head of the connecting rod. The piston, together with the piston rings, must prevent the emission of gases from the combustion chamber and oil consumption, and do this reliably and under all engine operating modes. The presence of oil on the contact surfaces helps sealing. BMW diesel engine pistons are made exclusively from aluminium-silicon alloys. So-called full-skirted autothermal pistons are installed, in which steel strips included in the casting serve to reduce installation gaps and regulate the amount of heat generated by the engine. To match the material to the cylinder walls of gray cast iron, a layer of graphite is applied to the surface of the piston skirt (semi-fluid friction method), due to which friction is reduced and acoustic characteristics are improved.

As part of the crank mechanism, the piston experiences loads:

• pressure forces of gases formed during combustion;
• moving inertial parts;
• side slip forces;
• moment at the center of gravity of the piston, which is caused by the location of the piston pin with an offset from the center.

The forces of inertia of moving reciprocating parts arise due to the movement of the piston itself, piston rings, piston pin and connecting rod parts. The inertial forces increase in a quadratic dependence on the rotational speed. Therefore, in high-speed engines, the low mass of pistons together with rings and piston pins is very important. In diesel engines, the piston crowns are subjected to a particularly high load due to an ignition pressure of up to 180 bar.
The deflection of the connecting rod creates a lateral load on the piston perpendicular to the axis of the cylinder. This acts in such a way that the piston, respectively, after bottom dead center or top dead center, is pressed from one side of the cylinder wall to the other. This behavior is called a change of fit or a change of side. To reduce piston noise and wear, the piston pin is often offset from the center by approx. 1-2 mm (non-axial), This creates a moment that optimizes the behavior of the piston when changing the fit.

Design

The piston has the following main areas:

• piston bottom;
• piston ring belt with cooling channel;
• piston skirt;
• piston boss.

BMW diesel engines have a combustion chamber cavity in the piston crown. The shape of the cavity is determined by the combustion process and the position of the valves. The area of ​​the piston ring belt is the lower part of the so-called firing belt, between the piston crown and the first piston ring, as well as the bridge between the 2nd piston ring and the oil scraper ring.