Crank mechanism of an internal combustion engine: device, purpose, how it works


In internal combustion engines, there are two mechanisms that make it possible to move vehicles. It is gas distribution and crank. Let's focus on the purpose of the KShM and its structure.

What is the engine crank mechanism

KShM means a set of spare parts that form a single unit. In it, a mixture of fuel and air in a certain proportion burns and releases energy. The mechanism consists of two categories of moving parts:

  • Performing linear movements - the piston moves up / down in the cylinder;
  • Performing rotational movements - the crankshaft and the parts installed on it.
Crank mechanism of an internal combustion engine: device, purpose, how it works

A knot that connects both types of parts is capable of converting one type of energy into another. When the motor works autonomously, the distribution of forces goes from the internal combustion engine to the chassis. Some cars allow energy to be redirected back from the wheels to the motor. The need for this may arise, for example, if it is impossible to start the engine from the battery. Mechanical transmission allows you to start the car from the pusher.


What is the engine crank mechanism for?

KShM sets in motion other mechanisms, without which it would be impossible for the car to go. In electric vehicles, the electric motor, thanks to the energy it receives from the battery, immediately creates a rotation that goes to the transmission shaft.

The disadvantage of electric units is that they have a small power reserve. Although the leading manufacturers of electric vehicles have raised this bar to several hundred kilometers, the vast majority of motorists do not have access to such vehicles due to their high cost.

Crank mechanism of an internal combustion engine: device, purpose, how it works

The only cheap solution, thanks to which it is possible to travel long distances and at high speed, is a car equipped with an internal combustion engine. It uses the energy of the explosion (or rather the expansion after it) to set in motion the parts of the cylinder-piston group.


The purpose of the KShM is to ensure uniform rotation of the crankshaft during rectilinear movement of the pistons. Ideal rotation has not yet been achieved, but there are modifications to the mechanisms that minimize the jerking resulting from sudden jolts of the pistons. 12-cylinder engines are an example of this. The angle of displacement of the cranks in them is minimal, and the actuation of the entire group of cylinders is distributed over a greater number of intervals.

The principle of operation of the crank mechanism

If you describe the principle of operation of this mechanism, then it can be compared with the process that occurs while riding a bicycle. The cyclist alternately presses on the pedals, driving the drive sprocket into rotation.

The linear movement of the piston is provided by the combustion of the BTC in the cylinder. During a microexplosion (the HTS is strongly compressed at the moment the spark is applied, therefore, a sharp push is formed), the gases expand, pushing the part to the lowest position.

Crank mechanism of an internal combustion engine: device, purpose, how it works

The connecting rod is connected to a separate crank on the crankshaft. Inertia, as well as an identical process in adjacent cylinders, ensures that the crankshaft rotates. The piston does not freeze at the extreme lower and upper points.

The rotating crankshaft is connected to a flywheel to which the transmission friction surface is connected.

After the end of the stroke of the working stroke, for the execution of other strokes of the motor, the piston is already set in motion due to the revolutions of the mechanism shaft. It is possible due to the execution of the stroke of the working stroke in adjacent cylinders. To minimize jerking, the crank journals are offset from each other (there are modifications with in-line journals).


KShM device

The crank mechanism includes a large number of parts. Conventionally, they can be classified into two categories: those who perform the movement and those that remain fixed in one place all the time. Some perform various kinds of movements (translational or rotational), while others serve as a form in which the accumulation of the necessary energy or support for these elements is ensured.

Crank mechanism of an internal combustion engine: device, purpose, how it works

These are the functions performed by all the elements of the crank mechanism.

Block crankcase

A block cast from durable metal (in budget cars - cast iron, and in more expensive cars - aluminum or other alloy). The necessary holes and channels are made in it. Coolant and engine oil circulate through the channels. Technical holes allow the key elements of the motor to be connected into one structure.

The largest holes are the cylinders themselves. Pistons are placed in them. Also, the block design has supports for the crankshaft support bearings. A gas distribution mechanism is located in the cylinder head.

Crank mechanism of an internal combustion engine: device, purpose, how it works

The use of cast iron or aluminum alloy is due to the fact that this element must withstand high mechanical and thermal loads.

At the bottom of the crankcase there is a sump in which oil accumulates after all the elements have been lubricated. To prevent excessive gas pressure from building up in the cavity, the structure has ventilation ducts.

There are cars with wet or dry sump. In the first case, the oil is collected in the sump and remains in it. This element is a reservoir for the collection and storage of grease. In the second case, the oil flows into the sump, but the pump pumps it out into a separate tank. This design will prevent a complete loss of oil in the event of a sump breakdown - only a small part of the lubricant will leak out after the engine is turned off.


The cylinder is another fixed element of the motor. In fact, this is a hole with a strict geometry (the piston must fit perfectly into it). They also belong to the cylinder-piston group. However, in the crank mechanism, the cylinders act as guides. They provide a strictly verified movement of the pistons.

The dimensions of this element depend on the characteristics of the motor and the size of the pistons. The walls at the top of the structure are facing the maximum temperature that can occur in the engine. Also, in the so-called combustion chamber (above the piston space), a sharp expansion of gases occurs after the ignition of the VTS.

To prevent excessive wear of the cylinder walls at high temperatures (in some cases it can rise sharply to 2 degrees) and high pressure, they are lubricated. A thin film of oil forms between the O-rings and the cylinder to prevent metal-to-metal contact. To reduce the friction force, the inner surface of the cylinders is treated with a special compound and polished to an ideal degree (therefore, the surface is called a mirror).

Crank mechanism of an internal combustion engine: device, purpose, how it works

There are two types of cylinders:

  • Dry type. These cylinders are mainly used in machines. They are part of the block and look like holes made in the case. To cool the metal, channels are made on the outside of the cylinders for the circulation of the coolant (internal combustion engine jacket);
  • Wet type. In this case, the cylinders will be separately made sleeves that are inserted into the holes of the block. They are reliably sealed so that additional vibrations are not formed during the operation of the unit, due to which the KShM parts will fail too quickly. Such liners are in contact with the coolant from the outside. A similar design of the motor is more susceptible to repair (for example, when deep scratches are formed, the sleeve is simply changed, and not bored and the holes of the block are grinded during the motor capitalization).

In V-shaped engines, the cylinders are often not symmetrically positioned relative to each other. This is because one connecting rod serves one cylinder, and it has a separate place on the crankshaft. However, there are also modifications with two connecting rods on one connecting rod journal.

Cylinder block

This is the largest part of the motor design. At the top of this element, the cylinder head is installed, and between them there is a gasket (why is it needed and how to determine its malfunction, read in a separate review).

Crank mechanism of an internal combustion engine: device, purpose, how it works

Recesses are made in the cylinder head to form a special cavity. In it, the compressed air-fuel mixture is ignited (often called a combustion chamber). Modifications to water-cooled motors will be equipped with a head with channels for fluid circulation.

Engine skeleton

All fixed parts of the KShM, connected in one structure, are called the skeleton. This part perceives the main power load during the operation of the moving parts of the mechanism. Depending on how the engine is mounted in the engine compartment, the skeleton also absorbs loads from the body or frame. In the process of movement, this part also collides with the influence of the transmission and the chassis of the machine.

Crank mechanism of an internal combustion engine: device, purpose, how it works

To prevent the internal combustion engine from moving during acceleration, deceleration or maneuvering, the frame is firmly bolted to the supporting part of the vehicle. To eliminate vibrations at the joint, engine mountings made of rubber are used. Their shape depends on the engine modification.

When the machine is driven on an uneven road, the body is subjected to torsional stress. To prevent the motor from taking such loads, it is usually attached at three points.

All other parts of the mechanism are movable.


It is part of the KShM piston group. The shape of the pistons can also vary, but the key point is that they are made in the form of a glass. The top of the piston is called the head and the bottom is called the skirt.

The piston head is the thickest part, as it takes on the thermal and mechanical stress when the fuel is ignited. The end of that element (bottom) can have different shapes - flat, convex or concave. This part forms the dimensions of the combustion chamber. Modifications with depressions of various shapes are often encountered. All these types of parts depend on the ICE model, the principle of fuel supply, etc.

Crank mechanism of an internal combustion engine: device, purpose, how it works

On the sides of the piston, grooves are made for installing O-rings. Below these grooves there are recesses for oil drainage from the part. The skirt is most often oval in shape, and its main part is a guide that prevents the piston wedge as a result of thermal expansion.

To compensate for the force of inertia, the pistons are made of light alloy materials. This makes them lightweight. The bottom of the part, as well as the walls of the combustion chamber, encounter maximum temperatures. However, this part is not cooled by circulating coolant in the jacket. Because of this, the aluminum element is subject to strong expansion.

The piston is oil cooled to prevent seizure. In many car models, lubrication is supplied naturally - the oil mist settles on the surface and flows back into the sump. However, there are engines in which oil is supplied under pressure, providing better heat dissipation from the heated surface.

Piston rings

The piston ring performs its function depending on which part of the piston head it is installed in:

  • Compression - the topmost. They provide a seal between the cylinder and piston walls. Their purpose is to prevent gases from the piston space from entering the crankcase. To facilitate the installation of the part, a cut is made in it;
  • Oil scraper - ensure the removal of excess oil from the cylinder walls, and also prevent the penetration of lubricant into the above-piston space. These rings have special grooves to facilitate oil drainage to the piston drain grooves.
Crank mechanism of an internal combustion engine: device, purpose, how it works

The diameter of the rings is always larger than the diameter of the cylinder. Due to this, they provide a seal in the cylinder-piston group. So that neither gases nor oil seep through the locks, the rings are placed in their places with the slots offset relative to each other.

The material used to make the rings depends on their application. So, compression elements are most often made of high strength cast iron and a minimum content of impurities, and oil scraper elements are made of high-alloy steel.

Piston pin

This part allows the piston to be attached to the connecting rod. It looks like a hollow tube, which is placed under the piston head in the bosses and at the same time through the hole in the connecting rod head. To prevent the finger from moving, it is fixed with retaining rings on both sides.

Crank mechanism of an internal combustion engine: device, purpose, how it works

This fixation allows the pin to rotate freely, which reduces the resistance to piston movement. This also prevents the formation of a working only at the attachment point in the piston or connecting rod, which significantly extends the working life of the part.

To prevent wear due to frictional force, the part is made of steel. And for greater resistance to thermal stress, it is initially hardened.


The connecting rod is a thick rod with stiffening ribs. On the one hand, it has a piston head (the hole into which the piston pin is inserted), and on the other, a knit head. The second element is collapsible so that the part can be removed or installed on the crankshaft crank journal. It has a cover that is attached to the head with bolts, and to prevent premature wear of parts, an insert with holes for lubrication is installed in it.

The lower head bushing is called the connecting rod bearing. It is made of two steel plates with curved tendrils for fixation in the head.

Crank mechanism of an internal combustion engine: device, purpose, how it works

To reduce the frictional force of the inner part of the upper head, a bronze bushing is pressed into it. If it is worn out, the entire connecting rod will not need to be replaced. The bushing has holes for oil supply to the pin.

There are several modifications of connecting rods:

  • Gasoline engines are most often equipped with connecting rods with a head connector located at right angles to the connecting rod axis;
  • Diesel internal combustion engines have connecting rods with an oblique head connector;
  • V-engines are often equipped with twin connecting rods. The secondary connecting rod of the second row is fixed to the main one with a pin according to the same principle as to the piston.


This element consists of several cranks with an offset arrangement of the connecting rod journals relative to the axis of the main journals. There are already different types of crankshafts and their features separate review.

The purpose of this part is to convert the translational motion from the piston into rotational. The crank pin is connected to the lower connecting rod head. There are main bearings in two or more places on the crankshaft to prevent vibration due to unbalanced rotation of the cranks.

Crank mechanism of an internal combustion engine: device, purpose, how it works

Most crankshafts are equipped with counterweights to absorb centrifugal forces acting on the main bearings. The part is made by casting or turned on lathes from a single blank.

A pulley is attached to the toe of the crankshaft, which drives the gas distribution mechanism and other equipment, such as a pump, generator and air conditioning drive. There is a flange on the shank. A flywheel is attached to it.


Disc-shaped part. The forms and types of different flywheels and their differences are also devoted to A separate article... It is needed to overcome the compression resistance in the cylinders when the piston performs the compression stroke. This is due to the inertia of the rotating cast iron disc.

Crank mechanism of an internal combustion engine: device, purpose, how it works

A gear rim is fixed at the end of the part. The starter bendix gear is connected to it at the moment the engine starts. On the side opposite to the flange, the flywheel surface is in contact with the clutch disc of the transmission basket. The maximum frictional force between these elements ensures the transmission of torque to the gearbox shaft.

As you can see, the crank mechanism has a complex structure, because of which the repair of the unit must be carried out exclusively by professionals. To extend the engine life, it is extremely important to adhere to the routine maintenance of the car.

Additionally, watch a video review about KShM:

Crank mechanism (KShM). The basics


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