Diesel engine.
Diesel cycle

Endothermic engine

Parts and components of a heat engine

Parts and components of a heat engine

In this section we describe the different parts of a thermal engine. Of thermal engines have many types, however we will focus on the diesel engine and the gasoline engine (otto cycle).

Despite being different engines share many parts in common, so the scheme presented will serve to indicate the nomenclature of each element.

The most important elements of internal combustion engines, common to diesel and explosion engines, we classify them in two large groups:

Schematic of a heat engine

1.- Bank
2.- Cylinder
3.- Tray, oil tank
4.- Crankshaft
5.- Connecting rod
6.- Plunger pin
7.- Piston
8.- Compression rings
9 .- Crankshaft
10.- Spark plug
11.- Valve spring
12.- Camshaft
13.- Seesaw
14.- Exhaust valve
15.- Vacuum valve
16.- Valve head

The cylinder is the cylindrical container in which the piston moves with reciprocating rectilinear motion. The cylinder is part of the block of cylinders or monoblock, as it was called formerly. This, in turn, is part of the bench, which we can consider as the fundamental structure of the engine. In many cases, the cylinder block is separated from the bedplate, to which it is attached by bolts.

The upper part of the cylinder is closed by the cylinder head.

The volume comprised in the cylinder between the cylinder head and the piston represents the combustion chamber, in which the mixture of air and fuel, ie the active fluid, is burned.


Fixed elements of a heat engine


The block is the element that contains the cylinders. Inside the engine block are the motor elements (pistons, connecting rods and crankshaft), which serves as support or bench. It is usually made of gray cast iron (iron alloy with a carbon content ranging between 2 and 4.5%) or aluminum alloy. The shape and dimensions depend on the number and arrangement of the cylinders.

On the outside of the block, the rest of the thermal engine's construction elements are fixed: the cylinder head on the upper part and the crankcase on the lower part. At one end are located the control elements of the distribution and auxiliary organs of the engine, such as the alternator, the air conditioning compressor, etc., and at the opposite end is the gearbox.

In the block there are also the elements of anchoring the engine to the chassis, supports that have elastic elements to absorb the vibrations of the engine, thus preventing them from being transmitted to the bodywork.


The cylinders are the most important part of the block. The piston travels between the extreme positions of the cylinder (top dead center PMS and bottom dead center PMI) which it occupies during its reciprocating movement.

According to the procedure used to obtain the cylinders, we distinguish three types of block:

  • Integral block: The cylinders are machined on the material of the same block.
  • Drywall Block: In this case, a thin-walled cylinder or sleeve is mounted in each hole in the block. These liners are snap-mounted, in contact with the wall of the block, so they can be cooled.
  • Blocks of wet shirts: The shirts are thick walled and in direct contact with the coolant, which constitutes the authentic cylinder. They are easily removable and are provided with gaskets to prevent the liquid from passing to the postman.

Cylinder head

The cylinder head is fixed by screws or studs on the upper plane of the block. The cylinder head serves as a tight cover for the cylinders, since it contains all or part of the combustion chamber, except in the case that it is formed in the head of the piston.

It installs, among other elements, the valves, in the 4-stroke engines, with the mechanisms to operate them; ignition and injection elements, intake and exhaust manifolds, etc. In the cylinder head, the coolant chambers are also machined.

The cylinder heads are usually made of aluminum alloy, material with a low specific weight and good thermal conductivity, which allows the heat to be evacuated quickly. The cylinder heads for 2-stroke thermal engines (small displacement motorcycles) are simpler, as they do not have distribution, and many are cooled by air.

To ensure a perfect seal between the cylinder head and the cylinder block and taking into account that it has to withstand high pressures and temperatures, a gasket, called cylinder head gasket, made of synthetic fibers to replace asbestos, is mounted between the two elements, very negative for health.


The crankcase is the container that contains the lubrication oil of the heat engine. It is mounted on the lower part of the block by means of screws and with the interposition of a gasket to facilitate sealing. Normally it is made of steel sheet, although it can also be made of aluminum alloy, due to the good thermal conductivity of this metal and the reduction of the sound level of the motor.

Inside the crankcase there are partitions to stop the movement of the oil during the movement of the vehicle. In the lower part, a screw cap is added to drain the oil.

Moving parts of a heat engine


the piston is the moving element that moves inside the cylinder, with an alternative rectilinear movement. This movement is given, at the time of the explosion, by the force of the gases and, during the other times, by the connecting rod. It consists of two parts: the head and the skirt. The head receives the pressure caused by the explosion and works at very high temperatures (from 300 ºC to 400 ºC). It has slots that house the segments that ensure airtightness. It is usually made of aluminum with copper, silicon and nickel alloys to harden it.

The piston is connected to the connecting rod through the bolt, which allows a movement of oscillation between the two elements. The bolt is made of cemented steel, material of great hardness and tenacity, and in the form of hollow cylinder to lighten its weight and, therefore, to reduce inertia.


They are elastic rings, located in a variable number on the grooves made in the head of the piston.

They fulfill the following functions:

  • Ensure tightness and lubrication of the cylinder.
  • Evacuate heat to the cylinder.
  • Prevent oil from passing into the combustion chamber.

There are two types: compression and inlet.

Compression segments: Generally two are mounted, the fire and the sealing:

  • Fire segment: It is mounted on the highest part of the piston head and is subjected to very hard working conditions due to pressures and high temperatures to which it is exposed. To increase its strength, it is usually coated with chromium.
  • Sealing segment: It is located behind the fire segment and is usually conical. The largest segment is below. It is subject to less severe conditions than the fire segment.

Outrigger segment: It is located after the sealing segment and has a series of

Slots that allow to collect the oil deposited in the walls of the cylinder during the displacements

From the piston and send it inside the piston to lubricate the bolt.

Connecting rod

The connecting rod transmits to the crankshaft the force with which the explosion of the gases pushes the piston. At the same time, it is part of the set that transforms the alternative linear movement into a rotary movement. It is usually manufactured from carbon steel alloy with chromium, manganese or molybdenum.

It consists of three parts: head, body and foot. The head is the part that goes attached to the crankshaft, through the interposition of the crankcase half-bearings, also called antifriction. The body has a double T or H profile; and is subjected to great efforts of traction, compression and bending, and the upper part, called connecting rod, constitutes the union with the piston through the bolt and with the interposition of a bronze bearing.


The crankshaft collects the forces originated during the explosion and converts, through the connecting rod, the reciprocating linear movement of the piston in rotary motion. It transmits the movement and the motive force to the transmission elements that are coupled to it. It is subject to torsion and bending stress, and has a strong and very resistant structure. The crankshafts, which can be obtained through melting or forging processes, are made of steel with alloys of Cr, Ni, Mo, etc.

The crankshaft consists of supports, normally five for an inline four-cylinder engine, which are attached to the block's bedplate. It also has some elbows called stumps where the connecting rods are fastened. In the extension of each elbow are the counterweights, which serve to balance the crankshaft. At one end of the crankshaft is mounted the timing gear and, in the other, the flywheel.


The flywheel has the function of storing kinetic energy to soften and regulate the rotation of the thermal engine. This energy accumulates it in the motor time (explosion) and yields it during the passive or resistant times (admission, compression and escape).

The flywheel is made of gray cast iron and on the door periphery, mounted under pressure, a crown gear of stamped and cemented steel which is where the electric starter motor of the vehicle is coupled. Its weight and dimensions are perfectly calculated for each type of motor.

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Last review: November 24, 2017