An internal combustion engine works by firing successively inside a hermetically sealed cylinder. For this to occur, components are needed that transform the reciprocating rectilinear movement of the pistons into circular movement on the crankshaft. All these components participating in these movements within the engine block are called Mobile Assembly. The mobile set consists of:
- Piston;
- connecting rods;
- Crankshaft.
Furthermore, the mobile assembly is housed in pressure-lubricated supports, called Bearings. The bearings cannot be in direct contact with the crankshaft, as a result of which their wear would occur prematurely. To prevent this from happening, bearings are used. Pistons, connecting rods, bearings and the crankshaft are made from materials with different characteristics. The big challenge in designing an internal combustion engine is that it works without friction and excessive temperatures.
Piston
Also called a piston, the piston has a cylindrical (actually elliptical) shape and moves within the engine cylinder in reciprocating movements. The pistons are made of gray cast iron, aluminum alloy or steel, require low specific weight, high resistance and rapid dissipation of heat acquired after combustion.
The piston must have a low specific weight, as its inertia must be as low as possible so that it has the least difficulty in executing its movement. It must be resistant to combustion that occurs inside the cylinder, absorb combustion heat and exchange it with the cylinder wall and lubricating oil. Its main function is to transmit the force generated by the combustion of the air-fuel mixture to the connecting rod. But in addition, the piston must seal the combustion chamber through the tracking rings, transfer the heat gained from burning the air/fuel mixture to the cooling fluid, determine the path of the connecting rod within the cylinder and serve as support for the applied normal force. against the cylinder wall, in this case completely transferring it to the connecting rod.
As it can be made from different types of materials, the piston clearance within the cylinder must be determined following important criteria, as each material has different attributes. For example, pistons made from cast iron are considerably more resistant, can work with tighter clearances and even in critical conditions, never suffer from seizure and consequent piston stoppage. While aluminum pistons are lighter, excellent heat conductors, work with larger clearances, but are not as resistant as their cast iron counterparts. The determining criteria for piston clearances within the cylinder are the diameter, raw material, piston size, engine rotation speed, cooling system, purpose of using the engine and the type of fuel to be used. .
Literally it would be wrong to say that the piston has a cylindrical shape, in reality the piston is designed to have an elliptical shape, but it is difficult to realize this, as they are millimetric in size. However, once the engine starts running and the components reach their working temperature, the piston assumes its cylindrical shape. Even so, the pistons do not remain at their constant measurements, the expansion of the pistons varies according to the driver’s requests, that is, with the activation of the accelerator. In fact, not just the pistons, the cylinders as well, but in this case the expansion variation occurs more slowly, and at this point we have the risk of seizure (It is at this point too, that the factories insist on informing in their manual care for the engine in its first moments of operation, known as running-in). Dimensionally, the pistons are wider at the top than at the skirt (bottom). The reason is the greater expansion of the top of the pistons due to the high temperatures reached in the combustion chamber. That’s why the gaps in the top and skirt are different.
When manufacturing pistons, some devices can be used to control their expansion. A widely used device is called the autothermal ring, a steel belt located below the piston head. Its mission is to control the expansion in that critical region, therefore it is possible to reduce the clearance at the top of the aluminum pistons. of pistons, the use of austenitic cast iron ring holders on the piston is common. This improvement aims to provide greater resistance to the channels that house the follower rings, both of which are subject to high temperatures and low lubrication, resulting in greater wear. Another alternative that aims to control piston expansion is the split skirt. In this case, an oblique cut is made in the piston skirt to reduce clearance in that location. Slotted skirt pistons are mounted with the cut positioned on the side towards which the connecting rod tilts in its downward movement.
Structurally, the piston has three defined parts, the top, the ring region and the skirt. The region at the top of the piston is the subject of much study in its design, as its distance from the first tracking ring determines the location where part of the air/fuel mixture is located at the moment of compression. However, after combustion, the mixture contained therein was not properly burned, resulting in hydrocarbons expelled during the exhaust time. Because of this, the reduction of the distance from the top of the piston to the first tracking ring has been implemented. Consequently, the channels and tracking rings must be reworked to withstand the greater demands of that environment. Furthermore, the piston pin is now closer to the piston head, the piston is shorter and more balanced in terms of load balance.
Heat conduction from the top of the piston to its bottom is due to its internal microstructure, the bottom part, that is, the skirt, is sometimes equipped with polymeric coatings (Teflon). Their function is to reduce friction in that region and minimize wear, consequently improving cold starting. During engine operation, we know that the speed of the pistons at the bottom and top dead centers is zero. But during the course it varies continuously, we can admit that a production scale automobile in the thousands per day has an engine capable of performing speeds of 70-75 km/h of the pistons in its cylinders. From this it is possible to understand why we have to use quality lubricants, or this would easily allow dry friction between piston and cylinder. The average speed of a piston can be calculated by the formula:
Vaverage = (2.s.n)/60
For this we have that s is the piston stroke and n is the number of rotations.
Tracking Rings
As the piston is smaller than the cylinder, it is necessary to use a component capable of sealing the combustion chamber at the bottom of the block. This component is called the tracking ring. Follower rings are manufactured from mild iron, also known as soft iron or mild steel. It is an iron with a high purity index, that is, steel with a low carbon content (less than 25%). Follower rings are not actually rings, they are almost circular in shape, as their ends do not meet. They are designed to have a larger diameter than the piston, and once mounted on the pistons, and these housed in the cylinders, the rings start to function as springs, maintaining contact with the cylinder wall.
Typically, in automotive applications, pistons have three tracking rings each. There are applications in which fewer tracking rings are necessary, such as, for example, in 2-stroke engines for Karts, in which the piston has only one tracking ring. The rings are housed in ring holders contained in the pistons, also called channels. The first channel houses the fire ring, and the second channel houses the compression ring. That one and this one are responsible for sealing the combustion chamber in relation to the bottom of the engine block. As it is directly in contact with the combustion chamber, the firestop ring requires a greater clearance than the others, and also more resistant raw material. The last ring, housed in the third groove from top to bottom, is called the oil scraper ring. Its function is to scrape off excess oil contained on the cylinder wall and prevent it from rising to the combustion chamber. When removing the oil from the cylinder, it is retained inside the scraper ring, and flows through holes into the piston, then falling into the crankcase.
Piston Pin
The connection between the connecting rod and the piston is carried out by a pin, the so-called piston pin, tubular in shape and made of case-hardened, tempered and ground steel. These treatments give the piston pin greater resistance, and its tubular shape allows for low weight. The piston pin can be mounted in three different ways:
- Floating pin;
- Semi-floating pin;
- Fixed pin.
In all ways of assembling the pin on the piston, the piston and connecting rod must be heated, since the gaps are small between the three components, the piston-connecting rod joint must be free.
The floating pin is the most used today, in this case the piston and connecting rod are heated to then insert the pin. The pin must not slip to the sides of the piston, which is why the piston is equipped with clips on both sides. In the case of semi-floating pins, the pin is fixed to the connecting rod, while in fixed pins, the pin is fixed to the piston. The pin has no movement at all. The piston pin is not in direct contact with the connecting rod, bearings or roller bearings are used to ensure minimum friction.
Connecting Rods
A phosphor bronze bushing is placed at the foot of the connecting rod, its purpose is to prevent direct contact between the pin and the connecting rod. In many cases, the connecting rod head is equipped with a calibrated hole in which the lubricant, under pressure, enters between the piston pin and connecting rod foot bushing, promoting wet friction between both parts. In fact, this hole in the foot of the connecting rod comes from a duct that runs throughout the connecting rod. The purpose of these holes is to promote the lubrication of the connecting rod, as well as the exchange of heat between the connecting rod and the lubricating oil. In automotive engines, it is standard to apply bearings to the connecting rod connections for durability reasons, but in smaller engines for different applications, such as karts, for example, roller bearings are frequently used. Bearings, although they also considerably minimize friction, wear out more quickly compared to bearings (also called bushings). It is no surprise that in kart engines used in competitions, roller bearings are preventive maintenance items. During vehicle operation, the connecting rod is subjected to traction, compression and bending efforts, so the shape of its body must be very resistant to these situations. The connecting rod body is often machined in an I-shape, but there are cases where A connecting rod with an H-shaped body is necessary. The latter type has greater resistance to buckling and other efforts.
The last part of the connecting rod body is the head, and this is also connected to the crankshaft journal through bearings, but in this case there are several materials that are used to manufacture these. The choice of bearing material is determined by the engine type. As a whole, the connecting rod has another important factor in its design, its length. The length of a connecting rod is determined by the engine stroke and the radius of the counterweight (crankshaft). From this, the r/l ratio is determined, the relationship between the radius of the crankshaft and the length of the connecting rod, which influences the maximum inclination that the connecting rod can reach. The ideal is to obtain the largest connecting rod possible, as this minimizes the lateral force of the piston on the cylinder due to the lower inclination of the connecting rod. However, long connecting rods have greater mass, which in itself is an obstacle to gaining engine power. On the other hand, short connecting rods generate greater inclination of the connecting rods, therefore greater support of the piston on the cylinder wall, which results in accelerated wear of the walls. There are measures to mitigate this wear, such as molybdenum disulfide or polytetrafluoroethylene film on the piston skirt, in addition to moving the cylinder axis away from the axis of the piston pin or crankshaft. The fact is that during the development of the engine there must be a compromise between the length of the connecting rod and the radius of the crankshaft, either a large connecting rod is adopted, with great inertia, or a short connecting rod is adopted, with less inertia, but which favors cylinder wear and engine vibration.
Crankshaft
The force produced during combustion is transmitted by the piston to the connecting rod, and from there to the crankshaft. This is also called the crankshaft or crankshaft. The crankshaft transforms the combustion force into useful torque. The torque is available at the ends of the crankshaft, but is transmitted to the gearbox through the end equipped with a flywheel. The crankshaft is manufactured in two ways: whole or in parts. The automotive line uses crankshafts made from a single piece, while small engines (2 strokes) use crankshafts made from several parts. However, there are cases of very large crankshafts (6 and 8 cylinders), in which it was necessary to manufacture them in parts (with movement transmission by gear) as the level of torsion would be destructive. The crankshaft material is steel, which can be made using a casting or forging process, in addition to undergoing treatments such as tempering and tempering, and containing alloys of copper, nickel, chromium, manganese and other components to provide greater resistance to the crankshaft.
The connecting rods are connected to the crankshaft through the journals, which have holes through which the lubricating oil comes out, which prevents dry friction in that area. The journals receive the connecting rods, which are fixed by the connecting rod caps through the bearings. The connecting rod head is shaped like the diameter of the journal, and this cannot have a high value, as this will increase internal friction, nor can it have an exaggeratedly small diameter, which would harm the crankshaft’s resistance. The crank radius is what determines the useful torque of the engine. When it starts to work, the crankshaft rotates with its journals supported on lubricated bearings, which are arranged along the crankshaft axis. The bearings are located in the engine block and their other halves are screwed into them. The bearing tightening sequence has the same principle applied to the heads, tightening from the center to the ends and loosening from the ends to the center.
In the crankshaft bearings there are calibrated holes into which lubricating oil is supplied under pressure from the oil pump. Internally in the crankshaft are lubrication channels that lead to holes contained in the journals and journals. Inside the crankshaft, the oil exchanges heat, as well as lubricating bearings, journals and journals. The crankshaft does not rest directly on the bearings, which would cause premature wear on both parts. To reduce friction between bearings and crankshaft, in addition to lubricating oil, bearings are used placed on the bearings. The design of a crankshaft aims to balance the vibrations, bending and torsion to which it is subjected, which occur due to combustion, more precisely the combustion occurring in cylinders at the end opposite to the flywheel. Whenever these cylinders enter engine time and soon after switch to unproductive times, the end opposite the flywheel suffers vibrations due to torsion at this same point. Thus, the design of a crankshaft can include counterweights on the trunnions, balancing shaft and vibration dampers, in addition to the number of bearings also helping to balance the mobile assembly. However, the more bearings the block has to support the crankshaft, the greater its length and friction will be. Which is why the number of bearings must be studied seriously.
Engine flywheel
The engine flywheel is a purposely heavy part, which can be made of cast iron or steel. Its main function is to absorb the kinetic energy of combustion and use it as potential energy during unproductive times. When combustion occurs, the energy produced rotates the crankshaft, on which the heavy flywheel is screwed. Because it is very heavy in relation to the crank, pistons and connecting rods, the flywheel suffers a lot of inertia to stop, and thus makes the mobile assembly continue to rotate even during unproductive times. The mass factor of the flywheel is inversely proportional to the number of cylinders, as the more the number of cylinders, the more cylinders will undergo combustion time after the two revolutions of the crankshaft, therefore more energy, and then the mass of the flywheel can be reduced. The flywheel is bolted to the crankshaft, more precisely at its rear end, using chrome-nickel steel screws tightened in the crankshaft collar. The crankshaft is installed according to the position determined in the project, but this is guaranteed by the frequent use of keys to place the flywheel in the right position.
It is through the flywheel that the engine receives the necessary rotation to start working, a steel crown is mounted around the flywheel. The teeth on the crown mesh with the teeth on the starter motor and enable the engine to start. Furthermore, it is through the flywheel that many electronic injection systems determine the engine rotation, as the flywheel crown is also a phonic wheel, which rotates very close to a rotation sensor. The flywheel is also used as an intermediary between the engine and gearbox, it is designed to be a contact surface for the clutch disc, thus transmitting torque from the engine to the gearbox. Furthermore, its primary shaft is supported by a small bushing in the center of the flywheel.
Bearings and bearings
Both bearings and bearings work with the aim of providing support and guidance for shafts. The bearings are located in the engine block, they have a circular shape in which half of them are detachable to enable the assembly of the crankshaft shaft. However, the bearings cannot be in direct contact with the shaft, which is why it is equipped with bearings. The bearings promote the reduction of friction, consequently reducing wear on moving parts.
There are two types of bearings used in automotive applications, rolling bearings and plain bearings (also called hydrodynamic bearings). The first is widely used in 2-stroke engines for kart competitions, the rolling bearings are also called grids. They are low cost, suitable for permanent lubrication and the small dimensions of the crankshaft. However, they are very sensitive to contamination in the lubricating oil, and wear out quickly due to sudden changes in engine load, in addition to being quite noisy. These characteristics disqualify rolling bearings for civil automotive applications, where high durability, smoothness and silence when running are required. In these points, hydrodynamic bearings are successful, as they are easy to assemble and replace, produce less friction and noise, consequently last longer, in addition to being very silent.
Rolling bearings suffer more from wear, as they are based on the principle that the rollers (or rollers) of the bearing can roll (ideally) or slide. In fact, slippage always occurs, generating wear, and at times when there is a lack of lubrication, this wear increases considerably. The rolling bearing races are made of chromium alloys, the rolling bearing has its structure made of steel or brass. Plain bearings use the principle of fluid dynamics to function, which is why they are also called hydrodynamic bearings. These are made up of the bearing itself, machined into the engine block and cylinder head, and its covers. As they are made from the same material as the block, it is necessary to use friction-reducing covers called bearings (also called bushings).
Bearings are made of steel, but are internally coated with bronze (hence the name bearings) or other special materials such as tin, copper, antimony, zinc or aluminum alloys. The shape of the bearing follows the bearing shape, many of them are composed of two semicircular parts that adjust to each other when tightening the crankshaft or valve control shaft in their bearings. The main function of the bearing is to support the loads imposed by combustion on the bearings, always reducing friction in any operating regime, consequently reducing wear on the shaft that rotates on the bearing. To perform this function, the bearing needs to fit perfectly into the bearing, and in no way can it rotate in conjunction with the shaft, otherwise it would quickly destroy the bearing. The bearing fits into the bearing and remains there due to the keyway contained in the bearing, a recess houses the shoulder (key) contained in the bearing. There are cases in which this shoulder is not on the bearing, but on the bearing so that the bearing has a hole to fit into the bearing shoulder.
To ensure lubrication of the shaft, the bearing has a channel and an oil hole, through which oil is supplied under pressure from the oil pump. The bearings are made of anti-friction metals, but in addition a sintered, porous material, which retains the lubricant in its pores, guaranteeing the oil film so desired by the engine’s moving parts, that is, wet friction. In automotive engines, bearings are present in the crankshaft bearings (bearing or bearing bearings), camshaft bearings, bearings of other axles (rocker or balancing arms), in the foot of the connecting rod head. The materials used in bearings vary depending on the engine application. In cases of simpler engines, which equip large production vehicles, bearings made of an alloy of tin, copper and antimony are used. The main characteristic of this bearing is its silent, smooth operation, but extremely noisy in cases of overheating.
When the application is aimed at high-performance engines, it is common to use pink metal bearings, that is, an alloy of aluminum, copper and zinc, as they support the greater load to which the moving assembly is exposed. In the absence of lubrication, the bearings and shaft wear quickly. For application in the diesel line, bronze (tin and copper alloy) or light metal (aluminum and zinc alloy) bearings are chosen, with an advantage for the latter. Bronze bearings are made entirely of this material, being different from the bearings used in Otto cycle engines. This is necessary because they must withstand the large torque that the engine can produce, in addition to the fact that parts in diesel engines are more robust (and heavier). In the absence of lubrication, both shaft and bearing (with bearing) are highly likely to wear out.
Light metal bearings expand more compared to bronze bearings, are just as resistant, and are also entirely made from the same material. However, its tolerance levels are greater, and when there is a lubrication deficiency, the tendency of this bearing is to lock the engine, and if it continues to operate, there will be great wear on the bearing and shaft. With the exception of bronze and metal bearings, the other materials mentioned are actually bearings made of steel, but with different materials internally. These materials give the bearing anti-friction and resistance characteristics, they are arranged in millimeter layers that can be easily worn out when periodic maintenance is neglected.
Maintenance
The moving assembly is the most critical part of the engine, it develops high speed and reaches high temperature. Thus, the lubricating oil has the tough task of reducing internal friction and helping the cooling system exchange heat with the engine. Because of this, changing the oil is the most basic and most important maintenance of a car engine. An expired oil will not withstand the pressure and temperature, which will be exposed to the bearings and cylinder walls. This way the oil stops lubricating, and exposes the surfaces, in addition to burning and turning into charcoal, literally.
The friction points in the moving assembly are between piston and cylinder, connecting rod and piston pin, connecting rod and crankshaft and crankshaft and bearings. These are critical points where any deficiency in lubrication, and the persistence of this deficiency, generates harmful wear on the engine. Therefore, the engine must have the cooling, lubrication and electronic injection system fully functioning. Bearing in mind that an internal combustion engine operates under critical conditions (low temperature and lubrication) after cold starting, it can be concluded that this moment is the peak of engine wear, and as the temperature increases and lubrication increases, This wear is severely reduced. Therefore, it is normal for pistons and bearings (in general) to present scratches, signs of wear from operation without lubricant, as long as these are not excessive to the point of reaching the internal layers of the material.
After the cold phase, the engine starts to operate at normal working temperature, lubrication is efficient and meets the demands imposed by the engine. However, the oil has an expiration date, and when reached, it requires changing the oil (and oil filter). The change is necessary due to its oxidation, the oxidation of the lubricating oil generates corrosion of internal components, reducing the lubricating power. The bearings, in fact, are severely penalized by the corrosion of oxidized oil, generating wear points called “pits”. The pistons work when there is a lack of lubrication when the oil pump fails or when the combustion chamber is washed (excessively rich air/fuel mixture). Washing dilutes the lubricating oil on the cylinder wall, and occurs due to problems with the electronic injection system or carburetor, consequently the piston suffers seizure in the top region and in extreme cases the seizure can reach the skirt region, or the occurrence of piston head erosion.
Failures in the oil pump can harm both pistons and bearings, which, when they start to operate with a lack of oil, have a shiny surface, and then wear. The scratches on the bearings overcome the anti-friction layers and reach the steel, the shaft over the bearings is also severely worn. Another important factor for the proper functioning of the mobile set is the cooling system. Knowing that the piston expands faster than the cylinder (lower expansion coefficient), the cooling system must be working perfectly to prevent the piston from reaching excessive expansion. Otherwise, the piston would seize inside the cylinder, especially in the region close to the piston pin.
Automotive Papers 