- ensures the transfer of mechanical forces to the connecting rod;
- is responsible for sealing the fuel combustion chamber;
- ensures timely removal of excess heat from the combustion chamber
The work of the piston takes place in difficult and in many ways dangerous conditions - at elevated temperatures and increased loads, therefore it is especially important that pistons for engines are distinguished by efficiency, reliability and wear resistance. That is why light but heavy-duty materials are used for their production - heat-resistant aluminum or steel alloys. Pistons are made by two methods - casting or stamping.
Piston design
The engine piston has a fairly simple design, which consists of the following parts:
Volkswagen AG
- ICE piston head
- piston pin
- Retaining ring
- Boss
- connecting rod
- Steel insert
- Compression ring one
- Second compression ring
- Oil scraper ring
The design features of the piston in most cases depend on the type of engine, the shape of its combustion chamber and the type of fuel that is used.
Bottom
The bottom may have different shape depending on the functions it performs - flat, concave and convex. The concave shape of the bottom provides a more efficient operation of the combustion chamber, however, this contributes to more deposits during the combustion of fuel. The convex shape of the bottom improves the performance of the piston, but at the same time reduces the efficiency of the combustion process fuel mixture in the chamber.
Piston rings
Below the bottom are special grooves (grooves) for installing piston rings. The distance from the bottom to the first compression ring is called the firing zone.
Piston rings are responsible for a reliable connection between the cylinder and the piston. They provide reliable tightness due to a snug fit to the cylinder walls, which is accompanied by an intense friction process. Engine oil is used to reduce friction. Piston rings are made from cast iron.
The number of piston rings that can be installed in a piston depends on the type of engine used and its purpose. Often, systems with one oil scraper ring and two compression rings (first and second).
Oil scraper ring and compression rings
The oil scraper ring ensures the timely removal of excess oil from the inner walls of the cylinder, and the compression rings prevent gases from entering the crankcase.
The compression ring, located first, receives most of the inertial loads during piston operation.
To reduce loads in many engines, a steel insert is installed in the annular groove, which increases the strength and degree of compression of the ring. Compression type rings can be made in the form of a trapezoid, barrel, cone, with a cutout.
The oil scraper ring in most cases is equipped with many holes for oil drainage, sometimes with a spring expander.
piston pin
This is a tubular part that is responsible for the reliable connection of the piston to the connecting rod. Made from steel alloy. When installing the piston pin in the bosses, it is tightly fixed with special retaining rings.
The piston, piston pin and rings together form the so-called engine piston group.
Skirt
The guide part of the piston device, which can be made in the form of a cone or barrel. The piston skirt is equipped with two bosses for connection with the piston pin.
To reduce friction losses, a thin layer of an antifriction agent is applied to the surface of the skirt (often graphite or molybdenum disulfide is used). The lower part of the skirt is equipped with an oil scraper ring.
A mandatory process for the operation of a piston device is its cooling, which can be carried out by the following methods:
- spraying oil through the holes in the connecting rod or nozzle;
- the movement of oil along the coil in the piston head;
- supplying oil to the area of the rings through the annular channel;
- oil mist
Sealing part
The sealing part and the bottom are connected in the form of a piston head. In this part of the device there are piston rings - oil scraper and compression. The channels for the rings have small holes through which the used oil enters the piston and then flows into the crankcase.
General engine piston internal combustion is one of the most heavily loaded parts, which is subjected to strong dynamic and at the same time thermal effects. This imposes increased requirements both on the materials used in the production of pistons and on the quality of their manufacture.
Rotary piston engine(RPD), or Wankel engine. Internal combustion engine developed by Felix Wankel in 1957 in collaboration with Walter Freude. In RPD, the function of a piston is performed by a three-vertex (trihedral) rotor, which performs rotational movements inside a complex-shaped cavity. After a wave of experimental models of cars and motorcycles that fell on the 60s and 70s of the twentieth century, interest in RPD has decreased, although a number of companies are still working on improving the design of the Wankel engine. Currently, RPDs are equipped with passenger cars Mazda. The rotary piston engine finds application in modeling.
Principle of operation
The gas pressure force from the burnt fuel-air mixture drives the rotor, which is mounted through bearings on the eccentric shaft. The movement of the rotor relative to the motor housing (stator) is carried out through a pair of gears, one of which, bigger size, is fixed on the inner surface of the rotor, the second, smaller support, is rigidly attached to the inner surface of the side cover of the engine. The interaction of gears leads to the fact that the rotor makes circular eccentric movements, in contact with the edges of the inner surface of the combustion chamber. As a result, three isolated chambers of variable volume are formed between the rotor and the engine housing, in which the processes of fuel-air mixture compression, its combustion, expansion of gases that put pressure on the working surface of the rotor and purification of the combustion chamber from exhaust gases take place. The rotational motion of the rotor is transmitted to an eccentric shaft mounted on bearings and transmitting torque to the transmission mechanisms. Thus, two mechanical pairs work simultaneously in the RPD: the first one regulates the movement of the rotor and consists of a pair of gears; and the second - converting the circular motion of the rotor into rotation of the eccentric shaft. The gear ratio of the rotor and stator gears is 2:3, so for one complete revolution of the eccentric shaft, the rotor has time to turn 120 degrees. In turn, for one complete revolution of the rotor in each of the three chambers formed by its faces, a complete four-stroke cycle of the internal combustion engine is performed.
RPD scheme
1 - inlet window; 2 outlet window; 3 - body; 4 - combustion chamber; 5 - fixed gear; 6 - rotor; 7 - gear wheel; 8 - shaft; 9 - spark plug
Advantages of RPD
The main advantage rotary piston engine is the simplicity of the design. The RPD has 35-40 percent fewer parts than a four-stroke piston engine. The RPD does not have pistons, connecting rods, crankshaft. In the "classic" version of the RPD there is no gas distribution mechanism. The fuel-air mixture enters the working cavity of the engine through the inlet window, which opens the edge of the rotor. Exhaust gases are ejected through the exhaust port, which crosses, again, the edge of the rotor (this resembles the gas distribution device of a two-stroke piston engine).
The lubrication system deserves special mention, which is practically absent in the simplest version of the RPD. Oil is added to the fuel - as in the operation of two-stroke motorcycle engines. The friction pairs (primarily the rotor and the working surface of the combustion chamber) are lubricated by the fuel-air mixture itself.
Since the mass of the rotor is small and easily balanced by the mass of counterweights of the eccentric shaft, the RPD is characterized by a low level of vibration and good uniformity of operation. In cars with RPD, it is easier to balance the engine, achieving a minimum level of vibration, which has a good effect on the comfort of the car as a whole. Twin-rotor engines are particularly smooth-running, in which the rotors themselves act as vibration-reducing balancers.
Another attractive quality of the RPD is its high specific power at high revs eccentric shaft. This allows you to achieve excellent speed characteristics from a car with RPD with relatively low fuel consumption. The low inertia of the rotor and the increased specific power compared to piston internal combustion engines improve the dynamics of the car.
Finally, an important advantage of the RPD is its small size. rotary engine less than a piston four-stroke engine of the same power by about half. And it allows you to make better use of space. engine compartment, more accurately calculate the location of the transmission units and the load on the front and rear axles.
Disadvantages of RPD
The main disadvantage of a rotary piston engine is the low efficiency of gap seals between the rotor and the combustion chamber. The RPD rotor having a complex shape requires reliable seals not only along the edges (and there are four of them on each surface - two along the top, two along the side faces), but also along the side surface in contact with the engine covers. In this case, the seals are made in the form of spring-loaded strips of high-alloy steel with particularly precise processing of both working surfaces and ends. The allowances for expansion of the metal from heating impair their characteristics - it is almost impossible to avoid gas breakthrough at the end sections of the sealing plates (in piston engines, the labyrinth effect is used by installing sealing rings with gaps in different directions).
In recent years, the reliability of seals has increased dramatically. Designers have found new materials for seals. However, there is no need to talk about any breakthrough yet. Seals are still the bottleneck of the RPD.
The complex sealing system of the rotor requires efficient lubrication of the friction surfaces. RPD consumes more oil than a four-stroke piston engine (from 400 grams to 1 kilogram per 1000 kilometers). In this case, the oil burns along with the fuel, which adversely affects the environmental friendliness of the engines. There are more substances hazardous to human health in the exhaust gases of RPD than in the exhaust gases of piston engines.
Special requirements are also imposed on the quality of oils used in RPD. This is due, firstly, to a tendency to increased wear (due to the large area of contacting parts - the rotor and the inner chamber of the engine), and secondly, to overheating (again, due to increased friction and because of the small size of the engine itself). Irregular oil changes are deadly for RPDs - since abrasive particles in old oil dramatically increase engine wear and engine hypothermia. Starting a cold engine and insufficient warming up lead to the fact that there is little lubrication in the contact zone of the rotor seals with the surface of the combustion chamber and side covers. If a piston engine seizes when overheated, then the RPD most often occurs during a cold engine start (or when driving in cold weather, when cooling is excessive).
In general, the operating temperature of the RPD is higher than that of piston engines. The most thermally stressed area is the combustion chamber, which has a small volume and, accordingly, an elevated temperature, which makes it difficult to ignite the fuel-air mixture (RPDs are prone to detonation due to the extended shape of the combustion chamber, which can also be attributed to the disadvantages of this type of engine). Hence the exactingness of RPD on the quality of candles. Usually they are installed in these engines in pairs.
Rotary piston engines, with excellent power and speed characteristics, turn out to be less flexible (or less elastic) than piston ones. They give out optimal power only at sufficiently high speeds, which forces designers to use RPDs in tandem with multi-stage gearboxes and complicates the design. automatic boxes gears. Ultimately, RPDs are not as economical as they should be in theory.
Practical application in the automotive industry
RPDs were most widely used in the late 60s and early 70s of the last century, when the patent for the Wankel engine was bought by 11 leading automakers in the world.
In 1967, the German company NSU produced a serial a car business class NSU Ro 80. This model was produced for 10 years and sold around the world in the amount of 37204 copies. The car was popular, but the shortcomings of the RPD installed in it, in the end, ruined the reputation of this wonderful car. Against the background of durable competitors, the NSU Ro 80 model looked “pale” - the mileage was up to overhaul engine with the declared 100 thousand kilometers did not exceed 50 thousand.
Concern Citroen, Mazda, VAZ experimented with RPD. The greatest success was achieved by Mazda, which launched its passenger car with RPD back in 1963, four years before the introduction of the NSU Ro 80. Today, Mazda is equipping RX series sports cars with RPD. Modern cars Mazda RX-8 are freed from many of the shortcomings of the Felix Wankel RPD. They are quite environmentally friendly and reliable, although they are considered “capricious” among car owners and repair specialists.
Practical application in the motorcycle industry
In the 70s and 80s, some motorcycle manufacturers experimented with RPD - Hercules, Suzuki and others. Currently, small-scale production of "rotary" motorcycles has been established only at Norton, which produces the NRV588 model and is preparing the NRV700 motorcycle for serial production.
Norton NRV588 is a sport bike equipped with a twin-rotor engine with a total volume of 588 cubic centimeters and developing a power of 170 Horse power. With a dry weight of a motorcycle of 130 kg, the power-to-weight ratio of a sportbike looks literally prohibitive. The engine of this machine is equipped with variable intake tract and electronic fuel injection systems. All that is known about the NRV700 model is that the RPD power of this sportbike will reach 210 hp.
As mentioned above, thermal expansion is used in internal combustion engines. But how it is applied and what function it performs, we will consider using the example of the operation of a piston internal combustion engine. An engine is an energy-power machine that converts any energy into mechanical work. Engines in which mechanical work is created as a result of the conversion of thermal energy are called thermal. Thermal energy is obtained by burning any fuel. A heat engine in which part of the chemical energy of the fuel burning in the working cavity is converted into mechanical energy is called a reciprocating internal combustion engine. (Soviet Encyclopedic Dictionary)
3. 1. Classification of internal combustion engines
As mentioned above, as the power plants of automobiles, internal combustion engines have been most widely used, in which the process of fuel combustion with the release of heat and its transformation into mechanical work occurs directly in the cylinders. But in most modern cars, internal combustion engines are installed, which are classified according to various criteria: By the method of mixture formation - engines with external mixture formation, in which the combustible mixture is prepared outside the cylinders (carburetor and gas), and engines with internal mixture formation (the working mixture is formed inside the cylinders) -diesels; According to the method of implementation of the working cycle - four-stroke and two-stroke; According to the number of cylinders - single-cylinder, two-cylinder and multi-cylinder; According to the location of the cylinders - engines with a vertical or inclined arrangement of cylinders in one row, V-shaped with an arrangement of cylinders at an angle (when the cylinders are located at an angle of 180, the engine is called an engine with opposite cylinders, or opposed); According to the method of cooling - for engines with liquid or air cooling; By type of fuel used - gasoline, diesel, gas and multi-fuel; By compression ratio. Depending on the degree of compression, there are
high (E=12...18) and low (E=4...9) compression engines; According to the method of filling the cylinder with a fresh charge: a) naturally aspirated engines, in which air intake or combustible mixture is carried out due to the discharge in the cylinder during the suction stroke of the piston;) supercharged engines, in which the air or combustible mixture is admitted into the working cylinder under pressure created by the compressor in order to increase the charge and obtain increased engine power; According to the frequency of rotation: low-speed, increased speed, high-speed; According to the purpose, engines are stationary, auto-tractor, ship, diesel, aviation, etc.
3.2. Basics of the piston engine device
Piston internal combustion engines consist of mechanisms and systems that perform the functions assigned to them and interact with each other. The main parts of such an engine are a crank mechanism and a gas distribution mechanism, as well as power, cooling, ignition and lubrication systems.
The crank mechanism converts the rectilinear reciprocating motion of the piston into the rotational motion of the crankshaft.
The gas distribution mechanism ensures the timely entry of the combustible mixture into the cylinder and the removal of combustion products from it.
The power supply system is designed to prepare and supply a combustible mixture to the cylinder, as well as to remove combustion products.
The lubrication system serves to supply oil to the interacting parts in order to reduce the friction force and partially cool them, along with this, the oil circulation leads to the washing off of carbon deposits and the removal of wear products.
The cooling system maintains the normal temperature regime of the engine, ensuring the removal of heat from the parts of the cylinders of the piston group and the valve mechanism that are very hot during the combustion of the working mixture.
The ignition system is designed to ignite the working mixture in the engine cylinder.
So, a four-stroke piston engine consists of a cylinder and a crankcase, which is closed from below by a pan. A piston with compression (sealing) rings moves inside the cylinder, having the shape of a glass with a bottom in the upper part. The piston through the piston pin and connecting rod is connected to the crankshaft, which rotates in main bearings located in the crankcase. The crankshaft consists of main journals, cheeks and connecting rod journal. Cylinder, piston, connecting rod and crankshaft make up the so-called crank mechanism. From above, the cylinder is covered with a head with valves, the opening and closing of which is strictly coordinated with the rotation of the crankshaft, and, consequently, with the movement of the piston.
The movement of the piston is limited to two extreme positions at which its speed is zero. The extreme upper position of the piston is called top dead center (TDC), its extreme lower position is bottom dead center (BDC).
The non-stop movement of the piston through the dead points is provided by a flywheel in the form of a disk with a massive rim. The distance traveled by the piston from TDC to BDC is called the piston stroke S, which is equal to twice the radius R of the crank: S=2R.
The space above the piston crown when it is at TDC is called the combustion chamber; its volume is denoted by Vс; the space of the cylinder between two dead points (BDC and TDC) is called its working volume and is denoted by Vh. The sum of the volume of the combustion chamber Vc and the working volume Vh is the total volume of the cylinder Va: Va=Vc+Vh. The working volume of the cylinder (it is measured in cubic centimeters or meters): Vh \u003d pD ^ 3 * S / 4, where D is the diameter of the cylinder. The sum of all working volumes of the cylinders of a multi-cylinder engine is called the working volume of the engine, it is determined by the formula: Vр=(pD^2*S)/4*i, where i is the number of cylinders. The ratio of the total volume of the cylinder Va to the volume of the combustion chamber Vc is called the compression ratio: E=(Vc+Vh)Vc=Va/Vc=Vh/Vc+1. The compression ratio is an important parameter of internal combustion engines, because. greatly affects its efficiency and power.
The most famous and widely used around the world mechanical devices- These are internal combustion engines (hereinafter referred to as internal combustion engines). Their range is extensive, and they differ in a number of features, for example, the number of cylinders, the number of which can vary from 1 to 24, the fuel used.
The operation of a piston internal combustion engine
Single cylinder internal combustion engine can be considered the most primitive, unbalanced and uneven stroke, despite the fact that it is the starting point in the creation of a new generation of multi-cylinder engines. Today they are used in aircraft modeling, in the production of agricultural, household and garden tools. For the automotive industry, four-cylinder engines and more solid devices are massively used.
How does it work and what does it consist of?
Reciprocating internal combustion engine has a complex structure and consists of:
- Housing, including a cylinder block, a cylinder head;
- gas distribution mechanism;
- Crank mechanism (hereinafter KShM);
- A number of auxiliary systems.
KShM is a link between the energy released during the combustion of the fuel-air mixture (hereinafter referred to as FA) in the cylinder and the crankshaft, which ensures the movement of the car. The gas distribution system is responsible for gas exchange during the operation of the unit: the access of atmospheric oxygen and fuel assemblies to the engine, and the timely removal of gases formed during combustion.
The device of the simplest piston engine
Auxiliary systems are presented:
- Inlet, providing oxygen to the engine;
- Fuel, represented by a fuel injection system;
- Ignition, which provides a spark and ignition of fuel assemblies for engines running on gasoline (diesel engines are characterized by self-ignition of the mixture from high temperature);
- A lubrication system that reduces friction and wear of contacting metal parts using engine oil;
- Cooling system, which prevents overheating of the working parts of the engine, providing circulation special liquids antifreeze type;
- An exhaust system that ensures the removal of gases into the corresponding mechanism, consisting of exhaust valves;
- A control system that provides monitoring of the operation of the internal combustion engine at the electronic level.
The main working element in the described node is considered internal combustion engine piston, which itself is a prefabricated part.
ICE piston device
Step-by-step operation diagram
The operation of an internal combustion engine is based on the energy of expanding gases. They are the result of combustion of fuel assemblies inside the mechanism. This physical process forces the piston to move in the cylinder. The fuel in this case can be:
- Liquids (gasoline, diesel fuel);
- gases;
- Carbon monoxide as a result of burning solid fuels.
Engine operation is a continuous closed cycle consisting of a certain number of cycles. The most common internal combustion engines are of two types, differing in the number of cycles:
- Two-stroke, producing compression and stroke;
- Four-stroke - are characterized by four stages of the same duration: intake, compression, working stroke, and the final - release, this indicates a four-fold change in the position of the main working element.
The beginning of the stroke is determined by the location of the piston directly in the cylinder:
- Top dead center (hereinafter referred to as TDC);
- Bottom dead center (hereinafter BDC).
By studying the algorithm of the four-stroke sample, you can thoroughly understand working principle of a car engine.
The principle of operation of a car engine
The intake occurs by passing from the top dead center through the entire cavity of the cylinder of the working piston with the simultaneous retraction of the fuel assembly. Based on the design features, the mixing of incoming gases can occur:
- In the intake manifold, this is true if the engine is gasoline with distributed or central injection;
- In the combustion chamber, if we are talking about a diesel engine, as well as an engine running on gasoline, but with direct injection.
First measure runs with open intake valves of the gas distribution mechanism. The number of intake and exhaust valves, their open time, their size, and their state of wear are factors that affect engine power. The piston at the initial stage of compression is placed at BDC. Subsequently, it begins to move upward and compress the accumulated fuel assembly to the dimensions determined by the combustion chamber. The combustion chamber is the free space in the cylinder that remains between the top of the cylinder and the piston at top dead center.
Second measure involves closing all the valves of the engine. The density of their fit directly affects the quality of fuel assembly compression and its subsequent ignition. Also, the quality of compression of fuel assemblies is greatly influenced by the level of wear of engine components. It is expressed in terms of the size of the space between the piston and the cylinder, in the tightness of the valves. The compression level of an engine is the main factor influencing its power. It is measured with a special device compression gauge.
working stroke starts when it is connected to the process ignition system that generates a spark. The piston is in the maximum upper position. The mixture explodes, gases are released that create increased pressure, and the piston is set in motion. The crank mechanism, in turn, activates the rotation of the crankshaft, which ensures the movement of the car. All system valves are in the closed position at this time.
graduation stroke is the final one in the considered cycle. All exhaust valves are in the open position, allowing the engine to “breathe” the combustion products. The piston returns to its starting point and is ready to start a new cycle. This movement contributes to the release of exhaust system and then into the environment, exhaust gases.
Scheme of operation of an internal combustion engine, as mentioned above, is based on cyclicity. Considering in detail, how does a piston engine work, it can be summarized that the efficiency of such a mechanism is not more than 60%. This percentage is due to the fact that at a given moment, the working cycle is performed in only one cylinder.
Not all the energy received at this time is directed to the movement of the car. Part of it is spent on keeping the flywheel in motion, which, by inertia, ensures the operation of the car during the other three cycles.
A certain amount of thermal energy is involuntarily spent on heating the housing and exhaust gases. That is why the engine power of a car is determined by the number of cylinders, and as a result, the so-called engine size, calculated according to a certain formula as the total volume of all working cylinders.
Definition.
piston engine- one of the variants of the internal combustion engine, which works by converting the internal energy of the burning fuel into the mechanical work of the translational movement of the piston. The piston is set in motion by the expansion of the working fluid in the cylinder.
The crank mechanism converts the translational motion of the piston into rotational motion of the crankshaft.
The working cycle of the engine consists of a sequence of cycles of one-sided translational piston strokes. Subdivided engines with two and four cycles of work.
The principle of operation of two-stroke and four-stroke piston engines.
Number of cylinders in piston engines may vary depending on the design (from 1 to 24). The volume of the engine is considered to be equal to the sum of the volumes of all cylinders, the capacity of which is found by the product of the cross section and the piston stroke.
IN piston engines different designs, the process of fuel ignition occurs in different ways:
Electric spark discharge, which is formed on spark plugs. Such engines can run on both gasoline and other types of fuel (natural gas).
Compression of the working body:
IN diesel engines working on diesel fuel or gas (with 5% addition of diesel fuel), air is compressed, and when the piston reaches the point of maximum compression, fuel is injected, which ignites from contact with heated air.
Compression model engines. The fuel supply in them is exactly the same as in gasoline engines. Therefore, for their operation, a special fuel composition (with impurities of air and diethyl ether) is required, as well as precise adjustment of the compression ratio. Compressor engines have found their distribution in the aircraft and automotive industries.
glow engines. The principle of their operation is in many respects similar to the engines of the compression model, however, it was not without a design feature. The role of ignition in them is performed by a glow plug, the glow of which is maintained by the energy of the fuel burning on the previous cycle. The composition of the fuel is also special, based on methanol, nitromethane and Castor oil. Such engines are used both on cars and on airplanes.
calorific engines. In these engines, ignition occurs when fuel comes into contact with hot parts of the engine (usually the piston crown). Open-hearth gas is used as fuel. They are used as drive motors in rolling mills.
Fuel types used in piston engines:
Liquid fuel– diesel fuel, gasoline, alcohols, biodiesel;
gases– natural and biological gases, liquefied gases, hydrogen, gaseous products of oil cracking;
Produced in a gas generator from coal, peat and wood, carbon monoxide is also used as a fuel.
Operation of piston engines.
Engine cycles described in detail in technical thermodynamics. Different cyclograms are described by different thermodynamic cycles: Otto, Diesel, Atkinson or Miller and Trinkler.
Causes of piston engine failures.
piston engine efficiency.
The maximum efficiency that could be obtained on piston engine is 60%, i.e. slightly less than half of the burning fuel is spent on heating engine parts, and also comes out with the heat of the exhaust gases. In this connection, it is necessary to equip the engines with cooling systems.
Classification of cooling systems:
Air CO- they give off heat to the air due to the ribbed outer surface of the cylinders. Are the
more on weak engines(tens of hp), or on powerful aircraft engines that are cooled by a fast air flow.
Liquid CO- a liquid (water, antifreeze or oil) is used as a coolant, which is pumped through the cooling jacket (channels in the walls of the cylinder block) and enters the cooling radiator, in which it is cooled by air flows, natural or from fans. Rarely, sodium metal is also used as a coolant, which is melted by the heat of a warming engine.
Application.
Piston engines, due to their power range (1 watt - 75,000 kW), have gained great popularity not only in the automotive industry, but also in the aircraft industry and shipbuilding. They are also used to drive combat, agricultural and construction equipment, power generators, water pumps, chainsaws and other machines, both mobile and stationary.