Main types of engines internal combustion and steam engines have one common drawback. It consists in the fact that reciprocating movement requires transformation into rotational movement. This, in turn, causes low productivity, as well as a rather high wear rate of the mechanism parts included in different types engines.
Quite a lot of people thought about how to create such a motor in which moving parts only rotated. However, only one person managed to solve this problem. Felix Wankel, a self-taught mechanic, became the inventor of the rotary piston engine. During his life, this man did not receive any specialty or higher education. Let's consider in more detail rotary piston engine Wankel.
Brief biography of the inventor
Felix G. Wankel was born in 1902, on August 13, in the small town of Lahr (Germany). In World War I, the father of the future inventor died. Because of this, Wankel had to quit his studies at the gymnasium and get a job as a sales assistant in a bookstore at a publishing house. As a result, he developed a passion for reading. Felix studied specifications engines, automotive, mechanics independently. He drew knowledge from books that were sold in the shop. It is believed that the Wankel engine scheme implemented later (more precisely, the idea of its creation) was visited in a dream. It is not known whether this is true or not, but it can be said for sure that the inventor had extraordinary abilities, a craving for mechanics and a peculiar
Pros and cons
Convertible reciprocating motion is completely absent in a rotary engine. The formation of pressure occurs in those chambers that are created using the convex surfaces of the triangular rotor and various parts of the body. The rotational movement of the rotor is carried out by combustion. This can reduce vibration and increase rotation speed. Due to the increase in efficiency thus brought about, the rotary engine is much smaller than a conventional piston engine of equivalent power.
The rotary engine has one main of all its components. This important component is called a triangular rotor, which rotates inside the stator. All three vertices of the rotor, thanks to this rotation, have a permanent connection with the inner wall of the housing. With the help of this contact, combustion chambers, or three volumes of a closed type with gas, are formed. When the rotational movements of the rotor inside the housing occur, the volume of all three formed combustion chambers changes all the time, resembling the actions of a conventional pump. All three side surfaces of the rotor work like a piston.
Inside the rotor is a small gear with external teeth, which is attached to the body. The gear, which is larger in diameter, is connected to this fixed gear, which sets the very trajectory of the rotational movements of the rotor inside the housing. The teeth in the larger gear are internal.
Due to the fact that together with the output shaft the rotor is connected eccentrically, the rotation of the shaft occurs in the same way as the handle will rotate the crankshaft. The output shaft will rotate three times for each rotation of the rotor.
The rotary engine has the advantage of being light in weight. The most basic of the blocks of the rotary engine has a small size and weight. At the same time, the handling and characteristics of such an engine will be better. He gets less mass due to the fact that there is simply no need for a crankshaft, connecting rods and pistons.
The rotary engine has dimensions that are much smaller conventional engine appropriate power. Thanks to the smaller engine size, handling will be much better, and the car itself will become more spacious, both for passengers and for the driver.
All of the parts of a rotary engine carry out continuous rotational movements in the same direction. The change in their movement occurs in the same way as in the pistons of a traditional engine. Rotary motors are internally balanced. This leads to a decrease in the vibration level itself. The power of the rotary engine seems to be much smoother and more uniform.
The Wankel engine has a convex special rotor with three faces, which can be called its heart. This rotor makes rotational movements inside the cylindrical surface of the stator. The Mazda rotary engine is the world's first rotary engine designed specifically for series production. This development began in 1963.
What is RPD?
In a classic four-stroke engine, the same cylinder is used for various operations - injection, compression, combustion and exhaust. In a rotary engine, each process is performed in a separate compartment of the chamber. The effect is not much different from dividing the cylinder into four compartments for each of the operations.
In a piston engine, the pressure generated by the combustion of the mixture causes the pistons to move back and forth in their cylinders. The connecting rods and crankshaft convert this pushing motion into the rotational motion required to propel the vehicle.
AT rotary engine there is no rectilinear motion that would have to be translated into rotational. Pressure builds up in one of the chamber compartments causing the rotor to rotate, which reduces vibration and increases the potential engine speed. The result is greater efficiency and smaller dimensions for the same power as a conventional piston engine.
How does RPD work?
The function of the piston in the RPD is performed by a three-vertex rotor, which converts the force of gas pressure into the rotational movement of the eccentric shaft. The movement of the rotor relative to the stator (outer housing) is provided by a pair of gears, one of which is rigidly fixed on the rotor, and the second on the side cover of the stator. The gear itself is fixedly fixed to the motor housing. With it in engagement is the gear of the rotor from the gear wheel, as it were, rolls around it.
The shaft rotates in bearings placed on the body and has a cylindrical eccentric on which the rotor rotates. The interaction of these gears ensures the expedient movement of the rotor relative to the housing, as a result of which three separated chambers of variable volume are formed. gear ratio gears 2: 3, so for one revolution of the eccentric shaft, the rotor returns 120 degrees, and for a full revolution of the rotor in each of the chambers, a full four-stroke cycle occurs. 
Gas exchange is controlled by the top of the rotor as it passes through the inlet and outlet ports. This design allows for a 4-stroke cycle without the use of a special gas distribution mechanism.
The sealing of the chambers is provided by radial and end sealing plates, which are pressed against the cylinder by centrifugal forces, gas pressure and band springs. The torque is obtained as a result of the action of gas forces through the rotor on the shaft eccentric
mixture formation
In theory, RPD uses several types of mixture formation: external and internal, based on liquid, solid, gaseous fuels.
Regarding solid fuels, it is worth noting that they are initially gasified in gas generators, as they lead to increased ash formation in cylinders. Therefore, gaseous and liquid fuels have become more widespread in practice.
The very mechanism of mixture formation in Wankel engines will depend on the type of fuel used.
When using gaseous fuel, its mixing with air occurs in a special compartment at the engine inlet. combustible mixture is delivered to the cylinders ready-made.
From liquid fuel, the mixture is prepared as follows:
- Air is mixed with liquid fuel before entering the cylinders where the combustible mixture enters.
- Liquid fuel and air enter the engine cylinders separately, and already inside the cylinder they are mixed. The working mixture is obtained by contact with residual gases.
Respectively, fuel-air mixture can be prepared outside the cylinders or inside them. From this comes the separation of engines with internal or external mixture formation.
Specifications of rotary piston engine
| options | VAZ-4132 | VAZ-415 |
| number of sections | 2 | 2 |
| The working volume of the engine chamber, cc | 1,308 | 1,308 |
| compression ratio | 9,4 | 9,4 |
| Rated power, kW (hp) / min-1 | 103 (140) / 6000 | 103 (140) / 6000 |
| Maximum torque, N * m (kgf * m) / min-1 | 186 (19) / 4500 | 186 (19) / 4500 |
| Minimum speed of the eccentric shaft per Idling, min-1 | 1000 | 900 |
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Engine weight, kg |
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Overall dimensions, mm |
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Oil consumption as % of fuel consumption |
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Engine resource up to the first overhaul, thousand km |
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appointment |
VAZ-21059/21079 |
VAZ-2108/2109/21099/2115/2110 |
models are produced
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RPD engine |
Acceleration time 0-100, sec |
Maximum speed, km \ h |
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Efficiency of rotary piston design
Despite a number of shortcomings, studies have shown that the overall Engine efficiency Wankel is quite tall by today's standards. Its value is 40 - 45%. For comparison, at piston engines internal combustion efficiency is 25%, modern turbodiesels - about 40%. The highest efficiency for piston diesel engines is 50%. To date, scientists continue to work to find reserves to improve the efficiency of engines.
final work efficiency The motor consists of three main parts:
Research in this area shows that only 75% of the fuel burns out in full. It is believed that this problem is solved by separating the processes of combustion and expansion of gases. It is necessary to provide for the arrangement of special chambers under optimal conditions. Combustion should take place in a closed volume, subject to an increase in temperature and pressure, the expansion process should occur at low temperatures.
- Mechanical efficiency (characterizes the work, the result of which was the formation of the torque of the main axis transmitted to the consumer).
About 10% of the engine's work is spent on setting in motion auxiliary units and mechanisms. This defect can be corrected by making changes to the engine device: when the main moving working element does not touch the stationary body. A constant torque arm must be present along the entire path of the main working element.
- Thermal efficiency (an indicator reflecting the amount of thermal energy generated from the combustion of fuel, which is converted into useful work).
In practice, 65% of the received thermal energy escapes with the exhaust gases into the external environment. A number of studies have shown that it is possible to achieve an increase in thermal efficiency in the case when the design of the motor would allow the combustion of fuel in a heat-insulated chamber so that maximum temperatures are reached from the very beginning, and at the end this temperature is reduced to minimum values by turning on the vapor phase.
Wankel rotary piston engine
The piston of the engine is a part that has a cylindrical shape and performs reciprocating movements inside the cylinder. It is one of the most characteristic parts for the engine, since the implementation of the thermodynamic process occurring in the internal combustion engine occurs precisely with its help. Piston:
- perceiving the pressure of gases, transfers the resulting force to;
- seals the combustion chamber;
- removes excess heat from it.
The photo above shows four strokes of the engine piston.
Extreme conditions dictate piston material
The piston is operated under extreme conditions, the characteristic features of which are high: pressure, inertial loads and temperatures. That is why the main requirements for materials for its manufacture include:
- high mechanical strength;
- good thermal conductivity;
- low density;
- insignificant coefficient of linear expansion, antifriction properties;
- good corrosion resistance.
Pistons can be:
- cast;
- forged.
The design features of the piston are determined by its purpose
The main conditions that determine the design of the piston are the type of engine and the shape of the combustion chamber, the features of the combustion process taking place in it. Structurally, the piston is a one-piece element, consisting of:
- heads (bottoms);
- sealing part;
- skirts (guide part).
Is the piston of a gasoline engine different from a diesel engine? The surfaces of the piston heads of gasoline and diesel engines are structurally different. In a gasoline engine, the surface of the head is flat or close to it. Sometimes grooves are made in it, contributing to the full opening of the valves. For pistons of engines equipped with a direct fuel injection system (SNVT), a more complex shape is characteristic. The piston head in a diesel engine is significantly different from a gasoline engine - due to the execution of a combustion chamber of a given shape in it, better swirl and mixture formation are provided.
The photo shows the engine piston diagram.
Piston rings: types and composition
The sealing part of the piston includes piston rings that provide a tight connection between the piston and the cylinder. The technical condition of the engine is determined by its sealing ability. Depending on the type and purpose of the engine, the number of rings and their location are selected. The most common scheme is a scheme of two compression and one oil scraper rings.
Piston rings are made mainly from special gray ductile iron, which has:
- high stable indicators of strength and elasticity at operating temperatures throughout the entire service life of the ring;
- high wear resistance under conditions of intense friction;
- good antifriction properties;
- the ability to quickly and effectively break in to the surface of the cylinder.
The main purpose of the compression ring is to prevent gases from the combustion chamber from entering the engine crankcase. Particularly heavy loads fall on the first compression ring. Therefore, in the manufacture of rings for the pistons of some forced gasoline and all diesel engines, a steel insert is installed, which increases the strength of the rings and allows for maximum compression. The shape of the compression rings can be:
- trapezoidal;
- barrel-shaped;
- tconical.
On the oil scraper ring the function of removing excess oil from the cylinder walls and preventing its penetration into the combustion chamber is assigned. It is distinguished by the presence of many drainage holes. Some rings are designed with spring expanders.
The shape of the piston guide (otherwise, the skirt) can be cone-shaped or barrel-shaped, which allows compensating for its expansion when high operating temperatures are reached. Under their influence, the shape of the piston becomes cylindrical. The side surface of the piston is coated with a layer of antifriction material in order to reduce losses caused by friction; graphite or molybdenum disulfide is used for this purpose. Lug holes in the piston skirt allow the piston pin to be secured.
A unit consisting of a piston, compression, oil scraper rings, as well as a piston pin is commonly called a piston group. The function of its connection with the connecting rod is assigned to a steel piston pin, which has a tubular shape. It has requirements for:
- minimal deformation during operation;
- high strength under variable load and wear resistance;
- good impact resistance;
- small mass.
- fixed in the piston bosses, but rotate in the connecting rod head;
- fixed in the connecting rod head and rotate in the piston bosses;
- freely rotating in the piston bosses and in the connecting rod head.
The fingers installed according to the third option are called floating. They are the most popular because their length and circumference wear is negligible and uniform. With their use, the risk of seizing is minimized. In addition, they are easy to install.
Removal of excess heat from the piston
In addition to significant mechanical stresses, the piston is also subjected to extreme high temperatures. Heat is removed from the piston group:
- cooling system from the cylinder walls;
- the internal cavity of the piston, then - the piston pin and connecting rod, as well as the oil circulating in the lubrication system;
- partially cold air-fuel mixture supplied to the cylinders.
- splashing oil through a special nozzle or hole in the connecting rod;
- oil mist in the cylinder cavity;
- injection of oil into the zone of the rings, into a special channel;
- oil circulation in the piston head through a tubular coil.
Video about a four-stroke engine - the principle of operation:
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.
AT 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:
AT diesel engines, running 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.
Piston group
The piston group forms a movable wall of the working volume of the cylinder. It is the movement of this “wall”, i.e. the piston, that is an indicator of the work done by the burnt and expanding gases.
The piston group of the crank mechanism includes a piston, piston rings (compression and oil scraper rings), a piston pin and its fixing parts. Sometimes the piston group is considered together with the cylinder, and is called the cylinder-piston group.
Piston
Piston Design Requirements
The piston perceives the force of gas pressure and transmits it through the piston pin to the connecting rod. At the same time, it performs a rectilinear reciprocating motion.
The conditions under which the piston operates:
- high gas pressure ( 3.5…5.5 MPa for gasoline and 6.0…15.0 MPa for diesel engines);
- contact with hot gases (up to 2600 ˚С);
- movement with change of direction and speed.
The reciprocating movement of the piston causes significant inertial loads in the areas of the passage of dead spots, where the piston changes the direction of movement to the opposite. Inertial forces depend on the speed of the piston and its mass.
The piston perceives significant forces: more 40 kN in gasoline engines, and 20 kN- in diesels. Contact with hot gases causes the central part of the piston to heat up to a temperature 300…350 ˚С. Strong heating of the piston is dangerous due to the possibility of jamming in the cylinder due to thermal expansion, and even burning the piston bottom.
The movement of the piston is accompanied increased friction and, as a result, wear of its surface and the surface of the cylinder (sleeve). During the movement of the piston from top dead center to bottom dead center and back, the pressure force of the piston surface on the surface of the cylinder (sleeve) changes both in magnitude and in direction depending on the stroke occurring in the cylinder.
The piston exerts maximum pressure on the cylinder wall during the stroke of the stroke, at the moment when the connecting rod begins to deviate from the piston axis. In this case, the gas pressure force transmitted by the piston to the connecting rod causes a reactive force in the piston pin, which in this case is a cylindrical hinge. This reaction is directed from the piston pin along the line of the connecting rod, and can be decomposed into two components - one is directed along the piston axis, the second (lateral force) is perpendicular to it and directed along the normal to the cylinder surface.
It is this (lateral) force that causes significant friction between the surfaces of the piston and cylinder (sleeve), leading to their wear, additional heating of parts and a decrease in efficiency due to energy losses.
Attempts to reduce the friction forces between the piston and the cylinder walls are complicated by the fact that a minimum clearance is required between the cylinder and the piston, which ensures complete sealing of the working cavity in order to prevent gas breakthrough and oil ingress into the working space of the cylinder. The clearance between the piston and the cylinder surface is limited by the thermal expansion of the parts. If it is made too small, in accordance with the requirements of tightness, then the piston may jam in the cylinder due to thermal expansion.
When the direction of movement of the piston and the processes (cycles) occurring in the cylinder change, the friction force of the piston against the cylinder walls changes its character - the piston is pressed against the opposite wall of the cylinder, while in the dead point transition zone the piston strikes the cylinder due to a sharp change in the value and load direction.
Designers, when developing engines, have to solve a set of problems associated with the above-described operating conditions of parts of the cylinder-piston group:
- high thermal loads causing thermal expansion and corrosion of metals of KShM parts;
- colossal pressure and inertial loads that can destroy parts and their connections;
- significant frictional forces causing additional heating, wear and energy loss.
Based on this, the following requirements are imposed on the piston design:
- sufficient rigidity to withstand power loads;
- thermal stability and minimum temperature deformations;
- the minimum mass to reduce inertial loads, while the mass of pistons in multi-cylinder engines should be the same;
- ensuring a high degree of sealing of the working cavity of the cylinder;
- minimum friction against the cylinder walls;
- high durability, since the replacement of pistons is associated with labor-intensive repair operations.
Piston design features
Modern pistons automotive engines have a complex spatial shape, which is due to various factors and conditions in which this critical part operates. Many elements and features of the piston shape are not visible to the naked eye, since deviations from cylindricity and symmetry are minimal, however, they are present.
Let us consider in more detail how the piston of an internal combustion engine is arranged, and what tricks designers have to go to in order to ensure that the requirements set out above are met.
The piston of an internal combustion engine consists of an upper part - a head and a lower part - a skirt.
The upper part of the piston head - the bottom directly perceives the forces from the working gases. In gasoline engines, the piston crown is usually made flat. In the piston heads of diesel engines, a combustion chamber is often made.
The bottom of the piston is a massive disk, which is connected by means of ribs or racks with tides having holes for the piston pin - bosses. The inner surface of the piston is made in the form of an arch, which provides the necessary rigidity and heat dissipation.
Grooves for piston rings are cut on the side surface of the piston. The number of piston rings depends on the gas pressure and average speed piston displacement (i.e., engine speed) - the lower the average piston speed, the more rings are required.
In modern engines, along with an increase in the frequency of rotation of the crankshaft, there is a tendency to reduce the number of compression rings on the pistons. This is due to the need to reduce the mass of the piston in order to reduce inertial loads, as well as to reduce friction forces, which take a significant share of engine power. At the same time, the possibility of gas breakthrough into the crankcase of a high-speed engine is considered a less urgent problem. Therefore, in the engines of modern cars and racing cars, you can find designs with one compression ring on the piston, and the pistons themselves have a shortened skirt.
In addition to the compression rings, one or two oil scraper rings are installed on the piston. The grooves made in the piston for oil scraper rings have drainage holes for draining engine oil into the internal cavity of the piston when removing it with a ring from the surface of the cylinder (sleeve). This oil is normally used to cool the inside of the piston crown and skirt and then drains into the oil pan.
The shape of the piston head depends on the type of engine, the method of mixture formation and the shape of the combustion chamber. The most common flat shape of the bottom, although there are convex and concave. In some cases, recesses are made in the bottom of the piston for valve plates when the piston is located at top dead center (TDC). As mentioned above, in the bottoms of the pistons of diesel engines, combustion chambers are often made, the shape of which may vary.
The lower part of the piston - the skirt directs the piston in a rectilinear motion, while it transfers the lateral force to the cylinder wall, the value of which depends on the position of the piston and the processes occurring in the working cavity of the cylinder. The magnitude of the lateral force transmitted by the piston skirt is much less than the maximum force perceived by the bottom from the side of the gases, so the skirt is made relatively thin-walled.
A second oil scraper ring is often installed in the lower part of the skirt in diesel engines, which improves cylinder lubrication and reduces the likelihood of oil entering the working cavity of the cylinder. To reduce the mass of the piston and friction forces, the unloaded parts of the skirt are cut in diameter and shortened in height. Inside the skirt, technological bosses are usually made, which are used to fit the pistons by weight.
The design and dimensions of the pistons depend mainly on the speed of the engine, as well as on the magnitude and rate of increase in gas pressure. So, high-speed pistons gasoline engines as light as possible, and diesel pistons have a more massive and rigid design.
At the moment the piston passes through TDC, the direction of the lateral force, which is one of the components of the gas pressure force on the piston, changes. As a result, the piston moves from one wall of the cylinder to another - occurs piston changeover. This causes the piston to hit the cylinder wall, accompanied by a characteristic knock. To reduce this harmful phenomenon, the piston pins are displaced by 2…3
mm in the direction of maximum lateral force; in this case, the lateral pressure force of the piston on the cylinder is significantly reduced. This misalignment of the piston pin is called desaxage.
The use of deoxidation in the design of the piston requires compliance with the rules for mounting the crankshaft - the piston must be installed strictly according to the marks indicating where the front part is (usually an arrow on the bottom).
The original solution, designed to reduce the effect of lateral force, was applied by the designers of Volkswagen engines. The bottom of the piston in such engines is not made at right angles to the axis of the cylinder, but is slightly beveled. According to the designers, this allows you to optimally distribute the load on the piston, and improve the process of mixture formation in the cylinder during the intake and compression strokes.
In order to satisfy the conflicting requirements for the tightness of the working cavity, which implies the presence of minimal gaps between the piston skirt and the cylinder, and to prevent the part from jamming as a result of thermal expansion, the following structural elements are used in the piston form:
- reduction of skirt rigidity due to special slots that compensate for its thermal expansion and improve cooling of the lower part of the piston. The slots are made on the side of the skirt that is least loaded with lateral forces pressing the piston against the cylinder;
- forced limitation of the thermal expansion of the skirt by inserts made of materials with a thermal expansion coefficient lower than that of the base metal;
- giving the piston skirt such a shape that, when loaded and at operating temperature, it takes the form of a regular cylinder.
The last condition is not easy to fulfill, since the piston is heated unevenly throughout the volume and has a complex spatial shape - in the upper part of its shape is symmetrical, and in the area of the bosses and on the lower part of the skirt there are asymmetric elements. All this leads to uneven temperature deformation of individual sections of the piston when it is heated during operation.
For these reasons, in the design of the piston of modern automobile engines, the following elements are usually performed that complicate its shape:
- the piston crown has a smaller diameter compared to the skirt and is closest in cross section to the correct circle.
The smaller cross-sectional diameter of the piston bottom is associated with its high operating temperature and, as a result, with a greater thermal expansion than in the skirt area. Therefore the piston modern engine in longitudinal section it has a slightly conical or barrel-shaped shape, narrowed towards the bottom.
The diameter reduction in the upper belt of the conical skirt for aluminum alloy pistons is 0.0003…0.0005D, where D is the diameter of the cylinder. When heated to operating temperatures, the shape of the piston along the length "levels" to the correct cylinder. - in the region of the bosses, the piston has smaller transverse dimensions, since metal arrays are concentrated here, and the thermal expansion is greater. Therefore, the piston below the bottom has an oval or elliptical shape in cross section, which, when the part is heated to operating temperatures, approaches the shape of a regular circle, and the piston approaches a regular cylinder in shape.
The major axis of the oval is located in a plane perpendicular to the axis of the piston pin. The ovality ranges from 0,182 before 0.8 mm.
Obviously, designers have to go to all these tricks in order to give the piston a regular cylindrical shape when heated to operating temperatures, thereby ensuring a minimum clearance between it and the cylinder.
The most effective way to prevent the piston from seizing in the cylinder due to its thermal expansion at a minimum clearance is to force the skirt to cool and insert metal elements with a low thermal expansion coefficient into the piston skirt. Most often, mild steel inserts are used in the form of transverse plates, which are placed in the boss area when casting the piston. In some cases, instead of plates, rings or half rings are used, which are poured into the upper belt of the piston skirt.
The bottom temperature of aluminum pistons must not exceed 320…350 ˚С. Therefore, to increase heat removal, the transition from the piston bottom to the walls is made smooth (in the form of an arch) and quite massive. For more efficient heat removal from the bottom of the piston, forced cooling is used, spraying onto the inner surface of the bottom engine oil from a special nozzle. Usually the function of such a nozzle is performed by a special calibrated hole made in the upper head of the connecting rod. Sometimes the nozzle is mounted on the engine housing at the bottom of the cylinder.
To ensure the normal thermal regime of the upper compression ring, it is located significantly below the edge of the bottom, forming the so-called fire or fire zone. The most worn ends of the piston ring groove are often reinforced with special inserts made of wear-resistant material.
Aluminum alloys are widely used as a material for the manufacture of pistons, the main advantage of which is their low weight and good thermal conductivity. The disadvantages of aluminum alloys include low fatigue strength, high coefficient of thermal expansion, insufficient wear resistance and relatively high cost.
The composition of alloys, in addition to aluminum, includes silicon ( 11…25% ) and additives of sodium, nitrogen, phosphorus, nickel, chromium, magnesium and copper. Cast or stamped blanks are subjected to mechanical and heat treatment.
Much less often, cast iron is used as a material for pistons, since this metal is much cheaper and stronger than aluminum. But, despite the high strength and wear resistance, cast iron has a relatively large mass, which leads to significant inertial loads, especially when the direction of piston movement changes. Therefore, cast iron is not used for the manufacture of pistons for high-speed engines.
Reciprocating internal combustion engines have found the widest distribution as energy sources in road, rail and maritime transport, in agricultural and construction industries (tractors, bulldozers), in emergency power supply systems for special facilities (hospitals, communication lines, etc.) and in many others areas of human activity. In recent years, mini-CHPs based on gas-piston internal combustion engines have become especially widespread, with the help of which the problems of supplying small residential areas or industries with energy are effectively solved. The independence of such CHPPs from centralized systems (such as RAO UES) increases the reliability and stability of their operation.
Reciprocating internal combustion engines, which are very diverse in design, are capable of providing a very wide range of powers - from very small (engine for aircraft models) to very large (engine for ocean tankers).
We repeatedly got acquainted with the basics of the device and the principle of operation of piston internal combustion engines, starting from the school course in physics and ending with the course "Technical thermodynamics". And yet, in order to consolidate and deepen knowledge, we will consider this issue very briefly again.
On fig. 6.1 shows a diagram of the engine device. As is known, the combustion of fuel in an internal combustion engine is carried out directly in the working fluid. In piston internal combustion engines, such combustion is carried out in the working cylinder 1 with a moving piston 6. The flue gases formed as a result of combustion push the piston, forcing it to do useful work. The translational movement of the piston with the help of the connecting rod 7 and the crankshaft 9 is converted into rotational, more convenient to use. Crankshaft located in the crankcase, and the engine cylinders - in another body part, called a block (or jacket) of cylinders 2. In the cover of cylinder 5 are the inlet 3 and graduation 4 valves with forced cam drive from a special camshaft kinematically connected to the crankshaft of the machine.
Rice. 6.1.
In order for the engine to work continuously, it is necessary to periodically remove combustion products from the cylinder and fill it with new portions of fuel and oxidizer (air), which is done due to piston movements and valve operation.
Piston internal combustion engines are usually classified according to various general features.
- 1. According to the method of mixture formation, ignition and heat supply, engines are divided into machines with forced ignition and self-ignition (carburetor or injection and diesel).
- 2. On the organization of the workflow - for four-stroke and two-stroke. In the latter, the work process is completed not in four, but in two piston strokes. In turn, two-stroke internal combustion engines are divided into machines with direct-flow valve-slot purge, with crank-chamber purge, with direct-flow purge and oppositely moving pistons, etc.
- 3. By appointment - for stationary, ship, diesel, automobile, autotractor, etc.
- 4. By the number of revolutions - for low-speed (up to 200 rpm) and high-speed ones.
- 5. According to the average piston speed d> n =? P/ 30 - for low-speed and high-speed (d? „\u003e 9 m / s).
- 6. According to the air pressure at the beginning of compression - for conventional and supercharged with the help of driven blowers.
- 7. According to the use of exhaust gas heat - for conventional (without the use of this heat), turbocharged and combined. In turbocharged cars, the exhaust valves open a little earlier than usual and the higher-pressure flue gases are sent to the impulse turbine, which drives the turbocharger to supply air to the cylinders. This allows more fuel to be burned in the cylinder, improving both the efficiency and performance of the machine. In combined internal combustion engines, the piston part serves in many respects as a gas generator and produces only ~ 50-60% of the machine's power. The rest of the total power is obtained from a gas turbine powered by flue gases. To do this, flue gases at high pressure R and temperature / are sent to the turbine, the shaft of which transfers the received power to the main shaft of the installation using a gear or fluid coupling.
- 8. According to the number and arrangement of cylinders, engines are: single, double and multi-cylinder, in-line, K-shaped, .T-shaped.
Consider now the real process of a modern four-stroke diesel engine. It is called four-stroke because a full cycle is carried out here in four full strokes of the piston, although, as we will now see, several more real thermodynamic processes are carried out during this time. These processes are clearly shown in Figure 6.2.
Rice. 6.2.
I - suction; II - compression; III - working stroke; IV - pushing out
During the beat suction(1) The suction (inlet) valve opens a few degrees before top dead center (TDC). The moment of opening corresponds to the point G on the R-^-chart. In this case, the suction process occurs when the piston moves to the bottom dead center (BDC) and proceeds at a pressure r ns less than atmospheric /; a (or boost pressure r n). When the direction of piston movement changes (from BDC to TDC), the intake valve does not close immediately either, but with a certain delay (at the point t). Further, with the valves closed, the working fluid is compressed (up to the point With). AT diesel cars clean air is sucked in and compressed, and in carburetors - a working mixture of air with gasoline vapor. This stroke of the piston is called the stroke. compression(II).
A few degrees of crankshaft rotation before TDC is injected into the cylinder through the nozzle diesel fuel, its self-ignition, combustion and expansion of combustion products occur. In carburetor machines, the working mixture is forcibly ignited using an electric spark discharge.
When air is compressed and heat exchange with the walls is relatively low, its temperature rises significantly, exceeding the self-ignition temperature of the fuel. Therefore, the injected finely atomized fuel warms up very quickly, evaporates and ignites. As a result of fuel combustion, the pressure in the cylinder is at first sharp, and then, when the piston begins its journey to the BDC, it increases to a maximum at a decreasing rate, and then, as the last portions of the fuel received during injection are burned, it even begins to decrease (due to the intensive growth cylinder volume). We assume conditionally that at the point With" the combustion process ends. This is followed by the process of expansion of flue gases, when the force of their pressure moves the piston to BDC. The third stroke of the piston, including the combustion and expansion processes, is called working stroke(III), for only at this time the engine does useful work. This work is accumulated with the help of a flywheel and given to the consumer. Part of the accumulated work is spent on the completion of the remaining three cycles.
When the piston approaches BDC, the exhaust valve opens with some advance (point b) and exhaust flue gases rush into exhaust pipe, and the pressure in the cylinder drops sharply to almost atmospheric. When the piston moves to TDC, flue gases are pushed out of the cylinder (IV - ejection). Since the engine exhaust path has a certain hydraulic resistance, the pressure in the cylinder during this process remains above atmospheric. The exhaust valve closes after TDC (point P), so that in each cycle a situation arises when both the intake and exhaust valves are open at the same time (they talk about valve overlap). This allows you to better clean the working cylinder from combustion products, as a result, the efficiency and completeness of fuel combustion increase.
The cycle is organized differently for two-stroke machines (Fig. 6.3). These are usually supercharged engines, and for this they usually have a driven blower or turbocharger. 2 , which during engine operation pumps air into the air receiver 8.
The working cylinder of a two-stroke engine always has purge windows 9 through which air from the receiver enters the cylinder when the piston, passing to the BDC, begins to open them more and more.
During the first stroke of the piston, which is commonly called the working stroke, the injected fuel is burned in the engine cylinder and the combustion products expand. These processes for indicator chart(Fig. 6.3, a) reflected by the line c - I - t. At the point t the exhaust valves open and under the influence of excess pressure, the flue gases rush into the exhaust tract 6, as a result
Rice. 6.3.
1 - suction pipe; 2 - blower (or turbocharger); 3 - piston; 4 - exhaust valves; 5 - nozzle; 6 - exhaust tract; 7 - working
cylinder; 8 - air receiver; 9 - purge windows
then the pressure in the cylinder drops noticeably (point P). When the piston is lowered so that the purge windows begin to open, compressed air from the receiver rushes into the cylinder 8 , pushing out the remaining flue gases from the cylinder. At the same time, the working volume continues to increase, and the pressure in the cylinder decreases almost to the pressure in the receiver.
When the direction of movement of the piston is reversed, the process of purging the cylinder continues as long as the purge windows remain at least partially open. At the point to(Fig. 6.3, b) the piston completely blocks the purge windows and the compression of the next portion of the air that has entered the cylinder begins. A few degrees before TDC (at the point With") fuel injection begins through the nozzle, and then the processes described earlier occur, leading to the ignition and combustion of the fuel.
On fig. 6.4 shows diagrams explaining the design of other types of two-stroke engines. In general, the operating cycle for all these machines is similar to that described, and design features largely affect the duration
Rice. 6.4.
a- loop slot blowing; 6 - direct-flow purge with oppositely moving pistons; in- crank-chamber purge
individual processes and, as a result, on the technical and economic characteristics of the engine.
In conclusion, it should be noted that two-stroke engines theoretically, they allow, ceteris paribus, to obtain twice as much power, but in reality, due to the worse conditions for cleaning the cylinder and relatively large internal losses, this gain is somewhat less.