In the cylinder-piston group (CPG), one of the main processes occurs, thanks to which the internal combustion engine functions: the release of energy as a result of the combustion of the air-fuel mixture, which is subsequently converted into a mechanical action - the rotation of the crankshaft. The main working component of the CPG is the piston. Thanks to him, the conditions necessary for the combustion of the mixture are created. The piston is the first component involved in the conversion of the received energy.
The engine piston has a cylindrical shape. It is located in the cylinder liner of the engine, it is a movable element - in the process of operation it performs reciprocating movements and performs two functions.
- With forward movement, the piston reduces the volume of the combustion chamber, compressing the fuel mixture, which is necessary for the combustion process (in diesel engines, the ignition of the mixture does occur from its strong compression).
- After the ignition of the air-fuel mixture in the combustion chamber, the pressure rises sharply. In an effort to increase the volume, it pushes the piston back, and it makes a return movement, transmitted through the connecting rod to the crankshaft.
What is a car internal combustion engine piston?
The device of the part includes three components:
- Bottom.
- Sealing part.
- Skirt.
These components are available both in solid pistons (the most common option) and in composite parts.
Bottom
The bottom is the main working surface, since it, the walls of the sleeve and the head of the block form a combustion chamber in which the fuel mixture is burned.
The main parameter of the bottom is the shape, which depends on the type of internal combustion engine (ICE) and its design features.
In two-stroke engines, pistons are used, in which the bottom of a spherical shape is the protrusion of the bottom, this increases the efficiency of filling the combustion chamber with a mixture and exhaust gases.
In four-stroke gasoline engines, the bottom is flat or concave. Additionally, technical recesses are made on the surface - recesses for valve plates (eliminate the possibility of a collision between the piston and the valve), recesses to improve mixture formation.
In diesel engines, the recesses in the bottom are the most dimensional and have a different shape. Such recesses are called piston combustion chambers and they are designed to create turbulence when air and fuel are supplied to the cylinder to ensure better mixing.
The sealing part is designed to install special rings (compression and oil scraper), the task of which is to eliminate the gap between the piston and the liner wall, preventing the breakthrough of working gases into the under-piston space and lubricants into the combustion chamber (these factors reduce the efficiency of the motor). This ensures that heat is removed from the piston to the sleeve.
Sealing part
The sealing part includes grooves in the cylindrical surface of the piston - grooves located behind the bottom, and bridges between the grooves. In two-stroke engines, special inserts are additionally placed in the grooves, against which the locks of the rings rest. These inserts are necessary to eliminate the possibility of the rings turning and getting their locks into the inlet and outlet windows, which can cause their destruction. 
The jumper from the edge of the bottom to the first ring is called the heat zone. This belt perceives the greatest temperature impact, so its height is selected based on the operating conditions created inside the combustion chamber and the piston material.
The number of grooves made on the sealing part corresponds to the number of piston rings (and 2 - 6 can be used). The most common design with three rings - two compression and one oil scraper.
In the groove for the oil scraper ring, holes are made for the stack of oil, which is removed by the ring from the wall of the sleeve.
Together with the bottom, the sealing part forms the piston head.
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Skirt
The skirt acts as a guide for the piston, preventing it from changing its position relative to the cylinder and providing only the reciprocating movement of the part. Thanks to this component, a movable connection of the piston with the connecting rod is carried out.
For connection, holes are made in the skirt for installing the piston pin. To increase strength at the point of contact of the finger, special massive influxes, called bosses, are made on the inside of the skirt.
To fix the pin in the piston, grooves for retaining rings are provided in the mounting holes for it.
Piston types
In internal combustion engines, two types of pistons are used, which differ in their design - one-piece and composite.
One-piece parts are made by casting followed by machining. In the process of casting, a blank is created from metal, which is given the general shape of the part. Further, on metalworking machines, working surfaces are processed in the resulting workpiece, grooves are cut for rings, technological holes and recesses are made.
In the composite elements, the head and the skirt are separated, and they are assembled into a single structure during installation on the engine. Moreover, the assembly in one piece is carried out by connecting the piston to the connecting rod. For this, in addition to holes for the finger in the skirt, there are special eyelets on the head.
The advantage of composite pistons is the possibility of combining materials of manufacture, which increases the performance of the part.
Manufacturing materials
Aluminum alloys are used as the manufacturing material for solid pistons. Parts made of such alloys are characterized by low weight and good thermal conductivity. But at the same time, aluminum is not a high-strength and heat-resistant material, which limits the use of pistons made from it.
Cast pistons are also made of cast iron. This material is durable and resistant to high temperatures. Their disadvantage is a significant mass and poor thermal conductivity, which leads to a strong heating of the pistons during engine operation. Because of this, they are not used on gasoline engines, since high temperatures cause glow ignition (the air-fuel mixture ignites from contact with heated surfaces, and not from a spark plug spark).
The design of composite pistons allows you to combine these materials with each other. In such elements, the skirt is made of aluminum alloys, which ensures good thermal conductivity, and the head is made of heat-resistant steel or cast iron.
However, composite type elements also have disadvantages, including:
- can only be used in diesel engines;
- greater weight compared to cast aluminum;
- the need to use piston rings made of heat-resistant materials;
- higher price;
Due to these features, the scope of use of composite pistons is limited, they are used only on large-sized diesel engines.
Video: The principle of operation of the engine piston. Device
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 Mazda cars. 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, of a larger size, is fixed on the inner surface of the rotor, the second, a support one, of a smaller size, is rigidly attached to the inner surface of the side cover of the motor. 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 of a rotary piston engine is its simplicity of design. The RPD has 35-40 percent fewer parts than a four-stroke piston engine. There are no pistons, connecting rods, crankshaft in RPD. 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 speeds of the 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. A rotary engine is about half the size of a piston four-stroke engine of the same power. And this allows you to more rationally use the space of the 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 due to 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 of automatic transmissions. 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 NSU Ro 80 business class passenger car. 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 before the overhaul of the engine did not exceed 50 thousand kilometers with the declared 100 thousand kilometers.
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 Mazda RX-8 cars are free 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 sportbike equipped with a twin-rotor engine with a total volume of 588 cubic centimeters and developing a power of 170 horsepower. 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.
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.
The oil scraper ring is responsible for removing excess oil from the cylinder walls and preventing it from entering the combustion chamber. 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 the negative effects of extremely 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:
- 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 can have a different shape depending on the functions it performs - flat, concave and convex. The concave shape of the bottom provides 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 of the 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) are installed.
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.
In general, the piston of an internal combustion engine 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.
A rotary piston engine or Wankel engine is a motor where planetary circular motions are carried out as the main working element. This is a fundamentally different type of engine, different from piston counterparts in the ICE family.
The design of such a unit uses a rotor (piston) with three faces, externally forming a Reuleaux triangle, carrying out circular movements in a cylinder of a special profile. Most often, the surface of the cylinder is made along an epitrochoid (a flat curve obtained by a point that is rigidly connected to a circle that moves along the outer side of another circle). In practice, you can find a cylinder and a rotor of other shapes.
Components and principle of operation
The device of the RPD type engine is extremely simple and compact. A rotor is installed on the axis of the unit, which is firmly connected to the gear. The latter is coupled to the stator. The rotor, which has three faces, moves along an epitrochoidal cylindrical plane. As a result, the changing volumes of the working chambers of the cylinder are cut off using three valves. Sealing plates (end and radial type) are pressed against the cylinder by the action of gas and due to the action of centripetal forces and band springs. It turns out 3 isolated chambers of different volume sizes. Here, the processes of compressing the incoming mixture of fuel and air, expanding the gases that put pressure on the working surface of the rotor and clean the combustion chamber from gases are carried out. The circular motion of the rotor is transmitted to the eccentric axis. The axle itself is on bearings and transmits the torque to the transmission mechanisms. In these motors, the simultaneous operation of two mechanical pairs is carried out. One, which consists of gears, regulates the movement of the rotor itself. The other converts the rotating motion of the piston into the rotating motion of the eccentric axle.
Rotary Piston Engine Parts
The principle of operation of the Wankel engine
Using the example of engines installed on VAZ vehicles, the following technical characteristics can be mentioned:
- 1.308 cm3 - working volume of the RPD chamber;
- 103 kW / 6000 min-1 - rated power;
- 130 kg engine weight;
- 125,000 km - engine life until its first complete repair.
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. The combustible mixture enters the cylinders in finished form.
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.
Accordingly, the 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.
RPD Features
Advantages
Advantages of rotary piston engines compared to standard gasoline engines:
- Low vibration levels.
In motors of the RPD type, there is no conversion of reciprocating motion into rotational, which allows the unit to withstand high speeds with less vibration.
— Good dynamic characteristics.
Thanks to its design, such a motor installed in the car allows it to be accelerated above 100 km / h at high speeds without excessive load.
- Good power density with low weight.
Due to the absence of a crankshaft and connecting rods in the engine design, a small mass of moving parts in the RPD is achieved.
- In engines of this type, there is practically no lubrication system.
Oil is added directly to the fuel. The fuel-air mixture itself lubricates friction pairs.
- The rotary piston type motor has small overall dimensions.
The installed rotary piston motor makes it possible to maximize the useful space of the engine compartment of the car, evenly distribute the load on the axles of the car and better calculate the location of the gearbox elements and assemblies. For example, a four-stroke engine of the same power will be twice the size of a rotary engine.
Disadvantages of the Wankel engine
— Quality of engine oil.
When operating this type of engine, it is necessary to pay due attention to the quality composition of the oil used in Wankel engines. The rotor and the engine chamber inside have a large contact area, respectively, the engine wears out faster, and such an engine constantly overheats. Irregular oil changes cause great damage to the engine. The wear of the motor increases many times due to the presence of abrasive particles in the used oil.
— The quality of the spark plugs.
The operators of such engines have to be particularly demanding on the quality of the composition of the spark plugs. In the combustion chamber, due to its small volume, extended shape and high temperature, the process of ignition of the mixture is difficult. The consequence is an increased operating temperature and periodic detonation of the combustion chamber.
— Materials of sealing elements.
A significant flaw in the RPD-type motor can be called the unreliable organization of seals between the gaps between the chamber where the fuel burns and the rotor. The device of the rotor of such a motor is rather complicated, therefore seals are required both along the edges of the rotor and along the side surface in contact with the engine covers. Surfaces that are subject to friction must be constantly lubricated, resulting in increased oil consumption. Practice shows that an RPD-type motor can consume from 400 g to 1 kg of oil for every 1000 km. The environmentally friendly performance of the engine is reduced, since the fuel burns together with the oil, as a result, a large amount of harmful substances are released into the environment.
Due to their shortcomings, such motors are not widely used in the automotive industry and in the manufacture of motorcycles. But on the basis of RPD, compressors and pumps are manufactured. Aeromodellers often use these engines to build their models. Due to the low requirements for efficiency and reliability, designers do not use a complex sealing system in such motors, which significantly reduces its cost. The simplicity of its design allows it to be integrated into an aircraft model without any problems.
Efficiency of rotary piston design
Despite a number of shortcomings, studies have shown that the overall efficiency of the Wankel engine is quite high by modern standards. Its value is 40 - 45%. For comparison, in piston internal combustion engines, the efficiency is 25%, in 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.
The final efficiency of the motor consists of three main parts:
- Fuel efficiency (an indicator characterizing the rational use of fuel in the engine).
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.
The current state of the rotary piston engine
Significant technical difficulties arose in the way of mass application of the engine:
– development of a high-quality work process in an unfavorable chamber;
- ensuring the tightness of the sealing of working volumes;
– designing and creating a structure of body parts that will reliably serve the entire life cycle of the engine without warping with uneven heating of these parts.
As a result of the huge research and development work done, these firms managed to solve almost all the most difficult technical problems on the way to the creation of RPDs and enter the stage of their industrial production.
The first mass-produced NSU Spider with RPD was produced by NSU Motorenwerke. Due to frequent overhauls of engines due to the above technical problems at an early stage in the development of the Wankel engine design, warranties taken by NSU led it to financial ruin and bankruptcy and the subsequent merger with Audi in 1969.
Between 1964 and 1967, 2375 cars were produced. In 1967 the Spider was discontinued and replaced by the NSU Ro80 with a second generation rotary engine; in ten years of Ro80 production, 37,398 cars were produced.
Mazda engineers have dealt with these problems most successfully. It remains the only mass manufacturer of machines with rotary piston engines. The modified engine has been serially installed on the Mazda RX-7 since 1978. Since 2003, succession has taken the Mazda RX-8 model, and it is currently the mass and only version of the car with a Wankel engine.
Russian RPDs
The first mention of a rotary engine in the Soviet Union dates back to the 60s. Research work on rotary piston engines began in 1961, by the relevant decree of the Ministry of Automotive Industry and the Ministry of Agriculture of the USSR. An industrial study with a further conclusion to the production of this design began in 1974 at the VAZ. specifically for this, a Special Design Bureau for Rotary Piston Engines (SKB RPD) was created. Since it was not possible to buy a license, the serial Wankel from NSU Ro80 was disassembled and copied. On this basis, the VAZ-311 engine was developed and assembled, and this significant event took place in 1976. At VAZ, they developed a whole line of RPDs from 40 to 200 strong engines. The finalization of the design dragged on for almost six years. It was possible to solve a number of technical problems associated with the performance of gas and oil seals, bearings, to debug an efficient workflow in an unfavorable chamber. VAZ presented its first production car with a rotary engine under the hood to the public in 1982, it was the VAZ-21018. The car was externally and structurally like all models of this line, with one exception, namely, under the hood there was a single-section rotary engine with a capacity of 70 hp. The duration of development did not prevent embarrassment from happening: on all 50 experimental machines, engine breakdowns occurred during operation, forcing the plant to install a conventional piston engine in its place.
VAZ 21018 with rotary piston engine
Having established that the cause of the malfunction was the vibration of the mechanisms and the unreliability of the seals, the designers undertook to save the project. Already in the 83rd, two-section VAZ-411 and VAZ-413 appeared (with a capacity of 120 and 140 hp, respectively). Despite the low efficiency and short resource, the scope of the rotary engine was still found - the traffic police, the KGB and the Ministry of Internal Affairs needed powerful and inconspicuous vehicles. Equipped with rotary engines, Zhiguli and Volga easily overtook foreign cars.
Since the 80s of the 20th century, SKB has been fascinated by a new topic - the use of rotary engines in a related industry - aviation. The departure from the main industry of using RPDs led to the fact that for front-wheel drive vehicles the VAZ-414 rotary engine was created only by 1992, and it was brought up for another three years. In 1995, the VAZ-415 was submitted for certification. Unlike its predecessors, it is universal, and can be installed under the hood of both rear-wheel drive (classic and GAZ) and front-wheel drive cars (VAZ, Moskvich). The two-section "Wankel" has a working volume of 1308 cm 3 and develops a power of 135 hp. at 6000 rpm. "Ninety-ninth" he accelerates to hundreds in 9 seconds.
Rotary piston engine VAZ-414
At the moment, the project for the development and implementation of the domestic RPD is frozen.
Below is a video of the device and the operation of the Wankel engine.
