As mentioned above, thermal expansion is used in an internal combustion engine. 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 motors. Thermal energy is obtained by burning any kind of fuel. A heat engine, in which part of the chemical energy of the fuel combusted in the working cavity is converted into mechanical energy, is called a piston internal combustion engine. (Soviet encyclopedic dictionary)
3. 1. Classification of internal combustion engines
As mentioned above, ICEs, in which the process of fuel combustion with the release of heat and its transformation into mechanical work, takes place directly in the cylinders, is the most widespread as power plants for cars. But in most modern cars, internal combustion engines are installed, which are classified according to various criteria: 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) - diesel engines; By the way of carrying out the working cycle - four-stroke and two-stroke; According to the number of cylinders - single-cylinder, two-cylinder and multi-cylinder; According to the arrangement 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 (with an arrangement of cylinders at an angle of 180, the engine is called an engine with opposite cylinders, or opposed); By cooling method - for engines with liquid or air cooled; By the type of fuel used - gasoline, diesel, gas and multi-fuel; By the compression ratio. Depending on the degree of compression, a distinction is made between
engines of high (E = 12 ... 18) and low (E = 4 ... 9) compression; By the method of filling the cylinder with a fresh charge: a) naturally aspirated engines, in which air or a combustible mixture is admitted due to vacuum in the cylinder during the suction stroke of the piston;) supercharged engines, in which air or a combustible mixture is injected 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, high-speed, high-speed; By purpose, stationary engines are distinguished, auto-tractor, ship, diesel, aviation, etc.
3.2. Fundamentals of piston internal combustion engines
Reciprocating internal combustion engines consist of mechanisms and systems that perform their assigned functions and interact with each other. The main parts of such an engine are the crank mechanism and the gas distribution mechanism, as well as the power supply, cooling, ignition and lubrication systems.
The crank mechanism converts the rectilinear reciprocating motion of the piston into rotary motion crankshaft.
The gas distribution mechanism ensures the timely admission of the combustible mixture into the cylinder and the removal of combustion products from it.
The power system is designed for the preparation and supply of a combustible mixture into the cylinder, as well as for the removal of 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 a normal operating temperature of the engine, providing heat removal 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 sump. Inside the cylinder, a piston with compression (sealing) rings moves, in the form of a glass with a bottom in the upper part. The piston through the piston pin and the connecting rod is connected to crankshaft, which rotates in the main bearings located in the crankcase. The crankshaft consists of main journals, cheeks and connecting rod journals. The 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 topmost position of the piston is called top dead center (TDC), its bottommost position is the bottom dead center (BDC).
The non-stop movement of the piston through the dead center 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 Vc; 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 = pD ^ 3 * S / 4, where D is the cylinder diameter. The sum of all the 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. Compression ratio is an important parameter for internal combustion engines because greatly affects its efficiency and power.
Rotary piston engine(RPD), or Wankel engine. Internal combustion engine designed Felix Wankel in 1957 in collaboration with Walter Freude. In RPD, the function of a piston is performed by a three-vertex (triangular) rotor, which makes rotational movements inside a cavity of a complex shape. After the wave of experimental car and motorcycle models in the 60s and 70s of the twentieth century, interest in RPDs declined, although a number of companies are still working to improve the design of the Wankel engine. Currently, the RPD is equipped with passenger cars Mazda... The rotary piston engine finds application in modeling.
Principle of operation
The force of the gas pressure from the burnt air-fuel 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, fixed on the inner surface of the rotor, the second, supporting, smaller, rigidly attached to the inner surface of the engine side cover. The interaction of the gears leads to the fact that the rotor makes circular eccentric movements, contacting the edges with the inner surface of the combustion chamber. As a result, three isolated chambers of variable volume are formed between the rotor and the engine casing, in which the processes of compression of the fuel-air mixture, its combustion, expansion of gases exerting pressure on the working surface of the rotor and cleaning 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 one converts the circular motion of the rotor into rotation of the eccentric shaft. The gear ratio of the rotor and stator gears is 2: 3, therefore, in one full 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 edges, a complete four-stroke cycle of the internal combustion engine is performed.
RPD scheme
1 - inlet window; 2 outlet window; 3 - case; 4 - combustion chamber; 5 - stationary gear; 6 - rotor; 7 - gear wheel; 8 - shaft; 9 - spark plug
Advantages of the 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. The RPD lacks pistons, connecting rods, and a crankshaft. In the "classic" version of the RPD, there is no gas distribution mechanism either. The fuel-air mixture enters the working cavity of the engine through the inlet window, which opens the edge of the rotor. The exhaust gases are discharged through the exhaust port, which again crosses the edge of the rotor (this is reminiscent of the gas distribution device of a two-stroke piston engine).
Special mention should be made of the lubrication system, which is practically absent in the simplest version of the RPD. The oil is added to the fuel, just like a two-stroke motorcycle engine. Friction pairs (primarily the rotor and the working surface of the combustion chamber) are lubricated by the fuel-air mixture itself.
Since the rotor mass is small and is easily balanced by the mass of the eccentric shaft counterweights, the RPD has a low vibration level and good uniformity of operation. In vehicles with RPD, it is easier to balance the engine, having achieved a minimum level of vibration, which has a good effect on the comfort of the car as a whole. Twin-rotor motors are particularly smooth running, in which the rotors themselves are vibration-reducing balancers.
Another attractive quality of the RPD is the high power density at high revs eccentric shaft. This makes it possible to achieve excellent speed characteristics from a car with a RPD with relatively low fuel consumption. Low inertia of the rotor and increased power density in comparison with piston internal combustion engines improve vehicle dynamics.
Finally, an important advantage of the RPD is its small size. A rotary engine is approximately half the size of a piston four-stroke engine of the same power. And this allows you to more efficiently 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 RAP
The main disadvantage of a rotary piston engine is the low efficiency of sealing the gap between the rotor and the combustion chamber. The RPD rotor of a complex shape requires reliable seals not only along the edges (and there are four of them on each surface - two on the top, two on the side edges), but also on 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 tolerances for metal expansion inherent in the design of seals 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, installing sealing rings with gaps in different directions).
In recent years, the reliability of seals has increased dramatically. The designers have found new materials for the seals. However, there is no need to talk about any breakthrough yet. Seals are still the bottleneck of the RPD.
The complex rotor sealing system requires effective 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 has a bad effect on the environmental friendliness of the engines. There are more substances hazardous to human health in the exhaust gases of the RPD than in the exhaust gases of piston engines.
Special requirements are also imposed on the quality of the oils used in the RPD. This is due, firstly, to the tendency to increased wear (due to the large area of contacting parts - the rotor and the internal chamber of the engine), and secondly, to overheating (again due to increased friction and due to the small size of the engine itself ). For RPD, irregular oil changes are deadly - since abrasive particles in old oil dramatically increase engine wear and engine hypothermia. Starting a cold engine and insufficient warming up leads 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 the piston engine jams due to overheating, then the RPD most often - during the start of a cold engine (or when driving in cold weather, when the cooling is excessive).
In general, the operating temperature of the RPD is higher than that of reciprocating engines. The most thermally stressed area is the combustion chamber, which has a small volume and, accordingly, an increased temperature, which complicates the process of igniting the fuel-air mixture (RPDs, due to the extended shape of the combustion chamber, are prone to detonation, which can also be attributed to the disadvantages of this type of engine). Hence the exactingness of the RPD to the quality of the 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 deliver optimal power only at high enough rpm, which forces designers to use RPDs paired with multi-stage gearboxes and complicates the design automatic boxes gear. Ultimately, RPDs are not as economical as they should be in theory.
Practical applications in the automotive industry
RPDs were most widespread in the late 60s and early 70s of the last century, when the patent for the Wankel engine was bought by 11 leading car manufacturers in the world.
In 1967, the German NSU company released serial a car business class NSU Ro 80... This model was produced for 10 years and sold around the world in the amount of 37,204 copies. The car was popular, but the shortcomings of the RPD installed in it, in the end, spoiled the reputation of this wonderful car. Against the background of durable competitors, the NSU Ro 80 model looked "pale" - mileage up to overhaul engine with the declared 100 thousand kilometers did not exceed 50 thousand.
Concern experimented with RPD Citroen , Mazda , VAZ... The greatest success was achieved by Mazda, which released its passenger car with RPD back in 1963, four years before the appearance of the NSU Ro 80. Today, Mazda is equipping RX series sports cars with RPDs. Modern cars Mazda RX-8 saved from many of the disadvantages 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 RPDs - Hercules, Suzuki and others. Currently, small-scale production of "rotary" motorcycles is established only at Norton, which produces the NRV588 model and prepares the NRV700 motorcycle for serial production.
Norton NRV588 is a sports 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 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.
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 piston's translational motion. The piston is set in motion when the working fluid expands in the cylinder.
The crank mechanism converts the forward motion of the piston into the rotational motion of the crankshaft.
The working cycle of the engine consists of a sequence of strokes of one-way forward strokes of the piston. Engines with two and four strokes are subdivided.
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 stroke of the piston.
V piston engines of various designs, the process of fuel ignition occurs in different ways:
Electrospark discharge that forms on the spark plugs. These engines can run on both gasoline and other fuels (natural gas).
By compressing the working fluid:
V diesel engines operating on diesel fuel or gas (with a 5% addition of diesel fuel), air is compressed, and when the piston reaches the maximum compression point, fuel is injected, which ignites from contact with heated air.
Compression engines... The fuel supply to them is exactly the same as in gasoline engines... Therefore, for their operation, a special composition of fuel (with admixtures of air and diethyl ether) is required, as well as precise adjustment of the compression ratio. Compressor engines have found their way into the aircraft and automotive industries.
Incandescent engines... The principle of their operation is in many ways similar to the compression model engines, but it was not without design features... The role of ignition in them is performed by a glow plug, the glow of which is maintained by the energy of the fuel that burns in 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.
Calorizing motors... In these engines, ignition occurs when the 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.
Fuels 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 the gasifier from coal, peat and wood, carbon monoxide is also used as a fuel.
The operation of piston engines.
Engine cycles detailed in technical thermodynamics. Different cyclograms are described by different thermodynamic cycles: Otto, Diesel, Atkinson or Miller and Trinkler.
Reasons for piston engine breakdowns.
Efficiency of a piston internal combustion engine.
The maximum efficiency that was 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 heat exhaust gases... In this connection, it is necessary to equip engines with cooling systems.
Cooling systems classification:
Air CO- give off heat to the air due to the ribbed outer surface of the cylinders. Are applied
more on weak engines (tens of hp), or on powerful aircraft engines which 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, but metallic sodium 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 aircraft and shipbuilding. They are also used to drive military, agricultural and construction equipment, power generators, water pumps, chainsaws and other machines, both mobile and stationary.
Piston ICEs are most widely used as energy sources in road, rail and sea 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-CHPPs based on gas-piston internal combustion engines, with the help of which the problems of power supply of small residential areas or industries, are effectively solved, have become especially widespread. 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 have 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 our knowledge, let us consider this issue very briefly again.
In fig. 6.1 shows a diagram of the engine device. As you know, 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 piston moving in it 6. The flue gases generated by 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 for use. The crankshaft is located in the crankcase, and the engine cylinders are located in another body part called the block (or jacket) of cylinders 2. The cylinder cover 5 contains the intake 3 and graduation 4 valves with a 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 the movements of the piston and the operation of the valves.
Reciprocating internal combustion engines are usually classified according to various general characteristics.
- 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. According to the organization of the working process - into four-stroke and two-stroke. In the latter, the working process is completed not in four, but in two piston strokes. In turn, two-stroke internal combustion engines are subdivided into machines with direct-flow valve-slotted blowing, with crank-chamber blowing, with direct-flow blowing and oppositely moving pistons, etc.
- 3. By appointment - for stationary, ship, diesel locomotive, automobile, auto-tractor, etc.
- 4. According to the number of revolutions - to low-speed (up to 200 rpm) and high-speed.
- 5. By the average piston speed d> n =? NS/ 30 - for low-speed and high-speed (th? „> 9 m / s).
- 6. By air pressure at the beginning of compression - for conventional and pressurized with the help of driven blowers.
- 7. According to the use of exhaust gas heat - into conventional (without using this heat), turbocharged and combined. On turbocharged cars, the exhaust valves open slightly earlier than usual and the higher-pressure flue gases are sent to a pulse turbine, which drives the turbocharger to supply air to the cylinders. This allows more fuel to be burned in the cylinder, improving both efficiency and specifications cars. 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 remainder of the total power comes from the flue gas turbine. For this, the flue gases at high pressure R and temperature / are sent to the turbine, the shaft of which, by means of a gear transmission or a fluid coupling, transfers the received power to the main shaft of the installation.
- 8. According to the number and arrangement of cylinders, engines are: one-, two- and multi-cylinder, in-line, K-shaped, T-shaped.
Let us now consider the real process of a modern four-stroke diesel engine. It is called a four-stroke cycle because a full cycle here is carried out in four full strokes of the piston, although, as we will now see, during this time, slightly more real thermodynamic processes are carried out. These processes are clearly shown in Figure 6.2.
Rice. 6.2.
I - absorption; II - compression; III - working stroke; IV - ejection
During the beat suction(1) The suction (intake) valve opens a few degrees before top dead center (TDC). The opening moment corresponds to the point G on R-^ -chart. In this case, the suction process occurs when the piston moves to the bottom dead center (BDC) and proceeds at a pressure p ns less atmospheric /; a (or boost pressure NS). When the direction of movement of the piston changes (from BDC to TDC), the intake valve does not close immediately either, but with a certain delay (at the point T). Further, when the valves are closed, the working fluid is compressed (to the point with). In diesel cars, clean air is sucked in and compressed, and in carburetor cars - a working mixture of air with gasoline vapors. This piston stroke is usually called a stroke. compression(II).
A few degrees of the angle of rotation of the crankshaft before TDC is injected into the cylinder through the nozzle diesel fuel, its spontaneous combustion, combustion and expansion of combustion products occurs. In carburetor machines, the working mixture is forcibly ignited using an electric spark discharge.
When the air is compressed and the 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 at first abruptly, and then, when the piston begins its way to BDC, increases with a decreasing rate to a maximum, and then, as the last portions of fuel supplied during injection are burned, it even begins to decrease (due to intensive growth cylinder volume). We will 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 the BDC. The third stroke of the piston, which includes the combustion and expansion processes, is called working stroke(III), because only at this time does the engine perform useful work. This work is accumulated by means of a flywheel and given to the consumer. A part of the accumulated work is expended during the execution of the remaining three cycles.
When the piston approaches BDC, the exhaust valve opens with some advance (point B) and the exhaust flue gases rush into exhaust pipe, and the pressure in the cylinder drops sharply to almost atmospheric. During the stroke of the piston to TDC, flue gases are pushed out of the cylinder (IV - ejection). Since the exhaust tract of the engine has a certain hydraulic resistance, the pressure in the cylinder remains above atmospheric pressure during this process. The outlet valve closes after TDC (point NS), so that in each cycle a situation arises when both the intake and exhaust valves are open at the same time (they speak of valve overlap). This makes it possible to better clean the working cylinder of combustion products, as a result of which the efficiency and completeness of fuel combustion increases.
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 ports 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 on indicator chart(fig. 6.3, a) reflected by the line c - I - t. At the point T exhaust valves open and, under the influence of excess pressure, 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 - worker
cylinder; 8 - air receiver; 9- purge windows
tate, the pressure in the cylinder drops noticeably (point NS). When the piston is lowered enough that the purge ports begin to open, compressed air rushes into the cylinder from the receiver 8 pushing the remaining flue gases out of the cylinder. In this case, 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 cylinder purging process continues as long as the purge ports remain at least partially open. At the point To(fig. 6.3, b) the piston completely overlaps the purge ports and the next portion of air that has entered the cylinder begins to compress. A few degrees before TDC (at the point with") fuel injection begins through the nozzle, and then the previously described processes occur, leading to the ignition and combustion of fuel.
In 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 only the duration
Rice. 6.4.
a- loop slot blowing; 6 - direct-flow blowdown with oppositely moving pistons; v- crank-chamber blowdown
individual processes and, as a consequence, on the technical and economic characteristics of the engine.
In conclusion, it should be noted that two-stroke engines theoretically, all other things being equal, they allow to obtain twice the power, but in reality, due to the worse conditions for cleaning the cylinder and relatively large internal losses, this gain is somewhat less.
