The main structure of engine
An engine is a machine that converts a certain kind of energy into mechanical energy. Its function is to convert chemical energy of combustion of a liquid or gas into thermal energy after combustion, and then convert thermal energy into mechanical energy through expansion and power output outside. The engine is a complex machine composed of many structures and systems. Its structure is different, but since basic working principle is same, its basic structure is similar. The general structure of engine is shown below.
A gasoline engine usually consists of a crank mechanism, a gas distribution mechanism, and five main systems for fuel supply, lubrication, cooling, ignition and starting. Diesel engines usually consist of two main institutions and four main systems (no ignition system).
1． Crank mechanism
The crank mechanism consists of a cylinder block, cylinder head, piston, connecting rod, crankshaft and flywheel. This is when engine generates power and converts linear reciprocating motion of piston into rotational motion of crankshaft to output power.
2． Valve mechanism
The valve train consists of an intake valve, an exhaust valve, a valve spring, a tappet, a camshaft and a gear wheel. Its function is to timely fill cylinder with fresh gas and timely release combustion exhaust gases from cylinder.
3． Fuel supply system
Due to different fuel used, it can be divided into gasoline engine fuel supply system and diesel engine fuel supply system.
Gasoline supply system is divided into two types: oil tank fuel supply system and direct fuel injection system. The commonly used carburetor type fuel supply system consists of fuel tank, gasoline pump, gasoline filter, carburetor, air filter, intake and exhaust manifolds . and exhaust mufflers, etc., its role is to supply combustible mixture to cylinder and control amount of combustible mixture entering cylinder to adjust power output and engine speed. Finally, exhaust gas exits cylinder after combustion.
Diesel engine fuel supply system is composed of fuel tank, fuel supply pump, high pressure fuel pump, diesel filter, inlet and outlet pipes and muffler. Some diesel fuel is injected to adjust engine power output and speed. , and finally, exhaust gases after combustion are ejected from cylinder.
4． cooling system
Cars are usually water-cooled. The water-cooled type is composed of a water pump, a radiator, a fan, a thermostat, and a water jacket (in case), and its function is to use cooling water circulation to dissipate heat from high-temperature parts to atmosphere through radiator, thereby maintaining normal operation of engine. temperature.
5． Lubrication system
The lubrication system is composed of oil pump, filter, oil passage, oil pan and so on. Its function is to distribute lubricating oil over friction surface of each respective moving part to reduce friction, slow down wear of parts, and clean and cool friction surface.
6． Ignition system
The ignition system of a gasoline engine consists of a power source (battery and alternator), an ignition coil, a distributor and a spark plug, etc. Its function is to ignite compressed combustible mixture in cylinder in time according to a predetermined time.
The starting system consists of a starter, a startingrelays, etc., which are used to start a stationary engine and put it into a self-starting state.
How engine works
The process of converting thermal energy of engine into mechanical energy is realized through four continuous processes of air intake, compression, work and exhaust, and each such process is called a work cycle. If crankshaft completes two revolutions and piston reciprocates four times to complete work cycle, such an engine is called a four-stroke. The crankshaft rotates once, that is, piston reciprocates in two strokes, making a duty cycle, which is called a two-stroke engine.
1. The principle of operation of a four-stroke gasoline engine:
(1) Intake stroke. The crankshaft drives piston from top dead center to bottom dead center, with intake valve opening and exhaust valve closing. As piston moves, internal volume of cylinder gradually increases with formation of a vacuum, so combustible mixture is sucked into cylinder through intake valve until piston reaches bottom dead center and ends when intake valve closes.
Due to inlet resistance of intake system, gas pressure in cylinder is lower than atmospheric pressure at intake end, which is about 0.075Mpa ~ 0.09Mpa. Due to heating of high temperature components such as cylinder walls and pistons and high temperature residual exhaust gases from previous cycle, temperature of gases rises to 370-440K.
(2) Compression stroke. At end of intake stroke, piston moves from BDC to TDC, driven by crankshaft, and volume in cylinder gradually decreases. At this time, intake and exhaust valves are closed, combustible mixture is compressed, and compression ends when piston reaches top dead center. During compression process, gas pressure and temperature rise at same time, and mixed gas is further mixed evenly. At end of compression, pressure in cylinder is about 0.6MPa-1.2MPa, and temperature is about 600K-800K.
(3) Working stroke. At end of compression stroke, spark plug generates electrical sparks to ignite gas mixture and quickly burn it, so that temperature and pressure of gas rise rapidly, thereby pushing piston to move from top dead center to top dead center. bottom dead center, and crankshaft does work through connecting rod until work is completed when piston reaches bottom dead center.
When work starts, pressure and temperature of gas in cylinder rises sharply, instantaneous pressure can reach 3MPa~5MPa, and instantaneous temperature can reach 2200K~2800K.
(4) Release stroke. As power stroke nears its end, exhaust valve opens, intake valve closes, and crankshaft pushes piston from bottom dead center to top dead center through connecting rod. The exhaust gas is expelled from cylinder under its own residual pressure and is driven by piston, when piston reaches top dead center, exhaust valve closes and exhaust stops. Due to exhaust drag in exhaust system, when exhaust stroke ends, cylinder pressure is slightly higher than atmospheric pressure, about 0.105Mpa ~ 0.115Mpa, and temperature is about 900K ~ 1200K.
2． The working principle of a four-stroke diesel engine:
Due to different nature of fuel used, formation and ignition of combustible mixture of a four-stroke diesel engine is very different from that of a gasoline engine. The following will mainly describe difference between duty cycle of a diesel engine and a gasoline engine.
(1) Stroke inka. During intake stroke, not a combustible mixture enters cylinder, but clean air.
(2) Compression stroke. During compression stroke, clean air entering cylinder is compressed. Due to high compression ratio of diesel, it is about 15-22. The temperature and pressure at end of compression are higher than gasoline engines. reach 800K-1000K.
(3) Working stroke. At end of compression stroke, fuel pump injects high pressure diesel oil into high temperature and high pressure air in cylinder through fuel injector in form of a mist, quickly evaporates and forms a mixture. with air. Since temperature in cylinder is higher than self-ignition temperature of a diesel engine (about 500K), diesel mixture will immediately ignite and burn out on its own, and then burn out during fuel injection, and pressure and temperature in cylinder will rise sharply, pushing piston down.
During working stroke, instantaneous pressure can reach 5MPa~10MPa, and instantaneous temperature can reach 1800K~2200K.
(4) Release stroke. This stroke is basically same as that of a gasoline engine.
According to duty cycle of above-mentioned four-stroke gasoline engine and diesel engine, main content of duty cycle of two engines is similar. Only power stroke produces power in four strokes, and remaining three strokes are auxiliary to power stroke to prepare for work, and they all consume part of energy. In first cycle, when engine starts, there must be an external force to crank crankshaft to complete intake and compression strokes. When a successful stroke begins, work energy is stored in flywheel through crankshaft to keep next cycle going.
3． The principle of operation of a two-stroke gasoline engine:
The duty cycle of a two-stroke engine also includes four stages of air intake, compression, power and exhaust, but it is completed within two reciprocating strokes of piston.
(1) First trip. The piston moves from bottom dead center to top dead center. When piston moves up, vent hole closes and outlet hole, combustible mixture that enters cylinder is compressed. When piston moves further up to top dead center, compression ends. At same time, when piston goes up, some degree of vacuum is formed in crankcase below it. When piston moves to inlet opening, a fresh combustible gas mixture is sucked into crankcase, this ends first cycle.
(2) Second stroke. As piston approaches top dead center, spark plug creates an electrical spark to ignite compressed combustible mixture. The high temperature, high pressure gas produced during combustion pushes piston down to do work. When piston descends to close inlet, mixed gas in crankcase is pre-compressed, when piston continues to descend until outlet opens, exhaust gas after combustion is expelled through outlet on its own. pressure; then vent hole opens, gas mixture pre-compressed in crankcase enters cylinder, and exhaust gases remaining in cylinder are removed. This process is called ventilation process, and it will continue until next stroke when piston goes up, closing vent and exhaust port. When piston drops to bottom dead center, second stroke ends.
From above two strokes, you can find out: in first stroke, air exchange and compression are performed above piston, and air is admitted below piston; The ventilation process consists of two cycles.
The main function of piston is to withstand gas pressure in cylinder and transfer it to crankshaft through piston pin and connecting rod. In addition, piston forms a combustion chamber together with cylinder head and cylinder wall,
Because upper part of piston is in direct contact with high temperature gas, it bears a large thermal load, piston is also affected by periodically changing gas pressure and inertia force, so piston must have sufficient strength and rigidity, and its mass should be as small as possible , thermal conductivity is better, it should have good heat resistance and wear resistance, and change in size and shape should be small with temperature change.
The piston material widely used in automotive engines is aluminum alloy, and some diesel engines also use alloy cast iron or high temperature steel for pistons.
The basic design of a piston can be divided into three parts: top, head and skirt.
1. Top of piston. The piston head is an integral part of combustion chamber and serves to resist gas pressure. According to different purposes and requirements, piston top is made in various shapes: most common are flat top piston, convex top piston, concave top piston and molded top piston.
(2) Piston head. The piston crown is part above piston ring groove. Its main function is to withstand gas pressure and transfer it to connecting rod, together with piston ring, it can seal cylinder and transfer heat absorbed by piston head to cylinder wall through piston ring.
Several annular grooves are cut into piston head for installation of piston rings. Gasoline engine pistons usually have 3 to 4 annular grooves, top 2 to 3 are used to install an air ring, and bottom one is used to install an oil scraper ring. Several small radial holes are drilled on bottom surface of oil ring groove so that excess oil scraped by oil ring off cylinder wall can drain back into oil pan through these small holes.
(3) Piston skirt. The part below piston ring groove is called piston skirt. Its function is to guide piston to reciprocate in cylinder and withstand lateral pressure.
Inline cylinder block
The cylinder block and top of crankcase are often cast into one body, which is called cylinder block-crankcase, or cylinder block for short. The top of cylinder block has one or more cylindrical cavities in which piston moves as a guide, called a cylinder; lower part is crankcase supporting crankshaft, and its inner cavity is space for crankshaft to move.
The cylinder block is assembly base for various engine mechanisms and systems and serves to maintain exact relative position of moving parts of engine.
To remove heat fromThe outside of cylinder has a water jacket (water-cooled engine) or a radiator (air-cooled engine).
The upper crankcase has front and rear walls and a middle baffle, on which main bearing mounting hole is made, and in some engines camshaft mounting hole is also made. To lubricate these bearings, main oil channels are drilled on side walls, and oil distribution channels are drilled on front and rear walls and middle bulkhead.
There are two common forms of engine cylinder arrangement: single-row and double-row: The cylinders of single-row (in-line) engines are arranged in a row, usually vertically. However, to reduce height of engine, cylinders are sometimes placed obliquely or even horizontally. A two-row engine with an angle γ<180° between axes of left and right rows of cylinders is called a V-engine.
Terms related to engine
(1) Top dead center - piston is farthest from center of rotation of crankshaft, usually highest position of piston.
(2) Bottom dead center is point at which piston is closest to center of rotation of crankshaft, usually lowest position of piston.
(3) Stroke is distance between top and bottom dead center.
(4) Stroke - a process in which piston moves once from one dead center to another dead center.
(5) Crankshaft radius - distance from center of connection between crankshaft and large end of connecting rod to center of rotation of crankshaft.
(6) Cylinder displacement is amount of space piston gives up from top dead center to bottom dead center.
(7) Engine displacement is sum of displacements of all engine cylinders, also known as engine displacement.
(8) Combustion chamber volume - when piston is at top dead center, space above piston crown is called combustion chamber, and its volume is called combustion chamber volume.
(9) Gross volume of cylinder - when piston is at bottom dead center, volume of entire space above top of piston is equal to sum of working volume of cylinder and volume of combustion chamber.
(10) Compression ratio - ratio of total volume of cylinder to volume of combustion chamber.