July 07 2023

Explain in detail structure and principle of operation of an automobile engine.

First, let's look at one of most common engine parameters - engine size. Engine displacement is sum of displacements of each engine cylinder, usually expressed in liters (l). Cylinder displacement refers to volume of gas moved by piston from top dead center to bottom dead center, also known as cylinder displacement, which depends on cylinder bore and piston stroke. Engine displacement is a very important parameter of an engine. It can reflect engine size better than cylinder diameter and number of cylinders. Many engine numbers are closely related to displacement. Generally speaking, larger displacement, greater power of engine.

Having dealt with displacement, let's look at other general parameters of engine. Many young racers report that they often see "L4", "V6", "V8", "W12" and other words in a car's engine information column and want to understand what it means. All of them indicate location and number of engine cylinders. The number of cylinders commonly used in automobile engines is 3 cylinders, 4 cylinders, 6 cylinders, 8 cylinders, 10 cylinders, 12 cylinders, etc.

Generally speaking, engines under 1 liter often use 3 cylinders, such as 0.8 liter Alto and Flair cars. An engine with a displacement of 1 to 2.5 liters is usually a 4-cylinder engine, and engines of ordinary economy cars and middle-class cars are mostly 4-cylinders. Engines around 3 liters are typically 6-cylinders such as Regal and new 3.0-liter Accord.

An engine with a displacement of about 4 liters is usually an 8-cylinder, for example, a Beijing Jeep's JEEP4700 with a displacement of 4.7 liters. Engines with a displacement of more than 5.5 liters usually use a 12-cylinder engine, for example, BMW 760Li with a displacement of 6 liters uses a V12 engine. With same cylinder diameter, more cylinders, larger displacement and higher power, and in case of same engine displacement, more cylinders, smaller cylinder diameter, higher engine speed and, therefore, higher engine speed. increase in power .

The above is knowledge of number of engine cylinders. Next, let's deal with important parameter "the shape of arrangement of cylinders." As a rule, cylinders of engines less than 5 cylinders are mostly in-line, and most common low-cost cars are L4 engines, i.e. in-line 4 cylinders. In addition, there are several more 6-cylinder engines arranged in line.

The cylinder block of an in-line engine is located in a line. The cylinder block, cylinder head and crankshaft are simple in structure, low manufacturing cost, good torque characteristics at low speed, low fuel consumption, compact size. in size, andwide application. The disadvantage is low power. Generally, sub-1 liter petrol engines use inline 3-cylinder engines, 1 to 2.5 liter gasoline engines mostly use inline 4-cylinder engines, and some four-wheel drive vehicles use inline 6-cylinder engines. width, superchargers, etc. can be located next to them. For example, JEEP4000 of a Beijing jeep uses an in-line 6-cylinder engine.

According to professionals, inline 6-cylinder engine has a better dynamic balance and relatively less vibration, which is why it is also used in some mid-range and high-end cars. Engines from 6 to 12 cylinders are usually V-shaped, and V10 engine is mainly found in racing cars. The V-shaped engine has small dimensions in length and height and is very convenient in layout. It is generally accepted that V-twin engine is a relatively advanced engine, so it has become one of symbols of car class.

The design of V8 engine is very complex and production cost is very high, so it is rarely used. The V12 engine is too big and heavy and is only used in a few high end cars like BMW. 760Li mentioned above. And recently, Volkswagen has developed a new W-type engine, which has two types, W8 and W12, that is, cylinders are arranged in four rows with offset angles, and shape is compact. The top-end Volkswagen Phaeton sedan has a W12 engine with a displacement of 6.0 liters.

The engine casing is skeleton of engine and is mounting base for various engine mechanisms and systems. All main parts and components of engine are installed inside and outside and carry various loads. Therefore, body must have sufficient strength and rigidity. The body group is mainly composed of cylinder block, crankcase, cylinder head, cylinder gasket and other parts.

1. Cylinder block

The cylinder block and upper crankcase of a water-cooled engine are often cast into one body called cylinder block - crankcase, which can also be called cylinder block. The cylinder block is usually made of gray cast iron. The cylindrical cavity at top of cylinder block is called cylinder, and lower half is crankcase supporting crankshaft, and its inner cavity is space for crankshaft to move. . Numerous stiffening ribs, water cooling jackets and lubricating oil passages are cast inside cylinder block.

The cylinder block must have sufficient strength and rigidity. According to position of cylinder block and installation plane of oil pan, cylinder blocks are generally divided into following three types.

(1) Conventional cylinder block. It differs in that mounting plane of oil pan and center of rotation of crankshaft are at same height. The advantages of this bThe cylinder blocks are low body height, light weight, compact design, easy processing, easy disassembly and assembly of crankshaft, and disadvantages are poor rigidity and strength

(2) Portal cylinder block Characterized by fact that oil pan installation plane is located below center of rotation of crankshaft. Its advantages are good strength and rigidity, ability to withstand large mechanical loads, and its disadvantages are poor manufacturability, bulky design, and difficulty in processing.

(3) Tunnel type cylinder block. The crankshaft main bearing bore of this type of cylinder block is an integral type, which uses rolling bearings, main bearing bore is relatively large, and crankshaft is installed at rear of cylinder block. Its advantages are compact structure, good rigidity and strength, and its disadvantages are high requirements for processing accuracy, poor workmanship, inconvenient disassembly and assembly of crankshaft.

For inside of cylinder to work properly at high temperatures, cylinder and cylinder head must be properly cooled. There are two cooling methods: water cooling and air cooling. The cooling water jackets are processed around cylinder and in cylinder head of water-cooled engine, and cylinder block and cylinder head water cooling jacket are connected. The cooling water circulates continuously in water jacket, removing some of heat, and cooling cylinder and cylinder head.

Modern cars mostly use water-cooled multi-cylinder engines. For multi-cylinder engines, cylinder arrangement determines size and design characteristics of engine, and also affects rigidity and strength of engine casing and is related to overall appearance of machine. According to cylinder arrangement, cylinder block can also be divided into three types: single-row, V-shaped and opposed.

(1) Inline

Engine cylinders are arranged in a row, usually vertically. The single-row cylinder block is simple in structure and easy to process, but length and height of engine are relatively large. Generally, engines with less than six cylinders use inline type. For example, engines used in Jetta, Fukang and Hongqi vehicles use this inline cylinder block. Some vehicles tilt engine at an angle to reduce engine height.

(2) V-shaped

The cylinders are arranged in two rows, and angle between axes of left and right rows of cylinders γ<180° is called a V-engine. Compared to an in-line engine, a V-engine shortens length and height of body and increases cylinder body. Rigidity reduces weight engine, but increases width of engine, and shape is more complex and difficult to process. Usually used for engines with more than 8 cylinders and 6-cylinder engines alsouse this form of cylinder block.

(3) Opposite

The cylinders are arranged in two rows, and left and right rows of cylinders are in same horizontal plane, that is, angle between axes of left and right rows of cylinders is γ=180°, which is also called opposed type. It is distinguished by its small height, convenient overall layout and possibility of air cooling. This type of cylinder is rarely used.

A cylinder drilled directly into cylinder body is called an integral cylinder. The one-piece cylinder has good strength and rigidity, and can withstand heavy loads. This type of cylinder requires high material requirements and high cost. If cylinder is made as a separate cylindrical part (i.e. cylinder liner), then it is installed in cylinder body. Thus, cylinder liner is made from wear-resistant high-quality materials, and cylinder block can be made from low-cost conventional materials, which reduces production cost. At same time, cylinder liner can be removed from cylinder block, which is convenient for repair and replacement, and can greatly extend service life of cylinder block. There are two types of cylinder liners: dry type cylinder liners and wet type cylinder liners.

The characteristic of dry cylinder liner is that after cylinder liner is installed in cylinder block, its outer wall does not directly contact with cooling water, but directly contact with surface of cylinder block wall, and wall thickness is relatively thin, usually 1-3mm . It has advantages of a one-piece cylinder block, with good strength and rigidity, but processing is more difficult, both inner and outer surface finishing is required, disassembly and assembly are inconvenient, and heat dissipation is poor.

A characteristic of a wet cylinder liner is that after cylinder liner is installed in cylinder block, its outer wall is in direct contact with cooling water, and cylinder liner is in contact with cylinder block only with upper and lower rings. , and wall thickness is usually 5-9 mm. It has good heat dissipation, uniform cooling and easy processing. Normally, only inner surface needs to be finished, and outer surface in contact with water does not need to be processed, so it is easy to disassemble and assemble. Leakage phenomenon. Some measures must be taken to prevent leakage.

The lower part of cylinder block used to install crankshaft is called crankcase, and crankcase is divided into upper and lower crankcases. Cast Upper Crankcase and Cylinder BlockThe one-piece lower crankcase is used to store lubricating oil and cover upper crankcase, so it is also called oil pan pattern (Figure 2-6). The oil pan has little force and is usually made from stamped sheet steel, its shape depends on overall engine layout and oil capacity. Installed in oil panAn oil-stabilizing baffle that prevents excessive fluctuations in oil level when car shakes. The bottom of oil pan also has an oil drain plug, usually with a permanent magnet to absorb metal chips in lubricating oil and reduce engine wear. A liner is installed between upper and lower mating surfaces of crankcase to prevent leakage of lubricating oil.

Three. Cylinder head

The cylinder head is mounted on top of cylinder block, sealing cylinder from above and forming a combustion chamber. It is often in contact with high temperature and high pressure gas, so it bears a large thermal load and mechanical load. The inside of cylinder head of water-cooled engine is formed on cooling water jacket, and cooling water hole on bottom end surface of cylinder head communicates with cooling water hole of cylinder body. Use circulating water to cool high temperature parts such as combustion chamber.

The cylinder head also has intake and exhaust valve seats, intake and exhaust valve guides, and intake and exhaust ports. The cylinder head of a gasoline engine is machined with holes for installing spark plugs, and cylinder head of diesel engines is machined with holes for installing fuel injectors. The camshaft bearing holes are also machined on cylinder head of an overhead camshaft engine to fit camshaft.

Cylinder heads are usually made of gray cast iron or alloy cast iron. Aluminum alloys have good thermal conductivity and improve compression ratio. Therefore, aluminum alloy cylinder heads have been increasingly used in recent years.

The cylinder head is an integral part of combustion chamber. The shape of combustion chamber has a great influence on performance of engine. Due to different combustion methods of gasoline and diesel engines, head cylinder parts that make up combustion chamber are completely different. The combustion chamber of a gasoline engine is located mainly on cylinder head, while in a diesel engine it is mainly in a hole on top of piston. Here, only combustion chamber of gasoline engine is introduced, and combustion chamber of diesel engine is introduced into diesel supply section.

Three common forms of combustion chambers in gasoline engines.

(1) Hemispherical combustion chamber

The hemispherical combustion chamber has a compact structure, spark plug is located in center of combustion chamber, and flame stroke is short, so combustion rate is fast, heat generation is small, and thermal efficiency is high. The structure of this combustion chamber also allows valves to be arranged in two rows, and inlet diameter is larger, sotherefore, inflation efficiency is higher. Although valve mechanism becomes more complex, this is beneficial for exhaust gas cleaning. widely used in car engines.

(2) wedge-shaped combustion chamber

The wedge-shaped combustor has a simple and compact design, small heat dissipation area and low heat loss, which can ensure good swirling of mixed gas during compression stroke, which contributes to better mixing quality. gas mixture Inflation efficiency. The valves are arranged in a row, so that valve mechanism is simple, but spark plug is located at height of wedge-shaped combustion chamber, and flame spread distance is greater. The Cherokee car engine uses this form of combustion chamber.

(3) Bowl-shaped combustion chamber

The bowl-shaped combustion chamber, cylinder head has good manufacturability and low manufacturing cost, but because valve diameter is easily limited, intake and exhaust effects are worse than those of hemispherical combustion chamber. The Jetta car engine and Audi car engine use a bowl-shaped combustion chamber.

Fourth. Cylinder Head Gasket

The cylinder head gasket is installed between cylinder head and cylinder block. Its function is to seal contact surface between cylinder head and cylinder block to prevent leakage of air, water and oil.

The material of cylinder head gasket must have a certain degree of elasticity, which can compensate for unevenness of joint surface to ensure sealing. At same time, it must have good heat resistance and pressure resistance, and not burn or deform under high temperature and high pressure. At present, copper leather and cotton cylinder head gasket is more widely used. Since there are three layers of copper leather on flange of copper leather and cotton cylinder head gasket, it is less prone to deformation than asbestos when compressed. Some engines also use woven wire mesh or perforated steel plate as a frame in center of asbestos and a cylinder gasket pressed on both sides with asbestos and rubber binders.

When installing a cylinder gasket, first of all check quality and integrity of cylinder gasket, while holes on all cylinder gaskets must be aligned with holes on cylinder block. Secondly, cylinder head bolts must be installed in strict accordance with requirements of manual. When tightening cylinder head bolts, it must be done 2-3 times in order of expansion symmetrically from center to edges, and tightened to specified torque for last time.

Four-stroke engine workflow: A four-stroke engine is a four-stroke reciprocating stroke of piston to complete work cycle, includingincluding four processes of air intake, compression, operation and exhaust. A four-stroke diesel engine goes through process of intake, compression, operation and exhaust just like a gasoline engine. But difference with a gasoline engine is that a gasoline engine has compression ignition, while a diesel engine has compression ignition.

Cooling system: usually consists of water tank, water pump, radiator, fan, thermostat, water temperature sensor and water drain switch. Automotive engines use two cooling methods, namely air cooling and water cooling. Generally, car engines are mostly water-cooled.

Lubrication system: The engine lubrication system consists of an oil pump, a filter, an oil filter, an oil passage, a pressure limiting valve, an oil gauge, a pressure sensor plug and an oil dipstick.

Fuel system: The fuel system of a gasoline engine is composed of a gas tank, a gas gauge, a fuel line, a gasoline filter, a gasoline pump, a carburetor, an air filter, an intake manifold, an exhaust manifold, etc.

Carburetor: This is a device that mixes gasoline and air in a certain proportion into atomized gas. This atomized gas is called a combustible gas mixture, which is supplied to cylinder in a timely manner and in appropriate amount.

The above is a detailed explanation of design and operation of an automobile engine. Hope it will be useful for everyone.

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