Edit this paragraph history
The engine was first born in Britain, so the concept of engine also originated in English. Its original meaning refers to "a mechanical device that generates power". With the progress of science and technology, people have continuously developed various engines for different purposes, but the basic premise of any engine is to generate power by burning a certain fuel. Therefore, the motor with electricity as energy source does not belong to the category of engine.
Looking back on the history of engine production and development, it has gone through two stages of development: external combustion engine and internal combustion engine.
The so-called external combustion engine means that its fuel burns outside the engine, and the engine converts the heat energy generated by this combustion into kinetic energy. The steam engine invented by Watt is a typical external combustion engine. When a large amount of coal is burned to generate heat energy to heat water into a large amount of steam, high pressure is generated, and then this high pressure pushes the machinery to do work, thus completing the conversion of heat energy into kinetic energy.
Understand what an external combustion engine is and you will know what an internal combustion engine is. The biggest difference between this engine and an external combustion engine is that its fuel is burned internally. There are many kinds of internal combustion engines. Our common gasoline engines and diesel engines are typical internal combustion engines. The unusual rocket engines and jet engines that we assemble on airplanes also belong to internal combustion engines. However, due to the different power output modes, there are great differences between the former two and the latter two. Generally speaking, the former is mostly used on the ground and the latter is mostly used in the air. Of course, some car manufacturers have installed jet engines in their cars in order to create a new world car speed record, but this is always a very special example and is not applicable to mass production.
In addition, there is a gas turbine. The working feature of this engine is that combustion produces high-pressure gas, and the high pressure of the gas drives the blades of the gas turbine to rotate, thus outputting power. Gas turbines are widely used, but because it is difficult to finely adjust the output power, gas turbines are rarely used in cars and motorcycles, and only some racing cars are equipped with gas turbines.
Human wisdom is endless, and various new engines are constantly being developed. However, due to the need of safe control, so far, our lovely motorcycle has only one choice-reciprocating engine.
Edit this paragraph parameter.
First, let's look at one of the most common engine parameters-engine displacement. Engine displacement is the sum of the working volumes of each cylinder of the engine, which is generally expressed in liters (L). The working volume of cylinder refers to the volume of gas swept by piston from top dead center to bottom dead center, also known as single cylinder displacement, which depends on cylinder diameter and piston stroke. Engine displacement is a very important engine parameter, which can represent the size of the engine better than cylinder diameter and cylinder number, and many indexes of the engine are closely related to displacement. Generally speaking, the greater the displacement, the greater the output power of the engine.
Knowing the displacement, let's look at other commonly used parameters of the engine. Many junior riders report that they often see the words "L4", "V6", "V8" and "W 12" in the engine column of automobile data, trying to find out what they mean. These all indicate the arrangement and number of cylinders of the engine. The number of cylinders commonly used in automobile engines is 3, 4, 6, 8, 10, 12, etc.
Generally speaking, engines with displacement less than 1 liter are generally three cylinders, such as 0.8-liter Alto and Fuller. 1 displacement up to 2.5 liters is generally a 4-cylinder engine, and the engines of common economy cars and mid-range cars are basically 4 cylinders. An engine of about 3 liters is generally 6 cylinders, such as Regal and the new Accord with a displacement of 3.0 liters.
Engines with a displacement of about 4 liters are generally 8 cylinders, such as the Beijing Jeep JEEP4700 with a displacement of 4.7 liters. Engines with a displacement of more than 5.5 liters generally use 12 cylinder engines. For example, the 6-liter BMW 760Li uses a V 12 engine. With the same cylinder diameter, the more cylinders, the greater the displacement and the higher the power. Under the condition of the same engine displacement, the more cylinders, the smaller the cylinder diameter, which can increase the engine speed and thus obtain greater lifting power.
The above is the knowledge about the number of engine cylinders. Let's continue to understand the important parameter "cylinder arrangement". Generally, the cylinders of engines with less than 5 cylinders are arranged in line, and the most common middle and low-grade cars are L4 engines, that is, in-line 4 cylinders. In addition, there are a few 6-cylinder engines arranged in a straight line.
The cylinder block of an in-line engine is arranged in a straight line. Cylinder block, cylinder head and crankshaft are simple in structure, low in manufacturing cost, good in low-speed torque characteristics, low in oil consumption, small in size and wide in use, with the disadvantage of low power. Generally, gasoline engines below 1 liter mostly use inline 3 cylinders, gasoline engines with 1 to 2.5 liters mostly use inline 4 cylinders, and some four-wheel drive models use inline 6 cylinders. Because of the small width, facilities such as supercharger can be arranged beside it. For example, the JEEP4000 of Beijing Jeep uses in-line 6 cylinders.
According to professionals, the in-line 6-cylinder engine has good dynamic balance and relatively small vibration, so it is also adopted by some middle and high-end cars. Engines with cylinders 6 to 12 are generally arranged in a V-shape, in which V 10 engines are mainly installed in racing cars. The length and height of the V-type engine are very small, so it is very convenient to arrange. It is generally believed that V-type engine is a relatively advanced engine, so it has become one of the symbols of automobile class.
V8 engine is rarely used because of its complicated structure and high manufacturing cost. The V 12 engine is too big and heavy, and there are only a few advanced vehicles, such as the BMW 760Li mentioned above. Recently, Volkswagen also has a newly developed W-type engine, including W8 and W 12, that is, four rows of cylinders are staggered and compact. Phaeton, Volkswagen's top car, has an engine W 12 and a displacement of 6.0 liters.
Edit this paragraph structure
The engine block is the skeleton of the engine and the installation foundation of various mechanisms and systems of the engine. The main parts of the engine are all installed inside and outside and bear various loads. Therefore, the car body must have sufficient strength and rigidity. Engine cylinder block is mainly composed of cylinder block, crankcase, cylinder head and cylinder gasket.
I. Cylinder block
The cylinder block and upper crankcase of water-cooled engine are often cast into a whole, which is called cylinder block-crankcase or cylinder block. The cylinder block is usually made of gray cast iron. The cylindrical cavity in the upper part of the cylinder block is called the cylinder, the lower part is the crankcase supporting the crankshaft, and its inner cavity is the space for the crankshaft to move. Many reinforcing ribs, cooling water jackets and lubricating oil passages are cast in the cylinder block.
The cylinder block should have sufficient strength and rigidity. According to the different installation plane positions of the cylinder block and the oil pan, the cylinder block is usually divided into the following three forms.
(1) The general cylinder block is characterized in that the installation plane of the oil pan is at the same height as the rotation center of the crankshaft. The advantages of this cylinder block are small height, light weight, compact structure, convenient processing and convenient disassembly and assembly of crankshaft; But its disadvantages are poor rigidity and strength.
(2) The gantry cylinder block is characterized in that the installation plane of the oil pan is lower than the rotation center of the crankshaft. Its advantages are good strength and rigidity, and it can bear large mechanical load. But its disadvantages are poor manufacturability, heavy structure and difficult processing.
(3) Tunnel cylinder block The crankshaft main bearing hole of this cylinder block is integral with rolling bearings. The main bearing hole is large, and the crankshaft is loaded from the back of the cylinder block. Its advantages are compact structure, good rigidity and strength, but its disadvantages are high machining accuracy, poor manufacturability and inconvenient disassembly and assembly of crankshaft.
In order to make the inner surface of the cylinder work normally at high temperature, it is necessary to properly cool the cylinder and cylinder head. There are two cooling methods, one is water cooling and the other is air cooling. Cooling water jackets are processed around the cylinder and cylinder head of water-cooled engine, and the cylinder block and cylinder head are communicated with each other. The cooling water continuously circulates in the water jacket, taking away some heat and cooling the cylinder and cylinder head.
Modern cars basically use water-cooled multi-cylinder engines. For a multi-cylinder engine, the arrangement of cylinders determines the overall size and structural characteristics of the engine, and also affects the stiffness and strength of the engine block, which is related to the overall layout of the automobile. According to the different arrangement of cylinders, the cylinder block can also be divided into three types: single row, V-shaped and opposite.
(1) embedded type
The cylinders of an engine are usually arranged vertically in a row. The single exhaust cylinder block is simple in structure and easy to process, but the length and height of the engine are large. Generally, engines with less than six cylinders are mostly single-row engines. For example, Jetta, Fukang and Hongqi cars all use this kind of in-line cylinder block. Some cars tilt the engine at an angle to reduce the height of the engine.
(2) V type
The cylinders are arranged in two rows, and the included angle between the center lines of the left and right rows of cylinders is γ < 180, which is called a V-type engine. Compared with the in-line engine, the V-type engine shortens the length and height of the engine block, increases the rigidity of the engine block and reduces the weight of the engine, but increases the width of the engine, which is complicated in shape and difficult to process. Generally used for engines with more than 8 cylinders, this type of cylinder block is also used for engines with 6 cylinders.
(3) objection
Cylinders are arranged in two rows, and the left and right cylinders are on the same horizontal plane, that is, the included angle between the center lines of the left and right cylinders is γ = 180, which is called opposition. It is characterized by small height and convenient overall layout, which is beneficial to air cooling. This kind of cylinder is seldom used.
A cylinder that directly drills holes in the cylinder block is called an integral cylinder. The integral cylinder has good strength and rigidity and can bear a large load. This kind of cylinder requires high materials and costs. If the cylinder is manufactured as a separate cylindrical part (i.e., cylinder liner) and then installed in the cylinder block. In this way, the cylinder liner is made of wear-resistant high-quality materials, and the cylinder block can be made of low-priced general materials, thus reducing the manufacturing cost. At the same time, the cylinder liner can be taken out of the cylinder block, which is convenient for maintenance and replacement and can greatly prolong the service life of the cylinder block. There are two kinds of cylinder liners: dry cylinder liners and wet cylinder liners.
The characteristic of dry cylinder liner is that after the cylinder liner is put into the cylinder block, its outer wall is not in direct contact with cooling water, but in direct contact with the cylinder block wall. The wall thickness is relatively thin, generally 1 ~ 3 mm, which has the advantages of integral cylinder block, good strength and rigidity, but complicated processing, inconvenient disassembly and assembly, and poor heat dissipation.
The characteristic of wet cylinder liner is that after the cylinder liner is put into the cylinder block, its outer wall is in direct contact with cooling water, and the cylinder liner is only in contact with the cylinder block up and down, and the wall thickness is generally 5 ~ 9mm. Good heat dissipation, uniform cooling and easy processing. Usually, only the inner surface needs to be finished, and the outer surface in contact with water does not need to be processed, so it is convenient to disassemble and assemble. However, its strength and stiffness are not as good as dry cylinder liner, which is easy to cause water leakage. Some measures should be taken to prevent leakage.
Two. crank case
The lower part of the cylinder used to install the crankshaft is called crankcase, which is divided into upper crankcase and lower crankcase. The upper crankcase is integrated with the cylinder block, and the lower crankcase is used to store lubricating oil and seal the upper crankcase, so it is also called the oil pan diagram (Figure 2-6). The oil pan bears little force and is generally stamped from thin steel plate. Its shape depends on the overall layout of the engine and the oil capacity. An oil stabilizing baffle is installed in the oil pan to prevent the oil level from fluctuating too much when the car bumps. There is also an oil drain plug at the bottom of the oil pan. A permanent magnet is usually installed on the oil drain plug to absorb the metal chips in the lubricating oil and reduce the wear of the engine. The gasket is installed between the joint surfaces of the upper crankcase and the lower crankcase to prevent the lubricating oil from leaking.
Three. cylinder head
The cylinder head is installed on the cylinder block to seal the cylinder from the upper part to form a combustion chamber. It is often in contact with high temperature and high pressure gas, so it bears great thermal load and mechanical load. A cooling water jacket is arranged in the cylinder head of the water-cooled engine, and the cooling water hole at the lower end face of the cylinder head is communicated with the cooling water hole of the cylinder block. Circulating water is used to cool high-temperature components such as combustion chambers.
The cylinder head is also equipped with intake and exhaust valve seats, valve guide holes for installing intake and exhaust valves, and intake and exhaust passages. The cylinder head of gasoline engine has holes for installing spark plugs, and the cylinder head of diesel engine has holes for installing fuel injectors. There are also camshaft bearing holes on the cylinder head of overhead camshaft engines for installing camshafts.
Generally, the cylinder head is made of gray cast iron or alloy cast iron, and the aluminum alloy has good thermal conductivity, which is conducive to improving the compression ratio, so the use of aluminum alloy cylinder heads is increasing in recent years.
The cylinder head is an integral part of the combustion chamber, and the shape of the combustion chamber has a great influence on the work of the engine. Because of the different combustion modes of gasoline engine and diesel engine, the parts of cylinder head that make up the combustion chamber are also very different. The combustion chamber of gasoline engine is mainly on the cylinder head, and the combustion chamber of diesel engine is mainly in the pit at the top of piston. Only the combustion chamber of gasoline engine is introduced here, and the combustion chamber of diesel engine is introduced in the diesel supply system.
Three common types of combustion chambers in gasoline engines.
(1) hemispherical combustion chamber
The hemispherical combustion chamber is compact in structure, and the spark plug is arranged in the center of the combustion chamber, which has short flame stroke, high combustion rate, low heat dissipation and high thermal efficiency. The structure of this combustion chamber also allows the valves to be arranged in two rows, and the diameter of the air inlet is larger, so the charging efficiency is higher. Although the valve train has become more complicated, it is beneficial to exhaust purification and is widely used in automobile engines.
(2) Wedge combustion chamber
The wedge-shaped combustion chamber has simple and compact structure, small heat dissipation area and small heat loss, which can ensure the mixture to form a good vortex motion in the compression stroke, which is beneficial to improve the mixing quality of the mixture, with small intake resistance and improved charging efficiency. The valves are arranged in a row, which makes the valve train simple, but the spark plug is placed at the height of the wedge-shaped combustion chamber, and the flame propagation distance is longer. This type of combustion chamber is used in Cherokee automobile engines.
(3) Basin combustion chamber
The pot-shaped combustion chamber has good cylinder head manufacturability and low manufacturing cost, but the intake and exhaust effect is worse than that of the hemispherical combustion chamber because the valve diameter is easily limited. Jetta engine and Audi engine use basin combustion chamber.
Four. cylinder gasket
The cylinder gasket is installed between the cylinder head and the cylinder block, and its function is to ensure the sealing of the contact surface between the cylinder head and the cylinder block and prevent air leakage, water leakage and oil leakage.
The material of the cylinder gasket should have certain elasticity, which can compensate the unevenness of the joint surface and ensure sealing. At the same time, it should have good heat resistance and pressure resistance, and it will not burn or deform under high temperature and high pressure. At present, the cylinder gasket with copper cotton structure is widely used. Because there are three layers of copper skin at the flanging of copper skin-cotton cylinder pad, it is not easy to deform when pressed compared with asbestos. Some engines also use cylinder pads made of asbestos with woven steel mesh or perforated steel plate as the skeleton and asbestos and rubber adhesive pressed on both sides.
When installing the cylinder gasket, first check the quality and integrity of the cylinder gasket, and all the holes in the cylinder gasket should be aligned with the holes in the cylinder block. Secondly, the cylinder head bolts should be installed in strict accordance with the requirements of the manual. When tightening the cylinder head bolts, they must be expanded symmetrically from the center to the periphery for 2 ~ 3 times, and finally tightened to the specified torque.
Edit the two-stroke engine in this paragraph.
Each working cycle of a two-stroke engine is completed in one rotation of the crankshaft, namely 360 degrees and two strokes of the piston.
The working process of a two-stroke diesel engine is similar to that of a two-stroke gasoline engine, except that pure air enters the cylinder of the diesel engine. Due to poor economy and serious pollution, two-stroke diesel engines have been eliminated from automobiles in recent years. Only the working principle of two-stroke gasoline engine is introduced here.
See 2004/12/23/13369.html.
It is a schematic diagram of the working principle of a two-stroke carburetor gasoline engine with crankcase ventilation. There are three holes in the engine block, namely, air inlet, air outlet and air exchange hole, which are closed by the piston within a certain period of time. The air inlet is communicated with the carburetor, the combustible mixture flows into the crankcase through the air inlet, and then enters the cylinder through the gas exchange hole; And exhaust gas is discharged from the exhaust hole. Its working cycle includes two strokes:
1. In the first stroke, the piston moves upward from the bottom dead center, and after the three air holes are closed, the mixed gas that has entered the cylinder is compressed above the piston; However, the crankcase below the piston has a certain degree of vacuum due to the increase in volume. When the air inlet is exposed, the combustible mixture flows into the crankcase from the carburetor through the air inlet.
2. When the piston is compressed near the top dead center in the second stroke, the spark plug ignites the combustible mixture, and the high temperature and high pressure gas expands to push the piston to do work downward. When the piston moves downward to do work, the air inlet is closed and the combustible mixture sealed in the crankcase is compressed; When the piston approaches the bottom dead center, the exhaust hole opens and the exhaust gas rushes out; Then, the gas exchange hole is opened, and the pre-compressed combustible gas mixture rushes into the cylinder to drive away the exhaust gas and carry out the gas exchange process. This process continues until the piston moves upward in the next stroke and the three air holes are completely closed.
In short, the piston goes up, exchanges air, and compresses the crankcase intake; When the piston descends, it performs power flight to compress the crankcase mixture and exchange air.
As can be seen from the working cycles of the above four-stroke and two-stroke engines, the two-stroke engine has the following characteristics:
(1) The crankshaft has a power stroke every revolution (360 degrees). So theoretically, the power of a two-stroke engine with the same displacement should be equal to twice that of a four-stroke engine.
(2) Compared with the four-stroke engine, its working frequency is faster, so its operation is more uniform and stable.
(3) simple structure and convenient use and maintenance.
However, due to the loss of fresh gas, the exhaust gas of the two-stroke engine is not completely discharged, and the air hole occupies a part of the piston stroke, so the energy loss is large and the economy is poor. So in fact, the power of a two-stroke engine is not equal to twice that of a four-stroke engine, but about 1.5- 1.6 times. Because of this shortcoming, two-stroke gasoline engines are rarely used in ordinary cars, only used in construction machinery such as motorcycles and a few mini-cars.
Edit this paragraph 1. basic theory
A gasoline engine converts the energy of gasoline into kinetic energy to drive a car. The simplest way is to get kinetic energy by burning gasoline inside the engine. Therefore, an automobile engine is an internal combustion engine-combustion occurs inside the engine.
There are two points to note:
1. There are other types of internal combustion engines, such as diesel engines and gas turbines, which have their own advantages and disadvantages.
2. There are also external combustion engines. Steam engines used in early trains and ships are typical external combustion engines. Fuel (coal, wood, oil) burns outside the engine to produce steam, and then the steam enters the engine to generate electricity. The efficiency of internal combustion engine is much higher than that of external combustion engine and much smaller than that of external combustion engine with the same power. Therefore, modern cars don't need steam engines.
In contrast, internal combustion engines are more efficient than external combustion engines, cheaper than gas turbines, and easier to add fuel than electric vehicles. These advantages make most modern cars use reciprocating internal combustion engines.
Edit paragraph 2. Combustion is the key.
The engine of a general automobile adopts four strokes. Mazda's rotary engine is not discussed here, but it is introduced in Automotive Illustrated. )
These four strokes are intake, compression, combustion and exhaust. After completing these four processes, the engine completes one cycle (2 cycles).
Understanding four strokes
The piston is connected with the crankshaft through the piston rod, and the process is as follows:
1. The piston starts from the top, the intake valve opens, and the piston moves downward to suck in the oil-gas mixture.
2. The piston moves to the top to compress the oil-gas mixture, which makes the explosion more powerful.
3. When the piston reaches the top, the spark plug sends out sparks to ignite the oil-gas mixture, and the explosion makes the piston move down again.
4. When the piston reaches the bottom, the exhaust valve opens, the piston moves upward, and the tail gas is discharged from the cylinder through the exhaust pipe.
Note: The ultimate motion of internal combustion engine is rotation, and the linear reciprocating motion of piston is finally converted from crankshaft to rotation, thus driving automobile tires.
Edit the third paragraph. Cylinder number
The core component of the engine is the cylinder, and the piston moves back and forth in the cylinder. What is described above is the movement process of a single cylinder, but in practical application, the engine has multiple cylinders (commonly 4, 6 and 8 cylinders). We usually classify engines according to the arrangement of cylinders: in-line, V-type or horizontally opposed (of course, there is also the W-type of Volkswagen Group, which is actually composed of two V's).
Different arrangements make engines have their own advantages and disadvantages in ride comfort, manufacturing cost and appearance, and they are equipped on corresponding cars.
Edit the fourth paragraph. water discharge
The mixture is compressed and burned in the combustion chamber, and the piston moves back and forth. You can see the change in the volume of the combustion chamber. The difference between the maximum value and the minimum value is the displacement in liters (L) or milliliters (CC). The displacement of a car is generally between 1.5L and 4.0L The displacement of each cylinder is 0.5L, and the displacement of four cylinders is 2.0L If there are six cylinders arranged in a V shape, it is V6 3.0. Generally speaking, the displacement represents the power of the engine.
Therefore, by increasing the number of cylinders or increasing the combustion chamber volume of each cylinder, more power can be obtained.
The engine runs by the mixed combustion of combustible gas and air. If the engine doesn't get enough fresh air, the combustion of combustible gas won't happen.
It will lead to poor fuel economy and reduced engine power. The speed of modern engines is very high, usually reaching 4500 revolutions per minute.
In fact, it only takes about 5 seconds to complete a working cycle, and the traditional two valves are not competent for ventilation in such a short time, so
The improvement of engine performance is limited, and the only solution is to expand the intake and exhaust space and win time with more space. Multi-valve technology is the best.
Solution, its appearance has improved the overall quality of the engine essentially.
The so-called multi-valve technology means that each cylinder of the engine has more than two valves, specifically, there are two arrangements: 2-in 1 out, 2-in and 2-out, and 3-in and 2-out. But the number of people counted.
More air intake will also reduce, and it will make the structure more complicated, the processing technology is extremely demanding, and the manufacturing cost will also increase, but it is not good. So today's engines are very common.
The structure of 3-5 valves is adopted, especially 4 valves are widely used, and almost all engines in modern middle and high-grade cars adopt multi-valves.
Structure, it has become a technical index of modern cars, for example, Jetta adopts 5-valve technology, which can make the engine at the same displacement.
, output more power.
Edit this paragraph v. Other parts of the engine.
Camshafts control the opening and closing of intake and exhaust valves.
Spark plug The spark generated by the spark plug ignites the oil-gas mixture and causes an explosion. Sparks must be released at the right time.
The inlet and outlet valves of the valve are opened at appropriate times to suck in the oil-gas mixture and discharge the tail gas. In compression and
During combustion, both valves are closed to ensure the sealing of the combustion chamber.
The piston ring provides a seal between the cylinder wall and the piston;
1. Prevent oil-gas mixture and tail gas from leaking into the lubricating oil tank during compression and combustion.
2. Prevent the lubricating oil from burning in the cylinder.
Most "oil-burning" cars are because the engines are old: the piston rings are no longer sealed (the exhaust pipe is smoking).
The piston rod connects the piston ring and the crankshaft, so that the piston and the crankshaft keep their respective movements.
Lubricating oil cases surround the crankshaft and contain a lot of oil.
Edit paragraph 6. Position and structural arrangement of engines in automobiles.
(1) front engine
1. Front wheel drive
The automobile drive system with front engine and front wheel drive is what we usually call FF. Except for some high-performance sports cars, the cars we see on the street at present generally use front engines. Why? Obviously, putting the engine in the front of the car can increase the space inside the car and make the ride more comfortable. Therefore, as long as it is not for the pursuit of high performance, supercars such as RVs or SUVs all adopt the layout of front engines.
And what are the advantages of using front drive? In the front drive structure, the power of the engine is directly transmitted to the front wheels, and there is no need for the transmission shaft to transmit the power from front to back, so that there is no protrusion in the center of the floor inside the carriage, which increases the leg space. Moreover, the front engine can be placed across the front of the car, and the gearbox and differential can be integrated into a whole. Compared with the rear-wheel drive car, the manufacturing process is relatively simple, and fewer parts are used, which can also reduce the manufacturing cost of the car.
The dynamic safety of front-wheel drive vehicles is higher than that of rear-wheel drive vehicles, and the stability of front-wheel drive vehicles on straight roads is better. The most common example is that in the case of high-speed cornering, the general driver can adapt to and deal with the phenomenon of understeer of front-wheel drive vehicles, because front-wheel drive vehicles will push their heads when cornering at high speed. At this point, as long as the driver releases the throttle to slow down, the turning angle of the car will be reduced and the car will return to the turning route. However, in the case of oversteer of a rear-wheel drive vehicle,
Another advantage of FF is that the crankshaft and transmission shaft of the engine are in a straight line, which shortens the distance from the engine power output to the wheels, improves the efficiency and helps to reduce unnecessary losses. However, if the front-wheel drive car with front engine concentrates its driving and steering functions on the front wheel of the car, it is easy to have torsional steering in the car with large power output. What is twist steering? It is the torsion generated near the steering shaft, and the position of the steering shaft deviates from the center of the wheel. When the car turns left or right, the "friction zone" will shift to the front and back of both sides, thus forming a "twisted state", which will affect the handling of the car. In addition, the center of gravity usually moves backward in auto start, which will make the tail heavier, the grip ability of the driving wheel (that is, the front wheel) will decrease, and the idling in situ will waste power, so the car can't start as fast as the rear-wheel drive. Another problem is the weight of the car body, because the front-wheel drive car concentrates the engine, gearbox, differential and transmission shaft on the front of the car, which will make the weight of the car body uneven and it is difficult to achieve a good balance in the dynamics of the car.
2. Rear wheel drive
The automobile drive system with front engine and rear wheel drive is what we usually call FR. Obviously, a car with this driving mode needs a long transmission shaft to transmit the power output from the engine at the front of the car to the driving wheels, that is, the rear wheels, so that for ordinary cars, such as vans, the body is relatively high, because the transmission shaft should be placed under the chassis of the car; For cars, in order to keep the characteristics of low chassis, the transmission shaft has to extend into the compartment, sacrificing internal space for comfort. Another problem is that the long drive shaft itself will consume some power, which is the shortcoming of FR car.
The advantage of FR is also obvious, that is, it is easier to balance the front and rear axles in the body weight distribution. Although the engine is above the front axle, the gearbox is located behind the front axle, and the rear axle has key components such as differential (that is, tail teeth), so it is easier to balance the whole vehicle than Mr, RR and FF. Alfa Romeo's 75 tried to install the gearbox and differential on the rear axle to balance the weight of the front and rear axles.
As we said in the last section, when the car starts, the center of gravity will naturally move backwards, so that the driving wheel is behind, which will be better than the front-wheel drive car. Although the engine is heavy at the front, the center of gravity will move backward when accelerating, so the center of gravity will return to the rear axle of the driving wheel, so the starting and acceleration will be much fresher. At the same time, the tracking performance of FR car will be better than that of FF car, because the power output of FR car is at the rear wheel and the steering is controlled at the front wheel. Both of them have their own duties, and there will be no torsion steering problem of FF car. When accelerating while turning, FF is prone to understeer.
(2) Center/Rear Engine
There are two places to put the car engine behind the passengers, in front of the rear axle or behind the rear axle. The difference between the two engines is not obvious, which can be distinguished by their names, that is, what we usually call MR mid-engine and RR rear engine. All the super sports car manufacturers in the world adopt the technology of rear engine. One of the purposes of doing this is to design cars according to the designer's design ideas as much as possible and make cars with unique shapes. In addition, the weight of the car can be directly pressed on the drive shaft.
Generally, when the FF car starts to accelerate, the rear center of gravity will reduce the adhesion of the front wheel, resulting in the front wheel spinning in place, which will waste power, while when MR and RR start, the rear center of gravity will increase the downward pressure on the rear axle, that is, the friction between the rear wheel and the ground will increase, which will effectively overcome the idling of the rear wheel. If the rear wheel idles, idling will only further move the center of gravity, which will soon stop the rear wheel idling.
In actual driving, tire idling affects power transmission in two situations: starting and cornering. The driver of FF car has the biggest headache for these two situations. One is to watch the engine continuously output power, but the car is spinning in the same place. Moreover, when turning, the inner tube rotates wildly, but there is no response when trying to speed up. For MR and RR cars, as long as the driver steps on the accelerator, the car will fly forward according to your idea and can withstand much more engine power than FF cars. When you drive a FF car with horsepower over 250, you will feel that the car is starting to get out of control, so MR and RR are the best choices for super sports cars that are crazy about horsepower and speed.
Take Porsche 9 1 1, the only RR car on the market, as an example. The official front-to-rear weight ratio is 39: 6 1, which is almost equal to a reverse FF car, while the weight ratio of MR car is relatively uniform, with Porsche's Boxster being 46: 54 and Ferrari's 360 Modena being 43: 57. Although some car manufacturers appreciate that the FR car can achieve a car-to-weight ratio of 50: 50, which makes it easier to handle, from the perspective of acceleration, MR and RR with light head and heavy tail are the most favorable. In order to make the front-to-rear weight ratio as small as possible, the sports car will adopt a body design with narrow head and wide tail, and adopt thicker rear wheels.