The engine is the power unit of the automobile. The performance directly affects the service performance of the automobile. There are many types of engines with different structures to meet the needs of different models.

1、 Classification by fuel used

According to the fuel used by the engine, the engine is divided into gasoline engine and diesel engine.

1. Gasoline engine

Small size, light weight and low price; Good starting performance, * high speed at high power; Low vibration and noise during operation; It is suitable for small and medium-sized vehicles, especially high-speed vehicles. Due to the limitation of deflagration, the compression *, thermal efficiency and economy of gasoline engine are not as good as diesel engine. Gasoline engine mixture is mainly formed in the exhaust pipe and enters the cylinder. When the compression is close to the end, it is ignited by the spark plug. The driver controls the load of the engine by controlling the air mixture into the cylinder through the accelerator pedal, which is called volume adjustment. The fuel supply system and ignition system of gasoline engine are the parts with higher failure ratio on gasoline engine. The harmful components carbon monoxide, carbon chlorine compounds and nitrogen oxides in the exhaust emission of gasoline engine are higher than those of diesel engine. However, with the current use of electronic controlled fuel injection system and other exhaust gas purification devices, this aspect has been greatly improved. In addition, the torque characteristics of gasoline engine are very suitable for the use of vehicles, which can significantly increase the labor intensity of drivers.

2. Diesel engine

Compared with gasoline engine, diesel engine has the advantages of large volume, heavy weight, high price and poor starting performance (especially at low temperature); Large vibration and noise during operation; It is easy to emit black smoke during overload operation. The features of diesel engine are:,

1) Due to the limitation of deflagration and the need of diesel spontaneous combustion, diesel engine compression *. Thermal efficiency and economy are better than gasoline engine.

2) Under the condition of the same power, the torque of the diesel engine is large, and the speed at high power is low, which is suitable for trucks.

3) The mixture of diesel engine is formed inside the cylinder. There is no throttle in the intake port and the intake resistance is small. The driver controls the fuel injection quantity by accelerating the road plate to change the load of the engine, which is called quality regulation. Because there is no hypoxia problem, the content of carbon monoxide and hydrocarbons in the exhaust gas is less than that of gasoline engine.

4) Due to the absence of ignition system and fuel supply device, the failure rate is low. Therefore, the fault of diesel engine is less than that of gasoline engine.

5) The torque characteristics of diesel engine are not suitable for the needs of vehicle driving conditions. Gears are used frequently during driving, which increases the driver's labor intensity.

Diesel engines are mainly used in medium and heavy vehicles.

2、 Number and arrangement of engine cylinders

The engine displacement is equal to the sum of the working volumes of each cylinder. Increasing the number of cylinders can not only increase the engine displacement and improve the engine output power, but also make the engine run smoothly and reduce vibration and noise. Modern cars use multi cylinder engines. The engines of mini cars are mostly 3 cylinders, and the engines of small trucks, buses and cars below medium size are mostly 4 cylinders; The engines of medium-sized trucks, large cars and buses are mostly 6 cylinders; Heavy truck - generally 6 ~ 8 cylinders. Most of the engine cylinders below 6 cylinders are single row in-line; The 8-cylinder engine is V-shaped; In order to reduce the engine height and shorten the length, some cars adopt V6 and V8 arrangement. Micro car engines mostly adopt the way of 3-cylinder oblique position. The in-line engine has simple structure and low price. The disadvantage is that the engine height is high and the length is long. It is a - kind of method that is used more. V-type engine has low height and short length, but its structure is complex and its price is expensive. It is suitable for large engines. The cylinder block of water-cooled engine is integrally cast. Small engines are made of aluminum alloy, and medium and large engines are mostly cast iron. The cylinder head is fixed on the upper plane of the cylinder block with bolts. In addition to closing the cylinder to form the combustion chamber, there are inlet and exhaust channels, and valves, spark plugs and valve distribution mechanisms are installed.

3、 Fuel supply mode of gasoline engine

1. Carburetor fuel supply system

The fuel supply system of gasoline engine is divided into carburetor type and fuel injection type. The working principle of the carburetor main oil supply device is that when the engine is working, the outside air enters the cylinder after being filtered by the air filter under the suction of the cylinder. When the air flows through the throat, the pressure drops due to the decrease of the section and the increase of the flow rate, forming a constant vacuum. The gasoline in the float chamber is sprayed from the main nozzle into the air inlet under the action of this vacuum. The sprayed gasoline is dispersed into a fog by the high-speed air flow, which is called atomization. Then, in the form of space evaporation and oil film evaporation, the oil volume is mixed with the air in the air passage to form a mixture into the cylinder. In order to achieve economy, the main oil supply device also adopts the scheme of air braking. The main nozzle is placed in the air chamber, and several rows of through holes are opened around to communicate with the air chamber. When the throttle opening increases gradually, the air hole is gradually connected with the air. It not only reduces the vacuum and thins the mixture, but also the air entering the main nozzle is conducive to the atomization of gasoline.

2. The carburetor type fuel supply device of the electronically controlled fuel injection type fuel supply system has the advantages of simple structure, reliable operation, low price and convenient maintenance. However, its major disadvantage is that it can not * control the concentration of the mixture, resulting in incomplete combustion and increased harmful components in the exhaust gas, which does not meet the strict requirements of today's environmental protection. In addition, due to the existence of the throat, the air inlet resistance increases. There are also phenomena such as uneven distribution of gasoline in each cylinder, gas resistance and icing. In order to solve these problems, electronic fuel injection system was used in car engine in 1980s It is more and more widely used in.

(1) Advantages of electronically controlled fuel injection system: electronically controlled fuel injection system (EFI)

It has the following advantages:

1) No matter under any environmental conditions and engine operating conditions, the concentration of mixture can be controlled to make gasoline burn completely and fully. This greatly reduces the content of harmful components in exhaust gas and makes the engine have excellent combustion economy.

2) It can centrally control oil supply, ignition and temperature, so as to improve the working performance of the engine, increase the output power of the engine and reduce the fuel consumption.

3) The utility model can keep the engine in a stable running state and make the vehicle run normally according to the requirements of the driver under various working conditions.

4) Due to the throat of the ring, the air inlet resistance is small. At the same time, it is not easy to produce gas resistance, and the gasoline is evenly distributed to each cylinder. The disadvantages of fuel injection system are high cost, complex structure and difficult maintenance.

(2) Classification of electronically controlled fuel injection systems:

1) According to the detection methods of air, it is divided into two categories: mass flow mode and velocity density mode.

2) According to the fuel injection mode, there are the following two categories. According to the injection position, it is divided into intake manifold junction (SPI) injection and intake manifold injection (MPI), also known as single point injection and multi-point injection respectively. At present, MPI is widely used.

There are three types of gasoline engine ignition systems: contact ignition system, electronic ignition system and computer-controlled ignition system