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 use 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.

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 will run for two weeks.

Understanding four strokes

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.

Three. 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 cylinders, 6 cylinders 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.

Four. 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.

Verb (abbreviation for verb) 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. During compression and combustion, both valves are closed to ensure the sealing of the combustion chamber.

Piston rings provide seals for cylinder walls and pistons;

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.

Classification of gasoline

No.90, No.93 and No.97 are three kinds of unleaded gasoline (gasoline has long since bid farewell to the lead age), in addition to No.95, 100 and so on. Different labels refer to the octane number of this brand of gasoline, such as: No.93 gasoline, indicating that the octane number of gasoline is 93, and the octane number indicates the anti-explosion performance of this brand of gasoline. The higher the gasoline label, that is, the higher the octane number, the less likely it is to deflagrate, which means the better the antiknock performance of the engine during combustion. Gasoline should be selected according to the compression ratio of the engine, and high-grade gasoline should be selected for vehicles with high compression ratio to ensure the best power output and the lowest cost of the engine without deflagration.

Compression ratio refers to the ratio of total cylinder volume (i.e. working volume+combustion chamber volume) to combustion chamber volume (compression ratio = total cylinder volume/combustion chamber volume). Compression ratio is a very important structural parameter of engine, which indicates the ratio of gas volume at the beginning of piston compression at bottom dead center to gas volume at the end of piston compression at top dead center. In terms of power and economy, the greater the compression ratio, the better. High compression ratio, good power, high thermal efficiency, vehicle acceleration and maximum speed will also be improved accordingly. However, the compression ratio of gasoline engine cannot be too large because of the restriction of cylinder material performance and gasoline combustion knock.

Usually, 90 # automobile gasoline should be used when the compression ratio is 7.5 ~ 8.0; When the compression ratio is 8.0 ~ 8.5, 90 ~ 93 motor gasoline should be selected; When the compression ratio is 8.5 ~ 9.5, 93 ~ 95 motor gasoline should be selected; When the compression ratio is 9.5 ~ 10, 95 ~ 97 motor gasoline should be selected. The general compression ratio can be found in the car manual. In addition to the instructions, some vehicle manufacturers will also mark the recommended fuel label on the inside of the fuel tank cover. Car owners should strictly follow the different compression ratios of the engine and choose the corresponding brand of automobile gasoline to make the engine play the best efficiency.

If unleaded gasoline below the specified requirements is used, the engine will knock. Generally speaking, detonation occurs during rapid acceleration and climbing. If the engine knocks for a long time due to the low gasoline grade, it will damage the engine, even the piston and cylinder block.

However, the higher the level, the better. The main basis for choosing gasoline grade is the compression ratio of the engine. Compression ratio, ignition advance angle and other parameters have been set in the engine computer, so there is no problem as long as the owner chooses gasoline in strict accordance with the requirements of the manual. Blind use of high-grade gasoline will not give full play to its advantages of high antiknock performance, and will also cause a waste of funds.

If you can't find the compression ratio of your car, please try to use high-grade gasoline to avoid hurting the car.

What is the difference between a gasoline engine and a diesel engine?

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Please refer to these two papers.