July 25 2023

Dry cargo exchange Principal analysis of a direct injection engine

People are becoming more and more aware of new energy and environmental protection, and replacement of cars is getting faster and faster, and engine must be constantly improved to meet people's requirements. For example, everyone is familiar with terms "direct injection into cylinder", "distributed combustion", "variable displacement", what are principles of their work? Let's find out together.

Pistons, Crankshaft Most "tired"?

Once engine is running, "head" of piston will be exposed to high temperature and high pressure, and it will continue to move up and down at high speed. The working environment is very harsh. We can say that piston is "heart" of engine, so very high requirements are placed on accuracy of piston material.

The crankshaft, advancing "under feet" of piston, is also inconvenient, and it has to constantly rotate at high speed. The crankshaft rotates thousands of times per minute, it is entrusted with heavy task of driving oil pump, generator, air conditioner compressor, camshaft and other mechanisms, it is intermediate shaft of engine power, so it is relatively "strong".

How does linear motion become rotational?

We all know that a piston in a cylinder moves up and down in a straight line, but how do you convert rectilinear motion into rotational motion to output rotational force that pushes wheels forward? In fact, it has a lot to do with structure of crankshaft. The connecting rod shaft and main shaft of crankshaft are not on same straight line, but opposite each other.

This principle of movement is actually very similar to how we pedal on a bicycle. Our two legs are equivalent to two adjacent pistons, pedals are equivalent to crankshaft, and big flywheel in middle is main shaft of crankshaft. . When we push down with left foot (the piston works or inhales to move down), right leg lifts up (the other piston contracts or exhales to move up). By walking in a circle in this way, linear motion is converted into rotational motion.

Why is engine flywheel so big?

We all know that only one of four piston strokes works. The three strokes of intake, compression and exhaust require a certain amount of force to run smoothly, and flywheel helps a lot in this process. , busy.

The reason flywheel is made relatively large is mainly to conserve kinetic energy of engine to keep crankshaft running smoothly. In fact, this principle is similar to toy top we used as children: after we spin it hard, it can keep spinning for a long time.

Engine size,compression ratio

The volume of space through which piston travels from top dead center to bottom dead center is called displacement of cylinder; sum of volumes of all cylinders in an engine is called displacement of engine and is usually expressed in liters (l). For example, volume of a car that we usually see is 1.6L, 2.0L, 2.4L and so on. In fact, volume of cylinder is a cylinder, and it is unlikely that this is exactly number of liters, for example, such numbers as 1998ml and 2397ml can be roughly denoted as 2.0l and 2.4l.

The compression ratio, that is, compression ratio of engine mixture, is expressed as ratio of total volume of cylinder to volume of compressed cylinder (that is, volume of combustion chamber) . Why compress gas mixture of cylinder? This provides easier and faster complete combustion of mixture, improving engine performance and efficiency.

What is variable displacement? How to change displacement?

It is usually necessary to increase displacement of engine to get more power, for example, 8-cylinder and 12-cylinder engines are very powerful. But price for this is increased fuel consumption. Especially when idling and other operating conditions do not require a large power output, fuel is wasted, and variable displacement can solve contradiction well.

Variable displacement, as name implies, means that displacement of engine is not fixed (ie number of cylinders involved in work changes), but can be changed according to needs of operating conditions. . How did engine achieve change in displacement? Simply put, it is turning on or off operation of a certain cylinder by controlling intake valve and oil circuit. For example, a 6-cylinder variable displacement engine can realize three operating modes: 3-cylinder, 4-cylinder, and 6-cylinder according to actual operating conditions to reduce fuel consumption and improve fuel economy.

For example, Volkswagen TSI EA211 engine uses variable displacement (cylinder closing) technology, mainly through an electromagnetic controller and a spiral groove bushing mounted on camshaft to close and open valve.

What is direct injection? What are benefits?

We know that in a traditional engine, fuel is injected into intake manifold and then mixed with air to form a mixed gas before it enters cylinder. During this process, since there is still a certain distance from combustion chamber in fuel injector, tiny oil particles will be adsorbed on pipe wall, and mixture of gasoline and air is highly dependent on intake air flow and valve closing.

Direct In-Cylinder Injection is direct injection of fuel into cylinder and direct mixing with air in cylinder. The ECU can accurately control fuel, injection quantity and injection time according to amount of air inhaled. The high pressure fuel injection system can make atomization and mixing efficiency of oil and gas more excellent, so that gas mixture that meets theoretical air-fuel ratio can be burned more completely, thereby reducing fuel consumption. Improve fuel consumption and increase engine power.

This set of diesel-based technologies is widely used in vehicles including Volkswagen (including Audi), BMW, Mercedes-Benz, and General Motors.

What is homogeneous combustion? Layered combustion?

The so-called "homogeneous combustion" can be understood as usual method of combustion, that is, fuel and air are mixed to form a combustible mixture of a certain concentration. With a longer mixture time, fuel and air can be fully mixed and combustion is more even, resulting in greater power output.

In stratified combustion, air-fuel mixture ratio in entire combustion chamber is different, and concentration of mixture near spark plug is higher than elsewhere, so that mixture around spark plug can burn quickly, thus moving more. The combustion of a leaner gas mixture at a distance is called "stratified combustion". The goal of homogeneous combustion is to obtain high power during high-speed movement and acceleration, stratified combustion is to save fuel at low speed and light load.

How to achieve stratified combustion?

For example, how is stratified combustion achieved in a TSI engine? First of all, when piston moves to bottom dead center on engine's intake stroke, ECU controls fuel injector to inject a small amount of fuel to form a lean mixture in cylinder.

The second injection of fuel occurs at end of compression stroke of piston, due to which an area with a relatively high mixture concentration is formed near spark plug (due to special design of piston crown), and then this part of rich mixture is used for ignition. The lean mixture in cylinder can realize lean combustion in cylinder, so that same combustion effect can be achieved with less fuel, and fuel consumption of engine can be further reduced.