Analysis of the Principle of Direct Injection in Automobile Engines

As the requirements for energy and environmental protection become increasingly stringent, engines must continue to upgrade and evolve to meet people's needs. For example, the terms "direct injection", "stratified combustion", "variable displacement" and so on are familiar to everyone. What are their working principles? Let's learn about them together.

Are the pistons and crankshafts the most "tired"?

Once the engine is running, the piston "head" will be subjected to high temperature and high pressure, and will keep moving up and down at high speed. The working environment is very harsh. It can be said that the piston is the "heart" of the engine, so the material manufacturing precision of the piston has very high requirements.

The crankshaft under the piston is also in a bad condition, as it has to rotate continuously at high speed. The crankshaft rotates thousands of times per minute, shouldering the arduous task of driving the oil pump, generator, air conditioning compressor, camshaft and other mechanisms. It is the transfer axis of the engine power, so it is also relatively "strong".

How to transform linear motion into rotational motion?

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

The principle of this movement is actually very similar to when we pedal a bicycle. Our two feet are equivalent to two adjacent pistons, the pedals are equivalent to the connecting rod shaft, and the large flywheel in the middle is the main shaft of the crankshaft. When we push down with our left foot (the piston does work or inhales and moves downward), the right foot will be lifted up (the other piston compresses or exhausts and moves upward). This cycle repeats itself, and the linear motion is converted into rotational motion.

Why are engine flywheels so big?

We all know that among the four strokes of the piston, only one is work. The three strokes of intake, compression and exhaust require a certain amount of power support to proceed smoothly, and the flywheel helps a lot in this process.

The reason why the flywheel is made relatively large is mainly to store the kinetic energy of the engine, so as to ensure the smooth operation of the crankshaft. In fact, this principle is similar to the gyroscope toys we had when we were young. After we rotate it hard, it can keep rotating for a long time.

Engine displacement and compression ratio

The volume of space that the piston passes through when it moves from the top dead center to the bottom dead center is called the cylinder displacement; the sum of the displacements of all cylinders in the engine is called the engine displacement, which is usually expressed in liters (L). For example, the displacement of cars we usually see is 1.6L, 2.0L, 2.4L, etc. In fact, the volume of the cylinder is a cylinder, and it is unlikely to be exactly a liter number. For example, numbers such as 1998mL and 2397mL can be approximately marked as 2.0L and 2.4L.

The compression ratio is the degree to which the engine's mixed gas is compressed, expressed as the ratio of the total cylinder volume to the compressed cylinder volume (i.e., the combustion chamber volume). Why do we need to compress the mixed gas in the cylinder? This allows the mixed gas to burn more easily and quickly, thereby improving the performance and efficiency of the engine.

What is variable displacement? How do you change displacement?

Usually, in order to obtain greater power, the engine displacement needs to be increased, such as 8-cylinder and 12-cylinder engines, which are very powerful. But the price paid is increased fuel consumption. Especially when high power output is not required in idling and other working conditions, fuel is wasted. Variable displacement can solve the contradiction well.

Variable displacement, as the name implies, means that the displacement of the engine is not fixed (that is, the number of cylinders participating in the work is changing), but can change according to the working conditions. So how does the engine achieve the change of displacement? Simply put, it is to open or close the work of a certain cylinder by controlling the intake valve and the oil circuit. For example, a 6-cylinder variable displacement engine can realize three working modes of 3 cylinders, 4 cylinders, and 6 cylinders according to the actual working conditions, so as to reduce fuel consumption and improve fuel economy.

For example, the Volkswagen TSI EA211 engine uses variable displacement (cylinder shutdown) technology, which mainly achieves the closing and opening of the valve through an electromagnetic controller and a spiral groove sleeve installed on the camshaft.

What is direct injection? What are the advantages?

We know that in traditional engines, fuel is injected into the intake manifold and then mixed with air before entering the cylinder. During this process, because there is a certain distance between the fuel injector and the combustion chamber, tiny oil particles will be adsorbed on the pipe wall, and the mixture of gasoline and air is greatly affected by the intake airflow and valve closing.

Direct injection directly injects fuel into the cylinder and mixes it with air. The ECU can accurately control the fuel, injection amount and injection time according to the amount of air inhaled. The high-pressure fuel injection system can make the atomization and mixing efficiency of oil and gas better, so that the mixed gas that meets the theoretical air-fuel ratio can burn more fully, thereby reducing fuel consumption and improving the power performance of the engine.

The so-called "homogeneous combustion" can be understood as a common combustion method, that is, the fuel and air are mixed to form a combustible mixture of a certain concentration, the air-fuel ratio of the mixture in the entire combustion chamber is the same, and it is ignited and burned by the spark plug. Since the mixture takes a long time to form, the fuel and air can be fully mixed and the combustion is more uniform, thereby obtaining a larger output power.

In stratified combustion, the air-fuel ratio of the mixture in the entire combustion chamber is different. The concentration of the mixture near the spark plug is higher than that in other places. In this way, the mixture around the spark plug can burn quickly, thereby driving the combustion of the thinner mixture farther away. This combustion method is called "stratified combustion". The purpose of homogeneous combustion is to obtain high power when driving at high speed and accelerating; stratified combustion is to save fuel at low speed and low load.

How to achieve stratified combustion?

How does the TSI engine achieve stratified combustion? First, when the piston moves to the bottom dead center during the intake stroke, the ECU controls the fuel injector to inject a small amount of fuel, forming a lean mixture in the cylinder.

A second fuel injection is carried out at the end of the piston compression stroke, forming an area with a relatively high concentration of the mixture near the spark plug. This richer mixture is then used to ignite the lean mixture in the cylinder, thereby achieving lean combustion in the cylinder. This can achieve the same combustion effect with less fuel, further reducing the engine's fuel consumption.