The power of high pressure burst: a detailed explanation of gasoline direct injection technology

For a gasoline engine, gasoline is fed into the cylinder and mixed with air, and then the oil-gas mixture is fully burned to obtain powerful power, so oil-gas mixing technology is also one of the keys to the engine. After going through the carburetor, single-point electronic injection, and multi-point electronic injection technology stages, oil-gas mixing technology has finally entered the direct injection era. More and more car models are beginning to use direct injection engines. So what are the technical key points of direct injection engines? Let's analyze them one by one.

High pressure fuel injection system

The high-pressure fuel injection system can be said to be the most critical system of the direct injection engine. Unlike the previous system where oil and gas are mixed in the intake manifold and then sucked into the engine by negative pressure, the direct injection engine uses a high-pressure fuel injector to inject fuel into the cylinder. Since the pressure in the cylinder is already very high, the fuel injection system needs to have a higher pressure. The high-pressure fuel injection system can be mainly divided into four parts: the engine control module (ECM), the high-pressure fuel rail, the high-pressure fuel pump, and the fuel injector. Among them, the ECM mainly collects engine data and controls the injection timing and injection amount according to the predetermined program to achieve the highest combustion efficiency; the high-pressure fuel pump is mainly responsible for the pressurization of the fuel, and the high-pressure fuel rail mainly plays the role of balancing the injection pressure of each fuel injector, and the final fuel injection task is performed by the fuel injector. In addition, there are multiple sensors that provide information such as fuel pressure to ensure the high efficiency of the entire system.

The four main parts of the high-pressure fuel injection system

ECM (or ECU) is not only a key part of direct injection engines, but also an important part of all newer internal combustion engines. This part involves multiple links such as chips, actuators, and software. If any of these links is missing, mass production and installation cannot be achieved. At present, ECM technology is still controlled by foreign companies and is relatively mature in technology. Although some domestic brands have initially acquired the ability to manufacture ECM, there are still many problems to be solved in aspects such as software matching and actuator reliability. However, just like transmission technology, once such key technologies are broken through, domestic brand manufacturers will benefit greatly.

The ECM of the powertrain displayed by FAW (right)

The high-pressure fuel pump is the key link in fuel pressurization. After the low-pressure fuel pump sends the fuel to the high-pressure fuel pump, the high-pressure fuel pump can pressurize the gasoline to a pressure of more than ten megapascals (this is thirty or forty times the pressure of an ordinary gasoline pump) and send it to the fuel rail. The high-pressure fuel pump is usually driven by a camshaft, and there are double-head or triple-head cams inside for pressurization (such as the "No. 9 Cam" of Ford's ECOBOOST series engine). The electronic fuel rail pressure regulator (FRP) is also integrated on the high-pressure fuel pump. It is a solenoid valve controlled by the ECM. The ECM controls the oil pressure regulator in a pulse width modulation manner. The oil pressure regulator controls the inlet valve of the high-pressure fuel pump, thereby controlling the fuel pressure. When the drive line fails, the high-pressure fuel pump enters the low-pressure mode and the engine can still run in an emergency.

The high-pressure oil pump used in the GM Ecotec series 2.0 direct injection engine is manufactured by Bosch

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Components such as high-pressure fuel pumps and fuel rails have very high requirements on the working environment and manufacturing precision. Some traditional diesel high-pressure equipment manufacturers such as Bosch have rich experience in this regard. Therefore, even for GM's direct injection engines, their high-pressure fuel pumps are provided by Bosch. As the first direct injection gasoline engine launched by an independent brand, the 2.0TGDI engine on the Riich G5 also uses Bosch's high-pressure fuel pump.

The 2.0TGDI engine used by Riich G5 also uses Bosch's high-pressure oil pump.

After being pressurized by the oil pump, the gasoline enters the high-pressure rail. After the pressure of the high-pressure rail stabilizes, due to the pressure difference between the rail and the combustion chamber, the gasoline is sprayed into the cylinder after the high-pressure pump is activated. There is also a solenoid valve inside the nozzle, which can control the injection amount and timing. The control accuracy is very high. At the same time, since the position of the nozzle is moved from the intake manifold to the cylinder, the working environment and temperature have changed greatly, and the reliability requirements are also greatly increased.

Basic structure diagram of high pressure oil rail

High-pressure fuel rail and nozzles used in GM Ecotec series 2.0 direct injection engines

Schematic diagram of the high-pressure fuel injection nozzle structure: ① polymer sealing ring; ② nozzle needle valve; ③ armature; ④ electromagnetic coil; ⑤ fine filter

Other engine component design

In addition to the fuel injection system, other engine components must also be designed accordingly for direct injection to ensure the high efficiency of the engine, especially the design of the piston top is very critical. According to the control method of the formation of the combustible mixture, the direct injection method in the cylinder can be divided into three categories: oil beam controlled combustion, wall controlled combustion and airflow controlled combustion.

In the oil beam control combustion system, the injector is placed in the center of the combustion chamber, and the spark plug is placed near the injector. The utilization rate of air by the oil beam control depends on the penetration depth of the oil beam, which is controlled by the injection pressure of the injector. This method can achieve good fuel economy in low-load stratified combustion, and when the engine is in medium and high load conditions, the ECM adjusts the high-pressure oil pump pressure to increase the penetration depth of the oil beam, thereby achieving homogeneous enriched combustion.

In the wall-controlled combustion system, the injector and the spark plug are far apart. The injector sprays the fuel into the piston pit, and then relies on the inertia of the intake air flow to send the oil-gas mixture to the spark plug. In order to avoid the temperature of the injector being too high, it is generally placed on the intake valve side, and the piston pit opening faces the intake valve side. After the oil and gas are mixed, they flow directly to the spark plug. This type of mixture takes a long time to form, and it is easy to form a large area of ​​combustible mixture.

The pit on the top of the piston mainly guides the air flow in the cylinder.

The vortex formed by the top surface of the piston can help the mixture burn more evenly and fully.

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In the airflow control combustion system, the airflow and oil beam in the cylinder formed by the special piston surface shape interact with each other. This system does not spray the oil mist toward the piston pit, but toward the spark plug. The specially shaped intake duct forms a certain angle with the injector, giving the mixture a certain swirl force in the cylinder. The airflow formed in the cylinder makes the oil and gas not spray directly toward the spark plug, but form a vortex in the cylinder to rotate around the spark plug. In this way, the proper mixture charge stratification and homogenization can be implemented in most working conditions.

Since direct injection engines have a higher operating temperature, the requirements for cylinder strength and cooling system are also higher. On the premise of ensuring strength, more new direct injection engines use aluminum alloy cylinders with better heat dissipation, and also use enhanced cooling systems to ensure higher thermal efficiency of the engine.

The aluminum alloy cylinder has better heat dissipation effect and is easier to achieve lightweight

The oil spray nozzle located at the bottom of the cylinder can cool the piston when it descends to the bottom dead center.

Although direct injection gasoline engines have obvious advantages, they are also limited by manufacturing technology and oil quality, so it is not realistic for them to be popularized in the short term. However, with its more efficient and economical characteristics, it is still the development trend of internal combustion engine technology in the future, and we are also expected to see more direct injection engines with excellent performance and high fuel economy.