A detailed introduction to direct in-cylinder injection technology

The technology of direct injection, or GDI (Gasoline Direct Injection) for short, has been in existence for more than 95 years since 1925. During this long development process, a number of car manufacturers have been steadfastly exploring its practicality and reliability in the midst of constant failures and lessons learned. For this reason, it is also necessary for us to make a detailed introduction to direct injection technology.


Demand gives rise to GDI technology

With the development of the automotive industry and energy shortage, contradictions are intensifying, how to pursue high power, the ultimate potential of the engine at the same time to respond to the tide of low-carbon environmental protection, even multi-point fuel injection technology can not meet the requirements of people. But, as the saying goes, if there is a demand, there is a market, which has given rise to the emergence of new technologies, more accurate injection technology was born, that is, direct-injection technology.

GDI vs. regular engine

Compared to those ordinary manifold injection engines and carburetor engines, the GDI engine can be eliminated throttle design, so the GDI engine has no throttle loss and reduces the pumping loss, greatly improving the efficiency of air intake. Engine speed is achieved by the engine control unit/Engine Management System (EMS) regulating the injection volume and timing, rather than indirectly controlling the engine speed by the throttle controlling the air intake volume. To achieve such a function requires the EMS to have considerable storage and computing power to control ignition timing and engine speed through very precise calculations to ensure good power and driving performance.

Unlike the practice of placing fuel nozzles on the intake manifold, in-cylinder direct injection is to place the nozzles inside the cylinder and inject the fuel into the combustion chamber precisely through a high-pressure fuel pump, making it more detailed and more evenly mixed with the air to ensure smoother and more efficient combustion of fuel.


The position of the nozzle, spray shape, air intake volume and pressure, and the design of the combustion chamber can change the ratio of fuel to gas mixture to achieve uniform or stratified combustion.

In general, direct-injection engine management systems offer a choice of three fuel combustion modes: lean-burn mode, theoretical ratio mode and full power mode. The difference between the different modes lies in the air-fuel ratio, with the theoretical ratio mode accurately controlling the air-fuel ratio at 14.7:1, but the air-fuel ratio in the lean-burn mode is as high as 65:1, and in some engines, the air-fuel ratio is even higher. Compared to conventional engines, the mixing accuracy of the GDI mixture is greatly improved, and fuel consumption is effectively reduced.

Ultra-thin combustion mode: Mainly used in light load operating conditions, uniform speed or deceleration conditions. Fuel is injected late in the compression stroke so that a small amount of fuel is mixed in the air near the spark plug, and a small amount of concentrated mixture surrounded by outer air, achieving stratified combustion while keeping the fuel and ignition spark away from the cylinder wall, minimizing heat loss and ensuring the lowest emissions. This mixture layered combustion mainly occurs in the piston surface and cylinder head composed of the ring combustion chamber, the combustion chamber also contains the nozzle to control the amount of fuel injection, to achieve such a combustion method, traditional carburetor or EFI engine is almost impossible.

Theoretical proportioning mode: mainly used for medium loads, fuel is injected at the beginning of the intake process, which is conducive to the uniform mixing of oil and gas, homogeneous mixture of complete combustion to ensure that the exhaust gas emissions are purer.

Full power mode: mainly used for rapid acceleration and high load conditions. The mixer ratio is slightly higher than the ratio of the theoretical proportioning mode in order to prevent the appearance of knocking cylinders. Fuel injection starts at the intake stroke, and direct injection is accompanied by other engine systems at the same time, such as variable timing valve VVT, variable intake manifold length VLIM/VIM, exhaust gas recirculation EGR, etc. The injectors may inject oil more than once during an operating cycle, and a supplementary injection occurs when the piston moves downward after ignition. This refueling action is good for improving power output and economy, but there is a risk that incompletely burned exhaust gases will erode the exhaust valves. For this reason, most manufacturers’ FSI systems suspend operation during normal engine operation.

Depending on the nozzle mounting position and nozzle port design, there are three injection modes of direct fuel injection technology: spray-guided direct injection, wall-guided direct injection, and airflow-guided direct injection.

Testing and cleaning of GDI fuel injectors

I have previously introduced you to the Autool CT150 injector cleaner and tester at How to clean or test the fuel injectors with CT150?, but it does not test GDI injectors. This time, we recommend the Autool CT400, also from Autool, which is the best product of the Autool CT series.

AUTOOL CT400 GDI Fuel Injector 6-cylinder Cleaner and Tester is a great upgrade, which adapts the latest GDI design. It not only can be used to clean and test the standard injectors, of note that it can carry out GDI injectors, including compatible for Benz, BMW, AUDI, Porsche. It comes with multi voltages available for GDI injectors, which enables to adapt for different types of injector testing. The machine is the necessary and preferred equipment for the automotive serving and maintenance, research, and teaching training departments.

  • [For GDI Injector Cleaning&Test]: AUTOOL CT400 can clean and test not only standard injectors but also GDI injectors, such as for Mercedes-Benz, BMW, Audi, Porsche, Ford, and other direct injection engines.
  • [Multi Voltage Selection]: AUTOOL CT400 GDI Fuel Injector 6-cylinder Cleaner and Tester designs with various types of voltage output (12V, 70V & 120V) to be selected to adapt for different types of injector testing.
  • [Upgrade Ultrasonic Cleaning]: Different from other cleaner& tester machines, AUTOO CT400 comes with a cooling fan and thermal protection, and with a power of 60w, its ultrasonic cleaning capacity is significantly enhanced.
  • [Various Parts equipped&Portable Drawer ]: It is equipped with a variety of injector accessories for different needs. A portable drawer at the bottom is designed to store a variety of parts and for easy access.
  • [6-Cylinder & 2 Draining Ports Design]: AUTOOL CT400 can test and clean 6 injectors at the same time for better efficiency; it also has both control valves for cleaning solution draining and test liquid recovery. Very easy and convenient to deal with and no worries about leaving a mess.

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