What is the meaning of CDI in Mercedes Benz?

In this article, we explain the meaning of CDi in Mercedes Benz and answer the following questions: what are the components of a CDI system? And How does the CDI system work? Lastly, we discuss some common problems and possible remedies.

What is the meaning of CDI in Mercedes Benz?

CDI stands for common rail diesel injection system in Mercedes Benz. It is a type of high-pressure fuel injection system employed specifically in Mercedes Benz diesel engines.

Common rail diesel injection is a widely and most common type of fuel injection method found on modern diesel engines simply because it offers improved fuel economy and more power from the engine. 

Different automotive manufacturers have their own nomenclature for this and there can be some changes in operating pressures but the basic principle behind the operation is the same.

Mercedes Benz started using Common rail diesel injection in 1997. It was first used in the OM611 engine which was a 2.2-litre diesel engine producing 140 bhp and used in models like the C class W202 and E class W210. 

Since its first implementation in the OM611 engine, the CDI system went through several modifications to improve reliability and reduce the cost of production. Currently, the CDI system consists of only 4 main components and it can work faultlessly for more than 100,000 km. 

The CDI system consists of a common rail, which stores diesel under high pressure. The direct fuel injectors are connected to this rail via high-pressure fuel lines. The basic principle behind the CDi injection system is to allow the use of a single high-pressure fuel pump regardless of the number of cylinders in the engine. 

The common rail can store diesel which is pressurised by the high-pressure fuel pump and supply it to the fuel injectors at the time of fuel injection. 

Previously, diesel engines had complex sequential pumps for each cylinder. And as the diesel engines are quality controlled, the throttle response was very poor. The diesel engine is called quality control because the amount of power generated depends on the quality of the air-fuel mixture in the engine. More diesel must be injected to get more power from the engine. Air is always present as diesel is a self-igniting engine.

What are the components of a CDI system?

The fuel system used in modern diesel engines is divided into low-pressure fuel systems and high-pressure fuel systems. The low-pressure fuel system is responsible for supplying fuel from the fuel tank to the high-pressure fuel pump. Following are the components of the low-pressure fuel system.

  • Fuel tank- fuel tank is usually made up of impact resistant polycarbonate shell that stores fuel required for the operation of the vehicle. Diesel does not form vapour hence there is no atmospheric vent in the fuel tank. It can be divided into two parts depending on the location of the vehicle. The fuel tank also houses the low-pressure fuel pump and a suction jet pump if the tank is divided into two halves.
  • Fuel pump (low pressure)- the low-pressure fuel pump supplies fuel to the high-pressure fuel pump. It is actuated by the fuel pump supply control unit (FSCU) depending on the signal received from the engine control unit. The FSCU and engine control unit are connected by CAN lines and a single wire discrete line in case CAN communication fails. The fuel pump can operate at maximum power in the limp home mode.
  • Fuel lines- Fuel lines are made up of steel and they connect the output of the low-pressure fuel pump to the input of the high-pressure pump via the fuel filter element. The return or leak oil line also goes back to the suction jet pump and delivers excess diesel back into the fuel tank.
  • Fuel filter- fuel filter consists of a fuel filter element to clean the diesel going into the high-pressure system. Depending on the car model it can have a fuel heater or water sensor integrated into the fuel filter.
  • Low-pressure fuel sensor- as the low-pressure fuel pump is regulated by the FSCU, a pressure sensor installed on the fuel pump is used to measure the fuel pressure.

Following are the components in the high-pressure fuel system:-

  • High-pressure fuel pump- it is the task of the fuel high-pressure pump to pressurise the fuel and send it to the fuel rail. It is connected to the fuel rail by a high-pressure steel line. The fuel high-pressure pump is driven by a timing chain connected to the camshafts and it is capable of producing 2500 bars of pressure.
  • Fuel rail- Fuel rail stores the diesel under high pressure and supplies it to the fuel injectors. Fuel rail also has a fuel pressure sensor on one end and a fuel pressure regulator on the other end. When the vehicle is turned off the pressure inside the rail is relieved by opening the pressure regulating valve. It is not possible to open the pressure regulating valve as both rail and pressure regulating valve are calibrated at the time of production.
  • Fuel pressure sensor – this fuel pressure sensor is located in the rail and it can measure the fuel pressure on the high-pressure side of the fuel system. It is a very crucial sensor as the engine control unit calculates how much fuel to compress and supply to the fuel rail.
  • Fuel injectors- Fuel injectors directly inject diesel into the combustion chamber under high pressure. Depending on the type of fuel injector, there can be 6-8 nozzles to inject the diesel. The fuel injectors are calibrated at the time of production and the data is stored in the form of an IMA code. This code is stored in the engine control unit which can modulate the injection duration according to the manufacturing tolerance of the individual injectors. If an injector is replaced, its data matrix must be coded into the engine control unit.
  • Quantity control valve- it is located on the fuel high-pressure pump, it controls the quality of fuel going inside the high-pressure pump. The engine control unit actuates the quantity control valve based on the actuation signal the quantity control valve can vary the cross-sectional area at the inlet of the high-pressure fuel pump. 
  • Pressure-regulating valve- pressure regulating valve is also on the fuel rail, it is calibrated at the time of manufacturing the rail. The regulating valve is used to release pressure inside the rail during the sudden deceleration phase or when the vehicle is turned off. The Pressure-regulating valve is naturally open when the ignition is switched off.

How does rail pressure regulation work in CDI engines?

The rail pressure is regulated in the fuel rail with the help of a quantity control valve and pressure regulating valve. However, there are specific conditions where the pressure is regulated by either one of the components.

  • Rail Pressure regulated by rail pressure regulator– apart from releasing pressure in rail during the deceleration phase, the pressure regulator is also used for 2 minutes after starting the engine. Regulating the rail pressure through this method allows the diesel to heat up faster which is extremely beneficial in cold temperatures.
  • Rail pressure regulation by quantity control valve- the rail pressure is regulated by the quantity control valve after 2 min of starting the engine and the outside temperature is greater than 20 degrees. The quantity control valve is completely open to allow maximum diesel into the high-pressure pump during starting.

There should not be any fluctuations in the rail pressure as this will lead to jerks and vibration of the engine.

What are some of the common problems in the CDI fuel injection system?

The most common problem comes when the vehicle is used with incorrect fuel. The CDI fuel system uses only diesel and using petrol will damage the high-pressure fuel pump. The high-pressure fuel pump is lubricated by diesel. Hence if petrol or water is in the fuel system damages the pump and fuel injectors.

Replacing the high-pressure components is the only option in such cases. The fuel filter with a water level sensor in some models can warn the driver of malfunctioning messages in the instrument cluster.

Any faults in the CDI fuel system can be diagnosed with ease. The xentry diagnosis tool available at the authorised dealership can perform individual tests of components and identify faulty parts.

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The common rail diesel injection systems allow for pressurised direct fuel injection in modern diesel engines. The CDI fuel system has helped to improve the overall output and throttle response of the vehicle. With the use of a piezo injector, the control unit can make preinjection, main injection and post-injection of fuel in the cylinder to reduce NOx.


FAQs: What is the meaning of CDI in Mercedes Benz?

What does CDI mean in cars?

CDI mean the vehicle is using a common rail diesel injection system. It is also the most common diesel injection system for passenger cars.

What does CGI stand for Mercedes?

CGI in Mercedes Benz stands for compressed gasoline injection. It represents a vehicle is powered by a petrol engine and the air is compressed by a turbocharger.

What is Mercedes-Benz BlueTEC?

Mercedes Benz Bluetec are diesel engines with a common rail diesel injection system and exhaust gases after-treatment systems like SCR & DPF