- Self-diagnosis of the engine management system allows for quick troubleshooting.
- Precise dosing of the injected fuel quantity ensures a reduction in the content of harmful substances in exhaust gases and low fuel consumption.
- Idle speed and rev limiting are controlled automatically.
When a diesel engine is running, clean air is sucked into its cylinders and compressed to high pressure. The air temperature rises to 700°C, which exceeds the ignition temperature of diesel fuel. The fuel is injected into the cylinder a little ahead of the ignition temperature and ignites. Thus, spark plugs are not used to ignite the fuel.
Fuel is supplied by the fuel pump under a pressure of 3.5 atm to the high-pressure fuel pump (HPFP). In the HPFP, a constant compression pressure of over 1300 atm is created already at low speeds.
The common fuel distribution line comes from the high-pressure fuel pump (Common Rail) to individual cylinders. The common line serves as a pressure accumulator and distributes fuel at a constant pressure to the injectors. The amount of fuel injected is dosed with the required precision by the engine control unit via electromagnetic injectors. If the microprocessor of the engine control unit closes, for example, the electromagnetic valves, the fuel injection stops. In other words, the pressure generation and fuel injection occur independently of each other. The advantage of this is that the injection can occur optimally, depending on the need and the composition of the exhaust gases, but regardless of the engine speed.
To optimize fuel combustion, multi-jet injectors open in two stages. First, a small amount of fuel is pre-injected, which creates favorable conditions for the ignition conditions of the main amount of injected fuel. This ultimately leads to a soft and silent combustion of the fuel mixture. When the injector opens, a small amount of fuel hits the internal components of the injector, lubricating them, and returns to the fuel tank.
Before the fuel enters the fuel pump and the high-pressure fuel pump, it is cleaned in the fuel filter from dirt and water. Therefore, it is important to regularly replace the filter as part of maintenance.
Material based on data from the portal: MercedesMAN
The fuel pump and high-pressure fuel pump do not require maintenance. All moving parts of the pumps are lubricated with diesel fuel.
Air is sucked into the engine or supplied by the turbocharger and passes through the air filter. The turbocharger compresses the air, which then enters the intercooler, where it is cooled after being heated by compression in the turbocharger. The cooling helps to better fill the cylinders with forced air, which in turn increases the torque and power of the engine.
To reduce the proportion of harmful substances in the exhaust gases, diesel engines have a diesel oxidation catalytic converter. At the same time, the recirculation system ensures a significant reduction in the content of nitrogen oxides in the exhaust gases. This is achieved by feeding the exhaust gases to the air sucked in by the engine, which reduces the oxygen concentration in the air entering the engine cylinders. This leads to a delay in ignition and a lower combustion temperature, which ultimately reduces the formation of NOx. The process of recirculation of exhaust gases must, however, be precisely dosed, since otherwise the soot content in the exhaust gases increases. For this purpose, the amount of sucked air is determined by a meter, which allows the electronic device to control the recirculation process.
Fuel is injected directly into the combustion chamber.
The engine is controlled by an electronic system similar to that of gasoline engines. The system controls the engine by analyzing information from a large number of sensors.
Information about the crankshaft position and engine speed is sent to the control unit by the crankshaft position sensor. The inductive head of the sensor is located opposite the flywheel and constantly scans special marks applied to its surface. When a mark passes the sensor head, it sends a pulse to the control unit. The marks are evenly applied to the flywheel surface, but one mark is missing. It should be located 90° before the TDC of the first cylinder. When the flywheel passes this point, the sensor does not send a pulse to the control unit. The unit recognizes this pause and accurately determines the TDC moment. The duration of this pause is used to determine the engine speed.
Information about the amount and temperature of air entering the engine comes from the absolute pressure sensor in the intake manifold and air temperature sensors. The absolute pressure sensor is connected to the manifold by a vacuum hose and measures the pressure in it. Two air temperature sensors are installed. One is installed before the turbocharger, and the other - after the intercooler. Air temperature and pressure are used to calculate the exact amount of fuel that must fall to the injectors.
The coolant temperature sensor measures the temperature and sends the received information to the control unit. By analyzing this information, the control unit adjusts the composition and injection moment of the fuel mixture, and also controls the cold engine warm-up system.
The brake light switch and brake pedal sensor inform the control unit of the current position of the brake pedal. Upon receiving signals from these sensors, the control system immediately switches the engine to idle speed until it receives a signal from the accelerator pedal position sensor.
The accelerator cable is missing. Instead, an accelerator pedal position sensor is installed. The sensor constantly informs the control unit about the pedal position, which in turn accurately calculates the injection parameters. Idle speed is also regulated by the control unit and cannot be adjusted manually. By analyzing information coming from various sensors, the control unit calculates the idle speed, adjusting it depending on the engine load and its temperature.
The fuel injection system is a direct injection system. The piston crowns contain swirl chambers that provide swirl of the fuel entering the combustion chambers.
The engine control unit controls the warming up of a cold engine. When the engine is cold, the injection timing is shifted by the control unit. The engine control unit, in turn, controls the operation of the glow plugs. The glow plugs are installed in each cylinder and are switched on before starting the engine, work during cranking of the engine by the starter and for some time after starting the engine. The plugs significantly facilitate starting a cold engine. After switching on the ignition, a lamp lights up on the dashboard (refer to Chapter User manual), signaling the activation of the glow plugs. As soon as the lamp goes out, you can start the engine. If the ambient temperature is very low, the glow plugs continue to work for some time after the engine has started. This ensures stable engine operation and a reduction in harmful impurities in the exhaust gases.
Due to the high starting properties of the direct injection engine in cold conditions, preheating is only required at temperatures below -10°C.
Fuel passes through the fuel filter. In the filter, fuel is separated from water and contaminants. Therefore, it is important to remove water from the fuel and replace the filter element in a timely manner.
Operation in winter
When the outside air temperature drops, the fluidity of diesel fuel decreases due to paraffin precipitation. Diesel fuel becomes similar in fluidity to honey and can clog the filter. For this reason, additives can be added to diesel fuel in winter to increase the fluidity of the fuel and ensure the possibility of starting the engine at outside air temperatures down to -22°C.
To prevent clogging of the fuel filter at low outside temperatures, the fuel is directed to a heat exchanger.