Diesel Engine Fuel System

Principle of Diesel Engine Operation

During diesel engine operation, clean air is drawn into the cylinders and compressed to high pressure. The air temperature rises to approximately +600°C, which exceeds the ignition temperature of diesel fuel. Fuel is injected into the cylinder slightly before top dead center and ignites. Therefore, spark plugs are not used to ignite the fuel.

When the engine is cold, the temperature of the compressed air is insufficient to ignite the fuel. In this case, the engine must be preheated. For this purpose, an electric glow plug is installed in each cylinder of the engine. To improve cold starting, the start of fuel injection is advanced. After the engine reaches operating temperature, the start of injection automatically returns to its normal timing. The start of injection is controlled by the DDE unit (DDE - Digital Diesel Electronics), as are the injected fuel quantity and the boost pressure control.

Fuel is drawn in by the fuel feed pump and delivered to the engine high-pressure fuel pump (HPFP). In the HPFP, fuel is pressurized to approximately 160 bar and injected in a sequence similar to the firing order of a gasoline engine. The fuel quantity injected is determined by the DDE unit according to accelerator pedal input. The accelerator pedal position is detected by a potentiometer. Fuel is injected into the cylinders through the injectors. Due to the presence of a prechamber or swirl chamber, the intake air acquires a specific motion, which ensures good mixing with the injected fuel.

Before fuel enters the HPFP, it passes through the fuel filter, where contaminants and water are separated out. The filter must be cleaned and replaced periodically.

The HPFP requires no maintenance. The moving parts of the pump are lubricated by diesel fuel. The HPFP is driven from the crankshaft by a chain.

To stop the engine, an electromagnetic valve is fitted on the HPFP. When a signal is applied to the valve, fuel supply to the HPFP stops and the engine shuts off.

The DDE unit detects and records malfunctions in engine operation. Malfunctions recorded by the unit can be read from memory using a special tester and then repaired.

Turbocharger

The engine is equipped with a turbocharger. The turbocharger has two turbine wheels located in separate housings. The turbocharger is driven by the energy of the exhaust gases. The rotational speed of the turbocharger reaches 120,000 rpm.

Because a larger quantity of air is supplied to the cylinders, engine power increases by up to 100 %. The boost pressure is from 0.4 to 0.8 bar (tire pressure is approximately 1.8 bar). If the boost pressure rises above the specified value, the relief valve opens.

Along with the increase in engine power, engine torque also increases, which improves its flexibility. Unlike a gasoline engine, the diesel engine, owing to its high compression ratio, uses fuel efficiently at low engine speeds.

The turbocharger is a high-precision component. Therefore, repair must be performed only by specialists. As a rule, if the turbocharger is damaged, it is replaced as a complete assembly.

The turbocharger is lubricated by the engine lubrication system. To ensure reliable lubrication of the turbocharger, observe the following conditions:
a) Use only the recommended engine oil.
b) Old engine oil may cause coking of the turbocharger. Therefore, engine oil and the oil filter must be replaced on schedule.
c) Even small contaminants may damage the turbocharger. Therefore, do not operate the engine without the air filter.

Exhaust Gas Recirculation System (EGR System)

The diesel engine fuel system is equipped with an exhaust gas recirculation system (EGR system), which reduces the content of nitrogen oxides (NOx) harmful to the environment in the exhaust gases. For this purpose, part of the exhaust gases is returned to the engine combustion chambers, which lowers the combustion temperature and, as a result, the content of harmful substances.

The system is controlled by the DDE unit, which regulates the injected fuel quantity and processes signals from the accelerator pedal sensor, engine speed sensor, atmospheric pressure sensor and coolant temperature sensor. The control unit regulates the vacuum signal to the EGR valve through the vacuum solenoid valve and monitors the correct metering of exhaust gas recirculation. The EGR valve is located in the exhaust manifold. At low coolant temperature and when increased engine power is required, the exhaust gases are not recirculated. The exhaust gas recirculation system requires no maintenance.

Fuel Filter Heater

As the outside temperature drops, the fluidity of diesel fuel decreases due to paraffin precipitation. Diesel fuel takes on the consistency of honey. For this reason, in winter special additives that improve fuel fluidity are added to the fuel, which allows the engine to start at temperatures down to -15°C and -22°C (gasoline is worse than AI-95).

These additives are not always sufficient for stable engine operation. Therefore, the fuel filter is equipped with an electric heater. This ensures that the fuel properties are maintained even at low outside temperatures.