The fuel for gasoline engines with direct injection requires higher fuel pressure. Pressure direct fuel injection engines varies from 5 to 130 bar, next-generation systems ranging up to 200 bar. The injection pressure is bigger atomizing (spraying) and penetration jet fuel into the cylinder is better. In contrast, the systems of indirect injection, in the gate valve, petrol pressure varies between 2.5 and 5 bar.
At a supply system gasoline, direct injection, pressure is classified into two categories:
- Low: generated by the fuel pump, the fuel pressure ranging from 1 to 5 bar…
- High: generated by the injection pump, fuel pressure ranging from 5 to 200 bar…
The injection pump is driven mechanically by a cam 2, 3 or 4 lobes located on the camshaft. If the engine is of V-rollers to provide the necessary flow rate of fuel may be used, and two syringe pumps. Working pressure is between 5 … 10 bar (idle mode) or 150 … 200 bar (full load).
Pump components are stainless steel to be compatible with engines using fuel with ethanol in variable proportion (15%, 85%, etc.)
The main functions of the injection pump are:
- to compress fuel
- Standing injectors deliver fuel required by computer injection , regardless of the operating point of the heat engine (load, speed)
- to limit to a maximum fuel pressure to avoid damage caused by overpressure
The piston is driven by a dedicated cam provided on the camshaft of the engine. The contact between the cam and the piston is permanently maintained under pressure by the return spring. In the intake phase of the piston moves downwards (pushed by the spring) and the fuel enters the pump through inlet valve. The compression phase, the cam starts acting piston compression phase. If the solenoid valve is energized (electrically powered) valve remains open and fuel is pushed back (discharge) in the low-pressure circuit.
The fuel pressure at the outlet of the pump is controlled by the computer of the injection. When it is necessary to increase pressure solenoid is energized on stage compression fitting furniture allow the closure of the intake valve and piston starts to compress the fuel. When the pressure in the pump cylinder is higher than the fuel pressure in the ramp, the exhaust valve opens and the fuel is pushed into the ramp.
The fuel pump further comprises a gas accumulator (at the top) which is intended to absorb the oscillations of the fuel pressure at the pump inlet.
It noted that solenoid valve controls the intake valve closing only implicitly cannot control the pressure ramp downwards. In other words, only the high-pressure pump, the pressure drop due to the injection of fuel is performed. In the case where the injectors get stuck, to limit the maximum pressure in the high pressure circuit, the pump is provided with a safety valve.
Direct injection has enabled substantial improvement of dynamic performance and consumption of petrol engines. Compared with gasoline indirect injection system, the disadvantage is the higher cost of high-pressure system, which is reflected in the final cost of the car. Also, the sensitivity to the quality components of high-pressure fuel is delivered. For this reason it is particularly important to follow the manufacturer’s instructions regarding fuel used and conditions for maintenance and service.