The Two Stage Fuel Injection system can be classified as hybrid system between Common Rail and cam driven systems.

In the first stage a constant pressure, adjustable between 5 and 500 bar (in newer designs up to 700 bar) is supplied to the cam actuated second stage plunger pump and the back side of the injector needle.

The backpressure on the injector needle increases the opening and closing pressure of the injector. While the injector closing pressure is increasing linear with the first stage pressure, the rise in opening pressure will be amplified acc. to the needle geometry.

Opening pressures up to 1300 bar are easily achievable at 500 bar first stage pressure, ensuring optimized spray formating during the complete injection cycle.

On actuation the second stage plunger pump produces a pressure wave controlling the injector needle, as known from conventional cam driven injection systems.

The diagram above shows the performance of the HVD-System at two different first stage pressures (black: 5bar, red: 500bar) at constant volumetric fuel flow.

With rising first stage pressure the folowing phaenomena are observed:

• Injector opening pressure is rising from 250 bar to app. 1200 bar
• Injector closing pressure is increased to above 500 bar.
• Peak injection pressure rises from 1200 bar to app. 1650 bar.
• The injector needle starts to float on the pressure wave, leading to  nearly constant injection rates during the first 20% of the injection, compared to injection with low first stage pressure. This is a stable process controlled by the characteristic of the pressure wave.
• Injection rate during the last 80% of the injection is increased due to the higher injection pressure
• Injection durration is reduced from 28°CA to 18°CA (-35%)