Bosch compact: Pressure in diesel engines

Outlook: “Over the next decade, the vast majority of diesel engines will manage with injection pressures of around 2,000 bar. Although 3,000 bar is not unrealistic, it will be limited to racing cars and high-performance diesel engines.”
(Dr. Markus Heyn, President of the Diesel Systems division at Robert Bosch GmbH)

Common-rail diesel: The CRS3-25 common-rail system features Bosch’s first piezo injector for passenger vehicles that works with an injection pressure of 2,500 bar. With their higher injection pressure, the new piezo models from Bosch are at the technological vanguard. The optimized fuel injection system atomizes the fuel more finely, improving combustion. Lower consumption is just one advantage of this technology.

By way of comparison: The pressure generated by a common-rail system is roughly equivalent to the pressure a 2,000-kilogram rhinoceros would exert standing on a fingernail. The compressed fuel is then finely dispersed at the speed of a supersonic jet.

Enhanced performance: A higher injection pressure generates greater specific power and increases torque. This is why increasing an engine’s injection pressure makes it more powerful: the time available for combustion is extremely limited as soon as an engine is running at full load and high engine speed. This means the fuel must be injected into the engine very quickly at high pressure in order to achieve optimum power yield.

Turbo: The more air there is in the combustion chamber, the higher the injection pressure must be. A large amount of fuel has to be introduced within a short space of time to achieve a combustible air-fuel mixture. Multiple turbocharged engines – particularly bi-turbo and tri-turbo models – benefit from injection pressures in excess of 2,000 bar.

Emissions: A higher injection pressure is a key factor in reducing an engine’s untreated emissions. Indeed, in compact-class vehicles it can often even help to avoid the need for exhaust gas treatment. The greater the injection pressure, the more finely both the injector and injection nozzle can be constructed. This improves atomization and results in a better air-fuel mixture, meaning that optimum combustion is achieved and no soot can form.

Systems competence: A higher injection pressure requires more than just a re-engineered injector. With its comprehensive diesel systems competence, Bosch is able to assemble a finely tuned system comprising not only the control unit, but also the fuel pump, the common-rail system and the injector.

Development of injection pressure – Bosch began with 100 bar 

up to 100 bar
Goal at the start of development in 1922

over 100 bar 
First series-production inline injection pump
(MAN truck, 1927)

300 bar
VE distributor injection pump (VW Golf D, 1975)

900 bar
Axial-piston pump (Audi 100 TDI, 1989)

1,500 – 1,750 bar
VP 44 radial-piston pump
(Opel Vectra, Audi A6 2.5 TDI, 1996; BMW 320d, 1998)

1,350 bar
Common rail (Alfa-Romeo 156 2.4 JTD, 1997)

2,050 bar
Unit injector system (VW Passat TDI, 1998)

over 2,000 bar
Common rail with piezo injector
(first deployed in the Audi A6 3.0 TDI, 2003/4)

2,500 bar
CRS3-25 common-rail system (available in series-production vehicles as of 2014)