Two of the largest gains of diesel injection development throughout the years have been electronic control and higher injection pressures.

The greater the injection pressure, the more finely both the injector and injection nozzle will be constructed giving a droplet less gas like injection mixture. This improves atomisation and results in a better air-fuel mixture, meaning that optimum combustion is very nearly achieved. The more fuel able to be burnt at this finer gas gives a specific power and torque increase and due to the more effective burn with less fuel is needed to achieve this greater power output.

Of course the higher pressures bring problems for the engineers to overcome in development. Many systems now run at 2000bar with a few even higher. At this pressure the nozzle injection exit holes can be worn by the fuel so a laser rifle cutting system with relief hole edges was developed. The injector control operational speed has to be able to cope with the higher requiements and also be able to fire multiple times per injection cycle. Piezo crystal controlled injectors are able to cope with these requirements over the traditional solenoid injector controlled systems.

Filtration on these systems is now even more important. With the injector exit holes being the same size as a hair on your head and the internal control valve tolerances being also very fine it takes only a little contamination to stop correct operation. Always use a quality fuel filter and fuel. A fuel additive such as FuelBright Diesel 1000 can also assist in keeping the fuel system working to its optimum.

Bosch development of injection pressure

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)

About the author

Graham Williams is the Thomas Injection Workshop Manager and has worked with diesel injection systems and auto electrical systems for over 45 years. He is a Bosch System Technician and a CAV diesel master repairer.