While the Japanese are leading in petrol direct injection technology, Germany's Bosch, working in conjunction with several European car makers, Have pioneered Common-Rail Direct Injection for diesel engines.
Compared with petrol, diesel is the lower quality ingredient of petroleum family. Diesel particles are larger and heavier than petrol, thus more difficult to pulverise. Imperfect pulverisation leads to more unburnt particles, hence more pollutant, lower fuel efficiency and less power. Common-rail technology is intended to improve the pulverisation process.
To improve pulverisation, the fuel must be injected at a very high pressure, so high that normal fuel injectors cannot achieve it.
In common-rail system, the fuel pressure is implemented by a very strong pump instead of fuel injectors. The high-pressure fuel is fed to individual fuel injectors via a common rigid pipe (hence the name of "common-rail").
In the current first generation design, the pipe withstands pressures as high as 1,350 bar or 20,000 psi. Fuel always remains under such pressure even in stand-by state. Therefore whenever the injector (which acts as a valve rather than a pressure generator) opens, the high-pressure fuel can be injected into combustion chamber quickly. As a result, not only pulverisation is improved by the higher fuel pressure, but the duration of fuel injection can be shortened and the timing can be more precisely controlled.
Benefited by the precise timing, common-rail injection system can introduce a "post-combustion", which injects small amount of fuel during the expansion phase thus creating a small scale combustion before the normal combustion takes place. This further eliminates the unburnt particles, also increase the exhaust flow temperature thus reducing the pre-heat time of the catalytic converter. In short, "post-combustion" cuts pollutants.
As a side effect the more precise timing reduces the charecteristic Diesel Knock we all hate so much.
According to PSA's press release, its new common-rail engine (in addition to other improvements) cuts fuel consumption by 20%, doubles torque at low engine speeds and increases power by 25%. It also brings a significant reduction in the noise and vibrations of conventional diesel engines. In emission, greenhouse gases (CO2) is reduced by 20%. At a constant level of NOx, carbon monoxide (CO) emissions are reduced by 40%, unburnt hydrocarbons (HC) by 50%, and particle emissions by 60%.
Not a bad result at all.