Three-phase fuel injection at up to 1600 bar

Another important feature of this second-generation common-rail technology is the newly developed seven-hole injection nozzle, which replaces the previous six-hole nozzle. The new injector makes it possible to reduce the hole diameter by around 20 percent, thereby reducing the flow rate. This means that the fuel is distributed more evenly in the cylinders, ignites more rapidly and burns more completely than before. The progress achieved thanks to this improved mixture formation is particularly noticeable when measuring the exhaust emissions.

The greater throttling effect of the smaller nozzle holes increases the injection time, however, which is particularly unfavourable when more performance is required. To compensate this effect the engineers at Mercedes have increased the injection pressure from the previous 1350 to 1600 bar, thereby shortening the injection time. As in all common-rail engines this high injection pressure is available at all times, i.e. also at low engine speeds.

The pilot-injection system has always been a special feature of common-rail engines by Mercedes-Benz. This ensures quieter combustion: a few milliseconds before the main injection process, a small quantity of diesel fuel flows into the cylinders, ignites and thereby pre-heats the combustion chambers. This creates more favourable conditions for the subsequent main injection: the fuel ignites more easily in the pre-heated cylinders, which means that the pressure and temperature no longer increase as suddenly as in an engine without pilot injection. This has a favourable effect on combustion noise.

In these second-generation CDI engines the engineers at Mercedes have perfected this principle by means of dual pre-injection. The highly-efficient solenoid valves in the new 1600-bar injectors permit even shorter time delays between pilot and main injection, and now small pilot quantities flow into the combustion chambers twice in succession within less than a millisecond, ensuring even better pre-heating. The result is audible, as combustion noise levels are once again significantly reduced.