For a few years before VW adopted common-rail technology a unique “unit injector” injection system was used. Read all about VW’s experiment, which engines used it and what the problems are.

Which TDIs Used the PD type Engines?

VW used the PD style in TDI engines for US model years 2004-2008, including:

• 2004-2006 Mk4 Jetta Wagon TDI , Golf TDI and New Beetle TDI – engine code BEW
• 2004-2005 Mk4 Jetta TDI (Sedan)- engine code BEW
• 2005.5-2006 Mk5 Jetta TDI (Sedan) – engine code BRM
• 2004-2006 B5.5 Passat TDI – ending code BHW
• 2004-2008 Touareg TDI with V10 Engine – engine codes BKW & BWF

What does PD, or Pumpe Duese Mean?

In a traditional diesel engine the fuel is injected into the cylinders using fuel injectors. The fuel sent to these injectors is pressurized to high pressure by an injection pump. One pump feeds all the injectors – in older rotary style systems each injector had its own feed line from the pump, while in modern common-rail designs all injectors share the same fuel rail.

The PD system was unique in that there was no injection pump. In the PD design each injector has its own dedicated fuel pump located in the injector itself.

The pressure generating pump inside the injector was driven by a rocker arm that was pushed by an extra set of lobes on the camshaft.

PD System Advantages

Compared to rotary style injection systems, the PD design allowed for far greater fuel pressure, up to 1,920 bar (about 28,000 psi) and precise control over injection timing and quantity. This allowed improved power and most notably much smoother operation through the pre-injection phase. For example, the ALH engine, which uses a rotary pump, and BEW engine are largely identical other than the injection system. However, the BEW engine exhibits far less diesel “knock” and makes more power – 100HP and 177 ft-lbs of torque versus 90/155 in the earlier ALH. Cold starting is also much improved.

PD versus Common-Rail

Common-rail technology was already available and being my other manufacturers when VW released the PD system. While they were considered competing technologies at the time, the common-rail system was less complicated (only on injection pump & simpler cam design), less expensive (injectors were cheaper without the pumping mechanism) and capable of handling much higher pressures – modern systems can utilize up to 2,800 bar. More importantly, however, is that there was doubt that the PD system could meet stricter emissions standards. Because of this VW ceased using PD injection systems when the 2009 2.0L Common-Rail “Clean Diesel” TDIs were announced.

Why did VW use the PD system instead of just using common-rail?

While there is no official announcement confirming this, it is suspected that VW wanted to create its own proprietary system to compete with common-rail. Common-rail technology was patented and largely controlled by Bosch. Having an in-house system would reduce VW’s costs to manufacture since they could avoid being exposed to Bosch licensing fees. In addition VW believed would have a unique advantage in the market if their PD technology could surpass common-rail. However, it became apparent quickly that the PD design was not competitive against common-rail, so VW had no choice but to abandon its development.