This article explains the differences between the three different fuel injection systems in TDIs available in North America and how they function.
TDIs, from the time they were introduced to the North American market to now, have had three different types of fuel injection systems, and these changes account for some of the major differences between the engines.
ROTARY PUMP: 1996-2003
The earliest TDIs (1997-1999) have an electronic injection pump called the Bosch VP 37. It is a rotary pump. The TDIs from 2000-2003 also have a Bosch injection pump, sized at either 10mm for a manual, or slightly larger at 11mm for an automatic. The injection pump is driven by sprocket on the timing belt, and in the earlier cars fueling is controlled mechanically, through a throttle. With the introduction of the AHU/ALH engines, fueling became drive by wire, or electronically controlled. Because the injection pump is controlled by the sprocket on the timing belt, doing a timing belt replacement job correctly is very important for engine timing.
Watch the video below to get an idea of how these pumps work. This isn’t exactly the model rotary pump on TDIs, but it’s close, and its mechanical function is similar.
In the injection pump, the fuel is pressurized to around 3300 psi and moves to the injectors. The injectors “pop” or fire once enough fuel pressure forces the needles to open for a short period of time. See the video below.
Pumpe Duse: 2004-2006
Pumpe-Duse (PD) or “Direct Injection” engines were designed directly by Volkswagen. There are significant differences between the earlier TDIs and the PD-equipped cars. These include cars with engine codes BEW, BRM, and BHW. The fuel follows this path:
- Via an in-tank lift pump, the fuel is placed under a small amount of pressure (5-7 psi) and moved towards the front of the car.
- There, it travels through the “tandem” pump, so named because it also creates vacuum pressure. At this point the fuel is further pressurized from 50 to 110 psi.
- Finally, the fuel travels to the four different injectors.
This is where things get interesting. The injection on PD cars is timed by specific lobes on the cam, which open and close valves to allow fuel into the injector. Once the fuel gets into the injector, its pressure skyrockets to about 27800 psi. Essentially, these little injectors act as their own little high-pressure fuel pumps. The fuel achieves its high pressure by being pressed into the injector by a plunger connected to the camshaft.
One downside to this type of injector system is that the camshaft wears easily because of the increased stress of having to control fuel injection.
Common Rail: 2009+
From 2009 through the present day, VW TDIs have been common rail cars. These include engine codes CBEA/CJAA, CATA, and all the newer EA288 engine codes. The fuel in these cars travels thusly:
- From an in-tank fuel pump just like in the PDs to
- A mid-pressure pump (pressures up to 73 psi)
- To a High Pressure Fuel Pump.
The high pressure fuel pump, or HPFP for short, works much like the rotary pump, except without the rotary part that pushes fuel into four separate lines. It pressurizes the fuel to 26107 psi and provides fuel to a “common rail”, a single fuel line that runs over all the injectors.
The injectors on the common rail, unlike their counterpart in the PD engines, do not further pressurize the fuel. They do have a significant technological edge, though, in their Piezoelectric actuators. To control the injection, these injectors have an electromagnetic piezoelectric actuator than can switch on an off (and therefore inject fuel) as many as four times faster than the solenoids in the old injectors. This allows the injectors to be much smoother and more precise than the earlier injectors.
More interesting info on the PD engines: