IDI Indirect Diesel Injection
Diesel fuel injectors in Volkswagen and Audi engines fall into four categories. The first version that started in the 1970’s was a single stage injector that injected indirectly, hence known as IDI or Indirect Diesel Injection. This refers to the mode of operation in which all the fuel for a stroke was injected at once, and the fuel was injected into a steel prechamber pressed into the aluminum head, then exited while already burning through a slanted hole aimed at the top of the piston. The steel prechamber successfully handled the diesel impact, but absorbed heat and made noise, which means it also wasted energy. These injectors typically operated at 130 bar or 1870 psi.
Testing and repair of these injectors is fairly straight forward. The needle and seat at the heart of the injection process is called the pintle. Allowing that the injector bodies are not corroded internally, the pintle section can be replaced separately. The injection pressure can be reset by changing a shim set. Larger pintles for increased power were made, but have become mostly unavailable.
Dual Stage External Tdi Injectors
The second version of diesel fuel injectors starts the series of the Tdi or Turbocharged Direct Injection engines. Another version known as SDI or Suction Direct Injection had no turbocharger, and was not sold in America. First appearing as the 1996 Passat Tdi, this series continued through the 2003 model year for the VW Golf, Jetta, Beetle and Passats. As the Tdi moniker infers, the fuel is injected directly into the combustion chamber. The combustion chamber mostly consists of an odd shaped cavity in the top of the piston, which puts the maximum cylinder pressure lower in the piston as well as helps the piston stay straighter in the cylinder as it descends during combustion. Tdi injectors are dual stage or 2 stage. An initial small pulse of fuel ignited in the cylinder, which assists in the burn process of the main injection volume. The added efficiency and noise reduction allows these engine significantly better power with even better mileage than the IDI engines. These injectors operate at 230 bar or 3321 psi. The higher pressure helps the atomization of the fuel, further improving efficiency. Larger pintles for performance increases generally do not affect the mileage unless the driver accelerates harder frequently.
Testing and repair of these dual stage injectors is a bit more complicated. The pintles can be replaced, but resetting both pressures is a project. The initial pressure can be reset by changing a rod to another length, but the main pressure cannot be checked on most equipment. Cheap aftermarket injector pintles generally have wildly varying main pressure characteristics, and large differences in combustion and smoke will occur. There are just a handful of facilities in the US that can perform a reset of both pressures. And they generally are equipped to first check the volume of injection of each pintle, and match them in a set. The net effect of properly setting up these injectors can be felt, heard and seen with better performance, less vibration and noise, and higher mileage. For most customers with a stock application, the difference may not be worth the expense. For high performance applications, the difference is tremendous and well worth the extra time and expense. Larger injector pintles always need to be accompanied by custom tuning to change the software that controls injection.
Pump Duse PD Internal Tdi Injectors
The third version of diesel fuel injectors starts the with the series PD or Pump Duse engines found in the 2004 VW Golf, Jetta, Beetle and Passat. The Volkswagen Touareg has an awe inspiring 10 cylinder version. These injectors are mechanically operated from a third camshaft lobe via a roller rocker assembly. The two stage injection is accomplished with an electrical solenoid that opens and closes to control the pressure. Pre-injection starts at 180 bar or 2611 psi, then the main injection can vary from 300 bar to 1920 bar, or 4351 to 27,846 psi. The resultant precision and high atomization generates greater power and efficiency than the same engine with the earlier externally mounted injectors. This PD system is also the first to have a fuel pump in the tank, and there is a camshaft driven fuel pump to feed the main injectors at pressures up to 8.5 bar, or 110 psi. This is called the tandem pump because it contains a fuel pump and a vacuum pump for the brakes, and for the EGR and turbocharger controls.
Testing and repair of these camshaft driven injectors is seldom done, as wear is just uncommon. Occasionally a fuel starvation or lack of lubrication issue will cause the tandem pump to grind up and shed metal flake into the injectors. The injectors must then be replaced. Larger injectors are available for racing use. Custom tuning software alone can yield performance and mileage increases by changing the injection duration. Better camshafts are available that increase both mileage and performance.
Common Rail CR Piezoelectric Tdi Injectors
The fourth version of diesel fuel injectors is the CR or Common Rail engines which start in California with the 2009 model year. Both Volkswagen and Audi use the CR technology. The VW Golf, Jetta and Passat series, and the Audi A3 and Q5 series have a 2 liter, 16 valve 4 cylinder engine. The Audi A6 and Q7 have a six cylinder version. These have piezo controlled injectors that are at least 4 times faster than a solenoid control, and have about one quarter the moving internal mass. As many as five separate pulses control the pre-injection, main injection volume and the intermittent regeneration cycles to clean the Diesel Particulate Filter or DPF. This system incorporates an electric fuel pump in the tank, and second auxiliary fuel pump that increases pressure to 5 bar or 73 psi, and a timing belt driven high pressure pump that varies system pressure up to 1800 bar or 26,107 psi.
Once again testing and repair of Common Rail injectors is simply not done. The only failures we have seen at Karmakanix have been due to metal flake ejected from a failed auxiliary pump, and the entire fuel system fills with metal flakes as the main fuel pump also grinds up. Larger injectors are not available or needed for street applications. Over 30% higher torque and better mileage can be had with custom software tuning from Malone Tuning.