TDI: Basic principles of technology (Articles)

The fundamental idea behind TDI was to optimize gasoline engine performance, fuel consumption, and emissions by incorporating electronic control systems into diesel engines, which traditionally used distributor injection pumps.
This initially led to the development of the TDI family, starting with the VP 37, which was based on proven mechanical distributor injection pumps.

In the VP 37, the throttle linkage, speed selector lever, centrifugal governor, cold start accelerator, load-dependent full-load limiter, and all the other mechanical components used to move the control slide via the engine control unit (ECU) and the fuel quantity control system have been replaced. For more information, see /viewtopic.php?t=3066.
The injection adjustment, which depends on the internal pressure of the pump, is fine-tuned by an electric injection adjustment valve, which also enables injection start based on the amount being dispensed. For more information, see /viewtopic.php?t=3195.

The electronics even allow for compensation of the diesel temperature: The warmer the diesel, the lower its density, which, with an unchanged injectionvolume, would lead to a loss of power.
Using a diesel temperature sensor in the pump (or, in common rail systems, in the return line to the tank), the signal from which is evaluated by the engine control unit (ECU), performance deviations caused by sealing issues are compensated for by adjusting the injection duration accordingly.

Regarding the injector nozzles, direct injection engines (unlike indirect injection engines) require a pre-injection of a small amount of diesel fuel, so that the main fuel charge can burn with acceptable noise levels and running smoothness. This is achieved through a new design of the fuel injectors, specifically using what are called dual-spring injector holder combinations.

Since the MSG requires precise information about the crankshaft speed and instantaneous position for its control functions, an inductive sensor scans a toothed wheel attached to the crankshaft. With this information, the MSG, together with the quantity control, simulates a more refined centrifugal governor and, for example, implements the typical diesel engine derating that occurs above approximately 4000 rpm.

The speed measurement is so precise that it can even capture the accelerations during the working cycles and the delays caused by the compressions.
This allows for the detection of speed differences between the working strokes of the individual cylinders. The result is used by the software components in the MSG responsible for idle speed control to adjust the injection quantity for each cylinder in such a way that the smoothest possible engine operation is achieved.

In addition to controlling the injection quantity, the engine control unit (ECU) also regulates the start of injection, exhaust gas recirculation (for more details, see /viewtopic.php?t=3095).
"the boost pressure (for more information, see /viewtopic.php?t=3101), and depending on the engine type, other actuators around the engine, such as the bypass valve, radiator fan after-run, etc."

While driving, the electric throttle position sensor transmits the driver's desired fuel injection amount to the engine control unit (ECU). The system first determines, based on the programmed boost pressure map, the amount of boost pressure required to combust the desired amount of fuel "cleanly" within the respective emission class, and then begins to adjust the boost pressure to the corresponding value.

At the same time, the engine control unit (ECU) calculates the actual mass of fresh air entering the cylinders based on the signal from the mass airflow sensor (MAF), the engine speed, and other correction factors.
The so-called... The black smoke limit map determines, for each engine speed and air mass per intake stroke, the maximum fuel injection quantity that can be combusted without exceeding the respective emission limits (this maximum quantity is also referred to as the soot limit).

In some TDIs, the soot reduction is calculated based on the current boost pressure (instead of the mass airflow sensor signal).

In any case, the engine control unit (ECU) compares this soot limit with the aforementioned driver demand and torque limit (which specifies the maximum injection quantity for each engine speed, at which no mechanical overload of the powertrain occurs). The ECU then determines the actual amount of fuel to be injected based on the lowest value of these three limits.

When the soot reduction system is active, insufficient fresh air enters the cylinders, preventing the desired fuel injection amount (as requested by the driver, or the maximum amount from the torque limitation) from burning cleanly and soot-free.
The most common causes of this operating condition are insufficient boost pressure (especially at low engine speeds or during the initial warm-up phase) or a faulty mass airflow sensor (MAF) that reports too little air intake, even though the cylinders are actually being filled adequately. For more details, see /viewtopic.php?t=3347 - however, the soot limit is calculated based on the boost pressure.

Sometimes, the air mass value setpoint in the measurement block for the EGR system can cause confusion.
It typically delivers 850 mg/stroke at full throttle, but this value is not always achieved, especially in VP-TDIs at higher engine speeds.
Possible reaction: "Oh, my mass airflow sensor is broken!" Replacing the mass airflow sensor is expected to result in a corresponding performance gain, but often nothing changes because the soot threshold was still above the torque limit, even though the perceived air mass was too low.
Conversely, in engines with high displacement (e.g., With an ARL engine (150 hp / 1.9L), even with 850 mg/stroke, the soot reduction system might still be active, meaning the engine isn't running properly, even though the target air mass value is being reached in the EGR control module.
Therefore, it cannot generally be determined from the (non-)achievement of the target air mass value in the EGR mass airflow measurement block whether the engine is still delivering its full power!

If the soot limit is calculated based on the mass airflow sensor (MAF) signal, then while increasing the boost pressure might often provide a solution when the MAF signal is too low, it could potentially cause the turbocharger to overspeed and fail, simply because...
- the LMM is defective.
- the LLK (medical device) is visibly soiled, or
- In the peak of summer, the TDI engine has to tow a caravan over an Alpine pass, and the intercooler becomes too hot due to a lack of airflow for cooling.
Therefore, the turbocharger boost pressure will not be increased if a lack of air is detected!
Conversely, even with a large excess of air (e.g., at arctic temperatures), the boost pressure will not be reduced below the target value specified in the boost pressure characteristic curve.
To also protect the turbocharger from over-revving, the boost pressure is reduced at low ambient pressure (e.g., in high mountain areas). For this purpose, the signal from an ambient pressure sensor, which is located in the MSG (Multi-Sensor Head), is evaluated.
The pressure sensor required for the actual pressure control is located, depending on the vehicle type, either in the engine control unit (ECU), separate from the intake manifold somewhere in the engine compartment, or directly in the intake manifold or at the outlet of the intercooler.

The software architecture, including features like driver requests, torque limitation, and soot reduction, has system-related reasons. In reality, the driver can almost always request a higher fuel injection amount with full acceleration than what the torque limitation allows. Then, the portion above the torque limit is simply ignored.

TDIs are designed such that, under normal conditions and with correctly adjusted boost pressure, the soot limit is set above the driver's desired torque output or the torque limit.
(Otherwise, soot buildup and boost pressure regulation may escalate.) For more information on this topic, please see /viewtopic.php?t=6529.

Therefore, simply increasing the boost pressure in a healthy TDI engine will not result in increased power. While the soot reduction system would increase (within the measurement range of the mass airflow sensor), the torque limitation is already at its lowest level when the accelerator is fully depressed, and it remains at its original value, which is not affected by the amount of air. For more information, see /viewtopic.php?t=3253.
Increasing the torque limit (which is essentially the power limiter in TDIs) can only be achieved through chip tuning!

For controlling the exhaust gas recirculation (EGR), the corresponding target air mass per intake stroke is read from the EGR characteristic map for each injection quantity and engine speed. If the air mass reported by the mass airflow sensor (MAF) (in the lower load range) is higher than this value, the EGR valve and the intake manifold control valve are adjusted accordingly, depending on the engine, so that the MAF value decreases to the target value. For more information, see /viewtopic.php?t=3095.

In a common rail diesel engine, the lines and nozzle volume between the distributor piston and the injector cause significant delays between the start of high-pressure generation and the actual start of injection, due to factors such as the transit time of the high-pressure wave and the elastic expansion of the lines.
Therefore, the start of injection (which is a key parameter for fuel consumption, emissions, etc.) is monitored in the VP 37 system using a signal from an inductive needle position sensor located in one of the injectors. The angular offset of the injection timing relative to the motor's top dead center (TDC) is compared by the engine control unit (ECU) with the current target value based on the injection start map, and this is then implemented in the control loop via an injection timing control valve in the injection pump.



The software architecture for PD engines largely corresponds to that of VP 37 engines, but instead of using a quantity control unit (MSG), the pump-injector units (PDE) are directly controlled.

These are filled and flushed with diesel by the pre-injection pump (a "stage" of the tandem pump driven by the camshaft, which also generates the vacuum for the control pneumatics) via their open solenoid valve.
If injection is required, the MSG activates the valve current for the relevant injector. The solenoid valve closes, thereby sealing the diesel supply in the injector. The pump plunger, actuated by the roller rocker arm, pressurizes the diesel, which causes the nozzle to open. Main and pre-injection starting.
The amount of fuel injected is determined by the opening duration of the injectors or the on-time of the injector current.

Once the required amount of fuel has been injected, the MSG switches off the valve current, and the solenoid valve opens again. The high pressure in the common rail is released towards the fuel supply line, and the injector spring closes the injector nozzle. The remaining volume of the pump chamber is also pumped back towards the fuel inlet.

During injection, the diesel fuel is compressed by several percentages of its volume due to the pressure, which can exceed 2000 bar(!), and releases heat to the PDE (pre-combustion chamber) body according to the principle of a bicycle pump.
To prevent the common rail injectors from overheating, they are flushed and cooled with diesel fuel by the tandem pump between injection cycles.
In the Pmax range, the diesel returning to the tank can become so hot that it would damage the tank. Therefore, PD vehicles have a diesel cooler in the return line, either as a diesel-water cooler with its own electric water circulation pump (which is controlled by the MSG depending on the return temperature), or as an unregulated diesel-air cooler on the underside of the vehicle, which resembles a huge heat sink for power electronics.

In common rail diesel engines, the high-pressure volume between the pump pistons and the injectors is so small that only a minimal (and sufficiently accurately calculable) delay occurs between the start of pressure buildup and the start of injection. Therefore, they do not have a needle lift sensor, and instead of measuring the start of injection, the start of fuel delivery is used in common rail systems. It occurs at the moment when the magnetic valve's flow monitoring system detects the valve needle reaching its limit and high-pressure generation begins.
However, there is inherently a short time interval between the activation of the PDE power supply and the impact of the valve needle, comparable to the activation time of a relay. Therefore, the valve flow must be activated slightly earlier than when the high-pressure pumping is intended to begin.
The activation time of the solenoid valve is measured for each process parameter and used to adjust the next power-on time of the valve, ensuring that the desired start of flow is achieved as accurately as possible (theoretically).

In addition, the activation time is used to monitor the functionality of each individual PDE: If the activation time falls outside a defined target range, an error is logged for the corresponding PDE.
However, such an error message does not always indicate a defective positive displacement element (PDE), but can also have trivial causes, such as air in the solenoid valve (which leads to a shorter engagement time) or operation with highly viscous biofuels (which causes a longer engagement time due to the throttling effect of the valve needle).
Then, normal variations in processing times, which are inherent to the manufacturing process, often result in the target range being exceeded for only 1 or a portion of the PDEs, and an error is only recorded for those specific PDEs.


The downside of PD (pump-injector) engines is their highly speed-dependent injection pressure. Because, after the injectors open, only the speed-dependent delivery rate of each individual PDE (piezoelectric direct injector) is forced through the injector nozzles, the commonly known 2000 bar is only reached in the range around 4000 rpm, while the pressure decreases significantly as the speed decreases.
In VP engines, on the other hand, the pump's delivery volume is pre-charged into the large high-pressure chamber of the injectors before the nozzles open, and is then depleted during injection. This results not only in higher injection pressures than what the pump's delivery rate would suggest, but also in a significantly less speed-dependent injection pressure in VP engines compared to PD engines.
The latter are optimized for high injection pressures in the combustion process, and therefore exhibit significantly more slack in the low RPM range compared to VP engines. A potential compensation through increased fuel injection would unacceptably worsen the exhaust emissions of PD engines.

An increase in injection pressure (and thus torque) at low engine speeds was achieved through an advancement in the solenoid-valve-controlled common rail system. However, this Generation 1.1 is only installed in a few engines: the 74kW AXR and the newer 1.9L 77kW TDI. Interestingly, the AXR actually has better acceleration from low RPMs than its predecessor, the ATD, which has the same maximum power output.
The partially redesigned common rail injection systems (PDEs) of the new 16V TDIs are based on the technology of the PDE 1.1 system.


Thank you to Bertil for his support .

Diagnosis of vehicles with TDI engines: