This article delves into the intricacies of using VCDS (Vag-Com Diagnostic System) to fine-tune a BRM TDI engine for improved fuel economy. Specifically, we’ll explore the relationship between cam timing, injection timing (SOI – Start of Injection), and their impact on MPG. We will analyze the function of the camshaft position sensor and discuss whether adjusting cam timing via torsion value impacts fuel efficiency.
Understanding the BRM TDI’s Cam and Injection System
The BRM TDI engine, a 16-valve double overhead camshaft (DOHC) PD (Pumpe Düse – Unit Injector) engine, differs significantly from its 8-valve single overhead camshaft (SOHC) predecessor. This difference raises questions about the effectiveness of traditional torsion value adjustments for optimizing fuel consumption.
While retarding cam timing can shift the power band higher and advancing it can move it lower, impacting fuel efficiency at different RPM ranges, the core question is the role of the camshaft position sensor. Does it solely determine TDC (Top Dead Center) for each cylinder, or does it influence the ECU’s injection timing calculations?
Example of a BRM TDI engine bay.
In the BRM engine, the camshaft position sensor resides on the inlet camshaft. However, the injector lobes are located on the exhaust camshaft, which lacks a dedicated sensor. This raises a crucial question: if we adjust the inlet camshaft using the torsion value, how can we ensure proper synchronization with the exhaust camshaft using VCDS?
The Impact of Cam Timing Adjustments
Several online forums suggest that altering cam timing on PD engines can affect the accuracy of the Multi-Function Display (MFD) fuel consumption readings. Therefore, manual tank-to-tank calculations are recommended for accurate measurements. Another theory posits that cam timing adjustments influence the amount of fuel injected due to changes in the injector lobe profiles. However, it’s debatable whether the relatively small adjustments of 2-3 degrees significantly impact the fuel delivery process.
Illustration of camshaft lobes and their interaction with injectors.
The fundamental questions remain:
- Does adjusting cam timing affect the ECU’s determination of injection timing?
- Does cam timing impact the quantity or pressure of fuel injected?
- Or is it primarily valve timing that influences fuel economy?
Exploring Start of Injection (SOI)
Observations using VCDS reveal that SOI timing varies significantly under different load conditions. At light loads (e.g., 2000 RPM), SOI is around 4-5 degrees BTDC (Before Top Dead Center). Under full load and boost, SOI advances to nearly 16 degrees BTDC. This behavior contradicts the typical practice in gasoline engines, where ignition timing is advanced under light loads for better efficiency. This raises the possibility that retarded SOI at light loads in the TDI engine is a strategy to minimize NOx emissions.
Example VCDS screenshot displaying SOI values.
Conclusion
Optimizing a BRM TDI engine for fuel efficiency using VCDS requires a deep understanding of the interplay between cam timing, injection timing, and various engine parameters. While adjusting the torsion value on the inlet camshaft is possible, the lack of a sensor on the exhaust camshaft presents a challenge. Further investigation is needed to determine the optimal synchronization between the two camshafts and their combined impact on fuel economy. The relationship between SOI and engine load also warrants further analysis to understand its role in emissions control and efficiency optimization. Using VCDS to monitor and log these parameters during real-world driving scenarios can provide valuable insights for fine-tuning the engine for optimal performance and fuel economy.
