The SOI (Start Of Injection) is the moment at which fuel injection into the cylinder begins, expressed in degrees of crankshaft rotation before Top Dead Center (TDC).
Types of modern injection
- Pilot: small injection 30° before TDC, softens combustion
- Pre-injection: 10-15° before PMS
- Main injection: the "main injection", during combustion
- Post-injection: after TDC, heats the DPF for regeneration
SOI and Tuning
Modifying SOI drastically changes the behavior of the engine:
- Anticipating = more power but more noise + mechanical stress
- Retarding = smoother but less power
- Eliminate pilot = more noise ("rough diesel")
Maps in ECU file
ECU files typically have 5-10 separate SOI maps for: pilot, pre, main, post, depending on RPM, load, temperature. Tuning Stage 1 optimizes all these maps in a coordinated way.
Effect on combustion
Injection timing determines where, relative to top dead center, energy release occurs. Anticipating increases power and pressure in the chamber but increases noise, temperatures and NOx; delaying softens combustion reducing performance. The pilot and pre injections serve precisely to prepare the main combustion, making it more gradual and silent.
Map coordination
The ECU file contains multiple SOI maps (for pilot, pre, main and post) depending on speed, load and temperature. Quality tuning optimizes them in a coordinated way with rail pressure and flow rate, avoiding intervening on just one parameter. Improper modifications produce noise ("rough diesel"), smokiness or increased emissions, as well as unnecessary mechanical stress.
Effect on driveability
Intervening on the timing changes the character of the engine in a perceptible way: a well-optimized combustion is quicker and more regular, a disorganized one becomes noisy or smoky. The pilot injections, in particular, must be preserved to maintain the smoothness and silence typical of modern diesels. For this reason, the SOI is not touched alone, but in harmony with rail pressure and flow rate, evaluating the result also in terms of comfort, not just power.