Start of Injection (SOI)

Key takeaways

Start of injection is the diesel’s combustion-phasing lever — the equivalent of ignition timing.
Advancing it can raise efficiency but also pressure, noise and NOx.
It is calibrated against emissions and mechanical limits, not just power.

Start of Injection (SOI) timing determines when fuel enters the combustion chamber relative to piston position. SOI directly affects power output, emissions, combustion noise and mechanical stress on the engine.

Overview

The ECU calculates optimal injection timing based on RPM, load, temperature and rail pressure. Earlier injection increases power but raises NOx and combustion noise; later injection reduces NOx but increases soot.

Controlled Signals

Crankshaft position
Camshaft position
Rail pressure
Intake air temperature
Coolant temperature

Maps Involved

Main SOI Maps vs RPM/IQ
Temperature Correction Maps
Altitude Correction Maps
Pilot Injection Timing Maps

Logic Sequence

IQ Request → Base SOI Lookup
         ↓
Temperature / Pressure Corrections
         ↓
Final SOI Command → Injector Driver

Calibration Objectives

Optimize power delivery
Control combustion noise
Balance NOx vs soot trade-off

Calibration Strategy

Advance SOI moderately for more power (1-3° max)
Monitor cylinder pressure rise rate
Coordinate with pilot injection timing
Verify EGT remains within limits

Diagnostics

Injection timing deviation faults
Excessive combustion noise complaints
High NOx readings during emissions test

Best Practices

Small SOI changes have large effects — move in 0.5° increments
Always verify cold start behavior after SOI changes

SOI in the Combustion Cycle

Main-injection timing is mapped in crank degrees relative to top dead center as a function of rpm and injected quantity, with corrections layered on top for coolant and air temperature, rail pressure and altitude. Typical diesel main timing sits a few degrees before TDC at idle and advances with rpm and load; the ECU also manages the separation between pilot, main and post injections so the pressure rise stays controlled.

Efficiency and Emissions Trade-offs

Advancing SOI raises peak cylinder pressure and thermal efficiency but also combustion noise and NOx; retarding it lowers pressure and NOx while pushing heat into the exhaust — raising EGT and soot. Emissions-oriented factory calibrations often run retarded timing in exactly the zones a performance file wants to clean up, which is why timing work must always be balanced against EGT margins and mechanical limits rather than simply maximized.

What does start of injection control on a diesel?

Combustion phasing: when the fuel event begins relative to the piston sets peak-pressure timing, efficiency, noise and NOx. It is the diesel analogue of ignition timing — a primary lever a quality diesel calibration tunes with care.

Why not just advance injection timing for more power?

Advancing raises peak cylinder pressure, combustion noise and NOx, and can exceed mechanical limits. The efficiency gain is real but bounded; a good map moves timing within emissions and durability margins, not to the edge for a dyno number.

Related: these maps and strategies are the daily work of our ECU remapping service. See also ECU Map Types Glossary in the knowledge base.