Tuning Wiki

Diesel smoke limits

How a diesel ECU limits fuel from the air that is actually available, and why that limit is what the driver feels as smoke, lag or a flat spot.

Updated by llandowtuninguk@gmail.com   Revision 1

What the limit is for

A diesel makes torque by injecting fuel into air that is already in the cylinder. If the fuel runs ahead of that air, the exhaust carries soot. Bosch describes an injected-fuel-quantity limit whose inputs include intake-air quantity, engine speed and coolant temperature, specifically to avoid excessive soot as well as mechanical and thermal overload. In the later torque-controlled description, the same idea is applied to the internal torque demand: too much fuel for the air is one of the reasons the demand is reduced. Bosch EDC, fuel-quantity limitation and torque limitation.

That is the calibration behind a smoke map. It is not a boost-pressure table, and it is not the driver's request. Desired boost is a target for the air system. The smoke limit cares about the air the engine can use. A leak, a slow turbo, a restricted filter or a sensor fault can leave the request high while the permitted fuel stays low.

Air mass, not the boost gauge

Boost pressure and air mass move together only while the rest of the intake path is doing what the calibration assumed. Altitude is the clearest documented case. Atmospheric pressure falls with height, the cylinder is charged with less combustion air, and the injected quantity has to be reduced or the engine emits excessive soot. Bosch measures that with the ambient-pressure sensor and also lets atmospheric pressure affect boost control and torque limitation. Bosch, altitude compensation.

The same gap appears in a tip-in on a turbocharged engine. The pedal can ask for torque before the compressor has delivered the air. If fuel is released against the request rather than against measured air, the visible result is a puff of smoke and then, often, heat. If fuel is held until the air arrives, the same moment feels like a hole or a delay. Neither reading means the pedal map is the thing that changed.

Where a broad-band lambda sensor is used, Bosch describes a fuller version of the same limit. Air mass and engine speed, together with a smoke-limitation map, set a desired lambda. That desired lambda and the air mass produce a preliminary maximum fuel quantity, and the measured exhaust oxygen can then correct it. The stated objective is the maximum fuel that can be injected without exceeding a given smoke value. This is a documented control structure, not a statement that every diesel file contains that sensor or that map. Bosch, full-load smoke limitation, printed pages 60–62.

What smoke is telling you

  • Smoke on a hard pull once boost is stable means the fuel that was actually injected was high for the air that was actually present. Compare the limited quantity with measured air, not with the boost target.
  • Smoke only while the turbo is still catching up is the transient case above. The steady-state limit can look fine in a snapshot taken later.
  • Smoke that appears only when hot, or only at altitude fits the temperature and ambient-pressure inputs Bosch lists. A calibration that was signed off at one condition is not automatically the same limit elsewhere.
  • A sudden loss of power with little smoke is often a different limiter: temperature, a fault substitute, a gearbox request or a speed limit. See which limit actually wins.

Exhaust gas recirculation changes the fresh air in the charge, so it moves the air the smoke limit can see. That interaction is covered in EGR and diesel part-load behaviour. How soot is formed and treated after it leaves the cylinder is covered in petrol and diesel emissions.

How this sits in the ECU families

EDC15 is described in quantity terms: a smoke or air limit can sit below the driver request and the full-load quantity. EDC16, EDC17 and diesel MD1 express more of the path in torque, then still have to turn the permitted torque into fuel the air can burn. The family pages are the place to see how each one is documented. They are not interchangeable map lists.

Raising the smoke limit without the air does not create reliable torque. It creates the soot and the exhaust temperature the limit was there to restrain. A sensor fault can do the same thing from the other direction: Bosch's fuel-temperature example uses a substitute chosen so that excessive soot is avoided, and that substitute can reduce power. Bosch, substitute functions.

What to log before changing it

Record pedal position, the requested quantity or torque, the quantity after limitation, measured air mass, requested and actual boost, ambient pressure, coolant temperature and engine speed. If a lambda signal exists, record the desired and measured values rather than assuming the map is the delivered mixture. Take the sample during the smoke, not only after the turbo has caught up. The wider checklist is in logging a drivability complaint.

Sources