What does the colour of the exhaust gases of a diesel generator mean?

Although it is perfectly normal for your generator to release some smoke, specially when it starts up or has to suddenly make power available, the smoke from the exhaust of a diesel generator should be colorless, under normal operating conditions. The fact that it has some color indicates that something is not right. There are three types of smoke in a diesel engine generator:



Black smoke is commonly released by diesel engines and is nothing more than unburned carbon particles, which is indicative of inefficient combustion, engine overload or an imbalance in the air / fuel ratio. The fuel system is supplying too much fuel or there is not enough clean air (oxygen) for combustion.

Possible causes for black smoke:

  • Incorrect injection timing. Engine/injection pump out of control (it can also have damages on the belt, currents, reels or distribution keys);
  • Injector with a defect, clogged or worn;
  • Injection pump with a defect;
  • Clogged air filter;
  • Air circulation in the genset room can be inadequate;
  • Turbo charged with a defect (insufficient air regards to the fuel);
  • Defect on the Intercooler;
  • Incorrect valve lash adjustment;
  • Problems in the engine head, clogged valves due to EGR (Exhaust Gas Recirculation) with a defect;
  • Low compression in the cylinder (joined or broken piston rings, components with wear or glazed cylinder);
  • Strangled admission system (small system or doubled intake pipe);
  • Other engine adjustments;
  • Poor quality fuel;
  • Excess of carbon (coal) in combustion areas and exhaust;
  • Operation at low temperatures;
  • High ambient temperature (less oxygen);
  • High altitude (rarefied air).



In a cold start-up of the generator, there may be a release of one or two puffs of blue-colored smoke, but more than that can be an indication of a serious problem. ‘Bluish’ smoke usually means that there is oil in the combustion, that is, the engine probably has an oil leak that has passed into the combustion chamber.

It is a very common problem in a generator that runs without load or with low power for a long time, as it does not reach normal operating temperature and will not have sufficient expansion in the pistons and segments. It is also convenient to pre-heat the engine before reaching the maximum load.

Possible causes for blue smoke:

  • Cylinder wear, segments or grooves in the segments;
  • Oil level of the engine above maximum level;
  • Oil too thin through the segments;
  • Stopping and restarting all the time for short periods of operation;
  • Fuel diluted in oil;
  • Operation under 30% of the load;
  • The oil can also come from an air filter (humid type) overloaded with oil;
  • Wear or defect in valves or valve stem packing;
  • Injection pump or defective supply pump, which can allow a mix of oil in the fuel;
  • Glazed cylinder;
  • Glued segment.



White smoke is a sign that there is water vapor or fuel that has not been burned coming out of the exhaust, indicating a possible problem in the injection system.

When you suspect that diesel is of poor quality, place separating filters in the generator set, the supply tanks and transfer systems.

Make at least one monthly start-up (to get the diesel circulating) and keep the daily deposit above 90%, in order to minimize water condensation. Preventively, add an antifungal to the fuel tank in generators in emergency service (little diesel renewal), in order to reduce contamination by a diversity of bacteria and fungi that, consequently, result in contaminated diesel and / or the creation of sludge in the tank.

Possible causes for white smoke:

  • Incorrect injection timing on the engine or injection pump;
  • Lack of fuel (what causes the pump to function in an incorrect timing);
  • Low compression in the cylinder (glued piston rings or broken or wear components);
  • Water or gasoline in the fuel;
  • Dirty diesel;
  • Damaged or defective injectors;
  • Incorrect injection timing (there can also have damages on the belt, current, reels or distribution keys);
  • Low ambient temperatures.


Bibliography and references: RYA-Diesel Engine, Motolusa, Lda, United Diesel Fuel Injection Services.

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