Understand the exhaust gas color of a Generator

Gas Generator

Understand the exhaust gas color of a Generator

Find out how to identify the type of malfunction through the smoke analysis!

The exhaust gas exit of a Diesel Genset can indicate that something is wrong because it shouldn’t have any color in normal usage conditions.

In the start-up of the genset or when it must provide power abruptly, it is perfectly normal that the engine releases a little bit of smoke, because of the turbo delay when trying to stabilize the airflow velocity that corresponds to the injected diesel volume inside the cylindric chamber. This smoke release will be inferior if the engine performance allows a quick stabilization.

There are three types of smoke in a genset:

  • Black;
  • Blue;
  • White.


Black Smoke

The black smoke is the most common smoke that is released from the genset engines and is nothing more than not burned carbon particles, which is a sign of a less efficient combustion, engine overload or an unbalance relation between air and fuel. The fuel system is providing too much fuel or there isn’t clean air (oxygen) enough for the combustion.

Possible causes for the 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;
  • Turbocharged 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).


Blue Smoke

In a cold start-up of the genset, it can generate some release of one or two puff smoke in a blue tone, but more than that can indicate that there is a severe problem. The blue smoke indicates the existence of oil in the combustion, which means that the engine probably has an oil leaking that passed to the combustion chamber.

This is a common problem in a genset with an extended function in idle or low power because the genset won’t reach a normal operating temperature and it will not have enough expansion in the pistons and segments.  It’s also to do an engine pre-eating before picking up the maximum load.

Possible causes for the 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

The white smoke is a sign that there is water vapor escaping through the exhaust or of fuel that wasn’t burned, which indicates that is possible that the problem is in the injection system.

When you suspect that the diesel fuel is of low quality, install separators filters in genset, fuel tanks cisterns and fuel transfer systems.

Do at least one start-up a month (so that the diesel circulates) and keep the daily tank above 90%, this way you can minimize the water condensation. Preventively, add an antifungal in the daily tank of generators in emergency service (less fuel renovation), so that you can reduce the contamination by the appearance of bacteria’s and fungus that can result in a contaminated diesel or mud in the deposit.

Possible causes of 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.

Technical Dep.: Nuno Vital Felício.

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