Why do diesel generators smoke on starting and stopping?

Like most engines, diesel generators produce smoke on starting and stopping. The amount of smoke produced depends on a number of factors, including fuel quality, temperature and emissions compliance level. Generators with a lower emissions compliance level will smoke more than generators that are emission compliant. These engines are often controlled by an ECU and this additional control helps to minimize smoke.

Engines produce black smoke, white smoke and blue smoke, depending on the particles in the exhaust gas.

Smoke on starting

Engine smoke on starting can vary and is often black and white in colour. White smoke comes from water or incompletely combusted fuel. Black smoke comes from fuel that has not combusted.

On starting the engine you will commonly see puffs of black smoke. This smoke is fuel that has not combusted - as the engine starts fuel is injected and it turns over trying to start. All the fuel that is injected during this cycle will be unburnt.

Then you will see some white smoke, this smoke is from partially unburnt fuel. Again, in the first few cycles until the engine gets up to speed combustion may not be optimal, hence the white smoke.

The smoke should clear after a few seconds. If it continues, refer to the articles on why engines produce black smoke, white smoke and blue smoke for more detail on how to fix these issues.

Smoke on Stopping

When the engine stops smoke is common. This can be black in limited quantities, but is more commonly white. As the engine slows down, the fuel combustion is less optimal and you therefore get black smoke and white smoke black smoke. This will only last a short period until the engine is at rest.