How to manage black exhaust smoke

Attention to load will improve power pack performance.

Power packs are the lifeblood of most reefer cargo operations. Keeping them in top shape through regular maintenance is a necessity.
Understanding load and how it affects power pack health is a critical part of this process.

Is your power pack underloaded?

It might seem obvious that overloading a power pack can have negative implications for a generator’s performance. Yet under-loading can be just as serious, if not more problematic to its long term health.

What does it mean to under-load a power pack?

An underloaded power pack is defined when the amount of load (Reefers) is far below the available power (Plugs) that the power pack is equipped to handle.

This situation may be unavoidable from time to time as there may only be limited numbers of refrigerated containers in your yard - as might be the case between shipments. Other times, it is a practice done intentionally under the misconception that this helps prevent the generator from having to work too hard.

The negative effects of under loading (wet-stacking).

Professionally built Power Packs are designed to precise specifications based upon the amount of load (or electricity) they are demanded to produce. Anything short of that, and the generator is not operating to its fullest potential. They will use more fuel, burn oil, and wet stack (produce black smoke) – issues that in time will lead to a shorter equipment life expectancy and sudden failure.

Wet-Stacking Power Pack Reefer Generator
Left: Black residue around the exhaust vent is a sign of an under-loaded power pack. Right: How an efficiently loaded power pack will look.

If you are experiencing black exhaust smoke issues, a rather common occurrence called wet-stacking is likely due to the power pack's lack of load.


To correct this situation, avoid running them on less than the optimal load for long periods of time. We understand that this may not always be practical given the number of reefers available. However, diesel-powered generators perform best when they are fully loaded up with reefer plugs. So be sure to manage them to full capacity periodically or consistently maintain them at an optimal load per generator.

What is the optimal load?

The number of actual reefers needed to obtain optimal load will vary from one size power pack to another. As a general rule of thumb, it is recommended to maintain a minimum of 70% capacity to reach the point at which the generator is working efficiently enough to burn cleanly and avoid maintenance concerns like wet-stacking (Black Exhaust Smoke).

As an example, the optimal load on a 40 plug power pack should be around 28 reefers. This is provided, of course, that the 40 plug power pack is built to proper specification.

* Care should be taken for hot boxes being brought down to temp not to overload the unit. Always follow the manufacturer's recommended operating instructions.


When possible, load up your Power Packs and let them run. Do this, and your black exhaust smoke issues will go away. But if problems persist, it could be signs of a bigger concern. Contact Power Pool Plus for further direction to help resolve.

Have other Power Pack performance questions? Contact us today.