You’ve cranked on your diesel generator, expecting to fix the power outage without issue. But after a few hours, black smoke starts pouring from the exhaust. And you notice black ooze collecting around the exhaust, as well. You wonder if the generator is damaged. But good news: it’s not … yet. You’re simply wet stacking your diesel generator. And that’s something you can fix.

Wet Stacking a Diesel Generator: What It Is

Wet stacking means that your diesel engine isn’t burning all the fuel being pumped into the cylinders and that the engine is running at suboptimal temperatures. It occurs when the engine runs with a light load (the resistance the engine must work against) for an extended period of time and doesn’t reach designated operational temperatures. Diesel engines operating with too light a load don’t get warm enough because they burn little fuel. So, engine combustion doesn’t get hot enough.

Thus, the engine doesn’t burn all the fuel, moisture, and carbon particles (soot) pumped into it. These unburned substances are either expelled — making the exhaust a sooty back color — or they collect in the turbo and exhaust systems.

Damaging Effects of Wet Stacking

If ignored, wet stacking can significantly damage your diesel engine over time. Unburned fuel will start to build up inside your engine, clogging injectors and decreasing engine performance. Deposits can likewise produce backpressure and reduce the efficiency of the engine’s turbo system. Worse, they’ll erode engine surfaces over time, decreasing the lifespan of the product.

Wet stacking also affects engine oil. Because the engine isn’t running as hot, the pistons don’t expand to meet the cylinder wall like they should. Because of this, gasses and unburned fuel lead into the oil pan under the cylinder, diluting the oil. This inevitably decreases the oil’s ability to protect your engine and causes increased wear.

Other negative effects include increased pollution and emissions, reduced power, and increased maintenance.

How to Tell if You’re Wet Stacking Your Generator

It’s fairly easy to tell if you’re wet stacking your generator. Signs include:

• Black ooze at the exhaust pipe
• Black ooze around the turbocharger
• Continuous, black exhaust
• Running light loads for extended periods of time

Fix Wet Stacking Before It Causes Damage

 

If have been wet stacking your diesel generator, don’t panic. You can remove light buildups from wet stacking by running your generator at optimal levels for a few hours. More serious buildups may require a complete engine overhaul.

To prevent wet stacking, take the following steps:

• Regularly exercise your generator per manufacturer guidelines. In other words, don’t let your generator sit idle for weeks or months. When you do exercise it, do so according to NFPA guidelines.
• Always run the generator at an electrical load that brings the unit up to the proper operational temperature. Usually, this requires the engine running at about 75 percent of its load capacity. If the buildings or operations which the generator is powering don’t require even that much from your generator, you may need an external load bank, which will allow your generator to run optimally under a larger load. Alternatively, don’t let your generator run at 60 percent or less of its rated output.
• Fill up the fuel tank. Too little fuel can also cause wet banking.
• If you’re using the diesel generator as your primary power source (even for a short period), you can set the generator to run at a higher load. You can set the generator to run at a higher load automatically, or you can adjust the load manually.
• Have a qualified technician maintain your generator.
• Make sure internal temperatures reach manufacture requirements when operating the generator in very cold conditions.

Taking these steps will likely prevent wet stacking from occurring in your diesel engine. However, if wet stacking is occurring and you haven’t been running your engine under light loads, there’s likely something wrong that needs addressed. In that case, contact a generator service provider to inspect and service your generator.