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Black Smoke & High Temperatures

Posted on December 19, 2019 in Engines, Highlighted, Investigation & Rectification | Comments Off on Black Smoke & High Temperatures

Black Smoke & High Temperatures

We had a request for technical support onboard a major shipping company’s vessel this week.

The Fleet Superintendent got in touch with us to report that one of their MAK 8M20 engines started experiencing high exhaust temperatures and producing black exhaust smoke.

As well as logging temperatures and pressures, they had already completed some of their own investigations, including:

• Check Scavenge Air Cooler – Found to be very dirty. The client has cleaned this but this made no difference, however, it may not be completely clean
• Valve timing checked – Inlet valve opening duration was 12 degrees different to the other engine

It’s always difficult to diagnose this type of issue without being on-site, but to support the client one of our engineers provided the following information and asked some specific questions to help get the engine operating properly:

“Black smoke is caused by

  1. Lack of air in the combustion chamber
  2. Too much load on the engine
  3. Too much fuel being injected
  4. Incorrect injection timing
  5. Incorrect valve timing
  6. Faulty injectors
  7. Dirty fuel

High exhaust temperatures are caused by

  1. Excessive back pressure in exhaust (blockage in pipe or silencer)
  2. Too much load on engine
  3. Lack of air in combustion chamber
  4. Lack of ventilation in engine room
  5. High engine inlet air temperature (intercooler/aftercooler)

As you can see, black smoke and high exhaust temperatures can be caused by the same issues.

I can see that the inlet valves are open for 12° less on this engine than Engine 1. Obviously, this would mean less air in the combustion chamber, but I do not know if it would be enough to cause your issues.

I would be more concerned that it is different at all. Valve timing is normally set deep in the engine and does not change during normal engine running.

Valve-lash-setting-on-a-Cummins

Have these symptoms appeared suddenly, or has it been a gradual increase in temperature? A sudden increase would indicate something breaking whereas a gradual increase might indicate the engine wearing or accumulating carbon deposits.

As you found the scavenge cooler dirty, I would also look at the complete induction system for carbon deposits. You could inspect using a borescope to save dismantling too much of the engine.

The running logs show that the exhaust temperature is around 440°C. This is not near the maximum allowed (we normally allow a max of 650°C for aluminium pistons) but as the engine seemed to be on no load, that is a very high temperature. On no load, I would normally expect temperatures of 100 – 200°C.

I assume that the engine is connected to an alternator? Have you checked that the indicated power is accurate? If possible you could run engine 1 and engine 4 on no load and compare the positions of the fuel racks.

This would tell you how much fuel each engine is consuming at the same power. If they are the same then that is ok but if they are different, engine 4 may have internal resistance either in the engine or alternator that would not show up on the generator power output gauge.”

I hope this insight is useful – if you are experiencing any similar issues or have any further questions, get in touch, we’d be happy to help.

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