20 Possible Causes for Reduction in Ship’s Auxiliary Engine Performance

Power generation is a continuous requirement on board ships. A blackout situation even for a few minutes can lead to devastating results such as grounding, collision etc., resulting in heavy losses and danger to the ship and its crew.

A ship is equipped with multiple generators/ auxiliary engines to supply continuous power at all times. These power generating units are watched over and maintained by ship engineers.

A sensible ship engineer will try to run a minimum number of generators at any given period of time and look for maximum load, which not only ensures goodwill for the auxiliary engines but also reduces overall fuel consumption and harmful emissions. This can happen when the auxiliary engine rated output is achieved without any deviation in the engine parameters (temperature, pressure etc.).

If the engine performance is deteriorating, it will result in reduced output with high exhaust temperature and low peak pressure. The reduced output of one generator will require another generator to run in parallel and compensate the power demand.


Ship engineers must know the probable reasons which can lead to a reduced power output of auxiliary engines.

Following are 20 possible causes, which will lead to reduced power output and performance of an auxiliary engine on board ship:

1. Fuel oil pressure too low: The fuel oil pressure is lower than the requirement, which can be due to faulty fuel oil attached pump or the fuel oil viscosity is very low

2. Type of Fuel Burned: The fuel oil pressure will also drop when the fuel grade is changed from HFO to MDO/ MGO/LSFO. This will lead to a decrease in engine performance

auxiliary engine

3. Fuel leakage: If the fuel pump parts i.e plunger and barrels are worn out, the fuel will leak out and the fuel pressure will drop at the discharge point

4. Fuel Temperature: If the fuel temperature is inadmissibly high (>60 deg C), the fuel viscosity will reduce and it will affect the fuel pressure

5. Firing Pressure Difference: If there is a high difference firing pressure in between individual cylinders, it will lead to a reduction in the overall output of the engine

6. Blocked Filter: Blocked or dirty line filters in the fuel oil system will reduce the oil pressure and hence the performance

7. Wrong Valve Clearance: The clearance between the intake/ exhaust valve and its guides is of extreme importance. If the clearance is more than required, the combustion mixture will leak out from this gap and reduce the engine performance

8. Damaged Exhaust Valve: A damaged exhaust valve or seat will not seal properly causing blow-by of exhaust gasses on combustion. This will increase the exhaust temperature and reduce engine output

9. High Exhaust Back Pressure: If there is a flaw in the exhaust piping installation or the silencer is fouled, it will lead to high exhaust gas pressure and an increase in exhaust temperature of all units

10. Contaminated Passages: A contaminated exhaust manifold will lead to hindrance in the exhaust gas flow and increase the exhaust temperature

11. Insufficient Fresh Air Supply: Correct amount of fresh air supply is a must for efficient combustion, if there is a reduction in the scavenge air supply, the combustion will be incomplete, which will reduce the engine output

12. High Suction Air Temperature to T/C: When a ship plies in hotter temperature regions (For e.g near the equator or Gulf regions in summers) the atmospheric air sucked by T/C compressor is already at a higher temperature. If this air is not cooled properly, it will decrease the performance of the engine

13. Charge Air Pressure Too Low: If the scavenge air pressure is low, the amount of air required for combustion to each the cylinder will not be sufficient, which will lead to incomplete combustion

14. Wrong Charge Air temperature: If the cooler controller is set at a higher temperature, the temperature of the output air supplied to the engine will be higher, which is not good for engine performance. Similarly, if the setting is on the lower side, the resulting combustion will not be efficient

15. Charge Air Cooler Contaminated: A contaminated and dirty charge air cooler will not allow seawater to cool the air properly, leading to an increase in exhaust temperature

16. Air Cooler S.W Temp High: If the seawater temperature is on the higher side, the air-cooled by it in the air cooler will be warmer than normal which will lead to high exhaust temperature

17. Air Cooler S.W Bypass Open: The air coolers provided with S.W bypass valve controls the cooling medium i.e seawater. If the bypass is open more than required, then the air supplied to the engine will have a higher temperature

18. Blower, Turbine or Nozzle Ring Worn/ damage: Fouled or damaged part of turbocharger will lead to either problem in the passage of exhaust gas or less amount of fresh air supply. These will lead to a sudden reduction in the output of the engine

19. Scavenge Air Leakage: Leaking scavenge air from the inlet pipe to the cylinder will result in less supply of air and incomplete combustion

20. Wrong Tappet Clearance Setting: Wrong tappet clearance will allow inlet/ exhaust valves to open more or less than required. This will result in either inappropriate combustion or the combustion mixture drawing out of the chamber at an earlier stage

Over to you..

Do you know any other important point that can be added to this list?

Let’s know in the comments below.

Do you have info to share with us ? Suggest a correction

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About Author

An ardent sailor and a techie, Anish Wankhede has voyaged on a number of ships as a marine engineer officer. He loves multitasking, networking, and troubleshooting. He is the one behind the unique creativity and aesthetics at Marine Insight.


  1. Did I miss out the ever so important Fuel Injection Valve. It is the first item that a young 4/E looks to when there is a fall in performance.
    While all the points leading to deterioration in performance are correct, from the standpoint of a engineer the reasons could have been better quoted. e.g. the high ambient temperature will cause less mass of air entering the cylinder. This will cause less mass per unit volume of fuel to be combusted leading to improper combustion, hence causing loss in performance… etc..etc..

  2. good day sir…how can i buy ur books if im in philippines? what kind of payment do u accept except credit card? pls reply thanks….

  3. Points were briefly described, fuel valve plays a major role in combustion, testing of fuel valve and it’s opening pressure are crucial for performance of the engines.the temperature of the fuel and the type of fuel are also important factors.

  4. There is only one thing is needed, always update your maintenance according to the manual maintenance running hours, such as calibration of fuel injector, tappet clearance, overhauling of cylinder head, changing of oil, and filters, general overhauling on required running hours, and ensure there is no leakage in the system,thats it, very simple and one more thing never use not original spare parts cause this will give you problems, Remember always update your maintenance plan, tnx

  5. Respected Sir…
    we have a daihatsu DK20 generator onboard. When it runs on Heavy fuel oil the exhaust temp of unit 1 & 2 is higher whereas the exhaust temp of Unit 5 & 6 is lower but when we change it over to diesel , the scenario gets exactly opposite. what could be the possible cause ? ( variation of 3-4 bar in peak pressure is noted, tappet clearance is within limit )

  6. Non treated cooling Fresh water will lead to deposits on liner wall and in cooling chambers at the cylinder head causing lower heat exchange then higher exhaust temperature and lower power output.

  7. good day
    i am working on Dihastu 6DK20 auxilary engine .idone all fuel pump overhaull ,after tring out the DG all 5 units have same exhaust temp, but one unit not not more than 80 dgree temp. Timing allready adjested . injection valve allready change . before overhaulling all exhaust temp was ok . pleast tell me what will be the problem of unit which is less exhaust ,thanks

  8. do check the condition of piston rings and liner. I might be due to the blow past from that unit

  9. Sir iam working in bulk carrier…our generator candle filer are giving diff lo or alarm very frequently..can u plz help..

  10. good day sir…how can i buy ur books if im in bangladesh? what kind of payment do u accept except credit card? pls reply thanks….

  11. @Sobuj: We only have ebooks (electronic books). Once you purchase the ebook using credit card/ PayPal, a digital copy will be sent instantly to the registered email id.

  12. Greetings

    Normally people check all components like piston rings, nozzles, plunger barrels etc.,, but never pay attention to the springs of delivery valve, injector.

    These springs when reduced its free length by 10% losses it’s sealing and damping action. This will drop the efficiency drastically and people keep changing the new components but do not focus on springs.

    This same applies to everywhere in compressor valves, injectors, deliver valves, cylinder head exhaust and inlet vales etc.,,

    Best of luck
    chary TSR

  13. I’ve been heavily interested in ships ever since I was a kid. The renewed interest now that I’m an adult is a result of watching a documentary about shipping disasters. One of the disasters shown in the show was the result of frequent blackouts in the ship. It’s interesting to know that could’ve been caused by fouled or damaged turbine because it will lead to a problem in the passage of exhaust gas or the amount of fresh air supplied in the ship. Perhaps the solution to the problem in the show could’ve been changing the gas turbine inlet silencers. Thanks for the informative article!

  14. DEAR.SIR
    I have an issue in our marine cat diesel generator model 3412C .V.TYPE .405 KW. 50HZ. 415V.
    during operation and diesel online once it has suddenly load like start main crane motor the blackout happens and diesel engine stop at same time , abnormal stop alarm on its monitor .
    we are going to check the diesel engine we have not any problem every thing was ok start diesel again and load it its ok but once again start main crane motor the blackout happened in all times when suddenly load only .
    please advice if any more check on engine or generator we can do it
    thanks in advance and your time

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