Case Study

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There was a vessel suffered from high exhaust temperature on unit No.4 for several weeks. The problem was started right after the replacement of exhaust valve No.2.

Following actions had been taken on unit No.4 but without improvement:

  • Replaced exhaust valve with ready spare.
  • Replace fuel injector valves.
  • Replace fuel pump suction valve.
  • Port inspection through scavenging receiver reviewed that the cylinder was in reasonable good condition.
  • Renewed the piston rings on the exhaust actuator and found the condition of oil cylinder was OK.
  • The engine speed had been limited to 95 rpm due to the high exhaust temperature (about 460 degree at 103 rpm).
  • The indicator cards taken by chief engineer shows that the exhaust valve No.2 close and open too early (high exhaust temperature, high compression pressure and longer "tail" on the PV diagram).
  • Based on the above information provided, It was decided to check the fuel and exhaust timing on unit No.2 and No.4.

The fuel and exhaust timing was checked:

Cylinder No. 2 4
Exhaust valve open at (A) 7.5 mm lift 85 108.4
Exhaust valve close at (B) 6.5 mm lift 226 249.5
Exhaust cam lead (measured) 24.5 1.05
Exhaust cam lead (shop trial record) 0.4 0.4
Fuel cam lead (measured) 18.2 mm 18.4 mm
Fuel cam lead (shop trial record) 18.5 mm 17.6 mm

It was very clear that the fuel cam lead has no change but the exhaust cam lead of No.2 unit was in the position of about 24 degrees advanced.

Sea trial was carried out after the correction of exhaust cam timing on No.2. The engine speed has increased to 110 rpm within short period of time.

It was confirmed that the engine was running in normal condition again with maximum exhaust temperature of 378 degree at 110 rpm.

Lesson Learned

The real reason behind the high exhaust temperature in unit No.4 was approved to be wrong exhaust timing on unit No.2. This situation seems very strange however, it can be explained as following.

The firing order of the engine is 1-5-3-4-2-6.

When the engine running in ahead direction unit No.2 fire right after unit No.4. The No.4 exhaust valve open at 288.4 degree and scavenging port start opening at 319 degree.

The exhaust gas flows out by it self between 288.4 and 319 degree and start to be forced out by scavenge air between crank angle 319 and 41 degree.

Due to the wrong exhaust timing, the No.2 exhaust valve open early at 324.8 degree instead of 349 degree. During this time the hot exhaust gas gash out from unit No.2 with higher pressure results in pressure pulsation the exhaust receiver which during a narrow range the scavenging angle of unit No.4 the exhaust pressure was even higher than scavenging pressure. The consequence is that the breath of unit No.4 was incomplete and bad combustion took place during next cycle due to lack of air.

Despite the pressure fluctuation, the temperature (where exhaust thermometer expose to) was also fluctuates. Based on the same theory, the No. 4 exhaust thermometer was exposed longer timing in the hot gas compared to other units.

Due to early closing of No.2 exhaust valve (Just 5 degree after the scavenging port closed), the compression pressure of unit No.2 was very high (56 bar instead of 45-47 bar in other units).

When the exhaust or fuel cam shifted, it is normally retarded as the engine normally running in ahead direction. However, it is possible that the cam shifted in advanced position if the engine was started in astern direction while the incident took place.

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