Case Study -1
The vessel main engine (6L67GFC) had suffered from high exhaust temperature on unit No.4 for several weeks. Following actions had been taken on unit No.4 but without improvement:
o Replaced exhaust valve with a ready spare.
o Replace fuel injector valves.
o Replace fuel pump suction valve.
o Port inspection through scavenging receiver reviewed that the cylinder was in reasonably good condition.
o Renewed the piston rings on the exhaust actuator and found the condition of the oil cylinder was OK.
o The engine speed was limited to 95 rpm due to the high exhaust temperature (about 460 degrees at 103 rpm).
o The indicator cards taken by the chief engineer shows that exhaust valve No.2 close and open too early (high exhaust temperature, high compression pressure and longer "tail" on the PV diagram).
o Based on the above information, we decided to check the fuel and exhaust timing on unit No.2 and 4.
The fuel and exhaust timing checked:
o Cylinder No. 2 & 4
o Exhaust valve open at (A) 7.5 mm lift 85 108.4
o Exhaust valve close at (B) 6.5 mm lift 226 249.5
o Exhaust cam lead (measured) 24.5 1.05
o Exhaust cam lead (shop trial record) 0.4 0.4
o Fuel cam lead (measured) 18.2 mm 18.4 mm
o Fuel cam lead (shop trial record) 18.5 mm 17.6 mm
o The fuel cam lead has no change, but the exhaust cam lead of the No.2 unit was in the position of about 24 degrees advanced.
o Sea trial has carried out after the correction of exhaust cam timing on No.2. The engine speed has increased to 110 rpm within a short period.
o It confirmed that the engine was running in normal condition again with a maximum exhaust temperature of 378 degrees at 110rpm.
The real reason behind the high exhaust temperature in unit No.4 was approved to be the wrong exhaust timing on unit No.2. This situation seems very strange. However, it can explain as follows.
The firing order of the engine is 1-5-3-4-2-6.
When the engine runs in an ahead direction unit No.2 fire right after unit No.4, the No.4 exhaust valve opens at 288.4 degrees and the scavenging port start opening at 319 degrees.
The exhaust gas flows out by itself between 288.4 and 319 degrees and start to be forced out by scavenging air between crank angle 319 and 41 degrees.
Due to the wrong exhaust timing, the No.2 exhaust valve opens early at 324.8 degrees instead of 349 degrees. During this time, the hot exhaust gas gash out from unit No.2 with higher pressure results in pressure pulsation of the exhausted receiver. During a narrow range, the scavenging angle of unit No.4, the exhaust pressure was even higher than the scavenging pressure. The consequence is that the breath of unit No.4 was incomplete, and poor combustion took place during the next cycle due to lack of air.
Despite the pressure fluctuation, the temperature (where exhaust thermometer exposes to) also fluctuated. Based on the same theory, the No. 4 exhaust thermometer exposed longer timing in the hot gas than other units.
Due to the early closing of the No.2 exhaust valve (Just 5 degrees 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 usually retarded as the engine normally runs ahead. However, the cam moved possibly in an advanced position if the engine started in an astern order while the incident took place.