DOOSAN-MANB&W, 5S50ME-B9.3-TII The main engine is a transitional product between MC-C and ME-C. This engine retains the exhaust valve cam and air distributor. ME-V Hydraulic Timing Unit (i.e., the exhaust valve timing control unit). This article mainly shares, through a case study, an understanding of the significance of the ME-V valve and exhaust valve timing control.
1. Symptoms of a ME-B main engine fault on a certain vessel:
When the main engine load rises above 55%, individual cylinders' ME-V valves often fail to actuate, causing the exhaust valve of that cylinder to open for a shorter time than the other cylinders. As a result, combustion in that cylinder is incomplete, and its explosion pressure is much lower than that of normally working cylinders, causing unbalanced output power among cylinders and seriously affecting the normal operation of the main engine.
2. Understanding the ME-V valve working principle:
1. Constant high-pressure oil from the HPS unit continuously acts on the reset piston of the ME-V valve, while high-pressure oil from the ELFI valve acts on the lower part of the ME-V valve's actuating piston. Since the lower piston area is larger than the upper piston area, by the differential piston principle, when the ELFI actuates, the ME-V valve's actuating piston moves upward, pushing oil from the upper chamber into the exhaust valve high-pressure oil pipe. At the same time, upon reaching the top, it closes the oil port shown in the figure (port A in the diagram) to hinder the closing of the exhaust valve. This appropriately prolongs the closing time, making the gas exchange process more complete and improving diesel engine economy. A feedback probe senses the piston movement and transmits a 4-20mA current signal to the CCU.
2. It starts to actuate during astern operation and ahead operation above 55% load: its main purpose is to delay the closing of the exhaust valve for more thorough exhaust (equivalent to increasing the valve overlap angle), so that more fresh air enters the cylinder and the combustion quality of the mixture in the cylinder is improved.
3. If the ME-V fails, the Pcomp/Pscav ratio increases (the exhaust valve closes early, causing compression pressure to rise), and the diesel engine's compression pressure at actual load increases by 10-30 bar, but the peak pressure generally does not increase because the ECS automatically delays the injection timing, reducing the pressure difference between Pmax-Pcomp.
4. The ME-V valve can only affect the closing timing of the exhaust valve, not its opening timing.
3. Fault cause investigation and handling:
1. After the engine manufacturer measured the oil pressure variations at various points of the ME-V valve, it was found that the ME-V valve control system had a defect. After the main engine ran astern, the ME-V control system parameters could not return to 0, and thus were affected during ahead operation, causing the ME-V activation control signal to end too early. This cut off the piston driving oil pressure of the timing unit prematurely, and the ME-V valve could not be activated normally.
2. The reset piston of the ME-V valve was sticking, causing the MEV
valve to fail to work normally.
3. The control program was modified to solve the problem of the signal ending too early; the piston clearance was increased to prevent sticking; and the problem was ultimately solved.
4. Experience sharing:
1. Failure of the MEV will not affect the safe operation of the main engine, but load fluctuations may occur during acceleration. When accelerating from55%to70%, if an alarm occurs, it can be cleared by the following methods:
1)manual decrease of Pcomp/Pscav ratio to maximum negative value on selected unit by adjusting the MOP / process adjustment / Cylinder press / Pcomp/Pscav Offset.
2) Manual increase of the hydraulic pressure set-point by 30 bar, by adjusting the MOP / auxiliaries Hydraulic System / setpoint with the HPS mode set to Manual.
2. During routine checks, pay attention to the MOP panel where ME-V ON and OFF alternate, and note that the current value is within the range of4-20mA.
3. It is advisable to keep a spare ME-V valve position sensor on board.
