Basic knowledge of marine diesel engines

1. What are the consequences if the intake or exhaust valve of a diesel engine is stuck in the fully open position during operation?

If the intake valve is stuck in the fully open position during operation, it will cause a backflow of the combustible mixture, causing local overheating of the scavenge air box and a throbbing sound. If the exhaust valve is stuck in the fully open position, exhaust gas will be drawn into that cylinder and discharged as a mixture of exhaust gas and oil mist. As a result, that cylinder will no longer produce power. If the diesel engine's speed control mechanism is fixed, since one cylinder is no longer producing power, the other cylinders will automatically increase their fuel injection rate, causing overload. This will result in unstable engine operation and vibration. For two-stroke, direct-flow scavenged superchargers, this may cause surge.

It is therefore crucial for engine management personnel to constantly ensure the airtightness and smooth movement of the intake and exhaust valves.

2. What are the consequences of exhaust valve opening too late or too early?

Valve opening or closing too early or too late is often caused by inaccurate valve clearance. If the exhaust valve opens too late at the end of the expansion phase of the power stroke and just before exhaust begins, the upward piston back pressure at the start of the exhaust stroke will increase, increasing power consumption. This problem is often caused by excessive clearance between the roller and the cam. Therefore, if the valve opens too late, it will inevitably close prematurely. This premature closing of the exhaust valve will significantly increase cylinder pressure at the end of the exhaust stroke. Since the intake valve generally opens before the end of the exhaust stroke, exhaust gas cannot be completely discharged at this time, which will affect the intake process. In severe cases, exhaust gas backflow may occur, filling the intake duct and then being drawn back into the cylinder as intake is formed. This results in a mixture of exhaust gas and air in the cylinder at the end of the intake stroke, which worsens combustion.

If the exhaust valve opens too early, power will be lost, and due to high exhaust temperatures, the exhaust valve face may also be burned. If the clearance between the roller and the cam is too small, premature opening will inevitably result in late closing. If closing is too late, it can cause exhaust gas to backflow through the exhaust pipe and into the cylinder during the intake stroke, significantly adversely affecting combustion.

3. Why can't intake and exhaust valves be used interchangeably? Why is clearance required? What precautions should be taken regarding valve seats, springs, etc. during maintenance?

To adapt to operating conditions, intake and exhaust valves are made of different materials. The intake valve handles cooler air, while the exhaust valve handles hot, corrosive exhaust gas. Using an intake valve instead of an exhaust valve will inevitably damage it quickly.

Valve clearance allows for expansion of the valve stem due to heat. Excessive clearance can cause knocking and affect timing. Too little clearance can prevent the valve from closing due to thermal expansion, potentially burning the valve and valve seat. During inspection, check the conical valve face for grooves; the axis of the cone and the axis of the valve stem should be aligned; measure the clearance between the valve stem and its guide tube to ensure it is within the specified range; the valve stem should be straight, and the slot for the clamping block should be intact; the valve line should not be too wide. If it is too wide, use two reamers or grinding wheels at different angles to narrow it. The valve housing should be leak-proof, and the valve seat should not be deformed. During inspection, remove any sediment or scale from the cooling water chamber. After the above cleaning and inspection, the trimmed valve and valve seat can be ground. Finally, check that the two end faces of the valve spring are parallel and perpendicular to the spring centerline, ensuring that the original spring force is maintained and there are no cracks. Finally, clean and oil the valve and carefully reassemble it.

Valve lathing only requires lathing if the valve line grooves are deep, and the metal removed should be minimized. Lashing the valve disc too thin will render the valve useless.

4. What should be paid attention to when installing the inlet and exhaust valves? What are the consequences of improper installation?

Inlet and exhaust valves must ensure good airtightness and smooth movement during operation. Therefore, the following points should be carefully considered during inspection and reinstallation.

1. Ensure the integrity of the valve line: A properly functioning valve should have a faint, round, narrow valve line at the contact point with the valve seat. This line forms the metal-to-metal contact between the valve head and the valve seat. The airtightness of the inlet and exhaust valves depends on this metal-to-metal contact. Therefore, this line should be continuous and uniform in width. If the valve line is nicked by the gas, it should be ground and trimmed before reinstallation. Deep nicks on the valve head can be removed using a lathe. Deep nicks on the valve seat can be smoothed with a tapered reamer. However, these methods are mostly used by shipyards during ship repairs. Self-repairs by crew members typically use abrasive sand to grind the valve line. Grinding the valve is generally done first with coarse abrasive sand, followed by fine abrasive sand. Finally, carefully grind with a small amount of lubricating oil until a uniform, glossy, gray valve line appears between the valve and the seat.

2. Ensure smooth upward and downward movement (opening and closing) of the air valve: To ensure smooth opening and closing of the air valve, check the clearance between the valve stem and the air valve guide, remove any carbon deposits or grease, and eliminate any factors that could cause the valve stem to tilt or become stuck.

3. Inspect the air valve spring, clamp, and lock ring: Before reinstalling the air valve, carefully inspect the air valve spring for breakage or end misalignment. If any are not as required, replace them. When reinstalling the air valve, carefully install the clamp and lock ring on the valve stem to prevent misalignment and valve stem tilt. If the clamp or lock ring falls off, the air valve could fall into the cylinder, causing serious damage to the piston, cylinder head, or cylinder liner. Therefore, carelessness is essential.

4. Check the copper pad for integrity. If it cannot maintain an airtight seal, replace it.

In addition, when installing the air valve, ensure that all joints between the valve housing and valve sleeve are clean. Tools and debris must not be left in the cylinder. Only after you believe everything is normal can you reinstall it.