Basic knowledge of marine diesel engines （Ⅱ）

3. What are top dead center, bottom dead center, and stroke?

A: Top dead center refers to the highest position the piston reaches within the cylinder.

Bottom dead center is the opposite of top dead center, meaning it's the lowest position the piston reaches within the cylinder.

When the piston is at its highest (or lowest) position, the corresponding crankpin on the crankshaft also reaches its highest (or lowest) position. At this point, the piston pin, crosshead, crankpin, and crankshaft centerline are all in the same vertical plane. Even if fuel is injected into the upper cylinder to generate gas pressure, the alignment of these three points often makes it difficult to rotate the crankshaft. In single-cylinder diesel engines, this can cause the engine to "stuck." Therefore, the highest position of the piston's movement is usually called "top dead center," and the lowest position is called "bottom dead center." In reality, diesel engines often consist of multiple cylinders. When the piston of one cylinder is at top dead center or bottom dead center, adjacent cylinders may be moving downward, performing work. Rotational torque is still present on the crankshaft, which can drive the piston of a cylinder past top dead center (or bottom dead center). Therefore, the top and bottom dead centers of all cylinders are not actually "dead points."

In single-cylinder diesel engines, the inertial force stored in the flywheel drives the crankshaft to cause the piston to pass through top and bottom dead centers, so there are no true "dead points."

The top and bottom dead centers of a diesel engine are important criteria for adjusting cylinder injection timing. The position of each cylinder at top dead center can generally be found on the diesel engine's flywheel.

When a piston reciprocates up and down in the cylinder, from top dead center to bottom dead center, the distance the piston travels from bottom dead center to top dead center is called a stroke (or travel). The length of a stroke is exactly equal to the diameter of the diesel engine's crankpin during one rotation.

4. What are compression volume, displacement, total cylinder volume, and compression ratio?

Answer: Compression volume: The cylinder space between the piston and the cylinder head when the piston is at top dead center is called the compression volume or combustion chamber volume. It is usually represented by the symbol .

Working volume: The cylinder space displaced by the piston as it moves from top dead center to bottom dead center is called the working volume. It is usually represented by the symbol .

Total cylinder volume: The total cylinder volume above the piston head when the piston is at bottom dead center. It is usually represented by the symbol . The total cylinder volume is the sum of the compression volume and the working volume. That is:

Compression ratio: The ratio of the total cylinder volume to the compression volume. Commonly expressed:

Compression ratio is a key performance parameter of a diesel engine. A high compression ratio increases the pressure and temperature of the compressed air, making it easier for the fuel to ignite and burn, making the engine easier to start. Conversely, a low compression ratio makes it difficult to ignite and start the engine. Therefore, the compression ratio has a certain impact on the diesel engine's combustion, efficiency, starting performance, and mechanical load. Generally, large, low-speed diesel engines have a compression ratio of around 11 to 13. Non-turbocharged diesel engines tend to have a higher compression ratio, while turbocharged diesel engines tend to have a lower compression ratio.

Although the compression ratio of a diesel engine is determined by the engine manufacturer, bearing wear, piston assembly sinking, or improper cylinder head gasketing or connecting rod lower plate gasketing during maintenance can also cause the compression ratio to change, affecting engine performance.

5. What are compression pressure and maximum burst pressure?

Answer: Compression pressure: The pressure at which the air entering the cylinder is compressed by the piston to top dead center. Compression pressure varies depending on the engine model. Older engines generally have a range of approximately 3 to 6 MPa, while modern marine main and auxiliary engines typically exceed 12.26 MPa.

Compression pressure can be measured using a dynamometer while the diesel engine is operating normally, by stopping fuel supply to each cylinder.

Maximum burst pressure: At the beginning of the engine's operating stroke at rated power, intense fuel combustion causes the pressure in the cylinder to rise sharply. This pressure is called maximum burst pressure. Maximum burst pressure varies depending on the engine model. Older engines are generally around 4.91 to 7.81 MPa or higher. Newer engines are above 12.75 MPa.

Maximum burst pressure can be measured using a dynamometer or burst pressure gauge under normal diesel engine operating conditions.

Compression pressure measurement can generally determine whether the piston rings are properly sealed. Piston ring leakage often causes a drop in compression pressure. Maximum burst pressure measurement can generally determine whether the fuel supply system is functioning properly. If the maximum burst pressure suddenly drops, the fuel supply system for that cylinder may be faulty.