Basic knowledge of marine diesel engines(Ⅴ）

What are the characteristics of the ventilation processes in two-stroke and four-stroke diesel engines?

Answer: For a diesel engine to operate continuously, the exhaust gases from the previous cycle must be expelled and replaced with an appropriate amount of fresh air. This process of replacing exhaust gases with fresh air is called the diesel engine's ventilation process.

In a two-stroke diesel engine, the piston completes four processes: intake, compression, expansion, and exhaust within one reciprocating stroke—that is, within one crankshaft rotation. This ventilation process generally takes no more than 130° to 150° of crank angle. A four-stroke diesel engine completes these four processes within two crankshaft rotations. The intake and exhaust processes each take a little more than one piston stroke, so the entire ventilation process takes approximately 400° to 450° of crank angle. Clearly, the ventilation time of a two-stroke diesel engine is much shorter than that of a four-stroke engine, and it is not as thorough as a four-stroke engine. Furthermore, for most of the ventilation time, the intake and exhaust processes of a two-stroke diesel engine occur within two separate strokes. Therefore, two-stroke diesel engines are prone to gas mixing, causing some fresh air to be expelled with the exhaust. Therefore, it's impossible for a two-stroke diesel engine to charge fresh air into the cylinders solely by atmospheric pressure during ventilation. To ensure fresh air can enter the cylinders at the end of exhaust, the intake pressure must be increased, using the pressure difference between the intake and cylinders to draw air into the cylinders. This necessitates a scavenging pump for two-stroke diesel engines. Modern two-stroke engines are equipped with exhaust gas turbines and blowers for starting and low-speed operation to supply fresh air for ventilation. Even so, the ventilation quality of a two-stroke engine is generally inferior to that of a four-stroke engine.

What components make up the valve train in a diesel engine? How does it operate?

Answer: To ensure the timely opening and closing of the intake and exhaust valves, and to coordinate their opening and closing timing with the crankshaft angle and the upward and downward movement of the piston, a valve train is typically driven by the crankshaft to promptly actuate the valves. This transmission mechanism generally consists of gears or chains, a camshaft, rollers, a push rod cylinder, rocker arms, and rocker arm seats. Some newer diesel engines also have a hydraulic push rod mechanism, such as the MC and RTA series large engines.

The operating principle is as follows:

The camshaft, driven by gears or chains, rotates synchronously with the crankshaft (a four-stroke engine rotates at half the crankshaft speed). The cam on the camshaft pushes the roller, causing the push rod to move upward, lifting one end of the rocker arm, causing it to swing around its rocker pin. The other end of the rocker arm presses downward on the exhaust valve, opening it. When the roller descends along the descending edge of the cam, the exhaust valve is lifted and closed by the force of the valve spring. Using a roller at the lower end of the push rod reduces wear between the push rod and the cam. An adjusting bolt is located at one end of the rocker arm, which adjusts the clearance between the valve cylinder and the rocker arm.

What types of camshaft transmission mechanisms are there? Why is the camshaft drive in large, low-speed diesel engines mostly chain-driven?

Answer: The transmission between the crankshaft and camshaft can be summarized as follows: gear drive, chain drive, and gear-chain drive.Gear drive is mostly used in small engines. Large diesel engines often use chain drive, but some also use a gear-chain drive, where the first stage is a chain drive and the second stage is a gear drive. For example, the original domestically produced 7DSZ75/160 diesel engine uses this type of cam drive mechanism.

The reason why chain drive is often used in the camshafts of large diesel engines is because of its following key features:

1. It allows for a compact transmission mechanism;

2. It has a simple structure;

3. It is not very sensitive to variations in axial parallelism and center distance.

Of course, chain drive also has disadvantages such as rapid wear, easy loosening, which can affect camshaft timing, and potential vibration. Therefore, chain drives are usually equipped with idler pulleys and tensioner pulleys to reduce chain vibration and adjust chain tension. Regardless of the form of transmission mechanism used, a certain speed ratio should be maintained between the camshaft and the crankshaft: in a four-stroke diesel engine, the camshaft speed is half the crankshaft speed; in a two-stroke engine, the crankshaft drives the camshaft at a speed ratio of 1:1.