Basic knowledge of marine diesel engines （Ⅳ）

1. What are rated power and continuous power?

Answer: Rated power (or rated power) is the power (i.e., the selling power, not the long-term operating power) determined by the machinery department or relevant unit during a test run of a machine. This is the rated power indicated on the nameplate. When a diesel engine undergoes full-load speed characteristic testing on a factory test bench, certain conditions are used as the rated operating conditions, such as a nominal atmospheric pressure of 133.322 Pa, an ambient temperature of 27°C, a cooling water temperature of 27°C, and a relative humidity of 60%. The rated power is measured under these conditions. However, when a diesel engine is installed on a ship, the ambient temperature is higher. Furthermore, the rated power values measured on the test bench are often measured under conditions of low exhaust backpressure and sufficient air volume, and are not affected by factors such as ship fouling and various navigation conditions. Therefore, the power output is higher. If a diesel engine installed on a ship is operated at rated power for an extended period, it will inevitably be overloaded. To ensure reliable long-term operation under a variety of practical operating conditions and to provide a certain amount of reserve power when necessary, the maximum power actually used by a diesel engine as a marine main engine should be slightly lower than its rated (or "continuous") power. This maximum power actually used, or the maximum output power permitted for long-term continuous operation, is called continuous power.

Continuous power is generally approximately 90% of the rated (or "continuous") power.

2. What is mechanical efficiency? What factors influence mechanical efficiency?

Answer: The mechanical efficiency of a diesel engine refers to the ratio of the effective power output from the flywheel to the indicated power output from the cylinder. It is generally expressed as a percentage and is essentially a factor that discounts the indicated power.

Why is this discount necessary? This is because the power transmission process involves the following losses:

①. Friction loss: This is the surface friction loss of the various moving parts of a diesel engine. Experimental results show that friction loss between the piston, piston ring, and cylinder is the greatest, accounting for approximately 55% to 65% of all friction losses, while friction loss in the bearings accounts for approximately 35% to 45%.

②. Losses caused by driving auxiliary machinery: To ensure normal operation, a diesel engine must drive the high-pressure oil pump, oiler, air distributor, valve train, scavenging pump, and other auxiliary mechanisms, which consumes some power.

③. Other losses: In a non-supercharged four-stroke diesel engine, the intake, exhaust, and compression cycles require the crankshaft to push the pistons up and down, which also consumes some power.

3. How does a four-stroke diesel engine work?

A: Any type of diesel engine must undergo four cycles of intake, compression, combustion, expansion, and exhaust during operation. If these four cycles are completed during the piston's four reciprocating strokes, it is called a four-stroke diesel engine.

4. How does a two-stroke diesel engine work?

A: During operation, the crankshaft completes one revolution, and the cylinders complete the four cycles of intake, compression, combustion, expansion, and exhaust. This means that each reciprocating stroke of the piston completes one cycle. This type of diesel engine is called a two-stroke diesel engine.

5. What are the characteristics of an exhaust gas turbocharged diesel engine? Answer: To improve diesel engine combustion, turbocharging technology is currently being incorporated into designs both domestically and internationally. Its operating principle is shown in Figure 3.

As mentioned earlier, the performance of a diesel engine depends largely on whether the fuel injected into the cylinder is completely and promptly burned. The more completely the fuel burns, the higher the engine's thermal efficiency and the greater the power output. However, the air entering the cylinder is essential for proper fuel combustion. After determining the piston stroke and the size of the intake port, the quality of the cylinder charge becomes a crucial factor influencing combustion. The density of fresh air is related to its temperature and pressure: the lower the air temperature and the higher the pressure, the higher its density. For a given cylinder volume, naturally, the more dense air is drawn in to increase the intake volume, which is beneficial for efficient diesel combustion. Therefore, most diesel engines currently utilize a waste turbine to drive a coaxial compressor to initially compress the fresh air entering the cylinder. This is the origin of the term "supercharging." In addition to supercharging the fresh air, the supercharged fresh air is typically cooled to increase its density. In this way, by introducing high-quality air that has been pressurized and cooled into the cylinder, more oil can be sprayed each time the fuel is injected, and better combustion of the fuel can be ensured, thereby greatly improving the power and economy of the diesel engine.