Everything you need to know about marine generator set

In recent years, diesel generator sets have been increasingly used on ships. In addition to being a conventional unit that provides marine electricity, it is also increasingly used for electric propulsion. Due to its unique advantages, the use of full or semi-electric propulsion has become the development trend of ships with more than thousands of tons, especially military ships. The diesel engine is mainly used as the prime mover of the generator set on the ship. The fuel cooler is very important for the diesel engine oil cooling of the diesel engine. The diesel engine cools the fuel through seawater to achieve the required performance of the diesel engine.

Working principle of diesel generator set

A marine diesel generator set manufacturers produces diesel engine used on ships. Its working principle is as follows:

A stream of fresh air is drawn or pumped into the engine cylinder, and then compressed to a high pressure by the moving piston. When the air is compressed, its temperature rises so that it can ignite the fine mist of fuel injected into the cylinder. The combustion of fuel adds more heat to the charged air, causing expansion and forcing the generator piston to do work on the crankshaft, which in turn drives the propeller of the transmission ship through other shafts.

The operation between two fuel injections is called a working cycle. In a four-stroke diesel engine, this cycle needs to be completed by four different strokes of the piston, namely suction, compression, expansion and exhaust. If we combine suction and exhaust with compression and expansion, a four-stroke engine becomes a two-stroke generator.

The two-stroke cycle begins when the piston rises from the bottom of its stroke (bottom dead center), when the intake port on the side of the cylinder is open. At this time, the exhaust valve is also opened, fresh air is filled into the cylinder, and the residual exhaust gas from the previous stroke is blown out through the opened exhaust valve. The valve blows out.

When the piston moves up to about one-fifth of its stroke in the morning, it closes the intake port and the exhaust valve closes, so the temperature and pressure rise to very high values.

When the piston reaches the top of its stroke (that is, the top dead center), the fuel valve injects fine mist of fuel into the high-temperature air in the cylinder, and the fuel burns immediately, and the heat quickly increases the pressure. In this way, the expanding gas forces the piston to move down during the power stroke.

When the piston moves down to a point past half of the stroke, the exhaust valve opens, and the high-temperature gas begins to flow out through the exhaust valve due to its own pressure, which is assisted by the fresh air entering through the air inlet. The air inlet opens as the piston moves further down. Then, another cycle begins again.

In a two-stroke engine, the crankshaft makes one power stroke per revolution, while a four-stroke engine requires two crankshaft revolutions to make a power stroke. This is why a two-stroke engine can do approximately twice as much as a four-stroke under the same size. The reason for the work done by the engine. In current actual use, the output power of a two-stroke engine with the same cylinder diameter and the same speed is about 80% higher than that of a four-stroke engine. This increase in engine power makes two-stroke generators widely used as the main engine of large ships.

Key specs for marine diesel generator set

Modern marine large-scale low-speed diesel engines have achieved more economic performance in the past decade than in the past few decades. Various energy-saving measures have appeared one after another and are becoming more and more perfect. These measures mainly include:

1. Adopt constant pressure turbocharging system and high efficiency exhaust gas turbocharger

The use of a constant-pressure turbocharging system instead of a pulse turbocharging system on a high-charge diesel engine is a significant feature of modern diesel engines, and it also helps to improve the efficiency of the lifter. The development and use of the new turbocharger has increased the efficiency of the turbocharger from 50-60% in the 1960s to 60-76%, which can significantly reduce the fuel consumption rate of diesel engines.

2. Increase stroke bore ratio S/D

This is to increase the proportion of diesel engines since the oil crisis. The main purpose of S/D is to greatly reduce the speed of the diesel engine while keeping the average piston speed Vm unchanged, so as to increase the efficiency of the propeller, thereby increasing the overall efficiency of the power plant. Therefore, since the late 1970s, S/D has grown rapidly, and the long-stroke and ultra-long-stroke diesel engine series of low-speed diesel engines have been gradually developed. The increase in S/D also makes the S/D value of MAN B&W’s SMC-C series diesel engine reached 4.0, while the S/D value of Wä rtsilä Swiss company’s SULZER RTA-T series diesel engine has even reached 4.17. However, the increase of S/D makes the structure of the diesel engine complicated and the cost increases, so the increase of S/D is limited.

3. Increase the ratio of the maximum burst pressure pz to the average effective pressure pe pz/pe

The theoretical cycle research and practice of diesel engines have proved that increasing pz/pe can significantly reduce fuel consumption. The study pointed out that when pz/pe is increased from 7.8 to 12, the fuel consumption rate can be reduced by about 12g/kW·h. Therefore, modern marine diesel engines adopt this measure to reduce fuel consumption. However, it is very difficult to increase pz by a large margin. It is limited by the load of the diesel engine, and corresponding measures must be taken to ensure the reliability of the diesel engine. Therefore, from the 1960s to the mid 1970s, although the pz of marine diesel engines gradually increased, the increase was not large (pz only increased by about 2.5MPa in the past 20 years). From the mid-1970s to the mid-1980s, the pz value of diesel engines has increased substantially, increasing by about 5MPa. The pz of some diesel engines has reached 15 MPa (such as Sulzer RTA engines), or even 18 MPa. Reducing the pe value while keeping pz unchanged can also reduce the fuel consumption rate, which is also a widely used energy-saving measure. Reducing pe is the use of diesel engine power reduction, such as maintaining the calibrated speed and choosing a lower pe (such as 80%), or choosing a lower pe when using a lower speed (such as 80%).