Basic knowledge of marine diesel engine

1. What is a diesel engine?

A diesel engine is a type of internal combustion engine.

In a diesel engine, diesel is not ignited by an external fire source, but by the movement of the piston. First, the external air is sucked into the cylinder, and then the air is compressed by the movement of the piston, which greatly increases the temperature and pressure and reaches the ignition temperature of the fuel. At this time, the diesel is directly sprayed into the cylinder in the form of atomization through the fuel injector. When the atomized oil encounters the high-temperature compressed air, it immediately ignites and burns. After the diesel burns, a large amount of heat energy is released. At this time, the temperature and pressure of the gas in the cylinder rise sharply, which pushes the piston to do work. In this way, as long as each cylinder is sprayed with fuel and ignited in turn, and the pistons of some cylinders are always driven by the gas pressure to drive the crankshaft to rotate continuously, it can ensure that the diesel engine continuously outputs mechanical energy. Therefore, in essence, a diesel engine is a power machine that converts the thermal energy of fuel into mechanical energy. Because it is not like a steam engine, which first lets the fuel burn in the boiler to produce steam, and then introduces the steam into the cylinder to do work, but directly sprays the fuel into the cylinder, and only undergoes one energy conversion, so that it burns and does work in the cylinder, so it is also called an internal combustion engine.

2. What is the main difference between a diesel engine and a gasoline engine?

The main difference is in the fuel delivery and ignition combustion.

The gasoline engine converts gasoline into gasoline vapor in the carburetor and mixes it with an appropriate amount of air, and then delivers it into the cylinder by the suction force of the machine's intake stroke. After that, the gasoline and air mixture is compressed (compressed to 1/5 to 1/7 of the original volume) through the upward movement of the piston. At this time, the matched electric spark plug ignites, causing the mixed gas to explode and burn, pushing the piston to do work. The diesel engine only inhales and compresses air, and the compression degree is as high as 1/12 to 1/18 of the original volume, and the compression temperature is as high as 600 to 700℃; at this time, the injector sprays atomized fuel into the cylinder on time, and it is compressed and ignited, pushing the piston to do work.

From the above description, it can be seen that the main differences between gasoline engines and diesel engines are:

1. Gasoline engine fuel must be vaporized before entering the cylinder, so gasoline with a very low volatility point must be used as fuel. Diesel engines directly spray fuel into the cylinder and rely on compression temperature to ignite spontaneously.

2. Another feature of gasoline engines is that the compression pressure is lower than that of diesel engines, and spark plugs are used for ignition and combustion. This is mainly because gasoline has poor anti-explosion performance and cannot be compressed with a large compression ratio.

For gasoline engines, spark plugs must be installed to ignite at a certain angle before the top dead center. In this way, a set of high-voltage electric ignition coils and distributors that circulate electricity to each cylinder in turn must be installed on the gasoline engine. Diesel engines do not require electric ignition, so ignition coils and distributors are omitted. However, diesel engines must pressurize the fuel to a very high pressure and spray it into the cylinder. Therefore, although diesel engines have omitted ignition coils and distributors, they must be equipped with a set of high-pressure oil pumps, injectors and other equipment.