Abstract: Diesel engines can output power during operation. In addition to the combustion chamber and crank connecting rod mechanism that directly convert the thermal energy of fuel into mechanical energy, they must also have corresponding mechanisms and systems to ensure their operation, and these mechanisms and systems are interconnected and coordinated. Different types and uses of diesel engines have different forms of mechanisms and systems, but their functions are basically the same. The diesel engine is mainly composed of body components and crank connecting rod mechanisms, valve distribution mechanisms and intake and exhaust systems, fuel supply and speed control systems, lubrication systems, cooling systems, starting devices and other mechanisms and systems.
1、 Composition and component functions of diesel engines
Diesel engine is a type of internal combustion engine, which is an energy conversion device that converts the heat energy released from fuel combustion into mechanical energy. The diesel engine is the power part of the generator set, generally composed of crankshaft connecting rod mechanism and body components, valve distribution mechanism and intake and exhaust system, diesel supply system, lubrication system, cooling system, and electrical system.
1. Crankshaft connecting rod mechanism
In order to convert the obtained thermal energy into mechanical energy, it is necessary to complete it through a crankshaft connecting rod mechanism. This mechanism is mainly composed of components such as pistons, piston pins, connecting rods, crankshafts, and flywheels,. When fuel ignites and burns in the combustion chamber, the expansion of the gas generates pressure at the top of the piston, pushing the piston to move back and forth in a straight line. With the help of the connecting rod, the crankshaft rotates to drive the working machinery (load) to do work.
2. Body group
The body components mainly include the cylinder block, cylinder head, and crankcase. It is the assembly matrix of various mechanical systems in diesel engines, and many parts of it are components of diesel engine crank and connecting rod mechanisms, valve distribution mechanisms and intake and exhaust systems, fuel supply and speed control systems, lubrication systems, and cooling systems. For example, the cylinder head and piston crown together form a combustion chamber space, and many parts, intake and exhaust ducts, and oil passages are also arranged on it.
3. Valve distribution mechanism
For a device to continuously convert thermal energy into mechanical energy, it must also be equipped with a set of air distribution mechanisms to ensure regular intake of fresh air and discharge of combustion waste gas.
The valve train is composed of a valve group (intake valve, exhaust valve, valve guide, valve seat, and valve spring, etc.) and a transmission group (tappet, tappet, rocker arm, rocker arm shaft, camshaft, and timing gear, etc.). The function of the valve train is to timely open and close the intake and exhaust valves according to certain requirements, exhaust the exhaust gas in the cylinder, and inhale fresh air, ensuring the smooth process of diesel engine ventilation.
4. Fuel system
Thermal energy must provide a certain amount of fuel, which is sent into the combustion chamber and fully mixed with air to generate heat. Therefore, there must be a fuel system.
The function of the diesel engine fuel supply system is to inject a certain amount of diesel into the combustion chamber at a certain pressure within a certain period of time, and mix it with air to do combustion work. It mainly consists of a diesel tank, fuel transfer pump, diesel filter, fuel injection pump (high-pressure oil pump), fuel injector, speed controller, etc.
5. Cooling system
In order to reduce the friction loss of diesel engines and ensure the normal temperature of various components, diesel engines must have a cooling system. The cooling system should consist of components such as a water pump, radiator, thermostat, fan, and water jacket.
6. Lubrication system
The function of the lubrication system is to deliver lubricating oil to the friction surfaces of various moving parts of the diesel engine, which plays a role in reducing friction, cooling, purifying, sealing, and rust prevention, reducing friction resistance and wear, and taking away the heat generated by friction, thereby ensuring the normal operation of the diesel engine. It mainly consists of an oil pump, oil filter, oil radiator, various valves, and lubricating oil passages.
7. Start the system
In order to quickly start the diesel engine, a starting device is also required to control the start of the diesel engine. According to different starting methods, the components equipped with the starting device are usually started by electric motors or pneumatic motors. For high-power generator sets, compressed air is used for starting.
2、 The working principle of a four stroke diesel engine
In the thermal process, only the expansion process of the working fluid has the ability to do work, and we require the engine to continuously generate mechanical work, so we must make the working fluid expand repeatedly. Therefore, it is necessary to try to restore the working fluid to its initial state before expanding. Therefore, a diesel engine must go through four thermal processes: intake, compression, expansion, and exhaust before it can return to its initial state, allowing the diesel engine to continuously generate mechanical work. Therefore, the above four thermal processes are called a working cycle. If the piston of a diesel engine completes four strokes and completes one working cycle, the engine is called a four stroke diesel engine.
1. Intake stroke
The purpose of the intake stroke is to inhale fresh air and prepare for fuel combustion. To achieve intake, a pressure difference should be formed between the inside and outside of the cylinder. Therefore, during this stroke, the exhaust valve closes, the intake valve opens, and the piston moves from top dead center to bottom dead center. The volume in the cylinder above the piston gradually expands, and the pressure decreases. The gas pressure in the cylinder is about 68-93kPa lower than atmospheric pressure. Under the action of atmospheric pressure, fresh air is sucked into the cylinder through the intake valve. When the piston reaches the bottom dead center, the intake valve closes and the intake stroke ends.
2. Compression stroke
The purpose of the compression stroke is to increase the pressure and temperature of the air inside the cylinder, creating conditions for fuel combustion. Due to the closed intake and exhaust valves, the air in the cylinder is compressed, and the pressure and temperature also increase accordingly. The degree of increase depends on the degree of compression, and different diesel engines may have slight differences. When the piston approaches the top dead center, the air pressure in the cylinder reaches (3000-5000) kPa and the temperature reaches 500-700 ℃, far exceeding the self ignition temperature of diesel.
3. Expansion stroke
When the piston is about to end, the fuel injector begins to inject diesel into the cylinder, mixing it with air to form a combustible mixture, and immediately self ignites. At this time, the pressure inside the cylinder quickly rises to about 6000-9000kPa, and the temperature reaches as high as (1800-2200) ℃. Under the thrust of high-temperature and high-pressure gases, the piston moves down to the dead center and drives the crankshaft to rotate, doing work. As the gas expansion piston descends, its pressure gradually decreases until the exhaust valve is opened.
4. Exhaust stroke
4. Exhaust stroke
The purpose of the exhaust stroke is to remove exhaust gas from the cylinder. After the power stroke is completed, the gas in the cylinder has become exhaust gas, and its temperature drops to (800~900) ℃ and pressure drops to (294~392) kPa. At this point, the exhaust valve opens while the intake valve remains closed, and the piston moves from bottom dead center to top dead center. Under the residual pressure and piston thrust in the cylinder, the exhaust gas is discharged outside the cylinder. When the piston reaches the top dead center again, the exhaust process ends. After the exhaust process is completed, the exhaust valve closes and the intake valve opens again, repeating the next cycle and continuously working externally.
3、 Classification and characteristics of diesel engines
A diesel engine is an internal combustion engine that uses diesel as fuel. Diesel engines belong to compression ignition engines, which are often referred to as Diesel engines after their main inventor, Diesel. When a diesel engine is working, it draws in air from the cylinder and is compressed to a high degree due to the movement of the piston, reaching a high temperature of 500-700 ℃. Then, the fuel is sprayed into high-temperature air in a mist form, mixed with the high-temperature air to form a combustible mixture, which automatically ignites and burns. The energy released during combustion acts on the top surface of the piston, pushing it and converting it into rotating mechanical work through the connecting rod and crankshaft.
1. Diesel engine type
(1) According to the working cycle, it can be divided into four stroke and two-stroke diesel engines.
(2) According to the cooling method, it can be divided into water-cooled and air-cooled diesel engines.
(3) According to the intake method, it can be divided into turbocharged and non turbocharged (naturally aspirated) diesel engines.
(4) According to speed, diesel engines can be divided into high-speed (over 1000 rpm), medium speed (300-1000 rpm), and low-speed (less than 300 rpm).
(5) According to the combustion chamber, diesel engines can be divided into direct injection, swirl chamber, and pre chamber types.
(6) According to the mode of gas pressure action, it can be divided into single acting, double acting, and opposed piston diesel engines.
(7) According to the number of cylinders, it can be divided into single cylinder and multi cylinder diesel engines.
(8) According to their usage, they can be divided into marine diesel engines, locomotive diesel engines, vehicle diesel engines, agricultural machinery diesel engines, engineering machinery diesel engines, power generation diesel engines, and fixed power diesel engines.
(9) According to the fuel supply method, it can be divided into mechanical high-pressure oil pump fuel supply and high-pressure common rail electronic control injection fuel supply.
(10) According to the arrangement of cylinders, it can be divided into straight and V-shaped arrangements, horizontally opposed arrangements, W-shaped arrangements, star shaped arrangements, etc
(11) According to the power level, it can be divided into small (200KW), medium (200-1000KW), large (1000-3000KW), and large (3000KW and above).
2. Characteristics of diesel engines for power generation
Diesel generator sets are powered by diesel engines. Compared with common power generation equipment such as thermal power generators, steam turbine generators, gas turbine generators, nuclear power generators, etc., they have the characteristics of simple structure, compactness, small investment, small footprint, high thermal efficiency, easy starting, flexible control, simple operating procedures, convenient maintenance and repair, low comprehensive cost of assembly and power generation, and convenient fuel supply and storage. Most diesel engines used for power generation are variants of general-purpose or other purpose diesel engines, which have the following characteristics:
(1) Fixed frequency and speed
The frequency of AC power is fixed at 50Hz and 60Hz, so the speed of the generator set can only be 1500 and 1800r/min. China and former Soviet power consuming countries mainly use 1500r/min, while European and American countries mainly use 1800r/min.
(2) Stable voltage range
The output voltage of diesel generator sets used in China is 400/230V (6.3kV for large generator sets), with a frequency of 50Hz and a power factor of cos ф= 0.8.
(3) The range of power variation is wide.
The power of diesel engines used for power generation can vary from 0.5kW to 10000kW. Generally, diesel engines with a power range of 12-1500kW are used as mobile power stations, backup power sources, emergency power sources, or commonly used rural power sources. Fixed or marine power stations are commonly used as power sources, with a power output of tens of thousands of kilowatts.
(4) Has a certain power reserve.
Diesel engines for power generation generally operate under stable operating conditions with high load rates. Emergency and backup power sources are generally rated at 12h power, while commonly used power sources are rated at continuous power (the matching power of the generator set should deduct the transmission loss and excitation power of the motor, and leave a certain power reserve).
(5) Equipped with a speed control device.
To ensure the stability of the output voltage frequency of the generator set, high-performance speed control devices are generally installed. For parallel operation and grid connected generator sets, speed adjustment devices are installed.
(6) It has protection and automation functions.
Summary:
(7) Due to the main use of diesel engines for power generation being as backup power sources, mobile power sources, and alternative power sources, market demand has been increasing year by year. The construction of the State Grid has achieved great success, and the power supply has basically achieved nationwide coverage. In this context, the application of diesel engines for power generation in China’s market is relatively limited, but they are still indispensable for the development of the national economy. With the continuous development of manufacturing technology, automatic control technology, electronic technology, and composite material manufacturing technology worldwide. Diesel engines for power generation are developing towards miniaturization, high power, low fuel consumption, low emissions, low noise, and intelligence. The continuous progress and updates of related technologies have improved the power supply guarantee capability and technical level of diesel engines for power generation, which will greatly promote the continuous enhancement of comprehensive power supply guarantee capabilities in various fields.
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Post time: Apr-02-2024