Principle and Composition of Diesel Engine and Generator

A diesel engine is an engine that burns diesel to obtain energy, while a diesel generator set is a power generation equipment that uses diesel as the main fuel. It uses the diesel engine as the motive force to drive the generator to generate electricity and convert kinetic energy into electrical energy and thermal energy. Its structural composition includes a chassis, a water tank, a diesel engine, a generator and a control cabinet on the generator. In this article, Cummins introduces the working principle of the diesel engine and the components of the synchronous alternator.

1. Working principle of diesel engine

The working process of a diesel engine is actually the same as that of a gasoline engine. Each working cycle also goes through the four strokes of intake, compression, power and exhaust. The work principle is shown in Figure 1.

1. Intake stroke

The working fluid entering the cylinder is pure air. Since the resistance of the diesel engine’s intake system is small, the intake end pressure pa= (0.85~0.95)p0 is higher than that of the gasoline engine. The intake end temperature Ta=300~340K, which is lower than that of gasoline engines.
The task of intake air is to fill the cylinder with fresh air. When the intake stroke begins, the piston is at top dead center and there is still some exhaust gas left in the combustion chamber inside the cylinder. When the crankshaft rotates, the connecting rod moves the piston from top dead center to bottom dead center. At the same time, the transmission mechanism connected to the crankshaft is used to open the intake valve. As the piston moves downward, the volume above the piston in the cylinder gradually increases: causing the air pressure in the cylinder to be lower than the pressure in the intake pipe, so outside air continues to fill the cylinder.

2. Compression stroke

Since the compressed working fluid is pure air, the compression ratio of a diesel engine is higher than that of a gasoline engine (generally ε=16~22). The pressure at the end of compression is 3000~5000kPa, and the temperature at the end of compression is 750~1000K, which greatly exceeds the auto-ignition temperature of diesel (about 520K).

3. Power stroke

When the compression stroke is near the end, under the action of the high-pressure oil pump, the diesel oil is sprayed into the cylinder combustion chamber through the fuel injector at a high pressure of about 10MPa, and it will ignite and burn by itself after mixing with air in a short period of time. The pressure of the gas in the cylinder rises rapidly, reaching a maximum of 5000~9000kPa, and the maximum temperature reaches 1800~2000K. Because the diesel engine ignites and burns by itself through compression, it is called a compression ignition engine.

4. Exhaust stroke

The exhaust of a diesel engine is basically the same as that of a gasoline engine, except that the exhaust temperature is lower than that of a gasoline engine. Generally Tr=700~900K. For a single-cylinder engine, its speed is uneven, the engine is not working smoothly, and the vibration is large. This is because only one of the four strokes does work, and the other three strokes consume power in preparation for doing work. To solve this problem, the flywheel must have a sufficiently large moment of inertia, which in turn leads to an increase in the overall engine mass and size. The use of multi-cylinder engines can make up for the above shortcomings. Modern cars mostly use four-cylinder, six-cylinder and eight-cylinder engines.
Modern engines are basically supercharged. Compared with naturally aspirated engines (the different principles are shown in Figure 2), turbocharged engines have added a turbocharger (blower) and an intercooler. The purpose is to Reduce the intake air temperature and increase the intake air volume. In addition, in order to delay heat dissipation after the engine is turned off and prevent the high-temperature turbocharger from being ablated, modern turbocharged engines also add an independent electronically controlled cooling system.

1. Generator Principle

An AC synchronous generator is a generator that operates synchronously with the power network and uses the mutual induction of the magnetic field between the rotor and the stator to generate electrical energy. Its working principle is as follows:
(1) The rotor rotates under the drive of the generator, and at the same time generates an electric field through the power supply to form a magnetic field.
(2) The coils on the stator also generate a magnetic field. The magnetic field between the rotor and the stator interacts, so that the electrons in the coils on the stator are moved by the electromagnetic force (as shown in Figure 3)
(3) These moving electrons generate an electric current in the stator coil, that is, alternating current (the circuit is shown in Figure 4). This alternating current is sent to the power network through the output port.
(4) The control system of the generator keeps the output voltage and frequency of the generator synchronized with the power network by adjusting the size and frequency of the rotor driving power. AC synchronous generators have the advantages of stable output voltage and frequency and high reliability, so they are widely used in power systems.

(1) Stator base and winding

stator base are welded design. The stator core is located in the center of the stator base and cannot rotate and move.
The stator winding is a double-layer coil, and the insulation class is H class. Insulation is made by a special manufacturing process, and the insulating material synthesized by mica and enamel is impregnated with casting resin. This can make the motor have high dielectric strength, moisture resistance, gas and water vapor resistance, high strength and long life.

(2) Rotor and winding

①The generator shaft is usually designed as a double bearing with a cylindrical shaft extension. The motor shaft is flanged.
② The rotor core of the main engine is installed on the shaft and pressed axially. Excitation windings and damping windings are also installed on the shaft. The damping winding strips are located in the slots of the rotor core and welded into a ring shape.
③ The fixed hub of the rectifier is installed between the two iron core layer groups on the shaft.
④ The rotor is dynamically balanced.

(3) AC exciter

The AC exciter is a rotating armature, three-phase, synchronous generator. Rotating armature generators are different from ordinary generators in that the stator and rotor are in the opposite relationship. The armature is installed at the non-connected end of the shaft to output alternating current, and the output of the static excitation device is connected to the excitation winding installed at the fixed end as shown in Figure 2.

(4) Rotary rectifier

The rotating rectifier is a silicon controlled rectifier, and the three-phase full-wave rectifier circuit shown in Figure 2 is installed on the rotor shaft. Mounting screw sizes are between 4.5Nm and 5.5Nm and contact screw sizes are between 2.5Nm and 3.5Nm.

(5) Excitation system

The excitation device and thyristor voltage regulator are combined into a THYRIPART (thyristor shunt) excitation system. The excitation system provides an excitation current that varies with load and is slightly higher than the rated voltage produced. The regulator reduces the field current when necessary to obtain a constant generator voltage. This load-dependent excitation system (complex excitation) enables the generator to have excellent dynamic response during a loaded short circuit.

(6) Shaft

The generator shaft should be made of cast iron approved by the classification society and designed to ensure sufficient strength when connected to the engine.

(7) Bearings

In accordance with the requirements of the designed operating conditions in the ordering specifications, the generator can be equipped with rolling bearings or sliding bearings with or without forced lubrication.

(8) Cooling fan

To ensure the required cooling air ventilation, a fan of cast iron or welded steel plate structure must be provided. Regarding the installation location, in any case, it needs to be installed on the engine side. This is a one-way ventilation system. The extracted cooling air enters the generator from the non-drive end, and the exhaust gas is discharged from the drive end.

(9) DE/NDE end protection

The shields at both ends of the generator are made into flat plates so that the bearings and shaft extension can meet the special structural type. The magnetic poles of the exciter are regularly distributed and fixed on the pole yoke ring with bolts. The pole yoke ring is welded to the non-driving end cover.

(10) Insulation method to prevent shaft current generation

① Bearing insulation Any connection to the bearing must be insulated to prevent bearing currents.
② In order to prevent the unbalanced reluctance of the magnetic circuit from generating shaft current, an insulator as shown in Figure 3 is installed at the non-driving end. Shaft voltage is a high frequency voltage, usually 1 volt or less and rarely produced in the several volt range.
③When there is shaft current, the shaft and journal will be damaged. In the worst case, dark spots can develop within minutes. Damage to localized oil thinning increases the likelihood of burnout.
④When disassembling and installing, be sure to measure the insulation resistance. The insulation resistance value is 1-3MQ to meet the requirements. Generally speaking, the shaft voltage of bearings is limited to the following range: <300mV Harmless 500-1000mV Harmful shaft current may occur o>1000mV Bearings may be damaged within one week to one year (unless insulation measures are taken)

(11) Thermometer

A mercury thermometer is installed to detect the bearing temperature.

(12) Anti-condensation heater

To prevent moisture and condensation when the generator is not running, an anti-condensation heater is installed in the stator of the generator. The anti-condensation heater can be easily removed from the outside of the generator housing. The heater is a nickel-chromium-iron alloy stainless shell, and the inside of the shell is filled with insulators and is U-shaped.

(13) Air/Water Cooler

If required, the generator can be supplied with a top-mounted air-water cooler upon special request. Chillers can use fresh or sea water and can be double piped. Due to the closed-circuit cooling system, the protection level of the generator is also raised from IP23 to IP44 and IP54. The electrical principle of the generator remains the same. If the cooling water system fails or the cooling element fails, the generator can be easily turned into an open circuit air cooling state for emergency operation.

(14) Terminal box

The connection of the excitation terminals +F1, -F2 of the terminals (U, V, W) of the main cable in the terminal box can be on the left or right side of the generator as required. The cable entry plate can be undrilled or drilled with stuffing box as required.

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