**Longhong Electromechanical** 

– A diesel engine is a type of internal combustion engine, a device that converts the thermal energy released from fuel 

combustion into mechanical energy. The diesel engine serves as the power unit of a generator set, 

typically consisting of the crankshaft and connecting rod mechanism along with the engine block assembly, 

valve train and intake-exhaust system, diesel fuel supply system, lubrication system, cooling system, and electrical system, among others. 

While the overall structure of a diesel engine generally includes these major systems, 

the specific design varies depending on factors such as the number of cylinders, cylinder arrangement, and cooling method.

**Working Principle of a Diesel Generator**  

When the piston moves within the cylinder, the highest position of the piston top inside the cylinder is referred to 

as Top Dead Center (TDC), while the lowest position is called Bottom Dead Center (BDC). The piston stroke is the 

minimum linear distance between TDC and BDC, typically denoted as **S**. The distance from the connection center of 

the crankshaft and the connecting rod's big end to the rotation center of the crankshaft is known as the crank radius.  

**Swept Volume**: The cylinder volume swept by the piston from TDC to BDC is referred to as the cylinder swept volume (or piston displacement).  

**Compression Ratio**: After fresh air is drawn into the cylinder, it fills the entire cylinder, occupying the total cylinder volume, 

which includes the combustion chamber volume and the cylinder swept volume. The compression ratio indicates how much 

the air (or air-fuel mixture) inside the cylinder is reduced in volume after compression, reflecting the degree of compression. 

A higher compression ratio means the gas is compressed more intensely during piston movement, resulting in higher gas temperature and pressure, 

thereby improving engine efficiency.  

In thermodynamics, only the expansion process of the "working medium" can perform work. For an engine to continuously produce mechanical work, 

the working medium must repeatedly undergo expansion. Therefore, the working medium must be restored to its initial state before undergoing expansion again. 

Thus, a diesel engine must complete four thermodynamic processes – intake, compression, expansion, and exhaust – to return to its starting state and 

continuously generate mechanical work. These four processes together constitute one working cycle. If a diesel engine completes one working 

cycle over four piston strokes, it is called a four-stroke diesel engine. If it completes one cycle over two strokes, it is called a two-stroke diesel engine. 

Currently, diesel engines used in generator sets are predominantly four-stroke engines.  

**Intake Stroke**: The purpose of the intake stroke is to draw in fresh air to prepare for fuel combustion. To achieve this, 

a pressure differential must be created between the inside and outside of the cylinder. During this stroke, 

the exhaust valve remains closed while the intake valve opens. The piston moves from TDC to BDC, 

gradually increasing the volume above the piston and reducing the pressure inside the cylinder. 

The pressure inside the cylinder drops to approximately 68–93 kPa below atmospheric pressure. Under atmospheric pressure, 

fresh air is drawn into the cylinder through the intake valve. When the piston reaches BDC, the intake valve closes, and the intake stroke ends.  

**Compression Stroke**: The purpose of the compression stroke is to increase the pressure and temperature of the air inside the cylinder, 

creating favorable conditions for fuel combustion. With both intake and exhaust valves closed, the air inside the cylinder is compressed, 

and its pressure and temperature rise accordingly. The extent of this increase depends on the degree of compression, 

which varies slightly among different diesel engines. As the piston approaches TDC, the air pressure inside the cylinder reaches 3,000–5,000 kPa, 

and the temperature reaches 500–700°C, far exceeding the auto-ignition temperature of diesel fuel.  

**Expansion (Power) Stroke**: As the piston nears the end of its upward movement, the fuel injector begins to spray diesel into the cylinder, 

mixing it with air to form a combustible mixture. The mixture ignites spontaneously at this point, 

rapidly increasing the cylinder pressure to approximately 6,000–9,000 kPa and raising the temperature to 1,800–2,200°C. 

Driven by the force of the high-temperature, high-pressure gas, the piston moves toward BDC, rotating the crankshaft and performing work. 

As the gas expands and the piston moves downward, the pressure gradually decreases until the exhaust valve opens.  

**Exhaust Stroke**: The purpose of the exhaust stroke is to remove waste gases from the cylinder. After the power stroke, 

the gas inside the cylinder becomes exhaust, with the temperature dropping to 800–900°C and the pressure falling to 294–392 kPa. 

At this point, the exhaust valve opens while the intake valve remains closed. The piston moves from BDC to TDC, 

and under the residual pressure inside the cylinder and the pushing force of the piston, the exhaust gases are expelled from the cylinder. 

When the piston reaches TDC again, the exhaust process ends. After the exhaust process is complete, the exhaust valve closes, 

and the intake valve opens again, repeating the next cycle and continuously performing work.  

In comparison, the working principle of a diesel generator is relatively complex. We hope this brief introduction proves helpful.

**Guangdong Longhong Electromechanical Equipment Co., Ltd.**  

www.longhongpower.com

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