Compressed Air Starting System for Large Diesel Generators

 
The compressed air starting system plays a critical role in large diesel generator sets, especially in situations where there is high starting resistance. Unlike smaller diesel engines, where electric starters may provide sufficient power, larger engines require more power to overcome the heavy starting load, making compressed air starting a common solution. This method not only overcomes the high resistance but also ensures the diesel generator can start smoothly under various conditions, making it widely used in ships, heavy machinery, power plants, and other fields.

Basic Principle of Compressed Air Starting

 
The principle behind compressed air starting involves introducing high-pressure air stored in cylinders into the diesel engine’s cylinders in sequence. This high-pressure air forces the pistons down, rotating the crankshaft and starting the engine. A key component in this process is the air distributor, which ensures that the air is delivered to each cylinder in the correct order, enabling each cylinder to fire in the pre-determined sequence. This method utilizes the energy from the high-pressure air, allowing the generator to start even without an external power source.

Components of the Compressed Air Starting System

 
The compressed air starting system in a diesel generator consists of several important components, each playing a vital role in the starting process. The main parts are as follows.

1. Air Storage Cylinder

 
This cylinder stores the high-pressure air and is usually located at the heart of the system. It is equipped with a starting valve and other control valves at the top. When the starting valve opens, high-pressure air is sent to the air distributor. The storage cylinder also features a dirt trap valve to remove impurities and a safety valve to prevent overpressure.

2. Air Distributor

 
The air distributor directs the high-pressure air to each cylinder in the correct order. It is driven by an internal rotating disc, which channels the air to the appropriate cylinder through specific pathways, ensuring the proper timing and synchronization during the starting process.

3. Starting Control Valve

 
This valve controls the flow of compressed air. It opens during the starting process to allow air through and closes after the engine has started, ensuring efficiency and safety.

4. Starting Valve

 
Mounted on the cylinder head of the diesel engine, this one-way valve opens when compressed air is directed to the cylinder, allowing the air to push the piston. Once the engine has started, the valve closes automatically to prevent air leakage.

5. Charging Valve

 
The charging valve is used to refill the storage cylinder with fresh compressed air for the next start. It is typically operated manually and is located on the first cylinder head. Once the required pressure is reached, the charging process stops.

6. Safety Valve

 
The safety valve prevents the air storage cylinder from becoming dangerously over-pressurized. When the pressure exceeds the safe limit, the valve automatically opens to release excess air, ensuring system safety.

Steps in Compressed Air Starting

 
The process of starting a diesel generator with compressed air involves several key steps: preparation, starting, and post-starting procedures. Each step must be carefully followed to ensure smooth and safe operation.

1. Preparation Phase

 
Check Air Storage Cylinder Pressure: Before starting, check that the air storage cylinder's pressure is within the range of 2450 to 2940 kPa. If the pressure is too low, refill the cylinder using the charging valve.
Turn the Crankshaft Manually: To conserve compressed air, manually rotate the crankshaft so that the piston in any cylinder is positioned 10°–15° past the top dead center. This positions the piston for optimal air intake.

2. Starting Phase

 
Open the Starting Control Valve: During startup, open the control valve on the air storage cylinder. The compressed air will flow through the air distributor to each cylinder, pushing the pistons down and starting the engine.
Adjust the Throttle: During the startup process, set the throttle to 1/2 or 1/3 of its range to allow the generator to enter the starting sequence. As the compressed air drives the pistons, the crankshaft will begin to turn.
Confirm the Start: When you hear the sound of combustion inside the cylinders, it means the engine has successfully started. Quickly close the starting switch and the control valve on the air storage cylinder, allowing the generator to operate at a low speed.

3. Post-Start Operations

 
Refill the Air Cylinder for the Next Start: After the engine has started, the air storage cylinder needs to be refilled. Open the return oil bolt on the first cylinder's injector and the charging valve on the air storage cylinder to start refilling. Once the cylinder reaches the required pressure, close the valves to prepare for the next start.

Conclusion

 
The compressed air starting system provides a reliable and efficient method for starting large diesel generators, especially those with high starting resistance. With components like the air storage cylinder, air distributor, and control valves working together, this system ensures smooth operation under various conditions while preventing overpressure risks, guaranteeing safe and reliable system performance.