The ignition system of a diesel engine fundamentally differs from that of a gasoline engine. Unlike gasoline engines that use spark plugs, diesel engines rely on a process known as compression ignition.
Understanding Compression Ignition
Based on the work of German engineer Rudolf Diesel, the diesel engine is defined as an internal combustion engine where ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to mechanical compression. For this reason, the diesel engine is often referred to as a compression-ignition engine (CI engine).
This means there is no external ignition source like a spark plug required. Instead, the engine creates the conditions necessary for the fuel to ignite spontaneously.
How Diesel Ignition Works
The process involves several key steps within the engine cylinder:
- Air Intake: The piston moves downwards, drawing fresh air into the cylinder.
- Compression: The piston moves upwards, rapidly compressing the air. Because gases heat up when compressed, this process significantly raises the temperature of the air inside the cylinder.
- Fuel Injection: Just before the piston reaches the top of its stroke, diesel fuel is injected into the cylinder under very high pressure.
- Ignition: The finely atomized diesel fuel is injected into the extremely hot, compressed air. The temperature of the air is well above the autoignition temperature of diesel fuel, causing the fuel to ignite immediately upon contact.
- Power Stroke: The resulting combustion creates high pressure, pushing the piston downwards and generating power.
Key Elements of the Diesel Ignition System
While there isn't a traditional 'ignition system' with coils and spark plugs, certain components are crucial for enabling compression ignition:
- Engine Cylinder & Piston: These form the chamber where compression occurs, generating the necessary heat.
- High Compression Ratio: Diesel engines are designed with much higher compression ratios than gasoline engines (typically 14:1 to 25:1 compared to 8:1 to 12:1 for gasoline). This high compression is essential to achieve the required air temperature for ignition.
- Fuel Injector: This is a vital component that sprays the fuel directly into the combustion chamber at the precise time and under immense pressure, ensuring fine atomization for rapid mixing and ignition.
- Glow Plugs (for Cold Starting): While not part of the running ignition process, glow plugs are often used to assist cold starting in diesel engines. They preheat the combustion chamber, making it easier for the compressed air to reach the autoignition temperature of the fuel when the engine is cold.
Why No Spark Plug?
Diesel fuel has different properties compared to gasoline. It is less volatile and requires a higher temperature to ignite. The high compression in a diesel engine effectively creates this high-temperature environment, making a spark plug unnecessary and allowing for a more efficient combustion process for diesel fuel.
Comparison: Diesel vs. Gasoline Ignition
| Feature | Diesel Engine (CI) | Gasoline Engine (SI) |
|---|---|---|
| Ignition Method | Compression Ignition (Air Temp) | Spark Ignition (Spark Plug) |
| Fuel Injected | Directly into Cylinder | Typically into Intake Manifold (or DI) |
| Fuel/Air Mixture | Air Compressed, Then Fuel Added | Mixture of Fuel & Air Compressed |
| Compression Ratio | High (14:1 to 25:1) | Lower (8:1 to 12:1) |
In summary, the ignition system of a diesel engine is inherent to its design, relying solely on the heat generated by compressing air to ignite the injected fuel, a characteristic that defines it as a compression-ignition engine.
