Stages of Combustion in CI Engines

If you're familiar with the differences between Spark Ignition (SI) engines and Compression Ignition (CI) engines, you might wonder whether these differences extend to the combustion process.  

The answer is, of course, yes! In one of our previous videos, we discussed the stages of combustion in SI engines. Today, we will dive into the combustion process in CI engines and break down its four distinct stages. 

Let’s get started! 

A Quick Comparison of SI and CI Engines 

In SI engines, the air-fuel mixture is drawn into the cylinder during the suction stroke, compressed, and ignited by a spark from the spark plug. In CI engines, however, only air is sucked into the cylinder during the suction stroke and is then compressed. Fuel is injected directly into the hot compressed air, causing spontaneous ignition due to the high temperature. This difference in operation leads to distinct combustion stages in CI engines. 

The Four Stages of Combustion in CI Engines 

The combustion process in CI engines can be divided into four stages: 

• Ignition Delay Stage 
• Rapid Combustion Stage 
• Controlled Combustion Stage 
• After Burning Stage 

Let's explore each stage in more detail. 

1. Ignition Delay Stage 

Also known as the "preparatory stage," the ignition delay is the time between the start of fuel injection and the actual ignition of the fuel. It can be further broken into two parts: 

• Physical Delay Period: After injection, the fuel is atomized and vaporized, then it mixes with the air and reaches its self-ignition temperature. This process, which depends on the fuel's properties, is called the physical delay. 
• Chemical Delay Period: Following the physical delay, chemical reactions take place, gradually accelerating until ignition begins. In most cases, this period lasts longer than the physical delay. However, at high temperatures, chemical reactions speed up, making the chemical delay shorter. 

The total ignition delay is the sum of these two delays. A prolonged delay can lead to knocking, a harmful engine condition caused by uncontrolled combustion. 

2. Rapid Combustion Stage 

Once the fuel ignites, the accumulated fuel in the combustion chamber from the delay period burns quickly. This rapid combustion results in a sudden and sharp rise in pressure. As this stage cannot be controlled, it is often referred to as the "uncontrolled combustion stage." The heat release is at its maximum, and the highest pressure is attained at the end of this phase. The longer the ignition delay, the more fuel accumulates, leading to higher pressure spikes. 

3. Controlled Combustion Stage 

During the controlled combustion stage, the high pressure and temperature from the previous phase ignite the newly injected fuel immediately. This stage, as the name suggests, can be regulated by controlling the fuel injection. The controlled combustion phase continues until the temperature reaches its maximum point, marking the end of this stage. 

4. After Burning Stage 

Combustion doesn’t stop when fuel injection ends. The leftover fuel in the combustion chamber continues to burn during the expansion stroke, which is known as the "after burning stage." This final stage ensures that the remaining fuel is fully combusted, contributing to the overall efficiency of the engine. 

Visualizing the Combustion Stages: The Pressure-Crank Angle Diagram 

To better understand these stages, we can visualize them in a pressure-crank angle diagram. Here’s how to interpret it: 

• X-axis: Crank angle 
• Y-axis: Pressure 

Key points on the diagram: 

• A: Fuel injection starts. 
• B: Fuel ignites. 
• C: Maximum pressure is attained. 
• D: Fuel injection ends. 
• E: Combustion ends. 

Each stage corresponds to different sections of the diagram: 

• A to B: Ignition delay stage 
• B to C: Rapid (uncontrolled) combustion stage 
• C to D: Controlled combustion stage 
• D to E: After burning stage 

Conclusion 

Understanding the combustion process in CI engines is crucial for optimizing engine performance and efficiency. The four stages—ignition delay, rapid combustion, controlled combustion, and after burning—define the unique combustion characteristics of CI engines. 

By comparing this to SI engines, we can appreciate the engineering differences between these two types of engines. Stay tuned for more insightful videos where we continue to explore engine technologies and their mechanics. 

Until next time, keep learning! 

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