In CI engines, the combustion is:
heterogeneous
Understanding Combustion in CI Engines
CI engines, which stand for Compression Ignition engines, are commonly known as diesel engines. The combustion process in these engines is quite different from that in Spark Ignition (SI) engines (petrol engines).
In a CI engine, air is drawn into the cylinder during the intake stroke and compressed to a very high pressure and temperature during the compression stroke. Towards the end of the compression stroke, fuel (diesel) is injected directly into this hot compressed air.
Why Combustion in CI Engines is Heterogeneous
The term heterogeneous means that the mixture is not uniform throughout the combustion chamber at the point of ignition and during combustion. Here's why this happens in CI engines:
- Fuel is injected as a spray into the air.
- Mixing of fuel and air occurs simultaneously with the heating, vaporization, and ignition of the fuel droplets.
- Combustion starts as soon as small pockets of the fuel-air mixture reach the self-ignition temperature of the fuel.
- As combustion progresses, different regions within the cylinder have varying fuel-air ratios. Some areas might be fuel-rich, others stoichiometric (chemically correct ratio), and many areas, especially away from the fuel spray, are lean (excess air).
- Combustion therefore occurs across a range of mixture strengths simultaneously, making the overall process heterogeneous.
Comparing CI and SI Engine Combustion
Let's compare the combustion types:
| Feature | CI Engine (Diesel) | SI Engine (Petrol) |
|---|---|---|
| Ignition Source | Auto-ignition (due to high temp/pressure) | Spark Plug |
| Fuel Introduction | Fuel injected directly into compressed air in cylinder | Fuel mixed with air before cylinder (carburetor/port injection) or mixed in cylinder before spark (direct injection) |
| Mixture State at Ignition | Heterogeneous (varying fuel-air ratios) | Ideally Homogeneous (uniform fuel-air ratio) |
| Overall Air-Fuel Ratio | Usually Lean (excess air) | Usually Stoichiometric or slightly Lean/Rich |
| Combustion Type | Heterogeneous | Homogeneous (ideally) |
Analyzing the Options
- Heterogeneous: As explained, the direct injection into compressed air leads to varying fuel-air ratios within the cylinder during combustion, resulting in a heterogeneous process.
- Very rich: While local pockets near the injector nozzle might be rich, the overall mixture strength in CI engines is typically lean, meaning there is excess air.
- Homogeneous: Homogeneous combustion occurs when the fuel and air are uniformly mixed before ignition, which is characteristic of ideal SI engine combustion, not CI engines.
- Chemically correct: Chemically correct refers to the stoichiometric air-fuel ratio where theoretically all fuel burns completely with the available air. CI engines typically operate with overall lean mixtures (excess air) and not necessarily at the chemically correct ratio, although some zones during heterogeneous combustion might approach it.
Based on the process of fuel injection and mixing in CI engines, the combustion is best described as heterogeneous.
Important Questions from Diesel Cycle
The compression ratio of diesel engines normally have the range:
Which of the following is NOT related to a spark ignition engine?
Which is the first stage of combustion in an SI engine?
An air standard Otto cycle consists of ______.
Where does mixing of fuel and air take place in case of diesel engine?
