Spark Ignition (SI) engines use spark plugs for ignition, providing higher speeds, lighter weight, and lower \(NO_{x}\) emissions, but lower thermal efficiency. Compression Ignition (CI) engines (diesel) use auto-ignition via high compression, delivering better fuel economy, higher torque, and lower hydrocarbon emissions, but higher \(NO_{x}\) and noise. [1, 2, 3]
Performance Characteristics
- SI Engine: Operates at higher speeds (\(>4000\text{–}6000\text{ rpm}\)) with a lower compression ratio (7–12). Known for higher power-to-weight ratio and smoother, quieter operation.
- CI Engine: Operates at lower speeds (\(<3000\text{–}4000\text{ rpm}\)) with a higher compression ratio (14–23). Offers superior thermal efficiency, higher torque at low speeds, and better fuel economy (lower Brake Specific Fuel Consumption - BSFC).
Combustion Characteristics
- SI Engine: Uses homogeneous charge (fuel-air mix) pre-mixed or injected before combustion. Combustion occurs via a flame front propagation initiated by a spark.
- CI Engine: Uses heterogeneous charge; air is compressed first, and fuel is injected at the end of the stroke. Combustion occurs via auto-ignition at multiple points, resulting in faster heat release and higher combustion temperatures.
Emission Characteristics
- SI Engine: Higher hydrocarbon (HC) and carbon monoxide (CO) emissions due to potential incomplete combustion. Lower \(NO_{x}\) compared to CI engines due to lower compression.
- CI Engine: Lower HC and CO emissions, but produces higher nitrogen oxides (\(NO_{x}\)) and soot (particulate matter) due to higher operating temperatures and pressures.
Key Differences Summary
| Feature | SI Engine (Petrol) | CI Engine (Diesel) |
|---|---|---|
| Ignition | Spark Plug | Compression |
| Compression Ratio | Lower (7–12) | Higher (14–23) |
| Speed | High | Low |
| Efficiency | Lower | Higher |
| Torque | Lower at low speed | Higher at low speed |
| \(NO_{x}\) Emissions | Lower | Higher |
| Particulates | Lower | Higher |