Factors that Increase Detonation in SI Engine

Detonation is an undesirable phenomenon that occurs in spark-ignition (SI) engines when the air-fuel mixture ignites spontaneously and simultaneously at multiple points within the combustion chamber. This can lead to engine knocking, decreased performance, and potential engine damage. Several factors can increase the likelihood of detonation in an SI engine.

1. Increased Spark Advance:
- Spark advance refers to the timing of the spark plug firing in relation to the piston position.
- Advancing the spark means firing the spark plug earlier in the compression stroke.
- Increased spark advance can lead to detonation because the flame front may collide with the expanding combustion gases, creating pressure waves that can initiate spontaneous combustion in other areas of the mixture.
- This can be particularly problematic at high engine speeds and loads.

2. Increased Compression Ratio:
- Compression ratio is the ratio of the cylinder volume at bottom dead center (BDC) to the volume at top dead center (TDC).
- Increasing the compression ratio increases the temperature and pressure of the air-fuel mixture before ignition.
- Higher compression ratios can lead to increased detonation because of the higher temperatures and pressures present during the compression stroke.
- Detonation can occur when the mixture auto-ignites due to these elevated conditions.

3. Increased Air-Fuel Ratio beyond Stoichiometric:
- The stoichiometric air-fuel ratio is the ideal ratio at which all the fuel and oxygen are completely consumed during combustion.
- Increasing the air-fuel ratio beyond the stoichiometric value (lean mixture) can increase the likelihood of detonation.
- Lean mixtures burn slower and can cause incomplete combustion, leading to the accumulation of unburned fuel and air pockets that can ignite spontaneously and cause detonation.

In summary, the factors that increase detonation in an SI engine are increased spark advance and increased compression ratio. While increased speed can also contribute to detonation, it is not listed as one of the factors in the given options. Similarly, increasing the air-fuel ratio beyond stoichiometric strength can increase the risk of detonation, as lean mixtures burn slower and can cause incomplete combustion. Therefore, the correct answer is option D, which includes both increased spark advance and increased compression ratio.