Ignition delay

Definition

Ignition delay is the time period between the start of fuel injection into a combustion chamber and the onset of combustion, or the point at which the fuel-air mixture begins to ignite. This delay is crucial as it influences the efficiency and performance of combustion engines, as well as the emission characteristics of the exhaust gases. Understanding ignition delay helps in optimizing engine designs and improving combustion stability.

5 Must Know Facts For Your Next Test

1. Ignition delay can vary significantly based on factors like fuel composition, temperature, pressure, and the presence of additives.
2. In low-temperature combustion engines, ignition delay plays a critical role in reducing nitrogen oxide (NOx) emissions by allowing more complete mixing of fuel and air before combustion.
3. Longer ignition delays can lead to increased emissions of unburned hydrocarbons and particulate matter due to incomplete combustion.
4. Understanding the mechanisms behind ignition delay helps in designing more efficient combustion systems that enhance performance and reduce emissions.
5. Research into ignition delay phenomena is essential for developing advanced combustion technologies like HCCI (Homogeneous Charge Compression Ignition) and PCCI (Partially Premixed Compression Ignition).

Review Questions

• How does ignition delay affect the efficiency and performance of combustion engines?
  • Ignition delay significantly impacts engine efficiency and performance as it determines when fuel begins to combust after injection. A shorter ignition delay often leads to more efficient fuel utilization and smoother engine operation, while a longer ignition delay can result in increased emissions due to incomplete combustion. By optimizing ignition delay through fuel selection and engine design, manufacturers can improve overall performance and reduce harmful emissions.
• Discuss how varying fuel properties can influence ignition delay in combustion processes.
  • Fuel properties such as chemical composition, volatility, and cetane number play a major role in determining ignition delay. Fuels with higher cetane numbers ignite more quickly under compression, resulting in shorter ignition delays. Conversely, fuels with lower cetane numbers tend to have longer delays. Additionally, factors such as temperature and pressure conditions in the combustion chamber can also affect how quickly a fuel will ignite after injection.
• Evaluate the implications of ignition delay on emissions produced by low-temperature combustion engines.
  • Ignition delay has significant implications for emissions in low-temperature combustion engines, particularly concerning nitrogen oxides (NOx) and unburned hydrocarbons. By carefully managing ignition delay through techniques like controlled mixing and optimization of operating conditions, it is possible to minimize NOx formation while enhancing combustion efficiency. This balance is critical for meeting stringent emissions standards and promoting cleaner combustion technologies, thereby addressing environmental concerns associated with traditional combustion engines.

Related terms

Autoignition: A process where a fuel ignites spontaneously without an external ignition source, typically due to high temperature and pressure conditions.

Combustion Phases: The distinct stages in the combustion process, including ignition delay, rapid combustion, and the cooling phase.

Cetane Number: A measure of a diesel fuel's ignition quality, indicating how quickly it ignites under compression; higher cetane numbers correspond to shorter ignition delays.