Understanding the Effects of Recycled Burnt Gases on the Controlled Autoignition (CAI) Combustion in Four-Stroke Gasoline Engines 2001-01-3607

Controlled Auto-Ignition (CAI) combustion, also known as HCCI or PCCI, has recently emerged as a viable alternative combustion process to the conventional spark ignition (SI) or compression ignition (CI) process for internal combustion (IC) engines, owing to its potential for high efficiency and extremely low emissions. One of the most effective and practical means of achieving CAI combustion in an engine is to retain or recycle the burnt gases. In order to understand better the effects of recycled burnt gases on CAI combustion, detailed analytical and experimental studies have been carried out. The analytical studies were performed using an engine simulation model with detailed chemical kinetics.

The five effects of the recycled burned gases studied include:

1. (1.)
   Charge heating effect: higher intake charge temperature due to hot burned gases;
2. (2.)
   Dilution effect: the reduction of oxygen due to the presence of the burned gases;
3. (3.)
   Heat capacity effect: due to higher specific heat capacity of burned gases;
4. (4.)
   Chemical effect: chemical reactions involving CO₂ and H₂O in the burned gases
5. (5.)
   Stratification effect: recycled burned gases are stratified from the fuel/air mixture.

And the main conclusions are:

1. (1.)
   The charge heating effect is mainly responsible for the advanced AI timing due to hot burned gases. It increases the heat release rate and shortens the combustion duration.
2. (2.)
   The dilution effect does not affect AI timing, but it extends combustion duration. Its slows down the heat release rate only when large amount of burned gases is present.
3. (3.)
   The heat capacity effect is mainly responsible for the reduction in the heat release rate, and it has a similar effect to extend the combustion duration as the dilution effect.
4. (4.)
   The chemical effect does not affect AI timing and heat release rate. It reduces the combustion duration slightly at high concentrations of burned gases.
5. (5.)
   The stratified burned gases facilitate the CAI combustion due to the presence of higher temperature region at the boundary between of the hot burned gases and combustible charge.