| Figure 7.5 |
The Fuel –air equivalence ratio –Temperature diagram for hydrocarbon - air mixture conditions for formation of soot and NOx. The conditions of normal SI and CI engine operation and target region
for HCCI/CAI for low NO and soot formation are shown. |
The HCCI/CAI combustion process has two main steps:
(i) Preparation of lean premixed, homogeneous fuel-air mixture, and
(ii) Autoignition leading to combustion of lean premixed charge.
Different approaches have been investigated to accomplish the above steps leading to HCCI/CAI combustion
CAI GASOLINE ENGINES
The objective of CAI combustion in gasoline engines is to obtain high fuel efficiency and low NOx emissions, the PM emissions already being insignificant. Fig. 7.6 compares P-V diagrams for a SI and CAI engine operation. It may be noted that the combustion in CAI engine is much faster and it approaches close to constant volume combustion.
| Figure 7.6 |
Comparison of P-V diagrams for SI and CAI combustion. |
Controlled autoignition (CAI) of homogeneous charge requires:
- Charge is kept at a high temperature for sufficient duration
- Composition of charge to ensure acceptable rate of combustion and low NOx formation
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