Methods to Obtain CAI

The following parameters have been used to obtain the required temperature, pressure and composition of charge for CAI operation:

• Residual gas content or EGR
• Compression ratio
• Intake mixture temperature
• Intake mixture pressure
• Fuel-air ratio
• Coolant temperature
• Injection timing in  gasoline direct injection engines

The practical engines have to operate in CAI/SI dual mode. The SI engine mode at high loads is required to have good specific power output. The autoignition of gasoline or natural gas requires CR > 20:1.Such high CR would be unacceptable at high loads when fuel-air ratio is to be increased as severe engine knock would result. The engine CR is to be limited to around 10:1. Hence, increase of CR to obtain CAI operation has not been a practical option. Intake mixture heating by using the waste heat of the exhaust gas although helpful but gives a further loss in volumetric efficiency as already a highly diluted charge is used to control the heat release rates subsequent to autoignition.
The most practical approach for CAI operation is use of large amounts of hot residual gases. Trapping of the residual gases inside the cylinder also termed as ‘internal EGR’ has been found a more acceptable approach.  A negative valve overlap period is used to retain high amount of residual gases in the cylinder. The exhaust valve is closed before the piston reaches top dead centre in the exhaust stroke and to prevent backflow of the burned gases in the intake system, the intake valves open well after TDC. The relative valve timings for a standard SI engine and for CAI operation are compared in Fig. 7.7.
To get maximum expansion work ideally the exhaust valves are to be opened at the usual time in the cycle but are to be closed earlier. The intake valve is to open late while they should close as in the normal SI engine operation. To obtain this flexibility in SAI-SI dual mode operation fully flexible, variable valve actuation systems are necessary which are very complex in construction and presently are not in production. In the practical engines, use of   low valve lift cam profile provides substantial reduction in valve opening period. This approach along with variable cam timing devices has been used in prototype engines.  By varying the exhaust valve closing time load can be varied.    As the exhaust valve closes earlier to trap more residual gases, less fresh charge is admitted resulting in lower engine output. The valve timings with lower valve lift are also shown on Fig. 7.7.
The trapping of residual gas in the cylinder results in high charge temperatures.. At high loads, the auto ignition however, may occur too early and high rates of pressure rise would result.