Methods of Charge Stratification and Combustion

The methods of charge stratification and combustion that have been studied and some of these employed in production engines may be grouped in the following three types. These  are also shown on Fig. 7.2.

Spray Controlled: The fuel spray characteristics primarily controls the charge stratification in this strategy. Ignitable mixture is formed at the boundaries of the fuel spray. The spark plug is placed close to the spray as it is there that the ignitable mixture is present at the time of ignition. Formation of good quality mixture becomes difficult at high engine loads. The combustion being highly sensitive to spray characteristics smoke formation is often observed at high loads. Wetting of the spark plug by liquid fuel causes frequent spark plug fouling.

Wall Controlled: In the wall-controlled concept, fuel injection is directed towards a specially designed piston cavity. The piston cavity is off centre. The spark plug is located away from the fuel injector on the side of combustion chamber. Fuel impinges on the piston cavity walls where it evaporates and mixes with air. An intense reverse tumble charge motion transports the mixture to spark plug electrodes.

Flow Controlled: Mixture is formed by interaction between fuel spray and suitably directed air motion like swirl or tumble. The spark plug and injector are generally widely spaced in these configurations. The air motion transports mixture to the spark plug such that the ignitable mixture is present at spark plug electrodes at the time of spark. When air motion is well organized, the combustion chamber walls do not get wetted by liquid fuel and a stable stratified charge operation is obtained over a wide range of engine operation.

The characteristics of combustion process obtained with the three charge stratification and combustion methods are compared in Table 7.1