Regime of CAI Operation

CAI operation was studied on a single cylinder engine using exhaust gas recirculation and heating the intake charge to  320 ± 1º C by an air heater. The fuel was injected at intake port . The CAI region is presented on a plot in Fig 7.8 with fuel-air equivalence ratio, and EGR mass rate as variables. The CAI region is enveloped by three engine operation regimes viz;

(i) Engine knock: It is experienced as increases. The knock limited increases as the rate of EGR increases

(ii) Misfire: Engine misfire of the charge results as EGR is increased and the engine bmep is decreased.

(iii) Partial burn:  For the fuel-air equivalence ratio less than about 0.2. As the fuel-air equivalence ratio decreases, the heat released also decreases lowering the average combustion temperatures resulting in partial burn and misfire.

Knock limited imep of 3.8 bar was observed. For an imep = 3.0 bar, depending upon the EGR in the range of 40 to 55% was required to obtain CAI operation.

The region of CAI operation forms only a small fraction of the total engine speed-load operation regime with the present status of development. CAI operation of a SI engine is typically obtained for engine speeds from 1000 to 3500 rpm. At 1000 rpm, knock free CAI operation using EGR is possible up to about 5 bar bmep that reduces to nearly 2 bar bmep at 3500 rpm. On the other hand the SI engine full load bmep is about 10 bar and the rated engine speed typically is around 6000-7000 rpm. The practical engines could be operated in mixed CAI-SI mode. At low and medium loads and at

vehicle cruising speeds engine operates in CAI mode but during cold starting, at idling and at high load and speeds the engine operates in conventional SI mode.