The emissions from an SI engine that operated in CAI mode at part loads are compared with the homogeneous stoichiometric SI engine on Fig. 7. 9. The engine used negative valve overlap ranging from 140-200º crank angle for CAI operation. Residual gas fraction varied from 40 to 70%. The intake air temperature was increased to 50º C by external heating. The NO_x emissions with CAI operation are extremely low. HC emissions were higher as use of very lean mixtures results in partial misfired combustion increasing HC emissions. CO emissions decreased by 80 % and the fuel consumption was lower by 15% at engine speeds of above 2000 rpm.

Figure 7.9

Comparison of indicated specific emissions of CAI ( =0.77)  and SI engine (= 1.0), IMEP = 2.6 bar, 3000 rpm..