emissions on the other hand. As shown in Fig. 5, the fuelto-
air equivalence ratio is the dominating factor for NOX
emissions in homogeneous operation. Apart from the
overall equivalence ratio, mixture homogeneity was found
to also strongly influence NOX emissions levels [34]. A
homogeneous mixture at an equivalence ratio 0:8
results in peak NOX emissions (see Fig. 5), whereas a
stratified mixture with the same global air–fuel ratio will
result in lower NOX emissions as the local equivalence
ratio will be either left or right of the peak. Thus, the
stratification of the fuel-air mixture that is possible with DI
allows both lowering NOX emissions and increasing
engine efficiency, compared to PFI. The increase in