We can use Taguchi method to find optimal combination (Input parameter) for an output parameter. Is it possible to find overall optimal combinations for overall performance (Performance, combustion and emission)?
Develop a polynomial equation used to fit your experimental data using, say Minitab, statistical software.
Then employ this model to optimize the required parameters using suitable optimization algorithm. You can perform uni-objective or multi-objective optimization as per your need.
Through grey relation analysis, data envelopment analysis is possible. More over you are considering emission parameters, performance parameters, it will come under multi response optimisation. Select proper factors, level and orthogonal array. Ref Applied Design of Experiments and Taugchi Method book by Dr krishnaiya and Dr Sharbudeen - Last Chapter Multi Response Optimisation
Use nature based inspired algorithm to yield the better optimal solution rather than using Taguchi method...Bcoz your optimized parameters available only in discrete points...