<p>This research study focuses on proposing a novel multivariate experimental data algorithm for optimization of water-air ratio of indirect water-injected automobile diesel engine. The proposed novel algorithm identifies the point of diminishing returns between two selected diesel engine performance parameters by plotting them simultaneously against varying water-air ratios. The point of diminishing returns pin-points the optimal water-air ratio for a given test configuration. In this way, a range of optimal water-air ratios can be determined by this proposed algorithm for any indirect water injected automobile diesel engine against its entire operational range. Fuel consumption, torque, NO_x emissions, and CO₂ emissions are set as performance parameters. For this study, the working of proposed algorithm was demonstrated through experimentation using four different engine operating conditions, where multiple water-air ratios were maintained in the water injection system. Measurements of torque, fuel consumption, NO_x and CO₂ emissions were taken for each test configuration. The proposed algorithm then conducted the graphical interpretation of the plotted data and identified the optimal water-air ratio which in this case was from 0.05 to 0.07 by balancing torque, NO_x and CO₂ emissions with fuel consumption demonstrating its practicality for determination of optimized water-air ratio.</p>