A nonlinear optimal (H-infinity) control approach is proposed for turbocharged diesel engines with potential use in ship propulsion. The dynamic model of the diesel engine undergoes approximate linearization round a temporary operating point. This is defined at each time instant by the present value of the system's state vector and the last value of the control input vector that was exerted on it. The linearization is based on Taylor series expansion and on the computation of the associated Jacobian matrices. For the linearized model an H-infinity feedback controller is computed. The controller's gain is calculated by solving an algebraic Riccati equation at each iteration of the control method. The asymptotic stability of the control approach is proven through Lyapunov analysis. This assures that the state variables of the diesel engine will finally converge to the designated reference values. Optimal functioning of the diesel engine signifies improved power, reduced polluting emissions and reduced fuel consumption.