Critical Design Issues of Heavy Fuel Turbogas Generators Derived from Automotive Turbochargers. Part II: The Improved Mach Method for off Design Performance Tuning

The improved Mach method is ideal to evaluate the variations of turboshaft power output with temperature, altitude and humidity.With this original method, the Mach number and the density are evaluated using the equations from acoustics.In this way, simulations are extremely accurate.This paper introduces an off-design example based on a very good turbocharger that reaches a maximum pressure ratio of 5 and is equipped with a high temperature turbine.Automotive turbochargers can be converted to small turbogas generator units.The cost-effectiveness of the solution is furtherly improved by the availability of hybrid turbo-generators with the electronic-converter already included.The design details to overcome the numerous issues of the modification were detailed in the first part of this paper.The resulting unity is a very economical, compact, reliable APU (Auxiliary Power Unit).Unfortunately, the efficiency is around 10% and the improvement through a recuperator makes the APU not convenient when compared with the ones based on CRDIDs (Common Rail Direct Injection Diesel).In fact, CRDIDs have a best thermodynamic efficiency that is above 50%.The off-design performance of turbochargerderived APUs differs substantially from traditional turboshafts equipped with axial compressors and turbines.Therefore, the performance tuning requires a different approach.