A computer program provides capabilities for numerical simulation and analysis of the thermodynamic performance of aircraft or automotive gas turbine engines. The program was developed to utilize the turboshaft-engine experience base accumulated in aerospace disciplines for designing automotive engines. Potential applications range from (1) small hybrid automotive power systems (power systems that include energy-storage subsystems) with power levels of about 25 kW to (2) heavy truck and earth-moving-machinery powerplants with megawatt power levels.

Given such inputs from the user as turbine and compressor inlet temperatures and performance characteristics of subsystems, the program calculates such powerplant thermal-performance characteristics as thermal efficiencies, specific fuel-consumption rates, cycle state points, and rates of flow of working fluids. The program also calculates data on fuel economy for a specified vehicle weight and an assumed driving cycle.

An Open Brayton Cycle is the theoretical basis for analysis of the performance of a gas turbine engine.

The program is based on a mathematical model of an open Brayton thermodynamic cycle (see figure). It was derived from a closed-Brayton-cycle program, "BRMAPS," developed previously for outer-space power systems energized by nuclear or concentrated solar heat sources. Written in a scientific programming language called "VSAPL," the program includes several interconnected subprograms that calculate the thermodynamic-performance quantities and that use empirical mass models to calculate the masses of essential subsystems and components. The code also computes the mass of the overall system, comprising the aforementioned components and subsystems plus interconnecting ducts and structures.

A key feature of the program is an iterative steepest-descent optimization routine that, for a given cycle temperature ratio (turbine-inlet-temperature/compressor-inlet temperature), rapidly converges to the optimum pressure ratios for maximum thermodynamic efficiency, minimum radiator area, and minimum overall system mass. Of course, performance figures can also be calculated for pressure ratios specified by the user, but by providing optimum values, the program enhances system analysis procedures drastically.

In its state of development at this writing, the program can be used to compare performances of alternative designs of automotive gas turbine engines and of hybrid systems that include gas turbine engines, under steady-state operating conditions. By use of this program, one can rapidly ascertain the payoff in fuel economy as a function of improvements in components or higher turbine inlet temperatures (made possible by use of advanced materials). Thus, the program can help to guide engine development along a most efficient and productive path. Subsequent versions of the program are expected to incorporate refinements of empirical models of component weights, plus models for volumes and costs.

This work was done by Albert J. Juhasz of John H. Glenn Research Center. LEW-16709


NASA Tech Briefs Magazine

This article first appeared in the September, 1999 issue of NASA Tech Briefs Magazine.

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