By accurately simulating the complicated reactive flow process in a hydrogen air bag inflator, TRW engineers gained useful design information that has already helped reduce cost and resolve design issues. Physical tests that are traditionally used to design air bag inflators show the performance of a particular design but provide almost no diagnostic information, making it difficult to make improvements. The challenge was accurately capturing both the stiff hydrogen-air reaction and the complex flow geometry without consuming inordinate computational resources. TRW engineers took advantage of a commercial computational fluid dynamics (CFD) software package that provides reliable solution algorithms and allows for substantial chemical reaction complexity. By creating the first accurate simulation of a hydrogen inflator, they were able to gain additional understanding of how the parts in the inflator influence combustion. This, in turn, has made it possible to fine-tune the critical combustion process according to the performance requirements of individual applications.