Fundamentals of Reciprocating Engine Airflow Part I: Valve Discharge and Combustion Chamber Effects 840337
Classical analysis of poppet valved reciprocating engine airflow requires the use of numerous simplifying assumptions that reduce the problem to a traditionally manageable level. At the time of development of classical theory, numerical solutions of nonlinear differential and integral equations were not practical forcing the use of assumptions that are not uniformly valid. Availability of modern numerical methods allows us to relax some of these “invalid” assumptions and place more of the physics into the analytical model. The intent of the present work is to discuss relaxation of several classical assumptions on airflow within the combustion chamber, and to integrally derive equations describing fluid motion and state within the chamber.
Significant modifications of classical theory include the addition of compressibility and wave motion in the theoretical model. For simplicity of application of the resulting theory, these effects are included in a new valve discharge coefficient. The new coefficient consists of a sum of a steady term and an unsteady term. The steady term is related directly to the classical discharge coefficient, while the unsteady term is considered novel and is a dynamic function of engine operating conditions. In addition to the new valve discharge number integral measures are derived to characterize inlet velocity profile and turbulent momentum losses. Several numerical examples illustrate implicit errors in the classical discharge coefficient and provide an understanding of the new measures.