Pressure Non-Uniformity and Mixing Characteristics in Stratified-Charge Rotary Engine Combustion

Studies are reported of stratified-charge combustion in rotary engines. They were performed with a three-dimensional model that computes intake, compression, liquid fuel injection, combustion, expansion, and exhaust. Comparisons are shown of computed and measured chamber pressures for two engines and seven conditions. They are the first comparisons of three-dimensional computations for rotary engines. The agreement is adequate for the purpose of interpreting the main features of the combustion flowfield. Then two subjects are considered: the mixing of injected-fuel and air, and the pressure non-uniformity within the combustion chamber. It is found that the TDC turbulence diffusivity of rotary engines in general is smaller than in corresponding reciprocating engines because of the longer time between intake and TDC. The pressure non-uniformity is shown to be caused by large fluid acceleration around TDC. It is concluded that slow mixing, due in part to low turbulence diffusivity, is responsible for the slow burning rate and that pressure non-uniformity can reduce indicated efficiency. Finally, it is argued that opportunities exist to improve efficiency by speeding up the completion of combustion and by controlling the pressure non-uniformity.