Influence of flight altitude on combustion dynamics and performance of spark-ignition Wankel rotary engines at part load

• Increasing altitude from 143 m to 3126 m caused about a 39% reduction in IMEP at 6500 rpm due to lower air density and a leaner charge. • Combustion stability remained high across altitudes, with COV_IMEP below 2%, even under high-altitude lean conditions. • A secondary heat-release peak appeared at high engine speeds, suggesting possible abnormal combustion phenomena. • Wiebe-model analysis indicated longer combustion durations than in reciprocating engines, reflecting distinct flame-propagation behavior in rotary engines. • The expansion polytropic index consistently exceeded the compression index, highlighting phase-dependent thermodynamic characteristics of Wankel rotary engines. This study examines how flight altitude affects the combustion dynamics and performance of spark-ignition Wankel rotary engines (WREs) operating under part-load conditions. Experimental evaluations were conducted at realistic altitudes ranging from 140 to 3126 meters, with an emphasis on key performance and combustion parameters, including indicated mean effective pressure (IMEP), the coefficient of variation of IMEP (COV IMEP ), heat release rate (HRR), and mass fraction burned (MFB). The results indicate a substantial decline in IMEP—approximately 39% at 6500 rpm—with increasing altitude, primarily due to reduced air density and inducted air mass. Despite the decline in power output, combustion stability remains unaffected, mainly as evidenced by relatively consistent COV IMEP values (1.4% ± 0.4% at 140 m and 1.7% ± 0.5% at 3126 m), demonstrating the WRE’s robustness under varying atmospheric conditions. The appearance of a secondary HRR peak at higher engine speeds suggests combustion irregularities specific to low-density environments. The Wiebe function, parameterized with an efficiency factor ( a = 2.1) and a shape factor ( m = 1.5), accurately models the combustion profile, with a lower shape factor indicating a more gradual combustion process than conventional reciprocating engines. Furthermore, refinements to the polytropic index during the expansion phase further enhanced thermodynamic simulations, resulting in more accurate predictions of HRR and MFB. Collectively, these findings improve our understanding of altitude's impact on WRE combustion and support the development of adaptive control strategies for optimizing combustion systems in high-altitude and UAV applications.