Performance of a Low-Blowby Sealing System for a High Efficiency Rotary Engine

The X engine is a non-Wankel rotary engine that allies high power density and high efficiency by running a high-pressure Atkinson cycle at high speeds. The X engine overcomes the gas leakage issue of the Wankel engine by using two axially-loaded face seals that directly interface with three stationary radially-loaded apex seals per rotor. The direct-interfacing of the apex and face seals eliminates the need for corner seals of the typical Wankel engine, significantly reducing rotary engine blowby. This paper demonstrates the sealing performance that can be achieved by this new type of seal configuration for a rotary engine based on dynamics models and experiments. The dynamics models calculate the displacement and deformation of the face and apex seals for every crank angle using a time implicit solver. The gas leakage is then calculated from the position of the seals and pressure in the chambers and integrated over a rotor revolution. An “effective leakage orifice” area can be determined, to compare blowby between different engine types. Model results show that the X engine equivalent leakage area could be around 35% that of the leakage area of a similarly sized Wankel engine obtained from the same modeling method, which brings the X engine leakage closer to the piston engine’s leakage range. Initial experimental results support the findings from the model, as the X engine shows an equivalent leakage area of about 65% that of a scaled Wankel engine. This result demonstrates the potential of the X engine to achieve gas sealing improvements through additional seal development. Introduction For applications that need a high power density, the rotary engine has been an interesting candidate since its debut in the 1950s. In addition to its impressive power density, it also features fewer moving parts and lower vibrations levels compared to piston engines. However, it has seen a decrease in popularity in recent years, notably in the automotive industry, with the last remaining Wankel engine powered car being manufactured in 2012. The withdrawal of rotary engines from the automotive market can be explained by the traditional drawbacks of rotary engines, along with increasing emission regulations around the globe. An important drawback of the rotary engines is the difficulty to seal the combustion chamber. The Wankel engine’s geometry requires radially loaded apex seals as well as axially loaded face seals, and the interface between these is not effectively sealed which leads to increased leakage. Various scientific work has been done on the Wankel type rotary engines since the 1950’s. The sealing performance of the Wankel engine was studied both experimentally and by modeling. Different methods were used to simulate the Wankel engine by the past, such as modifying piston engines commercial simulation softwares [1], by the use of CFD tools [2], or by analytical calculations [3, 4]. Throughout the literature, the seal leakage values are usually quantified as an equivalent orifice area, which is not directly linked to the dynamics of the seals. This leakage area is usually determined by fitting a full engine cycle simulation model to experimental in-cylinder pressure and other data, tuning parameters such as compression ratio, port flow coefficients, heat transfer multipliers, and leakage orifice areas. For a Mazda Wankel, Eberle and Klomp [5] determined a leakage area value of 2 mm2 per cell, while others [6]-[8] found 1 mm2. In an effort to better understand the behavior of the Wankel seals, models were created to study the dynamics of the seals [9]-[12]. The seal model by Picard [3, 4] successfully relates the sealing performance to the dynamics and deformation of the seals to predict leakage. The final results of the model predict an equivalent leakage area varying between 1 mm2 and 2 mm2 for the Renesis engine. The main conclusion of these studies is that the seals tend to leak near the extremities of the parts that interact with each other. In a piston engine, since the rings only have a single gap, they are able to achieve better sealing performances than Wankel engines. The X Engine is a rotary engine with an alternative architecture that has the potential to solve the rotary engine sealing Downloaded from SAE International by Alexander Shkolnik, Sunday, May 27, 2018