BALL PISTON ENGINE

A  new power machine concept has been designed and analyzed for production, and proof of principle subscale tests have been performed, with positive results. The machine design concept is applicable as a compressor, pump, motor, or engine. Simplicity of design based on spherical ball pistons enables a low moving part count, high power to weight ratio, elimination of valve train and water cooling systems, and perfect dynamic balance. 

The new design concept utilizes novel kinematic design to completely eliminate inertial loads that would contribute to sliding friction. Also, low leakage is maintained without piston rings by using a small clearance on the ball piston, resulting in choked flow past the ball. These features provide the potential for an engine with higher efficiency than conventional piston engines. The engine design utilizes existing recent technology to advantage, such as silicon nitride ball pistons, so a large development effort is not required. 

The machine having only a small number of moving parts, the design implements a modified version of the tried and proven thermodynamic Otto cycle when used as an engine. Although the small part count is an important advantage, other advantages exist that will give future engineers new-found freedom in tailoring the combustion process. One great advantage is that the stroke magnitude and rate can be different for different strokes in the cycle (i.e., intake, compression, power, and exhaust). This provides the possibility of converting more energy to shaft power by greater expansion during the power stroke compared to the compression stroke. 

Another advantage is the ability to complete any even number of strokes per revolution in a single rotor. This effectively multiplies the power output proportionally if the stroke is maintained constant. Current concentration is on 2 and 4 stroke designs, but 8 or 12 or more stroke rotors are feasible, only limited by centrifugal loads at high speeds for a given rotor size. In addition, rotors can be stacked axially to increase power. 

Recently, an engineering breakthrough has enabled the virtual elimination of inertial forces that contribute to friction in the ball piston machine. Friction losses are thus low and independent of operating speed, in contrast to conventional piston engines, where friction losses increase with speed. In addition, recent simulated compressor testing has shown that remaining friction can be nearly completely eliminated by hydrodynamic action of lubricant at the ball piston. 

INTRODUCTION

Efforts to develop rotary internal combustion engines have been undertaken in the past, and are continuing. One main advantage to be gained with a rotary engine is reduction of inertial loads and better dynamic balance. The Wankel rotary engine has been the most successful example to date, but sealing problems contributed to its decline. The Hanes rotary engine uses an eccentric circular rotor in a circular chamber with sliding radial vanes. This engine has never been fully tested and commercialized, and has a sealing problem similar to that of the Wankel. A more recent development, the Rand Cam engine , uses axial vanes that slide against cam surfaces to vary chamber volume. Currently under development, it remains to be seen whether the Rand Cam can overcome the sealing problems that are again similar to those of the Wankel.

In the compressor and pump arena, reduction of reciprocating mass in positive displacement machines has always been an objective, and has been achieved most effectively by lobe, gear, sliding vane, liquid ring, and screw compressors and pumps  but at the cost of hardware complexity or higher losses. Lobe, gear, and screw machines have relatively complex rotating element shapes and friction losses. Sliding vane machines have sealing and friction issues. Liquid ring compressors have fluid turbulence losses. The new design concept of the Ball Piston Engine uses a different approach that has many advantages, including low part count and simplicity of design, very low friction, low heat loss, high power to weight ratio, perfect dynamic balance, and cycle thermodynamic tailoring capability. These aspects will be discussed in more detail below. 

CONCLUSIONS

Analyses based on the design assumptions showed that the ball piston engine has potential for achieving higher efficiency than piston internal combustion engines. In addition, subscale tests have shown that critical leakage and friction characteristics are consistent with design assumptions. Thus, the feasibility of this new engine concept based on ball pistons has been proven. 

A new approach to kinematic design has been devised to eliminate friction contributions from inertial forces in the engine. On the other hand, conventional carburetion/induction and exhaust systems are applicable to the new engine. Some material problems were encountered in subscale testing, indicating that more detailed material selection was warranted. The material selection has been done in anticipation of additional subscale tests to extend the range of speed and duration of simulated operation. Baseline material for testing is M2 tool steel. 

Shortly after cylinder material selection is verified in subscale tests, fabrication and testing of a prototype engine will be undertaken. The prototype will be used to finalize design details such as thermal design, transient operation, starting, and cylinder wall treatments with actual combustion environment. The new design concept can be immediately applied to compressor and pump applications in parallel with further engine development. The concept holds immediate promise for high efficiency and low cost in these applications, where temperatures and loads are more benign and lower cost materials can be used.