Ball piston machines

Schematic Diagram of BALL PISTON ENGINE

BALL PISTON ENGINE

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 Wakelin 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.

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 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. 


 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. 

THE DESIGN CONCEPT

Although the design is applicable as a compressor, pump, motor, or engine, the engine implementation will be used for concept discussion. Figure 1and Figure 2 show end and side cross section views, respectively, of a four stroke engine design.

Mode of operation - The basis of the design is ball pistons rolling on an eccentric track. The balls exert tangential force on the cylinder walls which turn the rotor. Useful power is available at the rotor output shaft. The combustion chambers are within the spinning rotor. Chamber porting for intake, compression, power, and exhaust strokes is achieved by passage of the chamber tops across an internal stator with appropriate feeds as the rotor spins.
Beginning at top dead center (TDC) at 0 degrees rotation, the stator intake passage is open to the cylinder and a fuel/air charge is pulled into the cylinder as the ball piston moves radially outward for the first 90 degrees of rotation.

 Then the intake passage is closed off, and the ball reverses radial direction for the next 90 degrees of rotation, during which time the new charge is compressed (compression stroke).
Just past 180 degrees rotation, the compressed charge is ignited as the cylinder port passes a small ignitor port. Combustion ensues, and the high combustion pressure pushes radially outward on the ball piston for the next 90 degrees of rotation. The ball in turn pushes tangentially on the cylinder wall because of the "slope" of the eccentric ball track, which is now allowing the ball to move radially outward. The tangential force produces useful torque on the rotor (power stroke).
At 270 degrees of rotation, the spent combustion charge is allowed to escape through the exhaust passage as the cylinder port is uncovered. Exhaust is expelled as the ball moves radially inward for the next 90 degrees of rotation (exhaust stroke). Then the cycle repeats.
Important Design Features - The basic operation of the new design is conventional for an internal combustion engine, i.e. a piston reciprocates within a cylinder, and with porting, implements the four strokes of the Otto cycle. However, there are a number of features that make this engine design favorable for high efficiency and emissions control.
The porting required for four stroke operation is achieved with no additional moving parts, and no valve train losses. The porting mechanism is achieved with simple port clocking within the rotor/internal stator bearing interface. Thus, part count is low and the hardware is simple in geometry, with only the rotor and ball pistons as moving parts.
Note that cylinder induction and mixing are aided by centrifugal and coriolis accelerations, because the cylinders are within the spinning rotor.
Sliding friction sites are minimized by the use of a rolling ball piston. Friction at conventional piston rings, piston pin, and connecting rod/crankshaft bearing are eliminated. Sliding friction still exists at the ball/cylinder wall contact, but is minimized by special material selection and working gas hydrodynamics (and possibly local lubrication). The rotor/stator bearing is of a gas or fluid hydrostatic type, so friction is very low at that site.
The use of an eccentric ball track allows tailoring of the chamber volume vs. time to optimize the cycle from a thermodynamic and chemical kinetics standpoint. The only requirement is that the ball return to the starting radius at TDC before intake. For example, the expansion/exhaust stroke length can be made different than for intake/compression for more exhaust energy recovery, or the combustion can be held at constant volume for a certain period.
Multi-cycle rotors can be implemented. Instead of 4 strokes, 8, 12 or more strokes can be traversed in a single revolution. Compressors and pumps can use any multiple of 2 strokes (intake and compression only), either in parallel or staged arrangement. Provided that inertial forces are controlled (to be discussed later), power to weight ratio can therefore be made high. Other engine configuration options are also under investigation, including a dual rotor/intercooler configuration, diesel cycles, and 2 stroke cycles. The dual rotor option is attractive because it allows the compression and expansion ratios to be widely different (on separate rotors), but there are interstage pumping and intercooling losses that must be considered.
The use of many ball pistons, which each undergo the four strokes in clocked fashion, results in smooth power delivery and small net oscillatory forces. In fact, the total ball inertia forces are automatically balanced by symmetry if the number of balls is even. Further, combustion forces can be balanced by using an eight stroke rotor or stacking rotors axially with relative clocking. Also note that a four (or higher) stroke rotor compressor would be balanced.
Novel design of the ball track has been devised that will eliminate inertial forces on each ball that contribute to friction. As the ball moves in and out radially on the eccentric track while the rotor spins, coriolis and other acceleration forces are generated on the ball radially and tangentially. Net tangential inertial forces contribute to friction at the ball/cylinder wall contact point. By changing the ball
rolling radius using a widening/narrowing dual contact track in a prescribed manner, Figure 3, the net tangential inertial forces on the ball can be eliminated. In essence, the track design results in a balance of translational and rotational ball kinetic energy to eliminate tangential force. In other words, the ball track is designed so that the ball rolls around the track in synchronization with the rotor at constant rotation rate. Due to the form of the laws of motion, it is possible to maintain this condition at all rotation rates with a fixed track design. This allows the machine to be run at any high rpm desired, until the mechanical limits of the ball piston rolling on the track are reached (Hertzian stress fatigue). Engine power theoretically increases linearly with rpm. In actuality, intake flow dynamics may limit peak power at very high rpm, but that depends on the intake passageway details.
There is another interesting by-product of the rolling ball approach. The ball spins at very high rates around its own axis, while it is radially compressed by centrifugal forces of rotation about the rotor axis. These two sources of inertial load tend to cancel out in terms of generating internal ball stresses. This allows high engine speeds to be sustained with less ball fatigue damage.
Heat loss is kept low because the engine intake can be configured to flow through the outer stator/rotor cavity. Rotor heat loss is gained by the intake charge, with less loss to the outer stator.

Technical Challenges - The main concerns for operation of the new machine are being addressed in focused subscale testing.
First, leakage through the ball piston/cylinder gap is a significant factor for engine or compressor efficiency, especially at low speeds. Calculations show that the flow is choked during combustion due to high pressure differential and small clearance area. Choking is helpful in keeping leakage to acceptable levels. Engine efficiency predictions based on simple choked flow leakage models are very favorable. Leakage tests performed in subscale testing have shown that leakage is less than the simple models predict, and dependence on ball spin, pressure, and rpm have been and are being characterized.
Second, the friction and wear at the ball piston/cylinder wall sliding interface is important. Engine performance depends on the magnitude of the effective friction coefficient, and high relative sliding speed can contribute to wear. Engine efficiency predictions based on an average friction coefficient of 0.1 or less are very favorable. Subscale tests have proven that the coefficient of friction for a


silicon nitride ball piston on polished steel with no lubrication is about 0.075 +/- 0.03, about the same as estimated.
The wear issue must be proven out mainly by testing with a full range of operating conditions. Thus far, tendency for cylinder wall plasticity has indicated that cylinder material must be of high hot strength and hardness. Large reductions in "wear-in" plastic flow were achieved by changing cylinder walls from 1018 hot rolled steel to 17-4PH hardened to about R_c 44. A material with better hot hardness, such as achievable with M2 high speed tool steel, has been subsequently selected to resist high sliding flash temperatures and completely eliminate cylinder wall plastic deformation. Low cost production options include case hardening, plating over a hot hard substrate, coatings, and other surface treatment technologies.
It is intended to design the machine for no lubrication, except that available from the working gas or fluid. This is most feasible for compressor and pump applications. However, lack of lubrication is a driving consideration in cylinder wall material selection for the engine, based on subscale testing to date with air only. Extra lubrication is a secondary design option that may be best for some applications, especially the engine, where loads are higher. Lubrication can reduce friction coefficient and wear potential and provide hydrodynamic separation at the ball piston/cylinder wall, and also can reduce leakage flow past the ball piston. However, there will be a trade off for residue build up, emissions, and maintenance.

MATERIAL SELECTION

A series of tests to characterize friction and wear of various materials in a simulated ball piston engine/compressor were performed to aid in selection of production materials. Another goal of the tests was to explore the possibility that

                            

this machine could run without lubrication. If unlubricated operation was not possible, tests with instrumentation                                                                                                                                                 would be used to confirm the lubricated operation of the machine and the resulting internal loads. Two types of tests were performed. Simple drag sled static coefficient of friction tests were completed to evaluate sleeve materials and coatings. The materials with the lowest friction coefficient then became the focus of operational dynamic tests using a subscale tester.

TEST SETUP   

                                                                                                                                                                      The test setup was designed to simulate the conditions of a typical ball piston compressor. The key response parameters of interest for these tests were friction, wear, and leakage.

The test setup consisted of a test cylinder suspended on load cells, an electric motor driving an eccentric circular steel drive wheel with a simple ball track on the edge, a ball piston driven by the eccentric wheel, compressed air supplied to the cylinder, and a cylinder heater. The test cylinder had a removable sleeve so various material and coating combinations could be tested. The heater was used to change the sleeve temperature to partially simulate compressor thermal conditions and to adjust the ball piston clearance. A drip lubrication system for applying oil to the drive wheel track was fabricated to perform lubricated tests. This system deposited approximately five drops of 10 weight motor oil on to the drive wheel track per minute. A photograph of the test set up is shown in Figure 1.
The configuration of the eccentric drive wheel and its position relative to the cylinder axis results in simultaneous vertical oscillation and spinning of the ball within the cylinder and oscillating mechanical leverage angle that closely approximates the actual operation of a ball piston machine. Of greatest importance, given a cylinder internal pressure via supply air, ball/cylinder wall interaction forces (including sliding friction) are closely simulated.
Dynamic and static cylinder support loads, cylinder and tank pressure, and cylinder temperature were measured via a PC-based data acquisition system. The operating friction coefficient between ball piston and sleeve was indicated by correlation with one of the load cell signals. Leakage was calculated using tank pressure drop measurements over time. Wear was visually observed on the ball and sleeve after testing.

TESTING

First, static friction tests were performed on uncoated steel, cadmium plated steel, steel coated with Alumide, steel coated with Chromide, and steel coated with TiN, TiCN, TiAlN, or TiAlCN ion deposition treatments. In addition, tests were done after polishing TiAlN and TiAlCN samples with fine grit lapping compound. The polished TiAlCN coating was selected as the best sleeve coating for sub scale testing. It exhibited the lowest friction coefficient against a silicon nitride ball, about 0.06, comparable with that of smooth plain steel. This was a welcomed result, showing that the hard ceramic-like coating did not adversely interact with the silicon nitride ceramic. Polishing was necessary to remove sputter residue that increased roughness. The tests indicated that using a highly polished surface would minimize friction coefficient in production.

Next, operational tests were performed with a wide variety of sleeves and several ball types. Mild steel, Chrome plated steel, plain M2 tool steel, and TiAlCN coated M2 tool steel sleeves were tested. Silicon nitride, Alumina, chromium steel, Teflon, and Nylon balls were used. In addition, cryogenically treated plain M2 and TiAlCN coated M2 tool steel sleeves were tried. This cryo treatment was purported to improve wear resistance.

RESULTS

First, the Teflon and Nylon balls were eliminated from consideration due to insufficient dimensional stability. They deformed enough just in storage to preclude fitting the cylinder.

Although some of the remaining materials and coatings worked better than others, none of the unlubricated tests were successful. The heat generated at the ball piston/test sleeve interface, despite reasonable Hertzian stress levels, caused "smearing" failure of the sleeve wall shortly after the tests were started. Material was actually removed from the cylinder walls and redeposited. A distinct wear pattern was visible on the sleeve and most tests were stopped because the ball seized in the sleeve on the built-up material.
Lubrication solved the problem. The silicon nitride ball ran smoothly in the M2 tool steel sleeve. Test durations of 100 minutes at 200 psi and 800 RPM and 60 minutes at 400 psi and 800 RPM were achieved with no visible wear on sleeve or ball. Figure 2 shows the sleeve bore after the test. In addition, leakage was greatly reduced.


An Alumina ball was successfully run for 10 minutes at 400 psi and 800 RPM as well in a lubricated M2 tool steel sleeve. The sleeve and ball showed minimal signs of wear. Alumina is heavier than silicon nitride, but is much less expensive, and is a viable option for compressor, pump, and motor applications.
A 20 minute test at 400 psi and 800 RPM was run with a chrome steel ball on a lubricated M2 tool steel sleeve. The test was successful but indications of wear were observed. It is likely that the lower modulus of steel caused more deformation of the ball, resulting in a wider contact area, and some cylinder wear indicated as "fogging" of the shiny M2 sleeve surface. The ball also showed fine scratching in a directional pattern, and steel removed from the ball was seen in the lube oil. It is clear that the steel ball would not be acceptable for long term operation.

After this series of tests it is clear that lubrication must be used for the design to work as a compressor or engine with conventional materials. Without lubrication of some kind, the localized heat at the rubbing contact is too great even for high temperature resistant metals and coatings. It may be possible, for compressor/motor/pump applications, to use a ball piston made of some "high-tech" self-lubricating material, such as graphite-impregnated metal or a dimensionally stable and lubricious plastic. These materials may be investigated in the future. For an engine, however, the higher temperature requirements would preclude such materials, and oil lubrication is likely to be the best approach.

It was also found that oil lubrication can greatly improve machine efficiency by virtually eliminating rubbing friction and reducing leakage. Lubrication increases mechanical efficiency from approximately 85% to approximately 97%. Most of this improvement is due to friction reduction. Results indicate that the lube oil produces a hydrodynamic bearing layer at the contact region, resulting in negligible friction coefficient. If this is true, cylinder surface hardness is no longer a great concern, and special coatings and treatments are unnecessary.
Using a stiff ceramic ball with lubrication should work well in a compressor or an engine as long as the lubrication system is properly designed. High speed engines will require a silicon nitride ball to minimize centrifugal loads at high speeds. It is likely that materials other than M2 tool steel could be used as a cylinder material with proper lubrication, such as conventional cast iron and chrome plated iron or steel.
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.