• The SRAM engine consists of two concentric toroidal rings of different diameter. The engine shaft (power output) is located at the center of the two rings.

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• The inner ring forms the intake and compression chamber, while the outer one is the combustion and expansion chamber and the two chambers communicate through an intermediate chamber.

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• The gas exchange between the chambers is achieved with the use of sliding ports and valves.

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• A rotating moving arm (or disc) interconnects the pistons and the shaft of the engine. All the moving parts of the engine perform rotating motion.

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• Two other major components of the engine are the fuel injector and the spark plug. 

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• Additional peripheral components include the fuel supply lines, the electric/electronic components for operation and control, engine mounts etc. Although such components are necessary for the operation and control, they are not related with the basic operating concept of the engine. Therefore, they are not depicted and described further.

From the thermodynamic point of view, the operating cycle of the SARM engine is based on the Atkinson cycle, which can theoretically reach up to 20% higher efficiency than the Otto cycle. The major characteristic of an engine running on the Atkinson cycle is that the expansion ratio is higher than the compression ratio. This thermodynamic cycle is already applied in reciprocating piston engines; Toyota Prius has an effective compression ratio (CR) of 8:1 and an expansion ratio (ER) of about 13:1. After testing, Toyota concluded that the engine running on the Atkinson cycle can be 12%-14% more efficient than the corresponding engine running on the Otto cycle.

Compared to conventional reciprocating engines, where all processes take place within the engine cylinder (otherwise called, the combustion chamber), but during different strokes, in this engine, all processes occur simultaneously, but in separate chambers. These types of engines are called split engines. On the other hand, all chambers, pistons rotation and the shaft are located around the same axis and from this point of view, it could be said that the SARM engine presents some similarities with gas turbines. Both are concentric internal combustion split engines. The presence of the Pressure Chamber (PC), between the compression (CPC = Compression Chamber) and the combustion (CBC = Combustion Chamber) chambers, allows for the independent optimization of each thermodynamic process (intake, compression, combustion, expansion). Decoupling the processes and the volumes where they take place offers great flexibility, e.g. on the shape and the material of each chamber, or even on the surface treatment of each chamber.

From the dynamics point of view, the SARM engine also presents some particular features. Compared to conventional reciprocating engines, the SARM engine eliminates any reciprocating motion in all directions, limiting thus oscillations and confining vibrations, mechanical losses and additional inertial forces, which increase the thickness and weight of an engine’s component. Furthermore, the concentric operation of the SARM engine avoids the additional inertial forces developed by eccentricity in other rotary engines, permitting also higher piston speeds that increase the power density. Finally, the symmetric location of pistons balances the engine, neutralizing oscillations due to centrifugal and inertial forces, while in conventional reciprocating engines a system of counterweights is usually needed to balance the crankshaft. The generated symmetry of the SARM engine eliminates the mechanical vibrations and diminishes sound effects.

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Phases of operation

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1. The piston rotates with a speed of 3000 to 12,000 rpm inside the compression chamber (CPC), which has a toroidal shape.

2. The piston and the sliding port (SP) divide the CPC into two chambers; the intake chamber and the compression chamber. 

3. Atmospheric air is dragged through the intake port into the intake chamber, the trapped air between the piston and the SP is compressed.

4. The SP remains closed during the whole compression process, opening only to let the piston pass, as it moves. 

5. Valves open before the SP opening and the compressed air is delivered into the combustion chamber.

6. Inside the combustion chamber, the processes of fuel injection and mixing with the air take place. Combustion is initiated by a spark plug.

7. The expansion of the burning mixture, exercises a force on the CBC piston producing work and, through the rotating moving arm, transfers the rotating motion and the work to the engine shaft and the CPC piston. At the same time, the other side of the CBC piston pushes the burned gases of the previous combustion cycle to the atmosphere, through the exhaust port.

8. By this brief description of the SARM engine operation, a number of its special features become apparent, which form the basis of its advantages compared to the conventional reciprocating piston engines.

A major difference between the SARM engine and all conventional SI engines (either reciprocating or rotary) involves the operation of the pistons. While in conventional 4-stroke reciprocating engines, the same piston performs periodically the four different strokes (intake, compression, expansion, exhaust), in the SARM engine each piston performs continuously the same two individual strokes, and the same stroke takes place every time at the same side. The intake process takes place at the back side of the compression (CPC) piston, while the compression is realized with its front side. Similarly, the expansion takes place at the back side of the combustion (CBC) piston, while the burned gases are exhausted to the atmosphere by its front side. Thus, all four strokes (intake, compression, expansion, exhaust) occur simultaneously with one pair of CPC and CBC pistons. These mean that the CBC piston undergoes only the high pressure-high temperature processes, remaining “hot”, and the CPC piston operate in the low pressure-low temperature part of the cycle, remaining “cold”. This offers an extra flexibility to the SARM engine, which is the separate optimization of the materials for each piston, taking also into account that they rotate in significantly different radii.

The concentric design is another special feature of this rotary engine. At first, the rotary operation eliminates the mechanical losses imposed by the transformation of the piston’s linear motion the crankshaft’s rotational motion. In addition, one main moving part is responsible for all processes, which rotates concentrically with the engine shaft. This configuration minimizes the parts of the engine, reducing drastically its complexity and weight. On top of that, the SARM engine design has an additional positive characteristic. The pressure force applied on the piston during combustion and expansion is always vertical to the arm connecting the shaft and the piston. This means that this force is always transferred tangentially to the outer surface of the engine shaft and 100% of this force always produces torque. Contrary, in a conventional reciprocating engine, the force producing torque on the crankshaft is only a portion of the pressure force on the piston, depending on the instantaneous position of the crankshaft. Thus, for the same produced torque, the SARM engine needs to develop lower combustion pressures, enhancing higher efficiency and allowing for lighter and thinner combustion chamber walls. The higher efficiency is translated to lower fuel consumption, while the lower combustion pressures result to lower temperatures, too, with a subsequent positive effect on NOx formation.

Advantages of the novel engine

1. Simple design and less moving parts than the reciprocating engines. The fewer engine components reduce also the need for maintenance.

2. Up to 4 times smaller and lighter than the 4-stroke reciprocating engines.

3. There are no mechanical losses and inertial forces from the conversion of reciprocating to rotational motion. The absence of reciprocating motion eliminates oscillations.

4. No change in the direction of the piston motion, the characteristic piston “slap” occurring at the TDC of a conventional reciprocating engine, causing wear of the piston skirt and the bore, and noise.

5. Pressure force on the piston is always tangential and is fully exploited for power production. In addition, the applied forces are only in the direction of the flow allowing to the piston sleeve to be very short.

6. Concentric design that eliminates inertial forces developed by eccentricity in other rotary engines. 

7. Lack of lubricant inside the compression and combustion chambers. As it is well-known, the lubricant may also contribute to the unburned HC emissions of an engine.

8. No need for counterweight for crankshaft balancing, The SARM engine is self-balanced due to the symmetrical position of the pistons.

9. Up to 15% lower fuel consumption, taking advantage of the higher efficiency, as explained previously.

10. Great potential for lower NOx emissions, owing to the lower combustion temperatures. In addition, the compression chamber can be designed in such a way that the heat dissipation to the coolant (or the environment) is enhanced and the charge air remains at a lower temperature (thus higher density) before entering the combustion chamber.

11. Decoupling of the main engine processes. Although that this is an inherent characteristic of any split engine, and not only of the specific one, it is important to be mentioned here as it offers great advantages compared to the conventional reciprocating piston engines. As described previously, each process occurs at a separate volume and on a specific side of each piston, enabling thermodynamic optimisation, as well as selection of different materials and application of individual surface treatment.