Birotary Engine Advantages over a Traditional Four-Stroke Engine
Higher Power-To-weight Ratio
Higher power-to-weight ratio of the bi-rotary engine comes both from significant increase of the power per unit of volume and weight reduction, primarily from the following three design features:
High volumetric efficiency enabled by large intake ports
Compared with traditional four-stroke engines the bi-rotary engine’s intake and exhaust ports have significantly larger cross-section. In a traditional four-stroke engine, the intake and exhaust valves along with the spark plug and / or the fuel injector must all fit into the cylinder bore area. In the bi-rotary engine, the cylinder bore passes over the area of the intake port during the cylinder block rotation. Therefore, the cross-section of the port can cover a large share of the cylinder bore. A comparison with four-valve cylinder head on figure above clearly demonstrates this advantage. The port’s specially designed shapes enable much faster opening and closing than poppet valves. This contributes to the overall better flow characteristics and, ultimately, better volumetric efficiency of the bi-rotary engine.
Higher maximum RPMs allowed by very short stroke design
The short stroke design supports high RPMs while keeping mean piston speed within acceptable limits. The engine prototype has a 37mm stroke and a mean piston speed of 12.3 m/s at 10,000 relative RPM of the crankshaft versus cylinder block.
Higher peak torque with relatively flat torque curve
High volumetric efficiency translates into higher peak torque. The torque curve remains relatively flat at high RPMs. The slide valvetrain with large ports avoids deterioration in performance typical for poppet valvetrain at high RPMs.
Low Vibration
The bi-rotary engine has only one crankpin and its pistons move in a single plane. Therefore, the engine does not suffer from any longitudinal vibrations, unlike traditional in-line piston engines. The very short stroke design results in much lower amplitudes of inertial forces than traditional the long stroke design.
Centrifugal forces on the pistons reduce the maximum compressive force on the connecting rods and the Coriolis forces partially balance the forces from the inertial masses of the crank mechanism.
During one absolute crankshaft revolution, there are eight moments when the forces in the plane of the crank are zero. The resulting vector of all inertia forces acting on the engine have almost constant magnitude (depending on the crankshaft counterweight precision) and its direction is rotating around the axis of the engine with a frequency five times higher than the rotational frequency of the crankshaft.
This high frequency of rotation of the resultant force vector causes vibrations with very small deflections. The computer simulation of the engine in an environment without external influences (unattached engine, no gravitational field) showed maximum amplitude of 0.1 mm at operational rpm.
Additionally, if required, the engine could be perfectly balanced with a single balancing shaft. Details about simulations of kinematics and engine balance are the subject of a separate report covering computational analysis. The bi-rotary engine is characterized by a very smooth operation. The rotary movement at the engine’s output is characterized by a very small rotational unevenness. This is due to the high frequency of ignitions, small cylinder volume and the combined use of the inertia of the crankshaft and the rotating cylinder block. This will reduce vibrations by evening out the forces on the engine mounts.
Further, it is advantageous that the crankshaft of the motor spins three times faster and to the opposite side than the propeller, which partially compensates the gyroscopic moment of this propulsion device.
Compact Design
Small packaging dimensions relative to its performance is a major benefit of the bi-rotary engine. The engine is both small in diameter and short in the axial direction. The engine prototype dimensions are provided in Figure below. In aircraft applications, the small diameter results in lower drag of the engine housing. This considerably increases the performance of the propeller mounted in front of the engine. The compact size is also very important for electric vehicle range extenders.
Simpler Construction
The slide valve train is much simpler and easier to work with than the DOHC valve train of traditional piston engines. The bi-rotary engine valve train contains only gears that provide the transmission between the crankshaft and the engine block. There are no camshafts, rocker arms, valves, valve lash adjusting system, etc. This makes the engine lighter, cheaper to manufacture and easier to maintain and repair. At the same time, the slide valve system does not rule out variable valve timing.
Superior Combustion Chamber Design
The bi-rotary engine enables combustion chamber solutions that are not easily available in standard four-stroke engines. For example, there is space to place several spark plugs to ensure good burning of the mixture. The engine prototype uses twin spark plugs. The surface of the combustion chamber in the cylinder head can be simply and efficiently cooled. There are no hot spots, such as exhaust valves and thin bridges, between the valves, which are hard to cool. For these reasons, the engine can easier achieve a higher compression ratio with the resulting increase in efficiency. Direct injection or even a variable compression ratio system can be easily placed in the combustion chamber.
Dual Speed Output
The bi-rotary engine offers another advantage specific to its design. The engine can provide power output via both the crankshaft and the cylinder block. The cylinder block rotates at 1/3 of the speed of the crankshaft. In aircraft application, the cylinder block RPMs are better suited for matching modern high efficiency propellers without the necessity of an additional reduction gearbox.
Advantages Expected in a Serial Production Model
In addition to the advantages already demonstrated by the engine prototype, number of additional advantages are expected as the engine is further developed into a serial production model:
The bi-rotary engine should be cheaper to manufacture considering the engine’s simplicity and lower weight.
It should be also easier to maintain with likely longer time between overhauls. In piston engines, the requirement for overhauls is mainly driven by wear of the piston seals. Considering the very short stroke and thus the shorter distances travelled by pistons the engine should perform well in this respect. The high overhaul requirements in rotary Wankel engines are driven by the poor geometry of the sealing at the apex, as well as the high speed, with which the apex-sealing element moves. Both of these problems are resolved by patented sealing solution.
The bi-rotary engine has fewer parts, which reduces warehouse and supply chain requirements. Combined with the simple construction of the engine repair complexity and costs should be significantly reduced compared to traditional engines.
The engine can also potentially operate with fewer accessories. A starter/generator could be integrated into the engine’s block. For some applications, an optimized cooling system could be developed that use the engine oil to also cool the cylinder head. Since the engine already has a full-fledged oil cooling system for the cylinder block, the additional water cooling system at these applications can be completely eliminated.