In the ever-evolving world of ultralight aviation, finding a suitable power unit has always been a challenge. While electric solutions are often touted as the future of aviation, the reality is that current battery capacity and their weight do not meet even the basic requirements of ultralight aviation. This makes long flights impractical and increases the risk of crashes and fires. Traditional piston engines, on the other hand, are not optimal in terms of installation and vibrations. This is where the new Birotary Engine comes into play. Designed specifically for ultralight aircraft, unmanned systems, and similar devices.

Anyone in the industry knows that ultralight aircraft require power units that are:

Current electric solutions, although they offer environmental benefits, especially in reducing noise and vibrations, have significant limitations. The weight and energy density of today’s batteries make long flights nearly unfeasible. Moreover, the risk of fires increases, as current batteries are inherently unextinguishable in the event of a fire until completely discharged or burned out. For these reasons, traditional piston engines remain in use, whether as the main propulsion system or as so-called range extenders, due to their cost-effectiveness, reliability, and proven solutions.

The Birotary engine addresses these challenges and offers a solution tailored to the specific needs of today’s aviation industry.

The Birotary engine is a three-cylinder, four-stroke, spark-ignition reciprocating engine. The three cylinders are arranged in a “star” configuration at a 120° angle and move within a rotating cylinder block. The cylinder block rotates within a stationary engine casing, which houses the intake and exhaust channels and spark plugs. This casing also serves as the cylinder head and contains two combustion chambers. Each combustion chamber is equipped with its own intake and exhaust channels and a set of spark plugs. The rotating cylinder block also functions as a valve timing mechanism and ensures the exchange of contents in the cylinders.

As illustrated above, during one rotation of the cylinder block, each cylinder completes two full combustion cycles. The first image shows cylinder 1 at top dead center before the intake channel opens. The second image shows the completion of the intake cycle for cylinder 1 (bottom dead center) and the beginning of compression. The third image shows the start of the expansion process in cylinder 1 after ignition. The fourth image shows the beginning of the exhaust process in cylinder 1.

The higher power-to-weight ratio of the Birotary engine results from both the increase in performance per unit volume and the reduction in weight. This is achieved through the following design elements:

The sliding valve mechanism allows for significantly larger cross-sections for intake and exhaust channels compared to conventional engines. In a conventional four-stroke engine, intake and exhaust valves are located within the cylinder bore along with the spark plug and/or injector. In the Birotary engine, the entire cylinder bore passes over the channels as the cylinder block rotates. Thus, the channel cross-section can be much larger than the valve cross-sections in the cylinder heads of conventional engines. Moreover, the sliding valve opens and closes faster than a poppet valve due to its design, contributing to better flow characteristics. This sliding valve mechanism can inherently achieve much higher filling efficiency compared to conventional four-stroke engines. The valveless design is also suitable for burning hydrogen. Below is a flow parameter comparison with a comparable conventional piston engine.

The Birotary engine has only one crankshaft pin, and its pistons move in a single plane, avoiding any longitudinal vibrations unlike traditional inline piston engines. The very short stroke results in significantly lower amplitude of inertial forces compared to traditional long-stroke designs. The star three-cylinder arrangement has minimal unbalanced inertial masses.

The Birotary design inherently reduces vibrations due to its counter-rotating masses. Upon ignition, the crankshaft accelerates while the cylinder block accelerates in the opposite direction. This significantly compensates for the reaction moment in the engine mount. With a suitable torsional damper between the engine and the propeller, this power unit can achieve exceptional comfort and low vibrations during operation. This has been confirmed in the first flight tests by a test pilot. More information at knobgear.com/the-antivibration-properties-of-the-birotary-engine.

Sealing the combustion chamber is a crucial design task in rotary engine design. The Birotary engine has patented seals housed in the stationary casing. The sealing elements are not subjected to inertial forces and have surface contact with the cylinder block. Good heat removal from the sealing elements is ensured by the cooled stationary casing.

Thanks to further improvements made during development, we achieved a calculated torque, confirming successful sealing of the combustion chamber.

More information at knobgear.com/cs/new-birotary-engine-patents.

The development of the engine to date has taken place at KNOB Engines s.r.o., where we have reached important development milestones.

The Birotary engine has the potential to advance the ultralight aircraft and UAV industry. This engine offers a robust, efficient, and versatile solution that meets modern aviation requirements. Our development to date demonstrates the potential of our Birotary engine design.

For more information, please visit www.knobgear.com.