WANKEL ENGINE AND HYDROGEN

The Wankel engine, also known as a rotary engine, was invented in 1954 by engineer Felix Wankel while working at NSU. Although this engine was first used in the NSU Spider and achieved significant success in racing, it was not widely adopted due to inadequate sealing of the rotor segments, which led to excessive problem. However, with advances in materials and production technology and growing interest in alternative fuels and propulsion systems, it has regained attention in recent years.

Wankel engines have been used in automobiles such as the WATER Spider, NSU Ro-80, Mazda R-100, Mazda RX-7 and Mazda RX-8 such as.

Principle of Operation of Wankel Engines

The main advantage of the Wankel (rotary piston) engine is its mechanical simplicity in manufacturing. The Wankel engine has a much simpler structure than a conventional piston engine. It consists of a rotor (rotary piston) that rotates eccentrically within an oval body and an eccentric shaft (which performs the function of the crankshaft in four-stroke engines; designs may include two, three or four rotors).

Rotary piston; rotates on the eccentric shaft. While the eccentric shaft rotates in a circular motion about the center, the rotor mounted on it moves within the cylinder in an epitrochoidal path. These engines do not have valve mechanisms. Despite the absence of intake and exhaust valves, the air-fuel mixture enters through intake ports during the intake phase, and the toxic combustion gases exit through the exhaust port into the atmosphere. The torque generated by the rotary piston is transferred to the transmission via the eccentric shaft. The gears inside the rotary piston mesh with the gears on the eccentric shaft and rotate accordingly.

Relative motion reduction between the piston and housing is achieved by a pair of internal and external gears with a ratio of 2/3. The external gear is fixed to the housing and the internal gear rotates with the piston, rolling along the stationary external gear. Thus, the eccentric shaft also serves the function of driving the piston. When the eccentric shaft completes one revolution, the piston rotates 120° relative to the eccentric shaft. Therefore, for the piston to complete one full revolution relative to the housing and finish the combustion cycle, the eccentric shaft must rotate three times. That is, to produce one power stroke, the eccentric shaft gear must rotate 3 × 120° = 360° around the external gear.

Wankel engines can be described as either two-stroke or four-stroke.

They resemble two-stroke engines in that the combustion cycle is completed with each revolution of the rotor.

They resemble four-stroke engines in that each of the processes—intake, compression, ignition and exhaust—occupy equal angular intervals.

In this engine, two rotational speeds can be defined: one for the piston and one for the eccentric shaft. Since power is taken from the eccentric shaft and its output is measured at its ends, the rotational speed of the eccentric shaft is more suitable for comparison purposes.

Engine Operating Stages

In Wankel engines, the rotary piston completes all four strokes through its epitrochoidal motion within the housing. There are three ignition surfaces on the rotary piston, spaced 120° apart. Thus, as the rotor completes one revolution, it performs intake, compression and ignition. This principle allows high power output in a compact volume.

Intake Stroke

As in internal combustion engines, during the intake stroke the air-fuel mixture is drawn into the cylinder through the intake port.

 Compression Stroke

During this phase, the air-fuel mixture drawn in is compressed against the cylinder wall by the two edges of the rotor, increasing its pressure. As the rotor rotates, it further compresses the mixture and prepares it for ignition.

 Ignition (Power) Stroke

The compressed air-fuel mixture is ignited by spark plugs, initiating combustion. The resulting expansion and pressure wave cause the rotor to rotate, thereby producing the desired power output.

Exhaust Stroke

During the exhaust stroke, the burned gases produced by combustion of the air-fuel mixture are expelled through the exhaust port, via the exhaust manifold and pipes, into the atmosphere.

At this time, one edge of the rotor is expelling exhaust gases while the other edge seals off the intake port.

Disadvantages of Wankel Engines

- Sealing of the combustion chamber. In conventional piston engines, compression loss into the crankcase is prevented by circular piston rings. In Wankel engines, however, sealing is achieved by the cylinder head being tightly fastened to the cylinder block with special gaskets and bolts. The main problem in rotary piston engines is leakage of compression at the apex of the rotor every 120° and along the side surfaces of the rotor. Additionally, special oil seals or oil rings are required to prevent lubricating oil from entering the combustion chamber. The total number of sealing elements in a rotary engine is fewer than in conventional piston engines, but the manufacturing of these seals and rings is more complex and expensive due to the engine’s unique structure and efficiency requirements.

- Noisy operation occurs as the rotor passes rapidly over the opening exhaust port. However, in some Wankel engines, the exhaust system has been designed to disperse sound waves, improving the exhaust tone.

- In the combustion chamber, the compressed air-fuel mixture undergoes two-stage sequential combustion, causing heat and pressure waves to flow toward the exhaust ports through the spark plugs. This results in unwanted residual heat on the materials. Although a flow of cool, clean air entering through the intake ports helps cooling, the residual high temperature on the cylinder and rotor causes excessive thermal expansion of materials, leading to significant difficulties in maintaining sealing integrity.

Advantages of Wankel Engines

- The compression ratio in rotary piston engines is higher than in conventional piston engines, and the flame front generated after ignition travels a longer path. As a result, more combustion force is generated per ignition event. The combustion chamber in these engines is divided into two sections, with the second section being smaller. Thus, combustion initiated in the first section intensifies as it enters the narrower second section, increasing combustion pressure. At this point, the flame front exerts pressure on the surface of the rotary piston, driving its rotation.

- Compared to conventional internal combustion engines, Wankel engines offer numerous advantages in terms of weight, torque and performance. Their weight is lower and they produce circular motion directly, allowing more power to be extracted from the engine shaft with less fuel consumption.

- Wankel engines can achieve increased power output through the use of a turbocharger with an efficient intake system. With a twin-scroll turbocharger and intercooler, they produce higher torque than conventional engines. There are two exhaust ports: a primary and a secondary (wider). The intake vacuum closes the wider secondary port at low engine speeds, using hot exhaust gases to warm incoming air.

Wankel Engines and Hydrogen Applications

Hydrogen is a promising alternative fuel for the future. Wankel engines, considered engines of the future, are seen as ideal candidates for efficient hydrogen utilization. The rotating chamber of the Wankel rotor creates a moving combustion volume, and its larger surface area compared to other engines helps dissipate generated heat. Due to the distinct separation of intake, compression, expansion and exhaust zones, the moving flame front of hydrogen produces no detonation. It is widely believed that Wankel engines are highly compatible with hydrogen fuel. The Wankel engine is well suited for hydrogen use.

Mazda continues development efforts on hydrogen-blended hybrid engines for its Wankel-powered models such as the RX-7 and RX-8.

Use of Wankel Engines in UAVs

Wankel engines enable very high power output from a small engine volume, and offer numerous advantages including low vibration, light weight, simple construction, high power-to-weight ratio, high volumetric efficiency efficiency and reduced susceptibility to knocking. Due to these characteristics, they are applicable across a wide range of fields, from marine industries to light aircraft. Their use in UAVs is particularly advantageous, where high power-to-weight and power-to-size ratios are essential. Further improvements can enhance their operational advantages.

It is highly likely that Wankel engines experience misfiring issues at higher altitudes. If the combustion characteristics and high-altitude performance of Wankel engines are understood, appropriate measures can be taken to mitigate misfiring. Analyses have shown that these potential problems can be identified and addressed accordingly. Additionally, as altitude increases, the temperature, pressure and density of the intake air decrease, altering the properties of the air-fuel mixture entering the combustion chamber. Therefore, the impact of changes in the air-fuel ratio on Wankel engine performance must be studied.

Studies have demonstrated a significantly positive effect of hydrogen enrichment on Wankel engine performance. When 5% and 10% hydrogen was added to a gasoline-fueled reference Wankel engine, average effective pressure increased by 8.18% and 9.68% respectively, and indicated torque increased by 6.15% and 7.99% respectively, while specific fuel consumption decreased. Additionally, hydrocarbon (HC) and carbon monoxide (CO) emissions were reduced. However, due to higher operating temperatures resulting from hydrogen enrichment, nitrogen oxide (NOx) emissions increased compared to pure gasoline operation.