Abstract:The goal of this investigation is to propose a series/parallel hybrid electric power system for ultralight aviation designed to improve safety and, possibly, reduce fuel consumption. The power system consists of a Wankel engine, two electric machines, a battery, and a planetary gear set, all acquired from the automotive market. After a preliminary design based on takeoff power, the system is simulated over a typical flight mission and in case of engine failure for a first validation of the proposed powertrain. The investigation also shows a comparison in terms of performance and fuel consumption between the initial configuration (reciprocating piston engine), a non-hybrid Wankel arrangement, and the proposed hybrid electric configurations by using in-house simulation software. A heuristic energy management strategy is proposed as well. During a typical mission, the new powertrain works as a parallel hybrid during takeoff and climb, thus ensuring high performance and safety. During the cruise, the system behaves like a parallel hybrid with a continuously variable transmission that makes the engine work always at high efficiency. The battery is partially recharged during the descent by the extra power of the engine. The preliminary results reported in this work predict an improvement in fuel consumption by about 20% compared with the initial piston engine configuration and 28% compare with the non-hybrid Wankel powertrain, despite the larger takeoff weight.Keywords: safety; hybrid electric aircraft; series/parallel configuration; Wankel engine
The Wankel engine (ˈvaŋkəl̩, Vann-KELL) is a type of internal combustion engine using an eccentric rotary design to convert pressure into rotating motion. It was invented by German engineer Felix Wankel, and designed by German engineer Hanns-Dieter Paschke. The Wankel engine's rotor, which creates the turning motion, is similar in shape to a Reuleaux triangle, with the sides having less curvature. The rotor rotates inside an oval-like epitrochoidal housing, around a central output shaft. The rotor spins in a hula-hoop fashion around the central output shaft, spinning the shaft via toothed gearing.
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Due to its inherent poor thermodynamics, the Wankel engine has a significantly worse thermal efficiency and worse exhaust gas behaviour when compared against the Otto engine or the Diesel engine, which is why the Wankel engine has seen limited use since its introduction in the 1960s. However, its advantages of compact design, smoothness, lower weight, and fewer parts over the reciprocating piston internal combustion engines make the Wankel engine suited for applications such as chainsaws, auxiliary power units, loitering munitions, aircraft, jet skis, snowmobiles, and range extenders in cars. In the past, the Wankel engine has also been used in road vehicles such as motorcycles, and racing cars.
The Wankel engine is a type of rotary piston engine and exists in two basic forms, the Drehkolbenmotor (DKM, "rotary piston engine"), designed by Felix Wankel (see Figure 2.) and the Kreiskolbenmotor (KKM, "circuitous piston engine"), designed by Hanns-Dieter Paschke[1] (see Figure 3.), of which only the latter has left the prototype stage. Thus, all production Wankel engines are of the KKM type.
Felix Wankel designed a rotary compressor in the 1920s, and received his first patent for a rotary type of engine in 1934.[5] He realized that the triangular rotor of the rotary compressor could have intake and exhaust ports added producing an internal combustion engine. Eventually, in 1951, Wankel began working at German firm NSU Motorenwerke to design a rotary compressor as a supercharger for NSU's motorcycle engines. Wankel conceived the design of a triangular rotor in the compressor.[6] With the assistance of Prof. Othmar Baier [de] from Stuttgart University of Applied Sciences, the concept was defined mathematically.[7] The supercharger he designed was used for one of NSU's 500 cm two-cylinder two-stroke engines. The engine produced a power output of 110 PS (81 kW) at 8,500 rpm.[6]
In 1954, NSU agreed upon developing a rotary internal combustion engine with Felix Wankel, based upon Wankel's design of the supercharger for their motorcycle engines. Since Wankel was known as a "difficult colleague", the development work for the DKM was carried out at Wankel's private Lindau design bureau. According to John B. Hege, Wankel received help from his friend Ernst Höppner, who was a "brilliant engineer".[8] The first working prototype, DKM 54 (see figure 2.), first ran on 1 February 1957, at the NSU research and development department Versuchsabteilung TX. It produced 21 PS (15 kW).[9][10] Soon thereafter, a second prototype of the DKM was built. It had a working chamber volume Vk of 125 cm and also produced 21 kW (29 PS) at 17,000 rpm.[11] It could even reach speeds of up to 25,000 rpm. However, these engine speeds caused distortion in the outer rotor's shape, thus proving impractical.[12] According to Mazda Motors engineers and historians, four units of the DKM engine were built; the design is described to have a displacement Vh of 250 cm (equivalent to a working chamber volume Vk of 125 cm). The fourth unit built is said to have received several design changes, and eventually produced 29 PS (21 kW) at 17,000/min; it could reach speeds up to 22,000/min. One of the four engines built has been on static display at the Deutsches Museum Bonn (see figure. 2).[13]
Due to its complicated design with a stationary center shaft, the DKM engine was not practical.[3] Wolf-Dieter Bensinger [de] explicitly mentions that proper engine cooling cannot be achieved in a DKM engine, and argues that this is the reason why the DKM design had to be abandoned.[14] NSU engineer Walter Froede solved this problem by using Hanns-Dieter Paschke's design and converting the DKM into what would later be known as the KKM (see figure 5.).[3] The KKM proved to be a much more practical engine, as it has easily accessible spark plugs, a simpler cooling design, and a conventional power take-off shaft.[4] Wankel disliked Froede's KKM engine because of its inner rotor's eccentric motion, which was not a pure circular motion, as Wankel had intended. He remarked that his "race horse" was turned into a "plough horse". Wankel also complained that more stresses would be placed on the KKM's apex seals due to the eccentric hula-hoop motion of the rotor. NSU could not afford to finance the development of both the DKM and the KKM, and eventually decided to drop the DKM in favor of the KKM, because the latter seemed to be the more practical design.[15]
Wankel obtained the US patent 2,988,065 on the KKM engine on 13 June 1961.[16] Throughout the design phase of the KKM, Froede's engineering team had to solve problems such as repeated bearing seizures, the oil flow inside the engine, and the engine cooling.[17] The first fully functioning KKM engine, the KKM 125, weighing in at only 17 kg (37.5 lb) displaced 125 cm and produced 26 PS (19 kW) at 11,000 rpm.[18] Its first run was on 1 July 1958.[19]
NSU licensed the Wankel engine's design to companies around the world, in various different forms, with many companies implementing continual improvements. In his 1973 book Rotationskolben-Verbrennungsmotoren, German engineer Wolf-Dieter Bensinger describes the following licensees, in chronological order, which is confirmed by John B. Hege:[41][42]
According to Don Sherman, American Motors also obtained a license.[9] In 1961, the Soviet research organizations of NATI, NAMI, and VNIImotoprom commenced the development of a Wankel engine. Eventually, in 1974, development was transferred to a special design bureau at the AvtoVAZ plant.[45] John B. Hege argues that no license was issued to any Soviet car manufacturer.[46]
Wankel engines have a problem not found in reciprocating piston four-stroke engines in that the block housing has intake, compression, combustion, and exhaust occurring at fixed locations around the housing. This causes a very uneven thermal load on the rotor housing.[48] In contrast, reciprocating engines perform these four strokes in one chamber, so that extremes of "freezing" intake and "flaming" exhaust are averaged and shielded by a boundary layer from overheating working parts. The use of heat pipes in an air-cooled Wankel was proposed by the University of Florida to overcome this uneven heating of the block housing.[49] Pre-heating of certain housing sections with exhaust gas improved performance and fuel economy, also reducing wear and emissions.[50]
A peripheral intake port gives the highest mean effective pressure; however, side intake porting produces a more steady idle,[58] because it helps to prevent blow-back of burned gases into the intake ducts which cause "misfirings", caused by alternating cycles where the mixture ignites and fails to ignite. Peripheral porting (PP) gives the best mean effective pressure throughout the rpm range, but PP was linked also to worse idle stability and part-load performance. Early work by Toyota[59] led to the addition of a fresh air supply to the exhaust port, and proved also that a Reed-valve in the intake port or ducts[60] improved the low rpm and partial load performance of Wankel engines, by preventing blow-back of exhaust gas into the intake port and ducts, and reducing the misfire-inducing high EGR, at the cost of a small loss of power at top rpm. Elasticity is improved with a greater rotor eccentricity, analogous to a longer stroke in a reciprocating engine. Wankel engines operate better with a low-pressure exhaust system. Higher exhaust back pressure reduces mean effective pressure, more severely in peripheral intake port engines. The Mazda RX-8 Renesis engine improved performance by doubling the exhaust port area compared with earlier designs, and there have been studies of the effect of intake and exhaust piping configuration on the performance of Wankel engines.[61] Side intake ports (as used in Mazda's Renesis engine) were first proposed by Hanns-Dieter Paschke in the late 1950s. Paschke predicted that precisely calculated intake ports and intake manifolds could make a side port engine as powerful as a PP engine.[62] 2ff7e9595c
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