1.3 Classifications of Propulsion Systems
1.3.2 Internal Combustion
Massive amounts of protection used resulted in radiation levels low enough to consider continuing development. But, as in the US, development never continued past this point. Budgetary constraints and the development of new conventional aircrafts designs were cited as the main reason for the cancellation of the program in August 1966. Several other projects reached only design phase.
1.3.1.4 Final Comment
As described above, the three external combustion engines are not appropriate for employment in aviation field for different reasons. Steam engines are only appro- priate for small aircrafts while large ones need heavy boilers, piping and other accessories. Stirling engines generate also low power which is also improper for present aircrafts. Nuclear engines have two drawbacks regarding shielding of flight crew and passengers versus radiation, as well as the risk of crash in residence areas leading to catastrophic situation.
Wankel Engine
TheWankel engineinvented by German engineerFelix Wankelin 1950, is a type of internal combustion enginewhich uses arotary designto convert pressure into a rotating motion. Figure1.57illustrates Diamond DA20 aircraft powered by Wankel engine.Its four-stroke cycle takes place in a space between the inside of an oval- like epitrochoid-shaped housing and a rotor that is similar in shape to a Reuleaux triangle but with sides that are somewhat flatter [20]. This design delivers smooth high-rpm power from a compact size. The first Wankel rotary-engine aircraft was the experimental Lockheed Q-Star in 1968/1969. It was powered by a 185 hp Curtiss-WrightRC2-60 Wankel rotary engine. The compact size and quiet opera- tion of Wankel engine encouraged its usage inUAVs. Wankel engines are also becoming increasingly popular in homebuilt experimental aircraft being very cheap compared with certified aircraft engines, providing engines ranging from 100 to 300 horsepower (220 kW) at a fraction of the cost of traditional engines. Wankel engines operate at a relatively highrotational speedwith relatively low torque, thus, propeller aircraft must use a Propeller Speed Reduction Unit (PSRU) to keep conventional propellers within the proper speed range.
Piston Engine
APiston engine, also often known as areciprocating engine, is aheat enginethat uses one or morereciprocating pistonsto convertpressureinto arotating motion. It
Fig. 1.57 Wankel engine
is well known that piston engine powered the first ever piloted motorized flight made by Orville and Wilbur Wright on 17thDecember 1903 in Kitty Hawk, North Carolina, USA. Since then, continuous developments were enhanced. Piston engines may be classified into five groups as shown in Fig. 1.58. These are in-line, rotary, V-type, radial, and opposed. These engines are coupled to a propel- ler to furnish the forward flight of airplanes.
In-Line
An in-lineengine has cylinders lined up in one row. It typically has an even number of cylinders, but there are instances of three- and five- cylinder engines. Inline engines were common in early aircraft, including the Wright Flyer (12 horsepower), the aircraft that made the first controlled powered flight. An in-line engine may be air cooled but mostly liquid cooled.
The biggest advantage of an inline engine is that it allows the aircraft to be designed with a narrow frontal area for low drag. If the engine crankshaft is located above the cylinders, it is called an inverted inline engine, which allows the propeller to be mounted up high for ground clearance even with short landing gear. The disadvantages of an inline engine include a poor power-to-weight ratio, because the crankcase and crankshaft are long and thus heavy. Thus inline design was aban- doned, becoming a rarity in modern aviation.
Rotary Engine
Rotary enginewas extensively used in World War I as it is lightweight, powerful, cheap, and easy to manufacture in large quantities. Rotary engines have all the cylinders in a circle around the crankcase like a radial engine, but the difference is that the crankshaft is bolted to the airframe, and the propeller is bolted to the engine case. The entire engine rotates with the propeller, providing plenty of airflow for cooling regardless of the aircraft’s forward speed. Some of these engines were a two-stroke design, giving them a high specific power and power-to-weight ratio.
Unfortunately, the severe gyroscopic effects from the heavy rotating engine made the aircraft very difficult to fly. The engines also consumed large amounts of castor oil, spreading it all over the airframe and creating fumes which were nauseating to the pilots.
Piston Engines
In-line Rotary Opposed V-Type Radial
Fig. 1.58 Classification of piston engines
Radial Engine
Radial engine has one or more rows of cylinders arranged in a circle around a centrally located crankcase. Each row must have an odd number of cylinders in order to produce smooth operation. A radial engine has only one crank throw per row and a relatively small crankcase, resulting in a favorable power to weight ratio.
Because the cylinder arrangement exposes a large amount of the engine’s heat radiating surfaces to the air and tends to cancel reciprocating forces, radials tend to cool evenly and run smoothly. Wasp engine, completed in the Christmas eve of 1925, was a radial piston engine with a 1340 cubic inches displacement and manufactured by Pratt & Whitney company (P&W). Wasp engines dominated Navy and Army Air Force fighter planes as well as commercial transports. It powered approximately 100 different experimental and production airplanes including Boeing 40A, Boeing F2B-1 shipboard fighters, and Ford Tri-Motor. By the early 1930s, P&W worked on twin-row radial engines. Its twin Wasp (1830 cubic inches and 1350 horsepower) performed magnificently. A total of173,618 engines were produced that powered a large number of fighters, bombers (which participated later in WWII), and transports. The twin Wasp was followed by Double Wasp, which is an 18-cylinder twin-row radial with 2800 cubic inches of displace- ment. P&W engines from the wasp to the Double Wasp produced, licensed, and affiliated over 363,000 engines. Wasp major was the last P&W piston engine. It was 28-cylinder, 4360 cubic inch displacement and rated power up to 4300 horsepower.
The cylinders were four in rows, arranged for a spiral for better cooling. Its primary application was in heavy transport and bombers. Examples are Boeing’s giant double-decked Strato-cruiser, the 377 and Air Force B-50; both were powered by four Wasp Majors.
BMW 801 was the first German double-row radial engine manufactured in 1940/
1941. Figure1.59illustrates both rotary and radial piston engines.
Fig. 1.59 Rotary and radial piston engines
V-Type
Cylinders inV-type engineare arranged in two in-line banks, tilted 30-60! apart from each other. The vast majority of V engines are water-cooled. Perhaps the most famous example of this design is the legendary Rolls Royce Merlin engine, a 27-l (1649 in3) 60!V12 engine used in, among others, the Spitfires that played a major role in the Battle of Britain (Fig.1.60).
Opposed-Type
Anopposed-type enginehas two banks of cylinders on opposite sides of a centrally located crankcase; ULPower UL260i as an example. The engine is either air cooled or liquid cooled, but air cooled versions predominate. Opposed engines are mounted with the crankshaft horizontal in airplanes, but may be mounted with the crankshaft vertical in helicopters. Due to the cylinder layout, reciprocating forces tend to cancel, resulting in a smooth running engine. Opposed, air-cooled four and six cylinder piston engines are by far the most common engines used in small general aviation aircraft requiring up to 400 horsepower (300 kW) per engine.
Aircraft which require more than 400 horsepower (300 kW) per engine tend to be powered by turbine engines.
1.3.2.1.2 Continuous Combustion Engines
Continuous combustion engines are mainly turbine shaft engines. It includes turboprop turboshaft and propfan engines. They are featured with rotating elements
Fig. 1.60 The Rolls-Royce Merlin V-12 piston aero engine
known as turbomachines, as sub-modules. These modules may be fan, compressor(s), and turbine(s) as well as propellers/propfans.
Turboprop
Turboprop engines powers both civil and military transport aircrafts with a cruise speed less 450 mph (700 km). It is composed of a gas generator (compressor, combustion chamber and turbine) as well as a propeller. The turbine drives both compressor and propeller. Because gas turbines optimally spin at high speed, a turboprop features a gearbox to lower the speed of the shaft so that the propeller tips do not reach supersonic speeds. An alternative to the above turboprop engines, a second turbine is added which drives only the propeller either directly or via a gearbox. The first turbine in this case drives the compressor only. Thus it is free to rotate at its own best speed (referred to as a free- or power-turbine). The other turbine is identified as compressor-turbine. Recent turboprop engines generate thrust force from both propeller and exhaust jet stream. A fraction of 10–20 % of thrust is generated from jet stream. Consequently, some people classify turboprop as jet engine or reaction engine. Figure1.61illustrates two turboprop engines, the left is a single-shafted one with propeller coupled to compressor-turbine shaft and the right one is of the free turbine type.
Examples for turboprop engines are Rolls-Royce T56 (3460 shaft horsepower installed to P-3 Orion, C-130, C-2A aircrafts), Rolls-Royce AE2100 engine (3600–6000 shp installed to C-27 J Spartan, ShinMaywa US-1A Kai aircrafts), and Rolls-Royce TP400-D6 engine (11,000 + shp installed to Airbus Military A400M aircraft). The T34 is one of the earliest turboprop engines manufactured by Pratt & Whitney, which powered B-17 flying testbed, KC-97 J, and Douglas C-133A Cargomaster. PT6 turboprop engine (Pratt & Whitney of Canada) is the most popular power plant in its class (475–1650 hp). It has been selected for more than 130 different business, commuter, trainer, and utility aircraft applications.
Example for GE turboprop engines is CT7, which power Saab 340 and CASA- IPTN CN235.
Prop Gearbox Compressor
Shaft Combustion
chamber
Single Shaft Free Turbine
Turbine Exhaust
Fig. 1.61 Two types of turboprop engine
Turboshaft
Turboshaft engines are used primarily for helicopters and auxiliary power units. A turboshaft engine is very similar to a turboprop, with a key difference: In a turboprop the propeller is supported by the engine, and the engine is bolted to the airframe. In a turboshaft, the engine does not provide any direct physical support to the helicopter’s rotors. The rotor is connected to a transmission, which itself is bolted to the airframe, and the turboshaft engine simply feeds the transmission via a rotating shaft. The distinction is seen by some as a slim one, as in some cases aircraft companies make both turboprop and turboshaft engines based on the same design. An example for turboshaft engine is GE T700, which powers Seahawk helicopter (Fig.1.62).
Other examples for turboshaft engines are Rolls-Royce RTM322 (2100–2550 shp, which powers AgustaWestland WAH-64 Apache), Rolls-Royce Gnome (1175–1660 shp, which powers AgustaWestland Sea King and Kawasaki/Boeing Vertol 107), and T800 (1360–1680 shp, which powers AgustaWestland Super Lynx 300/3 CTS800) manufactured by LHTEC, a joint venture between Rolls-Royce and Honeywell. GE T700 is one of the historical turboshaft engines that powers the Marine Corps “SuperCobra” built by Bell, military Sikorsky H-60 and international versions of the Sikorsky S-70. The commercial version of T700, namely, CT7, powers Bell 214ST, Sikorsky S-70C, and Westland WS-70.
Propfan
A propfan or an unducted fan (sometimes denoted in former Soviet Union as turbopropfan) is a modified turbofan engine, with the fan placed outside of the engine nacelle on the same axis as the compressor blades. Propfans are also known as ultra-high bypass(UHB)engines and, most recently,open rotorjet engines. The design is intended to offer the speed and performance of a turbofan, with the fuel economy of a turboprop.
The propfan concept was developed to deliver better fuel efficiency than con- temporary turbofans. However, this achievement has noise penalty. Most propfans
Fig. 1.62 GE T700 turboshaft engine powering Seahawk helicopter
are experimental engines. Examples are General Electric’s GE36 Unducted Fan and Pratt & Whitney/Allison 578-DX. General Electric’s GE36 Unducted Fan (Fig.1.63) was a variation on NASA’s original propfan concept and appears similar to a pusher configuration for piston or turboprop engines.
McDonnell Douglas developed a proof-of-concept aircraft by modifying its MD-80. They removed the JT8D turbofan engine from the left side of the fuselage and replaced it with the GE36. The test flights conducted in Mojave, CA, USA, and ended in 1988 demonstrated a 30 % reduction in fuel burn over turbo-fan powered MD-80 and low-levels of exterior and interior noise/vibration. However, due to jet-fuel price drops and shifting marketing priorities, Douglas shelved the program the following year.
In the 1980s, Allison collaborated with Pratt & Whitney on demonstrating the 578-DX propfan. The 578-DX was successfully flight tested on a McDonnell Douglas MD-80. However, none of the above projects came to fruition, mainly because of excessive cabin noise (compared to turbofans) and low fuel prices.
The Ivchenko-Progress D-27 propfan developed in the USSR with the propfan blades at the front of the engine in a tractor configuration (Fig. 1.64). D-27’s propfans propelled the Antonov An-70. D-27 propfan engine is a three-shaft propfan engine with a propeller diameter of 4.5 m and dry weight of 1650 kg (3638 lb). Its gas generator is made up of an axial low-pressure compressor, a mixed-flow high-pressure compressor, an annular combustion chamber, a single- stage high-pressure turbine, and a single-stage low-pressure turbine. The SV-27 contra-rotating propfan is driven by a four-stage turbine via a shaft connected to a planetary reduction gear.
With the current high price for jet fuel and the emphasis on engine/airframe efficiency to reduce emissions, there is renewed interest in the propfan concept for Fig. 1.63 General electric GE-36 unducted fan (propfan) engine
jetliners that might come into service beyond the Boeing 787 and Airbus A350XWB. For instance, Airbus has patented aircraft designs with twin rear- mounted counter-rotating propfans.
1.3.2.2 Reaction Engines
The other main group of internal combustion engines, namely, the reaction engines is next subdivided into the athodyd (where athodyd stands forAeroTHermODY- namicDuct) or turbine types. Athodyd group includes ramjet, pulsejet, and scramjet engines. Turbine engines include engines that have turbomachinery modules, which combine all types of turbojet, turbofan, turbo ramjet, turbo rocket, and advanced ducted fan engines. In short, allreaction enginesdevelop its propulsive force as a reaction to the jet exhaust gases. Three essential modules are seen in all reaction types, namely, an entry duct (sometimes identified as inlet duct or intake), a combustion chamber or burner, and an exhaust nozzle. The exhaust nozzle (s) accelerate air/gases to greater speeds than flight speed, thus create thrust that pushes the aircraft forward.
Fig. 1.64 The Ivchenko-progress D-27 powering an-70 aircraft
1.3.2.2.1 Athodyd Types
Athodyd group includes ramjet, pulsejet, and scramjet engines, which do not have any major rotating elements or turbomachinery. The pulsejet operates intermit- tently and has found limited applications [21]. In ramjet engines, ram compression of the air becomes sufficient to overcome the need for mechanical compression.
Ramjet engine is also appropriate for supersonic flight speeds [22]. If the flight speed is so high, fuel combustion must occur supersonically, and then this ramjet is called a scramjet [23].
Ramjet Engine
Aramjet, sometimes referred to as astovepipe jet, is a form of jet engine using the engine’s forward motion to admit and compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill. It is composed of three modules: inlet duct, burner or combustor, and nozzle. It has two types: namely liquid- and solid-fuel ramjets.
Ramjet engines may be subsonic or supersonic (Fig.1.65). Subsonic ramjets do not need a sophisticated inlet since the airflow is already subsonic and a simple hole is usually used. For supersonic ramjets, supersonic flow is decelerated to subsonic speeds at the inlet through one or more oblique shock wave(s), terminated by a strong normal shock.
Thus air attains subsonic speeds at the entrance of combustion chamber. The combustor adds heat and mass to the compressed air by burning a fuel. The combustion chamber includes flame holders that stop the flames from blowing out. A ramjet combustor can safely operate at stoichiometric fuel to air ratios, which implies a combustor exit stagnation temperature of the order of 2400 K for kerosene. Products of combustion leaving the combustion chamber are reaccelerated through a nozzle, to supersonic speeds via a convergent-divergent nozzle to produce thrust. For a ramjet operating at a subsonic flight Mach number, exhaust flow is accelerated through a converging nozzle. Supersonic ramjet engines work most efficiently at speeds around Mach 3 and can operate up to speeds of at least Mach 5.
COMBUSTION CHAMBER
PROPELLING NOZZLE
Free Stream
Inlet Diffuser Fuel Nozzle
πd
Combustor τc πc
Lorin’s Ramjet (Subsonic) Supersonic Ramjet
0 2 4 6
FUEL SUPPLY AIR INTAKE
Fig. 1.65 Lorin’s and supersonic ramjet engines
Historically, ramjet was invented in 1913 by theFrenchinventorRene´ Lorin, who was granted a patent for his device illustrated in Fig.1.65. Attempts to build a prototype failed due to inadequate materials [24]. However, in 1949, the works of Rene´ Leducled to design of Leduc 010, which was one of the first ramjet-powered aircrafts that flew in 1949 and displayed in Fig.1.66. Later on, the French Nord 1500 Griffonreached Mach 2.19 in 1958.
In 1915, theHungarianinventorAlbert Fon!odevised a solution for increasing the range of artillery, comprising a gun-launched projectile by adding a ramjet propulsion unit. He submitted a German patent describing an “air-jet engine”
suitable for high-altitude supersonic aircraft. In an additional patent application, he adapted the engine for subsonic speed. The patent was finally granted in 1932 after 4 years of examination.
In theSoviet Union, a theory of supersonic ramjet engines was presented in 1928 by Boris S. Stechkin. The first successful ramjet engine, namely, GIRD-04, was designed by I.A.Merkulovand tested in April 1933. The GIRD-08 phosphorus- fueled ramjet was tested by firing it from artillery cannon. These shells may have been the first jet powered projectiles to break the speed of sound. In August 1939, Merkulov developed the first ramjet engine for use as an auxiliary motor of DM-1 aircraft. The world’sfirst ramjet powered airplaneflight took place in December 1939, using two DM-2 engines on a modified Polikarpov I-15. Merkulov designed a ramjet fighter “Samolet D” in 1941. Two of his DM-4 engines were installed on the YaK-7PVRD fighter, during World War II. In 1940, the Kostikov-302 experimental plane was designed, powered by liquid fuel rocket for take-off and ramjet engines for flight. In 1947, Mstislav Keldysh proposed a long-range antipodal bomber powered by ramjet instead of rocket.
Pulsejet Engine
A pulse jet engine (or pulsejet) is a very simple type of jet engine in which combustion occurs in pulses. Pulsejets use an intermittent combustion while ram- jets employ a continuous combustion process. Pulsejet engines are aunique type of jet engine, able to operate statically with few[25]or no moving parts[26]. They are very simple and cheap to construct. They feature an excellent balance of cost and Fig. 1.66 Leduc 010
function, as could run on any grade of petroleum and the ignition shutter system.
Their accompanying noise is unacceptable by modern standards. They have both a higher efficiency and very high thrust-to-weight ratio compared to other jet engines.
Pulsejet engines may be produced in many sizes with different outputs ranging from a few pounds to thousands of pounds of thrust. There aretwo main typesof pulsejet engines: valved (Fig. 1.67) and valveless (Fig. 1.68). Both types use resonant combustion and harness the expanding combustion products to form a pulsating exhaust jet, which produces thrust intermittently.
Valved
Valved enginesuse a mechanical one-way valve, which is a simple leaf-spring type of shutter. With the valve open, a fresh charge of air is admitted. The air mixes with the fuel and then an explosion takes place, which shuts the valve and forces the hot gas to go out the back of the engine through the tailpipe only, and allow fresh air and more fuel to enter through the intake (Fig.1.67). The superheated exhaust gases exit through an acoustically resonant exhaust pipe.
Fig. 1.67 Valved pulsejet operation, reproduced with the permission of Rolls-Royce plc, copy- right©Rolls-Royce plc [28]