Gas turbines offer a high-powered engine in a very small and light package. However, they are not as responsive and efficient as small piston engines over the wide range of RPMs and powers needed in vehicle applications. In series hybrid vehicles , as the driving electric motors are mechanically detached from the electricity generating engine, the responsiveness, poor performance at low speed and low efficiency at low output problems are much less important.
The turbine can be run at optimum speed for its power output, and batteries and ultracapacitors can supply power as needed, with the engine cycled on and off to run it only at high efficiency. The emergence of the continuously variable transmission may also alleviate the responsiveness problem. Turbines have historically been more expensive to produce than piston engines, though this is partly because piston engines have been mass-produced in huge quantities for decades, while small gas turbine engines are rarities; however, turbines are mass-produced in the closely related form of the turbocharger.
The turbocharger is basically a compact and simple free shaft radial gas turbine which is driven by the piston engine's exhaust gas. The centripetal turbine wheel drives a centrifugal compressor wheel through a common rotating shaft. This wheel supercharges the engine air intake to a degree that can be controlled by means of a wastegate or by dynamically modifying the turbine housing's geometry as in a VGT turbocharger. It mainly serves as a power recovery device which converts a great deal of otherwise wasted thermal and kinetic energy into engine boost.
Turbo-compound engines actually employed on some trucks are fitted with blow down turbines which are similar in design and appearance to a turbocharger except for the turbine shaft being mechanically or hydraulically connected to the engine's crankshaft instead of to a centrifugal compressor, thus providing additional power instead of boost. While the turbocharger is a pressure turbine, a power recovery turbine is a velocity one. A number of experiments have been conducted with gas turbine powered automobiles , the largest by Chrysler.
The first serious investigation of using a gas turbine in cars was in when two engineers, Robert Kafka and Robert Engerstein of Carney Associates, a New York engineering firm, came up with the concept where a unique compact turbine engine design would provide power for a rear wheel drive car. After an article appeared in Popular Science , there was no further work, beyond the paper stage. In , designer F. Bell and Chief Engineer Maurice Wilks from British car manufacturers Rover unveiled the first car powered with a gas turbine engine. The two-seater JET1 had the engine positioned behind the seats, air intake grilles on either side of the car, and exhaust outlets on the top of the tail.
The car ran on petrol , paraffin kerosene or diesel oil, but fuel consumption problems proved insurmountable for a production car. It is on display at the London Science Museum. While photos of the Firebird I may suggest that the jet turbine's thrust propelled the car like an aircraft, the turbine actually drove the rear wheels. Starting in with a modified Plymouth ,  the American car manufacturer Chrysler demonstrated several prototype gas turbine -powered cars from the early s through the early s.
Chrysler built fifty Chrysler Turbine Cars in and conducted the only consumer trial of gas turbine-powered cars. This vehicle, looking like an aircraft with wheels, used a unique combination of both jet thrust and the engine driving the wheels. The original General Motors Firebird was a series of concept cars developed for the , and Motorama auto shows, powered by gas turbines.
As a result of the U. Clean Air Act Amendments of , research was funded to developing automotive gas turbine technology. Toyota demonstrated several gas turbine powered concept cars, such as the Century gas turbine hybrid in , the Sports Gas Turbine Hybrid in and the GTV in No production vehicles were made.
The GT24 engine was exhibited in without a vehicle. In General Motors introduced the first commercial gas turbine powered hybrid vehicle —as a limited production run of the EV-1 series hybrid. The turbine design included a recuperator. American Ray Heppenstall joined Howmet Corporation and McKee Engineering together to develop their own gas turbine sports car in , the Howmet TX , which ran several American and European events, including two wins, and also participated in the 24 Hours of Le Mans. The cars used Continental gas turbines, which eventually set six FIA land speed records for turbine-powered cars.
The next year the STP Lotus 56 turbine car won the Indianapolis pole position even though new rules restricted the air intake dramatically. Chapman had a reputation of building radical championship-winning cars, but had to abandon the project because there were too many problems with turbo lag. AVS turbine hybrids were plagued with reliability and quality control problems, resulting in liquidation of AVS in The most successful design by Designline is now operated in 5 cities in 6 countries, with over 30 buses in operation worldwide, and order for several hundred being delivered to Baltimore, and New York City.
Brescia Italy is using serial hybrid buses powered by microturbines on routes through the historical sections of the city. Several locomotive classes have been powered by gas turbines, the most recent incarnation being Bombardier 's JetTrain. The Third Reich Wehrmacht Heer 's development division, the Heereswaffenamt Army Ordnance Board , studied a number of gas turbine engine designs for use in tanks starting in mid The first gas turbine engine design intended for use in armored fighting vehicle propulsion, the BMW -based GT , was meant for installation in the Panther tank.
The second use of a gas turbine in an armored fighting vehicle was in when a unit, PU, specifically developed for tanks by C. M1 Abrams tanks, among others. They are lighter and smaller than diesels at the same sustained power output but the models installed to date are less fuel efficient than the equivalent diesel, especially at idle, requiring more fuel to achieve the same combat range. Successive models of M1 have addressed this problem with battery packs or secondary generators to power the tank's systems while stationary, saving fuel by reducing the need to idle the main turbine.
Ts can mount three large external fuel drums to extend their range. Russia has stopped production of the T in favor of the diesel-powered T based on the T , while Ukraine has developed the diesel-powered TUD and T with nearly the power of the gas-turbine tank. The French Leclerc MBT's diesel powerplant features the "Hyperbar" hybrid supercharging system, where the engine's turbocharger is completely replaced with a small gas turbine which also works as an assisted diesel exhaust turbocharger, enabling engine RPM-independent boost level control and a higher peak boost pressure to be reached than with ordinary turbochargers.
This system allows a smaller displacement and lighter engine to be used as the tank's powerplant and effectively removes turbo lag. A turbine is theoretically more reliable and easier to maintain than a piston engine since it has a simpler construction with fewer moving parts, but in practice, turbine parts experience a higher wear rate due to their higher working speeds. The turbine blades are highly sensitive to dust and fine sand so that in desert operations air filters have to be fitted and changed several times daily.
An improperly fitted filter, or a bullet or shell fragment that punctures the filter, can damage the engine. Piston engines especially if turbocharged also need well-maintained filters, but they are more resilient if the filter does fail.
Gas turbines are used in many naval vessels , where they are valued for their high power-to-weight ratio and their ships' resulting acceleration and ability to get underway quickly. The first large-scale, partially gas-turbine powered ships were the Royal Navy's Type 81 Tribal class frigates with combined steam and gas powerplants. With their aft torpedo tubes replaced by antishipping missiles they served as missile boats until the last was retired in They were the fastest vessels in the Finnish Navy; they regularly achieved speeds of 35 knots, and The Turunmaa s were paid off in Karjala is today a museum ship in Turku , and Turunmaa serves as a floating machine shop and training ship for Satakunta Polytechnical College.
The next series of major naval vessels were the four Canadian Iroquois -class helicopter carrying destroyers first commissioned in Since then, they have powered the U. Navy's Oliver Hazard Perry -class frigates , Spruance and Arleigh Burke -class destroyers, and Ticonderoga -class guided missile cruisers.
The marine gas turbine operates in a more corrosive atmosphere due to the presence of sea salt in air and fuel and use of cheaper fuels. Up to the late s, much of the progress on marine gas turbines all over the world took place in design offices and engine builder's workshops and development work was led by the British Royal Navy and other Navies.
While interest in the gas turbine for marine purposes, both naval and mercantile, continued to increase, the lack of availability of the results of operating experience on early gas turbine projects limited the number of new ventures on seagoing commercial vessels being embarked upon. In , the Diesel-electric oil tanker Auris , 12, Deadweight tonnage DWT was used to obtain operating experience with a main propulsion gas turbine under service conditions at sea and so became the first ocean-going merchant ship to be powered by a gas turbine. Built by Hawthorn Leslie at Hebburn-on-Tyne , UK, in accordance with plans and specifications drawn up by the Anglo-Saxon Petroleum Company and launched on the UK's Princess Elizabeth 's 21st birthday in , the ship was designed with an engine room layout that would allow for the experimental use of heavy fuel in one of its high-speed engines, as well as the future substitution of one of its diesel engines by a gas turbine.
Following successful sea trials off the Northumbrian coast, the Auris set sail from Hebburn-on-Tyne in October bound for Port Arthur in the US and then Curacao in the southern Caribbean returning to Avonmouth after 44 days at sea, successfully completing her historic trans-Atlantic crossing. During this time at sea the gas turbine burnt diesel fuel and operated without an involuntary stop or mechanical difficulty of any kind. She subsequently visited Swansea, Hull, Rotterdam , Oslo and Southampton covering a total of 13, nautical miles.
Despite the success of this early experimental voyage the gas turbine was not to replace the diesel engine as the propulsion plant for large merchant ships. At constant cruising speeds the diesel engine simply had no peer in the vital area of fuel economy. The gas turbine did have more success in Royal Navy ships and the other naval fleets of the world where sudden and rapid changes of speed are required by warships in action.
It operated for 9, hours using residual fuel Bunker C for 7, hours. Fuel efficiency was on a par with steam propulsion at 0. This gave the ship a speed capability of 18 knots, up from 11 knots with the original power plant, and well in excess of the 15 knot targeted. The ship made its first transatlantic crossing with an average speed of Suitable Bunker C fuel was only available at limited ports because the quality of the fuel was of a critical nature.
The fuel oil also had to be treated on board to reduce contaminants and this was a labor-intensive process that was not suitable for automation at the time. Ultimately, the variable-pitch propeller, which was of a new and untested design, ended the trial, as three consecutive annual inspections revealed stress-cracking. This did not reflect poorly on the marine-propulsion gas-turbine concept though, and the trial was a success overall.
The success of this trial opened the way for more development by GE on the use of HD gas turbines for marine use with heavy fuels. To this end, the airliner of the future is likely to be an all-electric aircraft which has the capability to be autonomously operated. These two facets all electric and autonomous remove two of the most significant costs in the aviation industry, fuel and flight crew [ 1 ]. This then goes towards achieving the goal of economic sustainability.
Furthermore, by eliminating the need for fuel, the environmental impact from the aviation industry is also significantly reduced as the majority of emissions from aviation come directly from burning fuel. There needs to be incremental change in the evolution of aircraft in the aviation industry to develop and promote the benefits of aviation sustainability. Evolutionary steps towards all electric aircraft have provided incremental means to develop a stream of alternative propulsion technologies. This is clearly seen in the automotive industry. Hybridization of automobiles has made way for technology development and acceptance of hybrid vehicles.
One may expect that hybrid-propulsion systems for aerospace application will develop in the same manner and become an essential entity in the aviation industry.
The advent of the unmanned aviation industry has inadvertently helped the evolution of manned aviation by providing small scale but practical test beds for the iteration of aerospace technologies. The associated cost of the design and development life cycle means that unmanned aircraft can readily make use of novel and innovative technologies. The rate of development and application into unmanned systems results in more innovation at shorter time scales.
This of course further drives and grows the unmanned aviation sector. The rapid growth of unmanned aviation is essential to support the growth and expansion of the larger-scale commercial market. While unmanned aircraft in the past have commonly been associated with negative press with various news outlets reporting on predator drones launching hellfire missiles [ 3 ], it is becoming increasingly common to think of the almost ubiquitous quadrotor aircraft and their many developing applications. These applications include mail, package, and pizza delivery [ 3 ], as well as the ever-expanding photography and videography sectors.
There are also applications that require unmanned aircraft to achieve long endurance times and thus are specialty products that require specific expertise to operate legally [ 4 ]. Another identified growth sector is mining and agricultural survey applications. These two industries can involve the monitoring of exceptionally large areas, and hence any aircraft utilized needs to be capable of flying for many hours. When factoring in payload requirements sensors for the surveying , the current energy density of batteries means electric multi-rotor aircraft have limited usefulness in these long-endurance missions.
The increase in range and endurance could lie in the hybridization of the electric and internal combustion IC engine system to provide means to progress the technology and introduce it to the larger aviation industry.
The aim of this work is to review the general propulsion methods available to unmanned or remotely piloted aircraft systems RPAS and to present the current efforts into increasing the endurance of electric aircraft with the use of hybrid configurations. The hybrid configurations investigated and discussed in this work include the internal combustion engine ICE and conventional electric hybrid, the ICE and photovoltaic PV hybrid, and the fuel-cell hybrid.
To support and understand of each of these, the constituent propulsion systems are also described. The objective of this work is to present the associated improvements in performance characteristics in terms of fuel consumption and endurance of hybrid RPAS. Electric propulsion is a popular system for small and micro RPAS [ 5 , 6 ].
Electric propulsion is known to have a favorable electrical and mechanical efficiency while being reliable due to the simple mechanics and reduced moving parts compared to IC engines [ 7 , 8 , 9 , 10 ]. These systems are further divided into active and passive systems, which depend on the type of bus connection between the electric machine and the battery [ 9 ]. This is very similar to all other kinds of electric vehicles and has the following components [ 5 ].
This is primarily due to low energy densities of the batteries, and is the limiting factor in vehicle endurance. Research in battery technology has been essential in improving battery energy density for various applications [ 13 , 14 , 15 ]. There are multiple research efforts promising better battery technologies, including: Although the improvement of the efficiency of system components results in a more efficient propulsion design, energy density and battery technology remains the crucial element in the success of electric aircraft.
These battery technologies will be used in the first instance on unmanned aircraft before any certification onto manned aircraft. Table 1 shows the current battery specific energy density, as well as the predicted increases for the future, and the theoretical maximum achievable.
Also, since the specific energy is the energy per kilogram, Table 1 is indicative of the potential to reduce the overall weight and improve the performance of the RPAS. At present, most RPAS use Li-Ion batteries because they are a proven, reliable, and available battery technology [ 17 , 18 ]. A fuel cell is an electrochemical cell that converts chemical energy into electrical energy through an electrochemical reaction and is classified according to the kind of electrolyte it utilizes.
These include [ 23 ]: Proton exchange fuel-cells are the most commonly used fuel cell for aerial systems [ 24 ]. They directly produce direct current DC electric current, operate at low temperatures, and water is the only emission when utilizing hydrogen [ 25 , 26 ]. However, there is still a drawback with the PEM fuel-cell when exposed to fast load changes, which reduces the life span of the already expensive fuel-cell membrane [ 27 ].
The overall chemical reaction of the PEM is given as: The SOFC is also referred to as a high-temperature fuel cell, since it operates between temperatures ranging from to degrees Celsius [ 23 ]. This allows for potential for use in cogeneration or combined cycle applications where heat exchange is used to further improve efficiency [ 30 ]. This high temperature tends to give the flexibility of using a much denser propane fuel and gives advantage over the fuel storage space when compared to the PEM, where the less-dense hydrogen needs a heavier storage area [ 24 ]. This is a significant advantage in aircraft and aviation in general where weight is a critical factor.
The overall chemical reaction of the SOFC is given as: At the high temperatures within the cell, this gives: The reduction reaction occurs at the cathode air electrode at degrees Celsius, while fuel oxidation occurs at the anode. The anode should be porous to conduct fuel and transport the products of fuel oxidation away from the electrolyte and fuel-electrode interfaces [ 23 , 28 ].
This fuel cell has less power density than the SOFC and PEM; however, the advantage of the system is the use of a denser methanol fuel that is liquid in contrast to the gas state of the hydrogen and propane, which require bulkier gas storage systems [ 24 ]. In general, liquid fuel systems are simpler to use in aerospace vehicles as storage conforms to the aircraft structure and is not geometry-dependent. The overall chemical reaction of the DMFC is given as: Also, as indicated in Equation 9 , CO 2 is also produced as an emission byproduct.
Over the decade, advances of various technologies have increased the overall endurance from the 15 min to 48 h [ 24 ]. This represents a significant increase in the endurance capability of RPAS systems. A mission endurance of 48 h is sufficient for most RPAS applications, excluding those that require persistent on loitering mission scenarios. Some long-endurance aircraft utilize photovoltaic cells which convert solar energy into electrical energy.
The cells are made into solar panels and can be integrated into the wings of an aircraft. A solar cell system usually comprises a form of energy storage, such as batteries, to supply extra energy to the propulsion system or to store excess harvested energy away until it is operationally needed. Solar panels use a photovoltaic process to convert radiative solar energy to electrical energy through Photovoltaic Cells PVs [ 32 , 33 ].
Li-ion and Lithium polymer batteries are the most common batteries that are used in conjunction with PV. Endurance is reliant on the availability of solar activity sun light [ 34 ]. Solar panels for aerospace application are typically expensive due to geometry and weight requirements [ 35 ]. Improving the efficiency of individual components can improve the overall efficiency.
For example, high Power Conversion Efficiency PCE , weight, flexibility, mechanical resilience, and operational stability can all improve the overall efficiency of a PV system [ 36 ]. There are promising materials that will improve these conditions; these include advanced silicon [ 37 ], ultrathin kesterites [ 38 ], organic and inorganic semiconductors [ 36 , 39 ], organo-lead halide perovskites [ 40 , 41 ], and improved PCE [ 42 ].
There is further research investigating the implementation of PV panels with high power-to-weight ratio solar cells [ 36 ]. Such panels inhibit flexible material properties, allowing for a flexible wing design which would absorb turbulence and reduce aircraft perturbation.
Furthermore, it would allow shape conformity to maximize the PV-occupied wetted area and solar power yield. Figure 2 shows the increase in endurance of solar-powered aerial vehicles over almost 40 years, starting with research from through to [ 43 , 44 ]. Advances in PV technology and materials and battery technology contribute significantly to this [ 44 ].
The trend suggests that solar-based propulsion endurance has increased over the years by many orders of magnitude. Specifically, in the endurance was just 20 min [ 45 ]. In , the endurance was recorded as h, 22 min, and 8 s. So, from to there has been an increase by a factor of , that is, 3 orders of magnitude in less than 40 years.
The goal of persistent loitering with solar-powered aircraft is clearly achievable. View Instructor Companion Site. He has been teaching an Aerospace Propulsion class for the last 15 years and is the author of two books. His research interests include combustion, thermal-fluids, and propulsion systems and current projects include hypersonic inlets and supersonic reactors.
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