Electric/Hybrid-Electric Aircraft Propulsion Systems: E-Mobility in Aviation

Following are the topics that we covered in this Article:

• Importance of Hybrid Electric and all-Electric Propulsion systems for the aircraft
• A Brief History of the Electric Aircraft Revolutionary Periods
• Electric/Hybrid Electric Aircraft Propulsion Architecture:
• Role of Electrical machines in Electric Aircraft
• Battery Technologies for Electric Aircraft Propulsion Systems
  

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Do you know that the number of passengers using air transport would have doubled to 8.2 Billion by the year 2037?

Yes! You heard it right. International Air Transport Association (IATA) anticipates these statistics.

The bitter truth is that as the passenger count increases, then CO2 emission also increases. Air transportation contributes 2% of carbon dioxide and 3% of all greenhouse gases worldwide. Annually it is expected to grow by 4% to 5%. 

If there are no appropriate actions, these emissions will increase to 25% by 2050 globally. Too much greenhouse gas and carbon emissions will lead to the absorption of more and more heat by the earth’s atmosphere and the ocean, leading to global warming.

Moreover, the ocean water becomes more acidic. 

As a result, the ecosystem will be affected adversely. It’s high time that we need to preserve our mother nature and present it to our future generations.

The below graph shows the Carbon emission during the years 2004 – 2022.

Due to this, the global aviation community is attempting to minimize the adverse environmental effects of air transportation, primarily through the reduction of nitrous oxide and carbon emissions, as well as noise pollution. 

The below chart shows the greenhouse gas contribution of commercial aircraft globally.

Scientists and industrialists are working to develop technologically advanced state-of-the-art propulsion systems. A significant part of the framework is the development of all-electric and hybrid-electric propulsion systems for aircraft.

Importance of hybrid electric and all-electric propulsion systems for aircraft

Before discussing its significance and benefits, let us discuss what propulsion is all about.

The propulsion system of the aircraft comprises an engine and a thrust generator. The thrust generator is also known as a propeller or a propulsive nozzle. The aircraft propulsion has to serve two purposes.

Firstly, During flight cruising, the thrust from the propeller has to balance the drag of the airplane. Secondly, the thrust exerted by the propellor must be greater than the airplane’s drag for it to accelerate. The higher the difference between thrust and drag called the excess thrust, the higher the flight accelerates.

In Hybrid Electric Propulsion (HEP), to reduce the environmental impact, the Internal Combustion Engines (ICEs) and Electric motors are combined to form a propulsion system. Alternatively, this hybridization is also achieved by combining either Fuel cells or ICEs with Electric motors and Batteries.

The hybrid-electric propulsion system (HEPS) seems to be the most practical alternative for an energy-efficient, environmentally friendly, and noise-free aircraft propulsion. It combines the benefits of both conventional and all-electric propulsion systems.

HEPS is Advantageous in Several ways as compared against the Conventional Systems:

• Highly efficient.
• Highly reliable
• Improved power distribution quality.
• Higher flight range.
• Almost nil or lesser emission.
• Less noisy.
• Ability to expand markets to smaller airports.
• Highly flexible.
• Fuel and battery sources provide additional options for managing the propulsion system at different phases of a mission.
• Reduced energy usage.

On the other hand, recent advancements in electric motors, energy storage systems, and power electronics converters are allowing aircraft propulsion to become more electrical. More Electric Airplane (MEA) is a concept that intends to transform aircraft systems to be solely electrically powered.

Mainly monitoring and auxiliary systems are now powered by electricity, while the more sophisticated devices are still powered by hydraulic or thermal systems.

Despite this, progress with all-electrically driven aircraft has accelerated since 2010 and has expanded even more rapidly since 2016, particularly in the domain of urban air vehicles. Over the next three decades, aircraft propulsion will go from small all-electric urban air vehicles through medium-size hybrid-electric aircraft, and finally to HEP regional aircraft.

A Brief History of the Electric Aircraft Revolutionary Periods

The glider MB-E1 was the first piloted electric aircraft constructed and tested in 1973. There were four Ni-Cd batteries as energy storage in this aircraft, which provided 100V to power a dc motor. In 1979, they tested the first solar-powered piloted aircraft with Ni-Cd batteries.

Earlier developments in electrical drive trains and aircraft construction did not involve electrical systems. Instead, they modified commercial gliders by replacing them with electrical drive trains. After those few prototypes, there were tremendous advancements in solar power integration, storage systems, and lighter structure in small electric aircraft.

Solar Impulse 1, a more recent attempt, showcased an entirely solar-powered, long-range piloted airplane capable of being aloft for up to 36 hours. Solar Impulse 2 covered over 40000 kilometers without using any fuel in 2016. Many small full-electric aircraft have been successfully demonstrated globally, but the large full-electric aircraft are yet to be developed due to energy storage challenges.

Let’s look into the few of the manned electric aircraft that were demonstrated recently.

• Lange Antares 20E 

It is a 1 seater manned full electric aircraft launched in 2003 in Germany. It is 660kg in weight and It’s the world’s first commercially available glider.

• Pipistrel Taurus Electro G4

It is a four-seater manned full electric aircraft launched in 2011 in Slovenia. It is 1500kg in weight, and it is the world’s first four-seater electric airplane. It has won NASA’s green flight challenge.

• Bye Aerospace eFlyer 2

It is a 2 seater manned full electric aircraft launched in 2018 in the USA. It is 862kg in weight, and it uses Seimen’s SP70D motor for propulsion.

• Solar Impulse 2

It is a 1 seater manned full electric aircraft launched in 2015 in Switzerland. It is 2000kg in weight and it is the first piloted solar-powered aircraft to fly around the world.

• Airbus E- fan

It is a 1 seater manned full electric aircraft launched in 2014 in France. It is 550kg in weight and its flight time is 60 minutes. The production of this aircraft is stopped as they are focussing more on hybrid propulsion.

• Extra EA 330 LE- Siemens

It is a 1 seater manned full electric aircraft launched in 2017 in Germany. It is 1000kg in weight and is recorded as the world’s fastest piloted electric aircraft covering 340 km/h.

Unmanned ground and flight tests have been conducted on some of the electric aircraft listed below during 2020. But comprehensive demonstrations have yet to be completed.

Most short-term full-electric aircraft are for regional transit.

• Aviation Alice

It is an 11 seater planned full electric aircraft launched in Israel. It is 6350kg in weight and it uses a 3 X 260 kW electric motor for propulsion.

• NASA X-57 Maxwell

It is a 2 seater planned full electric aircraft launched in the USA. It is 1360kg in weight and it is NASA’s first electric aircraft that validates distributed propulsion.

• Rolls Royce ACCEL

It is a 1 seater planned full electric aircraft launched in the UK. It is 1200kg in weight and they are planning it as an all-electric racing airplane with the maximum speed.

• Ampaire Tailwind

It is a 6 seater planned full electric aircraft launched in the USA. They’re aiming for less than 300-mile regional air travel.

• XTI TriFan 600

It is again a USA-based planned full electric aircraft with 6 seats and 2857 kg. They are aiming for a range of 1200km air travel.

Electric/Hybrid Electric Aircraft Propulsion Architecture:

New modes of air travel enabled by electric power and motor technology is a future that’s much closer than you would realize. Electric aircraft are already in use for light sports and training aircraft. Electric commuter planes and air taxis are on their way, and they’ll be here by the middle of the decade. It’s a worthwhile endeavor for a connected future that protects our planet’s ecosystem for future generations.

All-electric and hybrid-electric architectures use an electric motor as the primary source of thrust. Or electric motors can be used in conjunction with a conventional engine to provide additional thrust or even a power boost to the propeller during critical periods of aircraft.

The complete electric propulsion system comprises other components such as motor controller, hardware and software, gearboxes, and cooling systems.   The entire system is known as an Electric Propulsion Unit (EPU).

HEP is a viable alternative to traditional short- and medium-range aircraft, with companies like Airbus, Siemens, Rolls-Royce, and Boeing all investing heavily in the technology. The five different architectures are Series hybrid, Parallel Hybrid, Series/Parallel hybrid, Turbo hybrid, and all-electric architectures.

Each architecture is unique in terms of energy efficiency, utility, complexities, and technological challenges. Let’s look into the architecture now.

Series Hybrid Configuration

In a series configuration, Mechanical coupling exists between the  Electric motor and the fans. 

The Internal combustion engine drives the electric generator, and the generator either runs the electric motor or charges the batteries through Power Electronics Converters. The energy generated by the generators might even recharge the batteries during different stages of flight that demand little propulsive effort (e.g., cruise phase).

Merits:

• Operates at maximum power and speed.
• Propulsion control is simple and easy.

Demerits:

• As the Electric Motor must provide all of the propulsion power on its own, it must be configured for maximum power, which adds to the drivetrain’s weight. 

Applications:

• High torque and low-speed applications.

Limitations:

Less efficient as compared with the parallel configuration and larger batteries and Electric Machines are required, increasing the powertrain’s mass and volume.

Parallel Hybrid Configuration

In a parallel hybrid configuration, two mechanically coupled parallel propellers exist. The battery-powered electric motor and internal combustion engine mounted on the same shaft control a fan or the propeller. This architecture allows the charging of batteries while the Internal Combustion Engine drives both the propellor and the  Electric motor via a coupler. In this configuration, Electric Motor runs as a generator.

Merits:

• Electrical Machines are smaller in size.
• Reduced weight.

Demerits:

• The mechanical coupling increases the mass.
• Involves more complex propulsion control systems.

Limitations:

Since it is engaged in thrust generation, the Internal Combustion Engine’s performance may be less optimum in various flight phases than in a series configuration.

Series/Parallel Configuration

As the name suggests, it combines both series and parallel configurations. It is the most widely used configuration in hybrid aircraft. All the components like Internal Combustion Engine, Electric Motor, Electric generator, and propellor are mechanically coupled.

The mechanical power of the Internal Combustion Engine can thus be utilized to operate the propeller or transformed into electric power via a generator to charge the batteries.

Merits:

• It enables fast topological changes while in operation.

Demerits:

• Highly complicated system configuration.
• Complexity in weight estimation.

Turbo Electric Hybrid Configuration

Turbo Electric hybrid configuration is similar to the series configuration and it does not fully dependent on the Batteries for propulsion. Power Electronic Converters with Electric Motors drive a fan or a propeller, while the Internal Combustion Engine drives an Electric generator.

There are no additional energy storage devices because all of the power comes from the fuel. Modern turboelectric innovations combine hydrogen-fueled Solid Oxide Fuel Cells that provide Electric Motors to drive fans with cryogenic superconducting components.

Merits:

• High-performance propulsion control.
• Flexibility in aircraft design.
• Internal Combustion Engines can perform at near to peak efficiency in terms of power vs speed, and they can be placed in the most productive areas in the aircraft.
• Electrical components are smaller in size and lesser in weight.
• The use of “green fuels” such as hydrogen is possible.

All-Electric Configuration

The fact that all-electric aircraft is a scalable technology, allowing for a gradual transition from 1-seat and 2-seat aircraft to 100-seat configurations, encourages their development.

At least 17 piloted electrical fixed-wing airplanes have been built since 2000, starting with the Antares 20E and progressing to the Electraflyer C, Yuneec 430, and Cri-Cri. The power density and efficiency of electric motors have increased in recent years, which facilitates this topology.

Battery power is the only source of propulsion in an all-electric architecture.

Merits:

• Higher energy conversion efficiency due to the presence of Electric motors and Power Electronic Converters.
• Managing a single power source necessitates simplified control systems.

Demerits:

• Low Battery Energy Density makes it not viable for most flights.
• Requires components capable of operating under higher temperatures.

Role of Electrical machines in Electric Aircraft 

Electrical motors are critical components in hybrid electric/electric propulsion systems. Electric motors have come a long way in the previous few decades.

Electrical motors can produce more than 500 horsepower, enabling them to compete with internal combustion engines. The following requirements should also be met when designing machines for safety-critical applications: 

• Isolation between lanes has to be provided by electrical, mechanical, magnetic, and thermal means; 
• high torque-to-weight and torque-to-ampere ratios; 

• phase inductance with a higher value, especially for permanent magnet motors; 
• exceptional efficiency across the whole speed spectrum. 

Electric Motors Used in more Electric Aircraft Propulsion

Due to various reasons like high maintenance requirements, low torque density, and lack of reliability, electrical machines with brushes or commutators are not considered. As a result, the candidate machines viable for electrical aircraft propulsion are limited to induction, reluctance, and permanent magnet motors.  

Let’s discuss all the Electric motors in detail as follows: 

(i) Induction motor

Squirrel-cage induction motors are widely used in electric aircraft propulsion because of their ease of use, robustness, low cost, and dependability. However, due to mutual coupling among all phases and the rotor, splitting the motor into magnetically separated modules is highly impossible.

Different multi-phase induction motor drives have indeed been designed in a modular structure that minimizes interphase electrical and magnetic couplings. These designs enhance fault tolerance to some level.

However, because of the safety need of independent control electronics and processing for each phase, there is a need for more intricate control approaches that may be difficult to execute and synchronize.

Although electric motors have been shown to continue working after an open-circuit failure, there are no designs available that can operate with a prolonged short-circuit. 

Difference Between AC motor and DC motor

Table 1: The difference between the ac motor and dc motor is given

(ii)  Reluctance Motors 

The rotor of the reluctance motor is made of a strong material that does not require windings or permanent magnets and can endure high temperatures and mechanical forces. As a result, it’s an excellent machine for low-cost applications. Rotor saliency is introduced in synchronous reluctance motors by adding internal flux barriers that guide the magnetic flux along the direct axis, or by adding salient poles.

The reluctance motor is more efficient than induction motors due to the absence of rotor joule loss as no emf will be induced when it runs at synchronous speed. Stator windings are arranged sinusoidally across the airgap, similar to induction motors, resulting in significant mutual coupling between phases and limiting its use in airplanes due to poor fault tolerance.

On the other hand, because of its inherent fault tolerance, the switched reluctance (SR) motor has now become prevalent in direct drive applications and is now making headway into aircraft prototypes.

On both the stator and rotor, the Switched Reluctance motor has salient poles and phase windings that are inherently independent. Due to the usage of unipolar excitation currents, a separate simple converter topology (two switches per phase) is typically utilized, providing additional phase independence.

The Switched Reluctance motor can still run with a correspondingly reduced mean torque capability if a short circuit develops on one phase. Another advantage of this motor is that the converter phase leg switches are connected in series with the motor phase winding, eliminating shoot-through failures produced by the converter switches.

Switched Reluctance motors have substantially smaller end windings, less phase coupling, and lower rotor loss than induction motors, which has led to their being preferred for fault-tolerant aerospace applications over induction machines.  

(iii) Permanent Magnet Motors 

Permanent Magnet (PM) Alternating Current motors are a general term for all brushless ac synchronous PM motors, comprising brushless dc and brushless ac motors fed with rectangular (or trapezoidal) and sinusoidal currents, respectively.

High power density and efficiency, high torque/inertia and torque/volume ratios, and increased reliability characterize them. Brushless dc and brushless ac motors are essentially the same in terms of hardware configuration, but they are supplied with various waveforms of power, which can be achieved by changing the control strategy in software. As a result, there is no need to distinguish the two in the analysis any longer. 

In terms of rotor PM configurations, PM motors can be used in a variety of ways. Surface-mounted permanent magnet motors typically have a small rotor diameter and low inertia (resulting in strong dynamic performance), but interior permanent magnet motors have higher per unit inductances and hence field weakening capabilities.  Deep PMs with a single-layer focused winding topology have typically been used in fault tolerance designs to reduce mutual inductance.

While it is widely acknowledged that PMAC motors have a higher torque density than reluctance and induction motors, they are less fault-tolerant by nature. Because of the continued presence of the magnet-induced back emf in traditional PM machine topologies, currents can flow in a failed lane even when it is unplugged from the electrical supply. They can, however, withstand such a mistake if the parameters are carefully chosen, and they are generally the preferred option. 

Power converter faults can cause an effective short circuit at the machine’s terminals. The generated current can be limited to not more than the rated current by properly selecting the PM motor inductance. Nonetheless, this results in large drag torque at low speeds.

To generate both the load torque and this drag torque, the remaining healthy phases must be overestimated. In short, enhanced PMAC motors can provide the smallest answer to actuation needs, but fault management is more difficult than in Switched Reluctance machines. Table 2 compares the performance of the three motor technologies for aeronautical applications. 

Comparative Evolution of Motors Based on Performance

 Table 2: comparative evolution of motors based on performance: 

Battery Technologies for Electric Aircraft Propulsion Systems: 

Lithium-sulfur battery

One of the essential technologies for supporting Hybrid Electric Propulsion for larger aircraft is energy storage. Batteries are the most common energy storage technology for aircraft.

On the other hand, Fuel Cells and supercapacitors are also being investigated. Solar photovoltaic cells or other harvesting devices can be added to any of the above-mentioned storage solutions. Batteries are currently the most widely used electrical energy storage mechanism.

Alkaline, lead-acid, nickel-cadmium (Ni-Cd), nickel-metal hydride (Ni-MH), lithium-ion (Li-ion), and lithium-ion polymer (Li-pol) are the current technologies, with the latter two being the most widely accessible.

The fact that batteries provide roughly 50 times less specific energy than liquid fuels is the problem for batteries to supply aircraft electric propulsion.

Read more: Top 5 Potential Battery Technology to Fuel the EV Industry

Because batteries do not lose mass as quickly as jet fuel during flight, they have a “hidden” weight that causes a drag penalty. There are prevalent constraints in battery designs like energy density in Wh/kg and specific power in kW/kg. 

With theoretical specific energy of 2600 Wh/kg and the ability to operate at low temperatures, the Li-S battery is a promising technology for aircraft use.

It can be deployed in thin-film layers along the wing, dispersing the weight and lowering the cost of sulfur. A novel lithium-metal technique has improved the poor Battery Energy Density (Wh/L) and short life cycle of Li-S batteries. Commercial Li-S batteries have specific energy ratings of up to 500 Wh/kg. In Juiz de Fora, Brazil, a full-scale industrial plant is being built. 

Summary 

Governments all over the globe are enacting strict restrictions to limit carbon emissions caused by air transportation. Nevertheless, as the number of people flying increases, greenhouse gas emissions from the aircraft industry are predicted to skyrocket by 2030.

The technology is remarkable, with significant implications for public safety and the environment. There is still plenty of room for growth in the broader ecosystem.

Airline companies and associated technological applications are only some part of the issue, and many stakeholders must participate in the shift for electrically-propelled aircraft to make it more conventional.

Despite the ambiguities at this point, one thing is certain: we are presently living in an era of aerospace and aviation innovation unlike any other in decades. Electrically propelled airplanes and their ecosystem are still in their infancy, but they are rapidly developing.