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The commercial aviation currently accounts for roughly 2.5 % of the global CO2 emissions and around 3.5% of world warming emissions, taking into account non CO2 effects on the climate. Its has grown faster in recent decades than the other transport modes (road, rail or shipping), with an average rate of 2.3%/year from 1990 to 2019, prior to the pandemic. Moreover, its share of Greenhouse (GHG) emissions is supposed to grow, with the increasing demand scenario of air trips worldwide. This scenario might threaten the decarbonization targets assumed by the aviation industry, in line with the world efforts to minimize the climate effects caused by the carbon emissions. In this context, hydrogen is set as a promising alternative to the traditional jet fuel, due to its zero carbon emissions.

The aviation industry (passenger and freight), which currently accounts for 2.5% of the global CO2 emissions (1.9% of global greenhouse gas (GHG) emissions), is continuously under pressure to reduce its environmental footprint, given its historical and forecasted environmental track, strongly affected by the remarkable air traffic volume increase rates, albeit with a slower growth in emissions, due to the massive aviation's efficiency improvements, driven by the in the design and technology(more efficient and larger) aircrafts; improved operational practices and increased load factors (more passengers and freight per flight). Nevertheless, it has not been enough to tackle the rapidly increasing CO2 emissions (26% in the 2013-2018 timeframe and expected to continue increasing), which ultimately could grow between 2.4 and 3.6 times by 2050.

The transport systems, as large energy consumers and important contributors to greenhouse (GHG), criteria pollutant and noise emissions worldwide, have been permanently challenged by the continuously increased stringent environmental standards to improve its energy efficiency, as well as to reduce its environmental footprint. Transit systems, which operates in urban areas, are particularly subjected to stringent environmental and efficiency regulations, given their proximity to large population concentrations, alongside the urban transport corridors. This is particularly true for bus transit systems, mostly powered by internal combustion engines (ICE), generally fueled with fossil diesel fuel.

Aviation industry currently accounts for almost 3% of worldwide greenhouse gas (GHG) emissions. Despite the continuous efforts to reduce this environmental footprint, with the use of technological efficiency driven solutions and operational changes to reduce climatic effects, such as engine improvements, fleet renewals and navigation operational improvements, the industry, which is permanently challenged by the continuously stringent standards, is aware of the need of additional measures to tackle, and even reduce, the GHG emissions, by decoupling the world's industry average growth (almost 4.1% annually) to the aviation's carbon emissions. Given its inherent operational features, the aviation sector requires fuels with high specific energy and energy density. This technical requirement makes the well known clean and efficient electrical propulsion technology to be limited to niche aviation segments (short range and low capacity airplanes) in the short and medium terms.

Turbofan engine technology has been widely used for propulsion of commercial airplanes in the subsonic range. Since the early 70's, turbofan's efficiency has greatly evolved, driven firstly by the core (thermodynamic) efficiency improvement, followed by a continuous increase in the bypass ratio (BPR), i.e. the rate between the engine fan airflow and the engine core airflow mass, with a focus on propulsive efficiency improvement. The core efficiency strategy, strongly supported on the engines' thermodynamic properties boost, i.e. using higher overall pressure ratios (OPR) and turbine inlet temperatures (TIT), has contributed decisively to the engine's thermodynamic efficiency improvement.

Environmental concerns have driven the scientific community in a continuous effort to set standards for emission and noise control in a diversity of industries worldwide. The aviation industry, which currently relies on fossil fuels, is strongly driven by a growing environmental awareness, which makes it one of those that are spending huge efforts to reduce its environmental footprint. Airplanes emit several types of pollutants, mainly Carbon Monoxide (CO), Unburned Hydrocarbon (UHC), Particulate Matter (PM), Nitrogen Oxides (NOx) and Sulfur Oxides (SOx), as well as Greenhouse Gases (GHG) and noise. These emissions, which can impact both the airport surroundings, as well as the high atmosphere layers, might affect the environment, through the modification of atmosphere’s chemical and physical properties, which ultimately might cause global, regional and local effects, as well as noise annoyance to the population near the airports.

The aviation industry has been strongly driven by stringent safety, efficiency and environmental requirements. In the efficiency field, engine efficiency, allied with aerodynamic design improvements, have played an important role to reduce the fuel burn, given that fuel expenses are currently one of the major cost drivers of the air carriers worldwide. Since the beginning of the jet engine era, engine efficiency has evolved significantly, through thermodynamic efficiency improvements, associated mainly with increased engine pressure ratio (EPR) and turbine inlet temperature (TIT). Another important optimization driver has been the propulsive efficiency, associated with increased bypass ratios (BPR). Material and aerodynamic properties have imposed limits for thermal efficiency improvements, which ultimately might hinder further progresses.

Heavy duty vehicle (HDV) segment, as an important source of emissions that strongly impact air quality and human health - especially in urban centers - has been continuously challenged by the increasingly stringent emission limits. The adoption of emission standards for the heavy duty industry was initially launched by the United States, followed by the European Union and Japan, and, subsequently, by other countries, like Australia, Brazil, China and India, among others, generally with a time lag. This continuous “cleaning” effort has led to the current rigorous emission limits - materialized by the so called U.S. EPA 2010 and Euro VI and their foreign variants - which have provided huge emissions reductions (HC, CO, NOx, PM and smoke and, more recently, CO2).

The aviation industry has been submitted to a set of environmental and commercial drivers that have led it to pursue engine technologies focused on the efficiency improvement, greenhouse (CO2) and pollutant (NOx and PM) emissions reductions, as well as noise abatement.The effort to comply with the ambitious long term environmental and efficiency targets set by the regulatory authorities has driven the aeronautic industry in a technological research effort. In the medium term, the aviation industry's strategy for commercial aviation is to focus on the advanced, but rather conventional propulsion systems (mainly turbofan engines). In this scenario, technological efforts have focused basically on enhancing thermal efficiency, through advanced core engines, as well as improving propulsive efficiency, through the use of low pressure systems (basically reduced pressure ratio and increased engine bypass ratio).

Despite the recent groundbreaking improvements in diesel engine technology, with its inherent improved emission performance (Euro VI, US 2010 and their equivalences), it is well known that there is a limit on cleaning diesel buses. At the same time, cities and transit operators have been permanently challenged for seeking for traction technologies to comply with the emissions’ reduction agenda. In this context, electric bus traction technologies appear as a promising alternative for cleaning the bus’ fleets, with their intrinsic potential to reduce environmental impacts caused by public transport, such as greenhouse gas and local pollutant, as well as noise emissions. Moreover, the use of electricity also contributes to reduce the transport system’s dependency on fossil fuels and their inherent price volatility.

Following the electrification trend observed in the automotive industry, the idea of an electric propulsion aircraft has also drawn attention and investments from a range of aviation industry stakeholders (including the world's largest aerospace companies) focused on both fuel burning reduction and environmental performance improvement (greenhouse gases (GHG), pollutants and noise emissions) potential of electric propulsion technology. Electric propulsion has the potential to provide more efficient, cleaner, quieter and more profitable aviation services, with potential benefits to both airlines and passengers. Furthermore, with its inherent quiet feature, it has also the potential to lead to a reassessment of the role of airports along the world cities, as well as revitalize regional short-haul flights and helps the launch of air service into underserved regions around the world.

The aviation industry currently holds a share of 2% global greenhouse gas (GHG) emissions. Although relatively small, estimated demand increase indicates an up to 350% emission rise in 2050, in the so called “no action scenario”. These emissions are injected into the upper atmosphere, with a potentialized stronger greenhouse effect than at ground level. In this context, ambitious emission reduction targets have been proposed into a global commitment, focused into a long term carbon emission reduction strategy, which would lead to net GHG emissions to peak in 2020, and then halves by 2050, based on 2005 levels, while accommodating increased air transport demand. To achieve this challenging goal, a multifaceted approach is required, ranging from technology oriented actions, like revolutionary aerodynamically driven design, new composite lightweight material and engine technology improvement, as well as improved ground and flight operational practices.

Environmental concerns and limited fossil fuels reserves have fostered an increased interest in alternative propulsion systems. In this scenario, electric traction, with its inherent zero local emissions, high efficiency and improved operational performance (acceleration and hill climbing potential), emerges as a desired option for public transport systems. Transit buses, the prevailing transport system in cities, and, hence, strong contributors to traffic environmental impact on urban areas, can reduce considerably their environment burden with the use of electric traction. This means less local pollutants, specially particulate matter - PM and nitrogen oxides - NOx, currently the “Achilles heel” of diesel engines, as well as CO2 greenhouse emissions - GHG.

Public transport has been considered the preferred strategy to reduce congestion and pollution from urban road traffic. For low to medium capacity, bus systems are considered the most affordable and flexible mode. Currently, diesel based systems still dominate transit bus market, due to their high productivity, low deployment costs, technological maturity, operational reliability and flexibility (high daily ranges, fast refuelling and no infrastructure requirement along the routes). However, although some important improvements in engine technology and aftertreatment devices, enforced by emission standards improvements (Euro VI, US 2010 and those related), have been achieved, it is well known that there is a limit to cleaning exhaust diesel buses exhaust. In this context, transit authorities and operators have been under pressure to shift for more environmental friendly technologies.

Emissions from motor vehicles have been a subject of concern in urban areas, as great amounts of population have been permanently exposed to large amounts of pollutants, with intrinsic adverse health effects. In this context, in the last two decades, stringent emissions standards have been developed to control the maximum emission limits of the so called regulated pollutants. This continuous reduction of emission targets has imposed a great effort to engine and vehicle manufacturer in the development of technological solutions for emission limits compliance, which can be done by reducing engine-out emissions through improvements in combustion process and fuel management system, as well as by using aftertreatment devices in the exhaust system.

Current massive urbanization process concentrates high amount of population and impose an increased demand on transport systems. In this context, transit bus system plays an important role, as the most dynamic and less capital intensive transit option available. At the same time, it is strongly dependant on fossil fuels, predominantly diesel fuel, with its intrinsic polluting and greenhouse (climate change) effects. This has boosted research and investments for alternative and renewable fuels. One solution currently receiving widespread recognition is biogas use in transit bus fleets, as it allows the use of a renewable fuel, made from substrates derived basically from waste and sewage that otherwise would produce methane released to the atmosphere.

Compression Ignition - CI or Diesel engines are currently considered the most fuel efficient combustion based drivetrain, and, for this reason, it has been historically used as the backbone for heavy duty markets, including transit bus fleets. At the same time, CI engines fueled by traditional crude oil based diesel fuel are facing the growing challenge of meeting the increasing stringent emission standards, specially on particulates matter, nitrogen oxides and greenhouse gases emissions limits. Moreover, petroleum based transport fuels are constantly faced by strategic and security concerns, due to the concentration of the main currently known reserves in political unstable regions. As such, it is both environmentally and economically important to find alternatives for crude oil based diesel fuel to be used in the transportation sector.

The growing concentration of population in world metropolis caused by increasing urbanization rates has pushed the demand for high capacity and efficient public transport systems. At the same time, environmental concerns have led to increasingly stricter emission standards. In this context, transit authorities have become strongly focused on making their bus fleets more efficient and cleaner, by incorporating new alternative fuels and clean propulsion technologies. This has led to increased interest in electric driven technologies, with their intrinsic efficient, quiet and environment friendly features. Trolleybuses, a well proven mature electric technology already adopted in some cities, although efficient and clean, are burdened by high infrastructure costs and operational inflexibility.

From the nineties there was a great interest in the use of compressed natural gas - CNG (predominantly composed of methane) on transit bus fleets around the globe. In a first moment, developed countries (US, EU and Japan) have focused their efforts to address serious urban air pollution problems caused by heavy duty diesel engines - since PM and NOx emissions were initially easier to control from natural gas engines than from conventional diesel engines - and also to offset growing oil imports. As such, for many years, dedicated methane fuelled city buses meeting emission requirements (Euro IV, V and EEV, US Federal and California, and Japan) either in a lean burn or stoichiometric technology, have been offered to the market.

Intensive use of fossil fuels in densely populated areas has caused adverse environmental effects in cities all over the world. This has fostered the evaluation of alternative technologies for transit applications, like hydrogen fuel cells - electrochemical energy conversion devices that operate with zero emission, quieter and with higher efficiencies than internal combustion engines, specially at part load regimes. Transit bus market is particularly well suited to technology innovations because they are i) centrally fueled and maintained, ii) professionally operated on fixed routes and schedules, iii) tolerate weight and volume requirement of new technologies and, finally, whenever necessary, iv) can be subsidized by government. In this scenario, considerable research, development and testing effort has been dedicated to hydrogen fuel cell bus technology, with the engagement of governments and transit authorities, bus industry and operators.