Modeling and Characteristics of a Novel Multi-fuel Hybrid Engine for Future Aircraft

Civil air transportation has undergone significant expansion over the past decades and is continuing to grow. Nevertheless, the tendency of energy depletion and the severe environmental problems yield challenges in its further development. To mitigate the climate impact of civil aviation, the Advisory Council for Aeronautics Research in Europe (ACARE) has set ambitious objectives for the year 2050 to reduce the CO2 emission by 75% per passenger kilometre, the NOx emissions by 90% and the perceived noise emission by 65% relative to the capacities of aircraft operating in the year 2000. The conventional approach of increasing Bypass Ratio (BPR), Overall Pressure Ratio (OPR), and Turbine Inlet Temperature (TIT) to improve the cycle efficiency, and thereby reducing the fossil fuel consumption and the associated emissions is unlikely to meet the ACARE goals. Moreover, the high OPR and TIT aggravate the NOx emissions for a given combustion technique. A novel multi-fuel hybrid engine for a Multi-Fuel Blended Wing Body (MFBWB) aircraft conceived in the “Advanced Hybrid Engine for Aircraft Development (AHEAD)” project brings to light promising solutions in this regard. The multi-fuel hybrid engine is a turbofan engine with the following added components: a Contra-Rotating Fans (CRF) system, two sequential combustors burning different fuels simultaneously, and a Cryogenic Bleed Air Cooling System (CBACS). The CRF can sustain the non-uniform flow ingested from the boundary layer of the airframe. The first combustor is the main combustor, where the Liquid Hydrogen (LH2) or the Liquid Natural Gas (LNG) is burnt to reduce CO2. The second combustor, Interstage Turbine Burner (ITB), is located between the high pressure and the low pressure turbine burning kerosene or biofuel in a Flameless Combustion (FC) mode. With the thermal energy provided by different fuel sources, the volume required to store cryogenic fuels is less; meanwhile, the FC technique is beneficial to reduce NOx. By introducing the CBACS, LH2 or LNG is used as a coolant to cool down the bleed air. According to fuel combinations, the hybrid engine is classified as LNG-kerosene version and LH2-kerosene version, where kerosene might be replaced by biofuel. By defining an “ITB energy fraction” as the ratio of the energy input of the ITB to the overall energy consumed, the fuel flow rates of two combustors are controlled. Using the developed model framework, the characteristics of the hybrid engine are studied and summarized in the following three aspects: Potentials of the ITB engine cycle:The sequential combustor configuration of the hybrid engine forms a reheat cycle. By distributing the energy into two combustors, the heat addition to each combustor decreases; therefore, the TIT is lower. Consequently, the turbine cooling air and the associated loss in the turbine efficiency reduces. Moreover, the NOx produced from the upstream combustor dissociates again in the ITB, which helps to lower the overall NOx emissions. These remarkable features are appreciable when the OPR and BPR are forced to continuously increase, which causes a substantial increase in the TIT of a classical engine. A turbine with very high inlet temperature has to be cooled substantially. Eventually, the gain in cycle efficiency might be canceled by the loss in the turbine efficiency. Moreover, when the TIT is increased beyond 1800 K, the NOx exhibits an exponential increase. Hence following the evolution of the engine technology, the reheat cycle would be an option for the next step. Characteristics of the multi-fuel hybrid engine:The features of the hybrid engine have been explored from various aspects. The isobaric heat capacity of the combustion products from LNG and LH2 is higher than that from kerosene, which is beneficial to the thermal efficiency. Using LNG and LH2 as a coolant, the bleed air temperature reduces substantially (maximum by more than 500 K), thereby, the turbine cooling air mass flow rate decreases by half. Moreover, the increase in fuel temperature is favourable to enhance the thermal efficiency. The hybrid engine has been optimized at cruise considering various ITB energy fractions. The optimized engine cycle is verified at critical operating points. The assessment of the standalone engine performance with baseline engines shows that the LH2-kerosene hybrid engine is superior to the LNG-kerosene hybrid engine in terms of the cycle efficiency and the CO2 reduction. However, the mission analysis shows conflicting results. Due to the stronger installation effect, the MFBWB together with the LH2-kerosene hybrid engine scores lower, implying that the LNG-kerosene BWB would have the least climate impact.Operating strategy of the multi-fuel hybrid engine:The operating strategy of the hybrid combustion system has been developed to enhance the steady state performance of the hybrid engine. The analysis exhibits that using an ITB is beneficial for the high pressure spool speed, the HPC exit temperature, and the HPT inlet temperature. However, the LPC surge margin and LPT inlet temperature conflict their limits as the ITB energy fraction increases. For the various thrust requirements at Sea Level Static (SLS) standard condition, a fuel control schedule together with a variable bleed valve schedule is proposed. Moreover, another fuel control strategy is suggested for the flat rating at SLS.Aircraft Noise and Climate Effect

The ahead project: Advanced hybrid engines for aircraft development

Aviation is an ever-increasing market and more passengers and cargo are carried each year. The world is becoming ever more connected. However, this does come at a price: aviation has a marked in!uence on the environment. If aviation is to thrive in the future, breakthroughs in aircraft design and propulsion systems are needed. The AHEAD project is an attempt at achieving such a breakthrough.Aerospace Engineerin

Performance assessment of a Multi-fuel Hybrid Engine for Future Aircraft

This paper presents performance assessment of the proposed hybrid engine concept using Liquid Natural Gas (LNG) and kerosene. The multi-fuel hybrid engine is a new engine concept integrated with contra rotating fans, sequential dual combustion chambers to facilitate “Energy Mix” in aviation and a Cryogenic Bleed Air Cooling System (CBACS). The current analysis focuses on three aspects: 1) effects of the CBACS on the HPT cooling air requirement and the associated effects on the cycle efficiency; 2) performance optimization of the hybrid engine; 3) assessment of the emission reduction by the hybrid engine. An integrated model framework consisting of an engine performance model, a turbine cooling model, and a Cryogenic Heat Exchanger (CHEX) model is used to perform the analyses. The parametric analysis shows that using the CHEX, the bleed air temperature can be reduced significantly (up to 600 K), which reduces the turbine cooling air requirement by more than 50%, while increasing the LNG temperature by 300K. Consequently, the cycle efficiency improves even further. Depending on the fuel flow distribution between two combustors. The CO2 emission from the hybrid engine is lower by 15% to 30%. The mission analysis along with the Multi-Fuel Blended Wing Body aircraft shows a reduction in NOx emissions by 80% and CO2 emission by 50% when compared to B-777 200ER.Aircraft Noise and Climate EffectsFlight Performance and Propulsio

A review of gas turbine engine with inter-stage turbine burner

Society is going through transformations at a rate that is unprecedented in human history. One such transformation is the energy transition, which will affect almost every facet of our society. Gas turbine engines are state of the art machines, a backbone of modern society, and used in various applications, right from power generation to propelling aircraft and ships. This paper reviews the possibilities offered by the Inter-stage Turbine Burner (ITB) configuration for both aviation and power generation with a view on sustainability and fuel flexibility. First, the thermodynamic characteristics of a Brayton-Joule cycle with ITB is elaborated, followed by discussions on the design and the off-design performance characteristics of such a gas turbine architectural variation. Finally, the viability of ITB architecture in reducing emissions and enabling “Energy Mix” in aviation is elaborated. The paper concludes with an outlook on the technological readiness ladder that the engineering community will have to address in the future.Aircraft Noise and Climate EffectsFlight Performance and Propulsio

Energy transition in aviation: The role of cryogenic fuels

Aviation is the backbone of our modern society. At present, around 4.5 Billion passengers travel through the air every year and aviation is responsible for around 5 % of anthropogenic causes of Global Warming (Lee et al, 2009). With the increase in global GDP, the number of travellers is expected to increase to 7.5 Billion by 2037 and to around 15 Billion by 2050. Even though the crude oil prices are low at the moment, with finite petroleum reserves available on our planet, it is expected that the Jet fuel prices will increase in the future. Moreover using kerosene causes several emissions which are bad for the environment. Liquefied Natural gas (LNG) and Liquid Hydrogen (LH2) can provide an attractive alternative for aviation.Aircraft Noise and Climate EffectsFlight Performance and Propulsio

Impact of hybrid electric aircraft on contrail coverage

Aviation is responsible for approximately 5% of global warming and is expected to increase substantially in the future. Given the continuing expansion of air traffic, mitigation of aviation’s climate impact becomes challenging but imperative. Among various mitigation options, hybrid-electric aircraft (HEA) have drawn intensive attention due to their considerable potential in reducing greenhouse gas emissions (e.g., CO2). However, the non-CO2 effects (especially contrails) of HEA on climate change are more challenging to assess. As the first step to understanding the climate impact of HEA, this research investigates the effects on the formation of persistent contrails when flying with HEA. The simulation is performed using an Earth System Model (EMAC) coupled with a submodel (CONTRAIL), where the contrail formation criterion, the Schmidt–Appleman criterion (SAC), is adapted to globally estimate changes in the potential contrail coverage (PCC). We compared the HEA to conventional (reference) aircraft with the same characteristics, except for the propulsion system. The analysis showed that the temperature threshold of contrail formation for HEA is lower; therefore, conventional reference aircraft can form contrails at lower flight altitudes, whereas the HEA does not. For a given flight altitude, with a small fraction of electric power in use (less than 30%), the potential contrail coverage remained nearly unchanged. As the electric power fraction increased, the reduction in contrail formation was mainly observed in the mid-latitudes (30° N and 40° S) or tropical regions and was very much localized with a maximum value of about 40% locally. The analysis of seasonal effects showed that in non-summer, the reduction in contrail formation using electric power was more pronounced at lower flight altitudes, whereas in summer the changes in PCC were nearly constant with respect to altitude.Aircraft Noise and Climate Effect

Performance analysis of an Aero Engine with Interstage Turbine Burner

The historical trends of reduction in fuel consumption and emissions from aero engines have been mainly due to the improvement in the thermal efficiency, propulsive efficiency and combustion technology. The engine Overall Pressure Ratio (OPR) and Turbine Inlet Temperature (TIT) are being increased in the pursuit of increasing the engine thermal efficiency. However, this has an adverse effect on engine NOx emission. The current paper investigates a possible solution to overcome this problem for future generation Very High Bypass Ratio (VHBR)/Ultra High Bypass Ratio (UHBR) aero-engines in the form of an Inter-stage Turbine Burner (ITB). The ITB concept is investigated on a next generation baseline VHBR aero engine to evaluate its effect on the engine performance and emission characteristics for different ITB energy fractions. It is found that the ITB can reduce the bleed air required for cooling the HPT substantially (around 80%) and also reduce the NOx emission significantly (>30%) without penalising the engine specific fuel consumption.Aircraft Noise and Climate EffectsFlight Performance and Propulsio

Impact on contrails coverage when flying with hybrid electric aircraft

iation is responsible for approximately 5% of global warming and is expected to increase substantially in the future. In the face of the continuing expansion of air traffic, mitigation of the aviation’s climate impact becomes challenging, but imperative. Among various mitigation options, hybrid electric aircraft (HEA) has drawn intensive attention due to its large potential in reducing the greenhouse gas emissions. The non-CO2 effects (especially the contrails) of the HEA on the climate change, however, remains ambiguous. As the first step to understand the climate impact of HEA, this research aims to investigate the impact on the formation of persistent contrails when flying with HEA. The simulation is performed using the Earth System model (EMAC) coupled with a CONTRAIL submodel, where the Schmidt Appleman Criterion (SAC) is adapted to estimate changes in the potential contrail coverage (PCC) globally. The analysis shows that the HEA forms contrails at relatively lower temperature than conventional aircraft. At the same altitude, the reduction in contrails formation is mainly observed in the tropical regions where the temperature is warmer. With a smaller fraction of electric power in use, the contrail coverage remains nearly unchanged. As the degree of hybridization increases further to 90%, an exponential reduction in the contrail formation is expected with a maximum value of about 40%.Aircraft Noise and Climate Effect

A Novel Engine Architecture for Low NOx Emissions

The fuel efficiency of turbofan engines has improved significantly, hence reducing aviation's CO2 emissions. However, the increased operating pressure and temperature for fuel efficiency cause adverse effects on NOx emissions. Therefore, a novel engine concept, which can reduce NOx emissions without affecting the cycle efficiency, is of high interest to the aviation community. This paper investigates the potential of an intercooler and inter-turbine burner (ITB) for the future low NOx aircraft propulsion system. The study evaluates performance and NOx emissions of four engine architectures: a very high bypass ratio (VHBR) turbofan engine (baseline), a VHBR engine with intercooler, a VHBR engine with ITB, and a VHBR engine with both intercooler and ITB. The cycles are optimized for minimum cruise Thrust Specific Fuel Consumption (TSFC), considering the same design space, thrust requirements, and operational constraints. The ITB is only used during take-off to minimize cruise fuel consumption. The analysis shows that using an ITB solely, with the energy split of 75% (the first burner) / 25% (ITB), reduces the cruise NOx emission by 26%, and the cruise TSFC slightly by 0.5%. The intercooler alone reduces the NOx emissions by 16% and the cruise TSFC by 0.8%. The combination of intercooler and ITB reduces the NOx emissions further by 38%. The analysis confirms that introducing an intercooler and ITB can potentially resolve the contradicting effects of fuel efficiency and NOx emissions for the future advanced turbofan engine. Aircraft Noise and Climate EffectsFlight Performance and Propulsio

Concept of robust climate-friendly flight planning under multiple climate impact estimates

The spatiotemporal dependency of aviation-induced non-CO2 climate effects can be incorporated into flight planning tools to generate climate-friendly flight plans. However, estimating climate impact is challenging and associated with high uncertainty. To ensure the effectiveness of such an operational measure, sources that induce uncertainty need to be identified and considered when planning climate-aware trajectories. The mismatch between different assessments of climate impact is an important indicator of uncertainty. This study introduces a concept aimed at planning robust climate-optimized aircraft trajectories under multiple climate impact estimates. The objective is to generate climate-optimal trajectories that achieve mitigation potential consistent with all available assessments. Case studies show that, even when there is a significant discrepancy between input models in specific regions, the proposed approach can effectively generate trajectories to mitigate the climate impact with a high level of confidence.Aircraft Noise and Climate EffectsAir Transport & Operation