BACKGROUND OF THE INVENTION

1. Field of the Disclosure

The present disclosure concerns aircraft propulsion systems in which electrical energy is used to supplement the operation of a combustion engine. The invention particularly concerns methods for aiding in the control of the negative climate impact resulting from operation of a combustion engine, for example via vapour trail formation.

2. Description of the Related Art

Vapour trails, also known as condensation trails or contrails, are line-shaped ice clouds that appear behind aircraft under certain circumstances. The formation of a contrail depends on a number of factors, including: ambient temperature, humidity and pressure; the efficiency of the aircraft's engines; and the properties of the fuel burned in the engines.

During the day, contrails reflect a proportion of incoming sunlight away, leading to a climate cooling effect which is at least partially offset against the climate warming impact associated with the absorption by contrails of heat radiating upward from the planet's surface. During the night, however, the cooling effect is not operative. For this reason, a contrail's climate warming impact (per unit time of contrail existence) is typically greater during the night than during the day. It is estimated that, globally, night-time flying accounts for some 40% of total flying miles, but for some 60% of aviation's total contrail-related climate warming impact.

The climate warming impact of a contrail is also influenced by its temperature such that a contrail forming in colder air will exert more of a climate warming effect than an otherwise equivalent contrail which forms in slightly warmer air. A contrail, once formed, will typically dissipate within a minute or so, unless the ambient air is supersaturated with respect to ice, in which case the contrail may persist. A persistent contrail may grow over time to resemble natural cirrus cloud, both in size and optical properties, and is then referred to as “contrail-cirrus”. Line-shaped contrails and contrail-cirrus are collectively referred to as “aviation-induced cloudiness” (AIC). Contrail-cirrus is thought to cause a majority of the negative climate impact of AIC due to it being spatially larger and longer-lived relative to a non-persistent line-shaped contrail.

Depending on the metric employed, the climate-warming impact of aviation induced cloudiness may be of a similar magnitude to that of the carbon dioxide (CO2) emitted by aircraft, and may therefore represent a significant element of aviation's total climate impact. The partial or complete suppression of contrail formation, and particularly the partial or complete suppression of persistent contrails, may represent an opportunity for a significant reduction in the overall climate warming impact of aviation. Here we use the term “partial suppression of contrails” to mean a reduction in the optical depth of formed contrails.

However, a number of potential techniques for reduction of contrail formation or modification of contrail properties by a gas turbine engine require the use of bespoke equipment and/or materials that are additional to those required for conventional engine operation. Any weight and/or energy penalties incurred in order to achieve contrail suppression or modification of contrail properties require careful scrutiny to determine whether such penalties outweigh the possible contrail reduction benefits on climate impact.

Conventional propulsion systems for civil aircraft typically comprise one or more turbofan engine placed under the wings of the aircraft. However, some studies have indicated that so-called distributed propulsion, which involves having numerous smaller propulsion units preferentially arranged around an aircraft, may provide some significant benefits in terms of noise reduction and fuel efficiency when compared with current state of the art propulsive arrangements.

The distributed propulsion units are typically electrically driven with the electrical energy being supplied from a power source mounted onto the airframe. The power source may be a battery but this is usually supplemented by an on-board generator powered using a reciprocating engine or, more usually, a gas turbine. It has been recognised that this arrangement can have a greater propensity for contrail formation than a conventional turbofan engine since the exhaust from the reciprocating or gas turbine engine has a relatively high water-vapour partial pressure relative to the temperature of the exhaust i.e. the “contrail factor”, which is used herein to refer to the gradient of a line representing the mixing of engine exhaust air with ambient air, when plotted on a chart using water-vapour partial pressure as the y-axis and temperature as the x-axis, is relatively high. A reduction in an engine's contrail factor reduces the range of ambient conditions under which the engine can form a contrail. Alternatively, at a particular ambient condition (characterised by pressure, temperature and humidity), a reduction in contrail factor may allow a transition from formation of a contrail to non-formation of a contrail.

It is an object of the invention to manage the formation of contrails by aircraft engines in a manner that reduces negative impact on the climate. It may be considered an additional or alternative object to provide a practical system for implementation of contrail control for an aircraft.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided an aircraft propulsion system comprising: an engine having an engine core comprising a compressor, a combustor and a turbine driven by a flow of combustion products of the combustor; at least one propulsive fan for generating a mass flow of air to propel the aircraft; an electrical energy store on board the aircraft; at least one electric motor arranged to drive the propulsive fan and the engine core compressor; and a controller arranged for control of the at least one electric motor to mitigate the creation of a contrail caused by the combustion products by altering the ratio of the mass flow of air by the propulsive fan to the flow of combustion products of the combustor, wherein control of the at least one electric motor comprises selectively driving both the propulsive fan and engine core compressor.

The controller may monitor conditions indicative of contrail formation. The controller may monitor conditions indicative of the formation of contrails having a negative climate impact.

The controller may reduce the flow of combustion products produced by the engine core to mitigate or suppress the creation of a contrail.

The controller may increase the power/torque supplied to the propulsive fan by the electric motor during contrail mitigation. The controller may increase/control the rotational velocity of the propulsive fan via the electric motor during contrail mitigation.

The controller may control operation of the electric motor so as to meet a thrust demand for the aircraft. Where the flow rate of combustion products is reduced by lowering the rate of fuel burn in the engine core this may reduce torque and/or thrust generated by the engine core. The reduced torque and/or thrust may be mitigated by increasing the torque applied to one or more propulsive fan by the electric motor.

The at least one electric motor may selectively assist the engine core compressor, e.g. supplementing the torque applied to the compressor due to the engine core combustion process.

The controller may control the operation of the at least one electric motor according to one or more operational variable for the engine core, e.g. the engine core compressor.

The controller may monitor operation of the engine core compressor. The controller may monitor an absolute or relative operational variable for the engine core compressor, such as any or any combination of rotational speed, torque, flow rate and/or pressure rise. Additionally or alternatively, the controller may monitor temperature and/or fuel flow rate for the combustor.

The controller may monitor the relative operation between the propulsive fan and the engine core compressor, e.g. according to relative values of the operational variable.

The controller may operate the electric motor to selectively drive the engine core compressor according to one or more predetermined threshold value of the operational variable. The threshold may comprise a safety threshold, such as a compressor surge margin.

The at least one electric motor may be controlled according to a control hierarchy wherein priority is given to driving the one or more propulsive fan or the engine core compressor for contrail mitigation, e.g. provided the threshold is not met or exceeded.

The at least one electric motor may comprise a first electric motor arranged to drive the propulsive fan and a second electric motor arranged to drive the engine core compressor. The engine core compressor may be assisted by the second electric motor selectively, e.g. only during operation of the first electric motor or only during a portion of the operational range of the first electric motor. Alternatively, a single electric motor may be provided, with a suitable selective transmission arrangement for driving either or both of the propulsive fan and engine core compressor.

The propulsive fan may be driven at least in part by a low pressure turbine, e.g. via a low pressure shaft. The propulsive fan may be coaxial with the engine core.

A gearing may be provided for altering the speed of the propulsive fan relative to the speed of an intermediate pressure turbine. The system may comprise a geared turbofan engine.

The engine may comprise an intermediate compressor (IPC), e.g. having one or more row of compressor blades, for compressing air to the engine core compressor. The propulsive fan may be drivingly connected to the IPC via a gearing.

A gearing may be provided between the at least one electric motor and the propulsive fan and/or engine core compressor.

A propulsive fan rotor may have one or more row of low pressure compressor blades for compressing air to the engine core compressor. The propulsive fan may comprise a booster. The compressor/booster may rotate in unison, e.g. at a corresponding rotational speed, with the propulsive fan.

The engine may comprise a two-shaft gas turbine engine. The engine may comprise a boosted turbofan engine.

The system may comprise one or more ambient condition sensor, e.g. for sensing any or any combination of ambient temperature, pressure and/or moisture/ice content.

The system may comprise one or more electrical generator for charging the electrical energy store. The generator may be driven by the engine. The first and/or second motor may operate as a generator, e.g. when not being used to drive the respective propulsive fan or engine core compressor. Accordingly, the term ‘electric motor’ as used herein may be considered to be a reference to an electric machine, including generator functionality, where applicable.

There may be one or more further propulsive units which are distributed around the fuselage and/or wings of the aircraft. The further propulsive units may be electrically driven fans connected to the electrical energy store via an electrical network, wherein upon sensing conditions indicative of contrail formation power supply to the further propulsive units from the electrical energy store may be initiated or adjusted, e.g. as either a power increase or decrease.

According to a second aspect of the invention, there is provided a method of operating an aircraft propulsion system having an engine with an engine core comprising a compressor, a combustor and a turbine; at least one propulsive fan for generating a mass flow of air to propel the aircraft; and an electrical energy store on board the aircraft, the method comprising: supplying fuel to the engine core combustor to produce a flow of combustion products for driving the engine core turbine and compressor; monitoring conditions indicative of contrail formation; selectively altering the ratio of the mass flow of air by the propulsive fan to the flow of combustion products of the combustor; and selectively assisting rotation of the engine core compressor by an electric motor drawing energy from the electrical energy store during the altering of said ratio.

The altering of said ratio may comprise reducing the rate of fuel consumption of the engine core and/or the rate of thrust/combustion products produced by the engine core. The rotation of the compressor may be assisted when the rate of combustion and/or rotational speed of the compressor reduce.

According to a third aspect of the invention, there is provided a data carrier comprising machine-readable instructions for the operation of an aircraft propulsion controller to: receive sensor readings for a plurality of engine operation variables; monitor conditions pertaining to adverse contrail formation by combustion products from the engine; output control instructions in response to a determination of adverse contrail formation to alter a ratio of the mass flow of air by a propulsive fan to a flow of combustion products of an engine core combustor; and concurrently output control instructions to selectively assist rotation of a compressor for the engine core by an electric motor drawing energy from an electrical energy store.

Any of the preferable features defined in relation to any one aspect of the invention may be applied to any further aspect. Accordingly the invention may comprise various alternative configurations and combinations of the features defined above.

