GB2524773A - Engine vapour trail mitigation system - Google Patents

Abstract

This invention concerns vapour trail mitigation system for a combustion engine 18 having a photon emitter 50 configured to impart energy into an exhaust flow from the engine so as to reduce or suppress condensation or ice nucleation sites in the exhaust flow prior to mixing with ambient air. The photon emitter may be configured to modify sulphur containing molecules in the exhaust flow prior to mixing with ambient air. The photon emitter may be configured to heat or eliminate at least some soot particles in the exhaust flow. The photon emitter may be arranged to emit photons of visible or ultra violet light. The photon emitter may be mounted to an aft portion of the engine to irradiate the exhaust flow as it passes through the engine exhaust. The photon emitter may be arranged to focus emitted photons into a jet portion of the exhaust flow. There may be a sensor 20 for detecting a condition indicative of contrail formation, and a controller for selective operation of the photon emitter in dependence upon a signal output from the sensor.

Description

TITLE OF THE INVENTION

Engine Vapour Trail Mitigation System

BACKGROUND OF THE INVENTION

The present invention relates to a vapour trail mitigation system for combustion engines, typically, although not exclusively, for aircraft engines.

Vapour trails are artificial clouds that are visible trails of condensed water vapour and/or ice formed as a result of the exhaust of vehicles' engines. They may be formed as warm, moist exhaust gas mixes with ambient air, and arise from the formation of microscopic water droplets or, if the air is cold enough, tiny ice crystals. The term "vapour trails" is intended to refer both to condensation trails (that is to say "contrails") from aircraft and to water and/or ice precipitation in or attributable to the exhaust plumes from engines of other machines and vehicles, such as ships.

It may be undesirable for some ships to produce vapour trails in certain situations.

For example, a military ship producing a vapour trail from its exhaust funnels is highly visible from the air and hence much easier to target.

It is understood that, depending on the choice of metric and the timescale considered, the climate-warming impact of aircraft exhaust vapour trails and resulting vapour trail-cirrus may be of a magnitude similar to, or perhaps even greater than, that of the CO2 emitted by aircraft, and therefore may represent a significant element of aviation's total climate impact. It is also understood that an aircraft vapour trail, once formed, will persist in ambient air which is supersaturated with respect to ice, leading to greater climate-warming impact as a result of the increase in longevity of the vapour trail.

IJS2O1O/0122519 describes the use of ultra-low sulphur aviation fuel as an alternative to conventional fuel to reduce sulphur by-product generation and hence reduce contrail formation. This document emphasises the need to retain the purity of the ultra-low sulphur aviation fuel, and hence the requirement to manage the supply chain which delivers the fuel, and to avoid mixing with other fuels.

The attempted suppression of vapour trail formation through the reduction of exhaust water vapour content through use of a heat exchanger and condenser arrangement is described in US2008/072577A. However such an arrangement incurs a significant weight penalty throughout the full duration of a flight, even though vapour trail suppression may only be required for a small percentage of the flight time.

Attempted suppression of vapour trail formation through the use of directed electromagnetic energy is disclosed in US2O1O/132330A. The electromagnetic energy is directed into the engine exhaust plume aft of the engine so as to warm the water vapour and air whilst mixing to avoid condensation, or to re-evaporate formed ice particles after mixing. The energy required to operate such system to prevent contrail formation could represent a significant portion of the engine power and thus incur an unfavourable fuel-burn penalty.

The attempted modification or suppression of vapour trails through the use of chemicals injected either into the engine (either with, or separately from, the fuel) or into the exhaust plume presents the prospect of additional pollution, incurs a weight penalty through the need to carry fuel additives with potentially little or no calorific value of their own, and may present challenges to engine reliability and/or component life.

The strategy of avoiding regions prone to vapour trail formation and/or persistence through the routing of aircraft around, above and/or below such regions has the disadvantage that it increases workload for air traffic control and/or pilots, reduces airspace capacity and, in the case of routing around regions prone to vapour trail formation or persistence, the length of the route followed by the aircraft is increased, resulting in a fuel-burn penalty. Additionally in the case of climbing so as to fly above regions prone to vapour trail formation or persistence, additional fuel is burned to provide the increased thrust necessary to perform the climb. If aircraft are scheduled to fly below regions prone to vapour trail formation or persistence, additional fuel may be burned subsequently if the aircraft is to return to its optimal cruising altitude once the aircraft has passed the avoided region.

It has been determined that the climate warming impact of a vapour trail of a given horizontal extent is determined, at least in part, by its optical depth. It has been proposed that a reduction in the number of soot particles emitted per unit mass of fuel burned by an aircraft's engine could reduce the initial optical depth of exhaust vapour trails. Reduction or elimination of aromatic and/or other non-paraffinic content in the aircraft fuel has been proposed as a way of reducing the soot produced by an engine. Biofuels are typically low in aromatics and/or other non-paraffinics but are typically much more expensive than conventional fuels and are in extremely short supply.

In view of the foregoing, it can be appreciated that there have been proposed a number of possible methods for reducing vapour trail formation or impact but none of those methods provide an ideal solution. The merits of any one solution must be juxtaposed against detrimental impact on engine operation cost, weight or efficiency in order to strike an improved balance for aircraft operators.

It is an aim of the present invention to provide an alternative system and/or method for contrail mitigation.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided an engine vapour trail mitigation system comprising a photon emitter configured to impart energy into the exhaust flow from the engine so as to modify sulphur-containing molecules in the exhaust flow prior to mixing with ambient air.

The photon emitter is typically arranged to impart energy into at least some of the constituent material of the exhaust flow, for example to transfer energy to the molecular and/or particulate material of the flow.

The photon emitter may be configured to cause photolysis of sulphur-containing molecules in the exhaust flow.

The engine may be a propulsion engine, such as an axial flow engine. The engine may be a gas turbine engine. The engine may be an aircraft engine.

The invention beneficially allows concentration of energy imparted into certain constituents within the exhaust flow such that the makeup of the exhaust flow is modified prior to mixing with ambient air. Suitable high power light sources may be used. In particular, it has been found that the reduction of sulphuric acid by photolysis has a beneficial impact on the threshold relative humidity level required for vapour trails to form when ambient air mixes with engine exhaust gas. This can thus suppress initiation of contrail formation.

Furthermore, even in the event that condensation does take place during mixing downstream of the engine, the invention may reduce the availability of condensation nuclei, thereby reducing the condensation rate and/or a characteristic of the vapour trail that is formed. The invention may serve to reduce the interaction between sulphuric acid and soot particles or else modify the soot particles in the exhaust flow, thereby lessening their effectiveness as condensation nuclei.

The invention may reduce the coating of soot particles by sulphuric acid in the exhaust flow.

Additionally or alternatively, the photon emitter may be configured to heat and/or eliminate at least some soot particles in the exhaust flow.

The invention may allow modification of the optical depth of a contrail that is formed, for example so as to reduce its negative/undesirable impact on climate.

The photon emitter may emit one or more beam. The photon emitter may comprise one or more light emitting diode and/or laser. The photon emitter may comprise a high power diode laser. A plurality of photon emitters per engine are typically used.

The photon emitter may be mounted relative to the engine exhaust. The photon emitter may comprise a photon source and a photon guide. The photons may be directed at the exhaust flow by the guide, for example exiting the guide in the direction of, or towards, the exhaust flow.

The photon emitter may be mounted to the engine, for example in the vicinity of an exhaust nozzle or duct.

A plurality of photon emitters may be mounted about, e.g. about a periphery of, an engine exhaust duct. The emitters may be arranged in a generally annular array so as to surround at least a portion of the exhaust flow. The emitters may be mounted about a core engine exhaust or bypass duct.

The emitters may or may not be directed inwardly towards a central axis of the engine or exhaust flow. The emitters may direct photons into the flow within engine exhaust and/or into the flow immediately downstream of the engine exhaust. The emitters may be arranged to direct photons or a beam of photons substantially radially inwardly or perpendicularly to the engine axis or a global direction of exhaust flow.

The exhaust flow may comprise a jet region and turbulent mixing region downstream of the jet region. The photon emitter may direct photons at the jet region of the exhaust flow. The jet region may be identified by higher velocity and/or higher concentration of exhaust gas products than the mixing region.

The invention may thus focus on the use of light sources onto a concentrated region of the exhaust flow in the vicinity of, or immediately downstream of, the engine exhaust such that the exhaust flow can be modified prior to mixing with ambient air. The focus of the invention is not therefore to influence the condensation process as it is occurring but rather to influence the factors that have been determined to affect condensation downstream. This tight focus allows energy to be imparted over a smaller upstream cross-sectional area or volume of flow than the downstream mixing region of the flow, thereby reducing the energy requirements to influence contrail formation.

The system may comprise one or more reflector, for example such that photons or a beam is reflected so as to cause a plurality of passes through the exhaust flow.

The reflector may comprise an internal wall of an engine exhaust or duct.

The system may comprise one or more receiver. The receiver may be arranged to detect light reflected from a vapour trail. The receiver may serve as a vapour trail sensor, for example for measuring the presence/absence and/or optical depth of a vapour trail downstream of the engine exhaust. The receiver may be directional. A plurality of receivers may be provided.

The system may comprise one or more sensor for detecting a condition indicative of contrail formation.

The system may comprise any or any combination of an ambient condition sensor, an engine operation sensor and/or a contrail detection sensor. The ambient condition sensor may comprise any or any combination of a pressure, temperature, humidity and/or ambient light sensor.

The system may comprise a controller, e.g. a control unit, arranged to control selective operation of the photon emitter. The controller may control the time/duration of operation of the photon emitter and/or the power supplied to the photon emitter and/or other characteristics such as the frequency spectrum of the illumination attributable to photons emitted by the photon emitter. The controller may be arranged to receive a sensor signal from one or more sensor and to control operation of the photon emitter in response to said received signal. One or more threshold sensor value may be determined or pre-determined for operation of the photon emitter.

The controller may comprise machine readable instructions, such as one or more module of machine code or algorithm, for control of the photon emitter in response to one or more variable input signal.

The controller may comprise a search algorithm, e.g. for implementing the search of trial control values and determining a resulting control value for implementation.

The controller may instigate a search or instruct a change to the photon emitter operation upon detection of a change, e.g. a material change, in an engine operating condition, ambient condition and/or contrail detection condition. The controller may instigate the search or change to emitter operation on a condition that the change in condition meets or exceeds a predetermined duration and/or a predetermined magnitude, for example so as to represent a material change in condition. The controller may instigate the search or change on condition that a sensed vapour trail characteristic meets or exceeds a predetermined threshold value, which may be a zero value.

The search performed by the controller may comprise a sweep through a range of photon emitter settings, such as power settings. The search may or may not comprise a continuous sweep through the range.

The controller may monitor, or be responsive to, one or more engine operation sensor. The controller may identify an engine operation regime based upon the engine operation sensor readings and/or ambient sensor readings. The engine operation parameter may or may not comprise soot emission index. Optionally, a representation (e.g. database, lookup-table, decision-tree, algorithm etc) may be used/accessed by the controller, from which can be inferred the regime of engine operation (defined below).

The controller may or may not comprise a usage policy, for example defining the circumstances under which the system should or should not be employed. The controller may determine whether or not to implement or cease emitter operation based upon an engine operation regime. Additionally or alternatively, the controller may decide whether or not to implement or cease emitter operation based upon a flight phase or altitude reading.

According to a second aspect of the invention, there is provided a data carrier comprising machine readable instructions for operation of an engine controller in accordance with the first aspect.

According to a third aspect of the invention, there is provided a method of engine vapour trail mitigation corresponding to the system of the first aspect. According to a fourth aspect of the invention, there is provided a controller arranged to perform the method of the third aspect.

According to a further aspect of the invention, there is provided a combustion engine vapour trail mitigation system comprising a photon emitter configured to impart energy into the exhaust flow from the engine so as to reduce or suppress condensation and/or ice nucleation sites in the exhaust flow prior to mixing with ambient air.

A reduction in nucleation sites may comprise a reduction in nucleation site numbers. A suppression of nucleation sites may comprise a suppression of the sites' function in fostering water and/or ice nucleation.

The term "mitigation" is used here to encompass the suppression of vapour trail formation, and/or the advantageous alteration of one or more vapour trail characteristic such as optical depth (aD).

Any of the preferable features defined above in relation to the first aspect may be applied to any further aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Practicable embodiments of the invention are described in further detail below by way of example only with reference to the accompanying drawings, of which: Figure 1 shows a vapour trail mitigation system according to the present

disclosure on an aircraft;

Figure 2 is a schematic rear view of an engine according to an example of the invention; Figure 3 is a schematic side view showing the general configuration of an engine according to another example of the invention; Figure 4 is a schematic rear view of an engine according to a further example of the invention with a different light source configuration; Figure 5 is a schematic side view of the arrangement of Figure 4; Figure 6 is a plan of a control system according to an example of the invention; Figure 7 is a schematic of a control unit for use in an example of the invention; Figure 6 is a flow chart of one control method for contrail mitigation; and, Figure 9 shows a flow chart of another control method for contrail mitigation.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows a machine 10, in this example an aircraft, which comprises a controller 12 and associated control system according to the present disclosure. In the example shown the aircraft comprises a fuselage 14 from which wings 16 extend, with engines 18 mounted to the wings. Other examples might involve alternative aircraft configurations, and different numbers of engines. Although the controller 12 is shown schematically as being centrally located in the airframe/fuselage 14, i.e. as a common controller, it may otherwise be located on any one engine and in signal communication with the other engine(s). In other examples of the invention a controller 12 could be provided for each engine which may or may not require communication between the engines for operation. Any such controller 12 may comprise a general engine controller comprising one or more bespoke algorithm to allow control in accordance with the invention, e.g. in addition to the conventional functions of such an engine controller.

The control system preferably comprises at least one vapour trail detection sensor 20. In the example shown in Figure 1: vapour trail detection sensors 20 are mounted towards the rear of the aircraft 10 facing aft. For example they are located at the tip of one or both wings 16 and/or at a trailing edge 22 of the fuselage. The, or each, vapour trail detection sensor 20 is mounted such that it has a field of view 24 directed towards a vapour trail formation region. That is to say they are positioned such that they have a field of view 24 in a direction downstream of the vehicle 10, which in operation will offer a view of vapour trails 32 formed within the exhaust plumes 26 downstream of the engines 18. Each of the vapour trail detection sensors 20 is configured to generate a first signal 28 (shown as a dotted line) which indicates, for example, an optical depth (OD) of the young vapour trail 32.

Each vapour trail detection sensor 20 is an optical device configured to deliver a signal indicative of the presence and/or one or more property of a vapour trail 32.

In this example, a characteristic property, e.g. optical depth, is used to determine both the presence/absence of a contrail and also its severity. A dedicated source of illumination 30 may be provided on the aircraft and directed towards at least one region downstream of the engines 18 to illuminate at least part of the field of view 24 of the sensor 20. In other examples, the illumination sources 30 may be avoided in favour of a source of illumination used for contrail mitigation as will be described below. The sensor 20 is configured to detect electromagnetic radiation of at least one wavelength emitted and/or reflected by the vapour trail in response to energy emitted from the source of illumination 30. In other embodiments, instead of illumination, an emitter/receiver of sound (or ultrasonic) waves could be provided. The sensor would then be configured to detect the sound returned from the ice particles in the young contrail.

In alternative embodiments, sensors for the direct detection of vapour trails may be replaced, or supplemented, with ambient condition and/or engine operation sensors. Suitable ambient condition sensors could comprise any or any combination of pressure, temperature and/or humidity sensors. Any such sensor(s) may be used to infer the presence/absence of a contrail and/or one or more contrail property as will be described below. A conventional engine operation sensor indicative of the engine operating point (e.g. thrust, throttle, engine speed, exhaust temperature, or one or more pressure sensor within the engine) may also be used in conjunction with one or more aforementioned sensor to provide further information for the purpose of determining whether or not to attempt contrail mitigation in accordance with the invention.

In further examples, the function of the vapour trail detection sensor could be performed by equipment remote to the aircraft, and the resulting information transmitted to the aircraft. Such equipment might include for example sensors mounted on the ground, on airships or balloons, on other aircraft, and/or on earth-orbiting satellites. In such situations, the operation of the non-aircraft mounted vapour-trail detection sensors may optionally be enhanced by use of an aircraft-mounted source of illumination.

Each engine is provided with one or more source of illumination to impart energy into certain constituents within the exhaust flow either within or directly behind the engine exhaust nozzle.

Turning to Figure 2, there is shown one example of a light source mounted so as to emit light into the exhaust flow as it exits the engine 18. The engine 18 is mounted in a conventional configuration beneath wing 16 by a pylon 33, although the invention is not specific thereto and may be applied to other engine mounting configurations as required.

The light source comprises a plurality of emitters 34 arranged in a circumferential array about the core engine exhaust 36, for example mounted to the wall of the core engine exhaust duct or nozzle. The emitters 34 are arranged to emit light generally radially inwardly as shown by arrows 37 towards the centre of the exhaust so as to concentrate or aim the energy within the exhaust or immediately downstream thereof. Thus, as the exhaust gases pass through the exhaust they are irradiated with photons such that energy is transferred to constituents within the exhaust flow.

Figure 3 shows a further embodiment, in which one or more illumination sources 38 are located elsewhere in the engine 18 and the light is directed to the desired location for emission into the exhaust flow by one or more light guide 40. For example, fibre optic cables or any other conventional light-directing medium may be used to convey light from the light source to the desired emission location. In this manner the light source may be located in a portion of the engine which offers a lower temperature environment or else a more convenient location within the engine architecture in comparison to that of the desired light emission site.

In the example of Figure 3, the light source 38 is located in the engine nacelle 42 and/or within the casing of the fan 44. Although a circumferential array of light sources are again shown in Figure 3, such an arrangement is not essential and other arrangements of light sources may be used. A plurality of light guide exits are arranged about the core engine exhaust 36 and typically define a circumferential emission zone or area about the exhaust duct 36. In this example, the light guide 40 may pass through the engine structure such that the light emission site is radially inside the engine bypass duct 46 relative to a central axis of the engine. The light guide 40 may generally follow a path, e.g. aft, along the core engine casing 48. The dashed line of the light guide 40 in Figure 3 shows the general path of the light between the source 38 and emission zone.

In Figures 4 and 5, there is shown an alternative illumination arrangement, in which the light sources 50 are arranged at the downstream end of the bypass duct 46. The light sources 50 may thus be mounted in the nacelle structure 42 and/or at the bypass duct exhaust, for example to direct light radially inwardly towards the core engine exhaust. In such an arrangement, as well as any of the embodiments described above, the direction of light emission may be obliquely angled (e.g. relative to the engine axis) such that light is emitted in a direction both aft and radially inwardly towards the core exhaust flow. As with the example of Figure 3, the light source may be mounted elsewhere in the engine and light guides may be used to emit the light into the engine exhaust flow at the desired location.

The gas turbine engine 18 operates in a conventional manner, such that air enters the engine at an upstream inlet and passes through the fan 44. A portion of the air from the fan passes into the core engine, where it is compressed by one or more core compressor 52 prior to entering a combustor 54, where the air is mixed with fuel and combusted within a combustion chamber. The hot combustion gases drive turbines 56 in a conventional manner and then exit the engine via the core engine exhaust 36 so as to provide a proportion of the propulsive thrust of the engine. The remainder of the thrust is provided by the air driven along the bypass duct by the fan 44.

In examples of the invention, the illumination sources 34, 38 or SO impart energy into certain constituents within the core engine exhaust flow as it exits the engine 18. By controlling the power output of the illumination sources and the location/direction of light emission into the exhaust flow and/or other characteristics such as the frequency spectrum of emitted light, the characteristics such as optical depth of contrails can be advantageously modified, in a manner which has not hitherto been disclosed in the prior art. In certain scenarios, it is possible that the emitted light may be sufficient to entirely prevent contrail formation, although it will be appreciated that the invention can offer a varying range of contrail modification for a range of engine operation and/or ambient conditions.

In other examples, it is possible that the light sources may be mounted on the airframe and directed towards the exhaust flow, rather than being engine mounted.

The invention derives from the general realisation that electromagnetic radiation can be imparted into certain constituents within the engine exhaust flow, not necessarily to control the temperature of the exhaust flow whilst mixing with ambient air] but rather to reduce the availability or effectiveness of condensation/ice nuclei within the exhaust flow. That is to say, an aircraft engine exhaust flow mixes with ambient air upon exiting the engine so as to follow a path from an initial temperature and water-vapour partial-pressure within the engine exhaust to a final temperature and water-vapour partial-pressure pressure matching ambient conditions. If the relative humidity over water and/or ice exceeds a certain value at any point during the mixing process, then contrail formation becomes possible. Contrails can in principle form homogeneously (not requiring condensation nuclei) but heterogeneous contrail formation (requiring condensation nuclei) is the most prevalent under normal circumstances. The more effective a condensation nucleus, then the lower the level of saturation over water and/or ice that must be reached during the mixing process in order for a contrail to form.

Within the heterogeneous formation route, the formation of an ice crystal can be achieved by direct deposition of ice onto the surface of the nucleus, or alternatively by the condensation of water followed by freezing. Under normal circumstances, the latter is the dominant route.

The present invention thus focuses on the imparting of energy into certain constituents within the exhaust flow so as to alter: * the effectiveness of condensation nuclei, thus requiring a higher relative humidity to be reached for condensation to occur during the mixing process with air; and/or * the number of condensation nuclei, thus advantageously altering the optical depth of a formed contrail In explaining the manner in which the invention can affect contrail nucleation, it is noted that sulphur content typically present in fuel is oxidised in the engine and in the exhaust plume to sulphur dioxide (SO2), sulphur trioxide (SO3) and sulphuric acid (H2S04). Sulphur may also originate in the air passing through the engine core (as SO2 or sulphate aerosol). Soot particles in the combustion gases are naturally hydrophobic, but become hydrophilic when they attach SO3 molecules which in turn become H2S04 molecules in the presence of water vapour. Thus sulphuric acid in a jet engine's exhaust increases the effectiveness of exhaust soot particles as water condensation nuclei, by coating them and allowing water to be absorbed and subsequently to freeze to form ice crystals containing included soot particles.

Instead of prior art techniques that attempt to reduce or eliminate sulphur content from the fuel, the present invention aims to affect potential nucleation sites by irradiating the exhaust flow so as to reduce the availability of sulphuric acid or its precursors, sulphur dioxide/trioxide, before or within the region where mixing with ambient air occurs. In order to achieve the invention, it is desirable that a reduction in the availability of nucleation sites is achieved upstream of mixing with ambient air. Thus electromagnetic radiation in the form of light is directed into the exhaust flow either within the exhaust or within the jet portion of the exhaust flow, i.e. where the majority of the flow comprises combustion gases rather than a mixture of the combustion gases and ambient air. This allows the energy to be focussed on a relatively narrow geometric region of the flow in an efficient manner rather than attempting to irradiate the flow over a larger geometric region in which mixing with ambient air occurs.

Imparting sufficient energy into the certain constituents within the exhaust flow prior to mixing with ambient air has the potential to bring about two potential contrail mitigating effects. The first concerns photolysis of the sulphuric acid present in the exhaust duct/plume, and/or its precursors such as SO2 and/or 503, into other species. Photolysis is achieved by illuminating the engine exhaust gases with light of an appropriate intensity and wavelength(s). These photolysis products are then expected to recombine over time to yield H2S04 and hence sulphate aerosol. However, by use of the invention, the formation of H2S04 is delayed until after the exhaust plume has mixed and diluted to the point where contrail formation is no longer possible. The role of H2S04 in enhancing contrail formation is therefore reduced or eliminated.

In a second potential contrail mitigating mechanism of the invention, a suitable light source is used to heat the surface of soot particles passing through the engine exhaust duct (and/or near4ield exhaust plume) to a very high temperature so that one or more of the following takes place: o H2S04 attached to the particle surface will decompose, the sulphur (VI) being reduced by the carbon to sulphur (IV) according to the reaction: H2S04 + C(s) -* SO2 + H20 + CO (the soot particle being represented as C(s), although it is not entirely solid carbon).

o soot burns reducing the total number of available condensation nuclei, o soot particles anneal to form more compact particles with less surface area available to attach water molecules Thus it can be seen that at least some potential water condensation nuclei in the flow may be eliminated or else modified to reduce susceptibility to condensation.

Accordingly the invention can achieve contrail mitigation by either or both of: 1. Reduction in the number density of condensation nuclei in the exhaust, leading to a reduction in the optical depth of any contrail formed as a result of water vapour condensing on particles.

2. An increase in the critical relative humidity that must be reached in order for condensation onto soot particles to occur by reducing the effectiveness of soot particles as water condensation nuclei.

In order to achieve either or both of the above contrail suppressing benefits, it is proposed that an intense light source. such as a laser, may be used. However other photon emitting or electromagnetic radiation emitting sources may be used provided they can impart the required energy to certain constituents within the flow.

Independently of the issue of contrails, sulphate aerosol arising from the combustion within the engine of sulphur contained within the fuel is known to exert a cooling effect on the climate, by reflecting incoming sunlight back into space, thus reducing the amount of solar energy incident upon the earth. The atmospheric residence time of the sulphate aerosol emitted by jet engines may be in the order of up to a week if the emissions take place within the troposphere, and can be significantly longer if emissions occur in the stratosphere. Thus, in contrast to a total reduction of fuel sulphur content, the present invention enables the potential benefit for climate cooling of sulphate aerosol arising from fuel sulphur content, since the aerosol will form eventually via solar induced photochemical oxidation of SO2, after the contrail mitigation benefits have been realised.

Furthermore it has been found that sulphuric acid also plays a role in reducing the ice (as opposed to water) nucleation efficiency of soot particles. A total reduction in fuel sulphur content may therefore increase the likelihood that an alternative contrail formation mechanism] namely the direct deposition of ice onto soot particle surfaces, will become active in place of the current water-based mechanism.

In order to ensure efficient use of the invention, there is also proposed a control scheme for selective operation of the contrail mitigation system as will be described below. It is desirable that the light source for contrail mitigation can be switched on and off according to a requirement for contrail mitigation, such that contrail mitigation can be targeted to those contrails with the greatest warming impact. Any engine efficiency/fuel-burn penalty can therefore be controlled and/or minimised. The control system thus allows real-time control of contrail mitigation.

The aim of the control system is to identify scenarios in which contrails are of greatest climate warming impact and to selectively suppress contrail formation. In any example of the invention, if complete contrail suppression cannot be achieved, the invention may still be employed to reduce the climate warming effect of a contrail by reducing its optical depth. Furthermore, even if complete contrail suppression is possible, the controller may elect to reduce contrail optical depth without eliminating the contrail altogether on the basis of an operational cost/energy assessment.

Figure 6 shows a basic schematic of a control system in which the control unit 12 receives data signals from any or any combination of the contrail detection sensor(s) 20, the ambient condition sensor(s) 58 and/or engine operation sensor(s) 60. The control unit 12 determines whether or not to operate the light source 34 (or 38 or 50) according to any embodiment of the invention and/or the extent or power level of operation and/or other characteristics such as the frequency spectrum of emitted light. For example a variable light source may be used or else the controller may select only a subset, rather than all, of the light sources 34 to be powered to allow for a plurality or range of operating conditions.

The control unit 12 determines how to operate the light source according to a deployment policy. If the sensors 20 and/or 58 indicate that contrail formation is not possible even in the absence of illumination from the light source, the light source will be switched off. In a simple embodiment, the light source will always be switched on upon detection of contrail or ambient conditions indicative of contrail formation. However it is envisaged in a working example of the invention that controller would offer improved operation of the light source by reference to a deployment policy or control scheme that takes into account one or more property of the contrail.

For example, with reference to Figure 7, the control unit 12 comprises one or more processor 66, which serves as a calculation or determination unit for controlling the light sources 34. The processor 66 accesses one or more data store comprising data used by the controller in determining a usage condition of the light source 34. The data store comprises a deployment policy 62, for example which may allow an engine/aircraft operator to set one or more variable values for contrail mitigation according to environmental considerations and/or cost.

The likely persistence and/or climate warming impact of a contrail are determined by the control unit 12 in response to received sensor signals. For example, 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 will grow over time to resemble natural cirrus cloud, both in size and optical properties, and is then referred to as "contrail-cirrus". It is proposed that a significant majority of the climate warming impact caused by contrails is attributable to contrail-cirrus because the contrail cirrus is spatially larger and longer-lived. The issue of whether or not a contrail will persist is therefore of considerable relevance when deciding how much effort, inconvenience or cost should be incurred in suppressing the formation of the contrail. Thus the controller may only operate the light source 34 in response to a contrail forming condition in which the ambient sensors indicate that ambient air is supersaturated with respect to ice.

Furthermore, during the day, contrails reflect a proportion of incoming sunlight away from the earth leading to a climate cooling effect which is offset against the climate-warming impact associated with the absorption by contrails of heat rising from the planet's surface. During the night, however, the cooling effect is not operative. For this reason, for a given optical depth and spatial extent, a contrail's climate warming impact, per unit time of contrail existence, is typically greater during the night than during the day. Accordingly the control unit may attempt mitigation of contrails determined to subsist at night time. An ambient light sensor may be used as one type of ambient condition sensor for this purpose. Additionally or alternatively, the controller may have access to a clock/calendar data input and/or a day/night cycle table to establish the relevant conditions.

The climate warming impact of a contrail is also influenced by its temperature. All other things (including persistence and time of day) being equal, a contrail forming in colder air will exert more of a climate warming effect than one which forms in slightly warmer air. An ambient temperature sensor may thus be used as a factor in determining whether to attempt contrail mitigation.

Additionally or alternatively, the controller may take into account the influence of ambient temperature upon the ability of ambient particulates and/or aerosol particles to act as condensation nuclei in the absence of effective soot particles. If it is determined by the controller that ambient temperature lies in such a range that a significant reduction in soot particle effectiveness and/or number density would enable an increase in contrail optical depth due to the action of said ambient particulates and/or aerosol particles as condensation nuclei, then the controller could apply a predetermined threshold to the extent of modification that would be applied to soot particle effectiveness and/or number density. The predetermined threshold would take a value dependent upon the ambient temperature and could be obtained from for example a pre-populated lookup-table or database. In such an example the controller could adjust the intensity and/or other characteristics of the light source in order to ensure that the extent of modification of soot particle effectiveness and/or number density lay within the predetermined threshold.

Turning now to Figures 8 and 9, there are shown two examples of processes for controlling operation of the light source. In Figure 8, an initial loop is performed involving checking current sensor readings against previously stored sensor readings and determining whether a material change in operating conditions (e.g. ambient conditions, engine operating point and/or contrail detection readings) has occurred. If not, then the current contrail mitigation settings can be retained. If a change in conditions is determined, then the controller 66 may refer to the deployment policy 62 as well as the relevant sensor data in order to determine whether contrail mitigation action is required. If contrail mitigation action is not required, the controller 66 will ensure that the light source is switched off, i.e. a default condition, before reinitiating the control loop.

Different examples or scenarios of calculation and implementation of a suitable illumination level or power are discussed below. In one simple example, the level of illumination is not variably adjustable, and so the light source is either on" or off'. Thus when the sensor readings and/or deployment policy indicate that contrail mitigation is required, illumination is activated, and when such is not required, it is switched off. The light source would output a predetermined wavelength, or wavelength range, and power/intensity level, typically deemed suitable as an average setting to best accommodate a spectrum of potential contrail forming conditions.

In determining whether or not action is warranted by the controller, a calculation of the engine efficiency penalty in using the light source may be performed. This may be compared to a climate warming penalty associated with the contrail, e.g. according to any or any combination of optical depth, temperature, persistence and/or time of day, and a decision taken as to whether the energy and hence fuel used in contrail mitigation would lead to a net climate detriment, once the advantages of contrail mitigation have been taken into account. In any example, the use of the light source may be modelled as an operational cost, such as a fuel cost, engine efficiency cost and/or financial cost according to corresponding parameters used in the deployment policy. Such an operation cost may be compared to a climate impact or cost of not using the light source.

In another example, the level of illumination is variable and can be tailored to the current operating conditions, by calculations performed by the controller 12. The controller may thus determine a least-cost" solution for the current conditions by comparing the engine operation cost (e.g. a sliding scale of operational cost over the range of available light levels). A solution offering the greatest reduction on contrail optical depth per unit of operational cost, e.g. per additional unit of fuel burned to achieve the contrail mitigation effect, may be sought. A solution would be characterised by factors such as the intensity and wavelength of illumination employed.

In another example, the level of illumination is variable and can be tailored to the current operating conditions, but is not instantly calculable by the controller 12 due to unknown or uncertain values of required operating variables. For example, there may be uncertainty concerning the precise levels of sulphuric acid and/or its precursors in the engine exhaust, and their relative proportions or the resulting photolysis, ice deposition and/or water-condensation response to illumination. In such a scenario, or others, it may be preferable to implement a trial-and-error approach to illumination level settings, using the contrail-detection sensor as a feedback mechanism. Thus light levels may be adjusted over a trial range of operation, whilst sensing the optical depth of the young contrail formed behind the aircraft. Such an approach is shown in Figure 9, in which, after a determination to attempt contrail mitigation, illumination settings may be increased or decreased until the minimum level for adequate contrail mitigation is found. Another method of identifying suitable illumination settings is to use an optimisation algorithm to explore an available illumination settings search space according to a computational model, rather than physical implementation of the trial values.

Alternative modes of operation involve reference to a lookup-table or database by the controller in order to identify the most appropriate illumination settings for the current operating conditions. In this way a relationship between operating conditions and the contrail mitigation effect of a particular illumination setting would be based on empirical understanding, according to previous instances of use or test results, rather than detailed calculations.

When altering the characteristics of the illumination according to any of the above examples, any or any combination of the following may be implemented: i. Altering the level of electrical power supplied to the sources of illumination so as to alter its intensity, at substantially the same or similar frequency spectrum.

ii. Varying the frequency spectrum of the illumination so as to excite different pads of the absorption spectrum of individual species.

Hi. Varying the frequency spectrum of the illumination so as to emphasise photolysis of different species, e.g. preferentially photolysing SO2 or H2S04.

The above examples concern the use of a light source to irradiate the exhaust flow. However it will be understood that the precise wavelengths of electromagnetic radiation used will be determined based upon the absorption spectrum of the gases and particulate materials in the exhaust flow. The wavelengths used may thus accommodate ultraviolet or infrared rays as well as the visible light spectrum. The lowest wavelength used may be in the order of 10-nm but it considered likely to be in the range 100-300 nm.

502 has a relatively complicated temperature-dependent, but fundamentally broad, absorption spectrum around 300nm, and thus conventional industrial light sources can be used. SO3 is also amenable to photolysis using wavelengths below 300nm, whereas H2S04 is most easily photolysed using visible red light around 740nm. Whilst photolysis of H2S04, SO3 and SO2 is possible by sunlight, the intensity of sources of illumination contemplated by the present invention greatly exceeds that of sunlight incident on the atmosphere due to the available time in which photolysis is to occur. Soot has a very broad absorption spectrum and can absorb light over a wide range of wavelengths.

H2504 reduction on soot surfaces is thermodynamically allowed, but does not occur with 100% yield at aircraft exhaust temperature (-700 K). Increased particle temperature will favour the process, for example wherein AG (i.e. a change in Gibbs Free Energy) at 700 K = -149 KJ/mol and AG at 1700 K = -485 KJ/mol.

Particles with a surface temperature of 1700 K would be expected to ignite in an air flow. Other sources of electromagnetic radiation such as arc lamps, discharge lamps, fluorescent lamps etc could in principle be contemplated, but would typically offer lower life, higher weight, higher power consumption and possibly higher space requirements.

The illumination requirements for this invention may be best met by light-emitting diodes (LEDs) or high power diode lasers. Although current LEDs typically offer lower power, current focus on LED technology is considered likely to result in much higher power, and with a frequency spectrum that could be tailored to this application.

Kilowatt light sources may be used, for example in the order of 1-20 kW or more.

It is envisaged that a source of illumination used to assist contrail optical depth detection, such as light sources 30 in Figure 1, will differ from the sources of illumination used to photolyse the exhaust flow and/or to heat soot particles.

However, it is possible that there may be some overlap of the wavelengths used and so it is possible that the light sources 34 could be arranged such that at least a portion of the light is emitted towards the contrail 32 such that the contrail sensor 30 could determine contrail optical depth thereby.

In any of the embodiments discussed above, it may be preferable to limit operation of the invention to a particular flight phase only, e.g. cruise, and/or to set one or more altitude threshold, below which the light source will be inactive.

Whilst the above discussed embodiments will incur a fuel efficiency reduction whilst the light source is active, it is envisaged that the light source will require activation for less than, on average, 15% or 10% or possibly even 5% (dependent on of the deployment policy) of flight duration and, as such, the overall impact upon fuel-burn will be small.

Claims (18)

1. CLAIMS: 1. A combustion engine vapour trail mitigation system comprising a photon emitter configured to impart energy into the exhaust flow from the engine so as to reduce and/or suppress condensation and/or ice nucleation sites in the exhaust flow prior to mixing with ambient air.
2. 2. A vapour trail mitigation system according to claim 1, wherein the photon emitter is configured to modify sulphur-containing molecules in the exhaust flow prior to mixing with ambient air.
3. 3. A vapour trail mitigation system according to claim 1 or 2, wherein the energy imparted by the photon emitter is sufficient to reduce or retard formation of sulphuric acid and/or the interaction between sulphuric acid and soot in the exhaust flow.
4. 4. A vapour trail mitigation system according to any preceding claim wherein the photon emitter causes photolysis of any or any combination of sulphur dioxide] sulphur trioxide and/or sulphuric acid.
5. 5. A vapour trail mitigation system according to any preceding claim wherein the photon emitter is configured to heat and/or eliminate at least some soot particles in the exhaust flow.
6. 6. A vapour trail mitigation system according to any preceding claim wherein the photon emitter is arranged to emit photons of visible and/or ultra-violet light.
7. 7. A vapour trail mitigation system according to claim 6, wherein a significant portion of the emitted photons are of wavelength 100-400 nm.
8. 8. A vapour trail mitigation system according to any preceding claim, wherein the photon emitter is mounted to an aft portion of the engine to irradiate the exhaust flow as it passes through the engine exhaust.
9. 9. A vapour trail mitigation system according to any preceding claim, comprising a plurality of photon emitters arranged in a circumferential array about an engine duct.
10. 10. A vapour trail mitigation system according to claim 8 or 9, wherein the photon emitter is arranged to emit photons in a direction radially inward towards a centre of the exhaust flow.
11. 11. A vapour trail mitigation system according to any preceding claim, wherein the photon emitter is arranged to focus emitted photons into a jet portion of the exhaust flow.
12. 12. A vapour trail mitigation system according to any preceding claim, wherein the photon emitter comprises a photon source and a photon guide, wherein the photon source is mounted remotely of the exhaust flow and the guide is arranged to direct the light from the source to the exhaust flow.
13. 13. A vapour trail mitigation system according to any preceding claim, comprising one or more sensor for detecting a condition indicative of contrail formation and a controller for selective operation of the photon emitter in dependence upon a signal output from said sensor.
14. 14. A vapour trail mitigation system according to claim 13, comprising one or more ambient condition sensor, the controller being arranged to control variable operation of the photon emitter based upon the ambient condition sensor output.
15. 15. An aircraft engine comprising a vapour trail mitigation system according to any preceding claim.
16. 16. A data carrier comprising machine readable instructions for a controller of a combustion engine vapour trail mitigation system to receive one or more signal indicative of vapour trail formation by mixing of an engine exhaust flow with ambient air and to output a control signal for operation of a photon emitter to impart energy into certain constituents within the engine exhaust flow so as to modify sulphur-containing molecules and/or soot particles in the exhaust flow prior to mixing with ambient air.
17. 17. A method of suppressing vapour trails produced by an exhaust flow of a combustion engine, comprising receiving one or more input indicative of vapour trail formation and operating a photon emitter to impart energy into certain constituents within the exhaust flow from the engine so as to modify sulphur-containing molecules and/or soot particles in the exhaust flow prior to mixing with ambient air.
18. 18. A vapour-trail mitigation system or method substantially as hereinbefore described with reference to the accompanying drawings.