Aircraft propulsion system
An aircraft propulsion system includes first and second engines and first and second electrical machines coupled to the first and second engines respectively. The first electrical machine is arranged to be driven by electrical power in order to at least partially drive the first engine. The second electrical machine is arranged to be driven by the second engine to generate electrical power. An electrical network allows transmission of electrical power between the electrical machines. A controller is arranged to selectively reduce fuel flow to the first engine and provide electrical power from the second electrical machine to the first electrical machine to drive the first electrical machine, based on expected properties of contrails formed in an exhaust plume of the first engine and/or the second engine. The system allows the formation of contrails by the engines to be managed in order to mitigate climate-warming effects of the contrails.
1. An aircraft propulsion system comprising:
at least two engines, each engine having: a propulsive fan and an engine core, the engine core comprising a compressor, a combustor and a turbine driven by a flow of combustion products from the combustor, the propulsive fan configured to generate a mass flow of air that bypasses the engine core to generate thrust;
a first electrical machine coupled to a first engine, the first electrical machine arranged to be driven by electrical power in order to at least partially drive the first engine;
a second electrical machine coupled to a second engine, the second electrical machine arranged to be driven by the second engine to generate electrical power;
an electrical network arranged to allow transmission of electrical power between the first electrical machine and the second electrical machine; and
a controller arranged to, based on expected properties of contrails formed in an exhaust plume of the first engine and/or second engine, selectively reduce fuel flow to the first engine, and provide electrical power from the second electrical machine to the first electrical machine, to drive the first electrical machine.
2. The aircraft propulsion system of claim 1 , wherein the controller is arranged to selectively change fuel flow to the second engine concurrently with reducing the fuel flow to the first engine.
3. The aircraft propulsion system of claim 2 , wherein the amount of electrical power provided to the first electrical machine and the change in fuel flow to the second engine is arranged to maintain the total thrust of the aircraft as unchanged.
4. The aircraft propulsion system of claim 2 , wherein the controller is arranged to change the fuel flow to the second engine such that the thrust of the second engine remains unchanged, and wherein the electrical power provided to the first electrical machine causes the thrust of the first engine to remain unchanged, and optionally wherein the change in fuel flow to the second engine is either (i) of the opposite sign to the change in fuel flow to the first engine or (ii) is of the opposite sign and of a different magnitude to the change in fuel flow to the first engine.
5. The aircraft propulsion system of claim 2 , wherein the thrust of the first and second engines is changed when the electrical power transfer occurs.
6. The aircraft propulsion system of claim 1 , wherein the first and second engines are provided at different distances from a centreline of the aircraft, such that an exhaust plume of one of the engines is entrained in a wingtip vortex, to a greater degree than an exhaust plume of the other engine.
7. The aircraft propulsion system of claim 1 , comprising three or more engines each having a corresponding electrical machine coupled to the engine, wherein the engines are arranged in groups, each group having one or more engine; and wherein the controller is arranged to, based on expected properties of contrails formed in exhaust plumes of engines in a first group of engines and/or engines in a second group of engines, selectively reduce fuel flow to the engines in the first group of engines, and provide electrical power from the electrical machines coupled to the engines in the second group of engines to the electrical machines coupled to the engines in the first group of engines, to drive the electrical machines coupled to the engines in the first group of engines to at least partially drive the engines in the first group of engines.
8. The aircraft propulsion system of claim 1 , wherein each engine comprises one or more spools, and wherein the electrical machines are coupled to the same spool of each engine.
9. The aircraft propulsion system of claim 8 , wherein the electrical machines are coupled to a first spool of each engine, the first spool arranged to rotate at a slower speed than at least one other spool, optionally wherein the first spool is arranged to rotate at a slower speed than any other spool.
10. The aircraft propulsion system of claim 8 , wherein each engine comprises an electrical machine coupled to a first spool of the engine and an electrical machine coupled to a second spool of the engine.
11. The aircraft propulsion system of claim 1 , wherein:
the second electrical machine is arranged to be driven by electrical power in order to at least partially drive the second engine;
the first electrical machine is arranged to be driven by the first engine to generate electrical power; and
the controller is arranged to, based on expected properties of contrails formed in an exhaust plume of the first engine and/or second engine, either selectively reduce fuel flow to the first engine, and provide electrical power from the second electrical machine to the first electrical machine to drive the first electrical machine to at least partially drive the first engine; or selectively reduce fuel flow to the second engine, and provide electrical power from the first electrical machine to the second electrical machine to drive the second electrical machine to at least partially drive the second engine.
12. The aircraft propulsion system of claim 1 , wherein the controller is arranged to:
determine a cost function associated with each possible change of fuel flow and thrust of the first engine;
select a combination of changes to fuel flow and thrust for the first engine; and
control the propulsion system based on the selected combination of changes to fuel flow and thrust for the first engine.
13. The aircraft propulsion system of claim 12 , wherein the controller is arranged to:
for each possible change of fuel flow and thrust of the first engine, determine a corresponding change in fuel flow and thrust to the second engine, the corresponding change in fuel flow and thrust to the second engine incorporated in the cost function; and
control the propulsion system based on the selected combination of changes to fuel flow and thrust for the first engine and the corresponding change in fuel flow and thrust to the second engine.
14. The aircraft propulsion system of claim 12 , wherein the cost function includes one or more of:
a cost associated with changes in contrail properties;
a cost associated with extra fuel use; and
a cost associated with changes to carbon dioxide emissions.
15. The aircraft propulsion system of claim 12 , wherein the change in thrust for the first engine is set to zero, based on one or more of the aircraft configuration, ambient conditions and operating conditions.
16. The aircraft propulsion system of claim 1 , wherein the controller is arranged to promote or reduce contrail formation in dependence on ambient and operational conditions.