Patent Application
20200386405
The application related generally to aircraft engines, and more particularly to gas path configurations thereof.
Aircraft turbine engines operate at a variety of design points, including takeoff and cruise, and are also designed in a manner to handle off-design conditions. Some aircraft can have large power differences between operating points, such as between takeoff and cruise for instance, which can pose a challenge when attempting to design an engine which is fuel efficient. Indeed, some aircraft engines are over-designed when viewed from the cruise standpoint, to be capable of handling takeoff power, which can result in operating the engine during cruise in a less than optimal regime from the standpoint of efficiency. Accordingly, there remained room for improvement.
In one aspect, there is provided an aircraft engine having a core gas path having a first combustor, a second gas path parallel to the core gas path, the second gas path having a second combustor, a turbine driven by the second gas path, a gearbox driven by the turbine, and a valve configured for selectively opening and closing the second gas path.
In another aspect, there is provided a method of operating an aircraft engine having a core gas path having a first combustor, a second gas path parallel to the core gas path, the second gas path having a second combustor, a turbine driven by both the core gas path and the second gas path, the method comprising: driving the turbine at a takeoff power level including simultaneously operating the first combustor and the second combustor in relation with the core gas path and the second gas path; subsequently to said driving the turbine at a takeoff power level for a given duration, closing the second gas path, shutting down the second combustor, and driving the turbine at a cruise power level solely via the core gas path.
In a further aspect, there is provided a turboprop or turboshaft engine comprising a core gas path having a first combustor, a second gas path parallel to the core gas path, the second gas path having a second combustor, a turbine driven by both the core gas path and the second gas path, and a valve configured for selectively opening and closing the second gas path.
Reference is now made to the accompanying figures in which:
The fluid path extending sequentially across the compressor 12, the combustor 14 and the turbine 16 can be referred to as the core gas path 18. In practice, the combustor 14 can include a plurality of identical, circumferentially interspaced, combustor units. In the embodiment shown in
Turboshaft engines, similarly to turboprop engines, typically have some form of gearing by which the power of the low pressure shaft 22 is transferred to an external shaft 26 bearing the blades or propeller. This gearing, which can be referred to as a gearbox 24 for the sake of simplicity, typically reduces the rotation speed to reach an external rotation speed which is better adapted to rotate the blades or propeller for instance.
Some applications, such as helicopters to name one example, can have large power differences between Take-Off (TO) and cruise. A typical helicopter can require less than 50% power to cruise versus its highest power rating, and this can result in the engine running in off-design condition for the majority of its mission, leaving a want for better fuel efficiency.
At takeoff, for instance, the second gas path 126 can be open, and the second combustor 128 can be activated, in a configuration shown in
During cruise, the flow through the second gas path 126 can be reduced or stopped by the valve 130, while the core gas path 118 can continue to operate at a comparable rate, reducing the power available at the turbine 132 to a second power level, which can correspond to a cruise power requirement for instance.
It will be noted that the selective operation, or closing, of the second gas path 126 can be performed without substantial impact on the operation of the core gas path 118. Accordingly, during a typical flight, the same engine can be operated in two or more operating modes which can produce a significantly different power level while always operating at a relatively high level of efficiency, and without requiring an additional engine altogether. It will also be noted that the two different power levels can be achieved without a significant change of rotation speed of the turbine shaft, for instance.
For instance, at takeoff, the turbine 132 can be driven while simultaneously operating the first combustor 114 and the second combustor 128 in relation with the core gas path 118 and the second gas path 126. Then, after operating the turbine 132 at the takeoff power level for a given duration, the second gas path 126 can be closed and the second combustor 128 can be shut down, while the turbine 132 can continue to be driven solely via the core gas path 118, at a cruise power level.
In the context of a helicopter, for instance, it can be desired for the rotation speed of the turbine's shaft not to vary too much between the different power levels. The rotation speed of the turbine at the takeoff power level can be less than 140% of the rotation speed of the turbine at the cruise power level, for instance, possibly less than 130% (e.g. for turboprop), possibly less than 110% (e.g. for turboshaft), and even possibly less than 105%. This while the amount of power generated at the cruise power level can be less than ¾ of the amount of power generated at the takeoff power level, possibly less than ⅔rd, and even possibly less than ½. In some embodiments, the second combustor will be at least 10% smaller than the first combustor. In some embodiments, the second combustor will be at least 20% smaller than the first combustor.
In an example where the OEI power level is higher than the takeoff power level, an aircraft engine can be designed in a manner for the OEI power level to be reachable by operating the core gas path and the second gas path at full power simultaneously, for instance.
If an engine with a single gas path was designed to reach such an OEI, the engine can rely on overall pressure ratio and temperature to generate the power required for its OEI condition, but then have components running off-design at cruise power, reducing engine efficiency. Moreover, in some cases, it is not possible to design the engine both for cruise condition, and in a manner to meet the power requirements for take-off or OEI, due to performance limitations of the components (temperature margins, compressor operating lines etc).
Designing a specific engine to meet both of these requirements—high power and cruise—with satisfactory efficiency at both conditions, but with only a single gas path, may not be feasible. It could be easier, based on the power requirements, to use two smaller engines at TO power and revert to a single powered engine in cruise. However, such a second engine may add weight, complexity, can reduce the reliability of the overall package, and can introduce subsequent challenges such as cold engine start times and OEI if one engine is turned off in flight (cruise).
A boost compressor can be used to increase the power output of the engine. However, if the additional flow and pressure entering is pushed through the core, it influences the operating characteristics and limits the optimization of core components ultimately effecting the off boost performance in terms of power and specific fuel consumption (SFC).
The design shown in
The use of the second combustor 128 can increase power (for takeoff), without significantly increasing the shaft speed of the common power turbine 132.
The example presented in
In
The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. Indeed, various modifications and adaptations are possible in alternate embodiments.
Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.
