GB2468143A

GB2468143A - Gas generator comprising a positive displacement gas motor with a controlled outlet valve

Info

Publication number: GB2468143A
Application number: GB0903289A
Authority: GB
Inventors: Matthew Moxon
Original assignee: Cranfield University
Current assignee: Cranfield University
Priority date: 2009-02-26
Filing date: 2009-02-26
Publication date: 2010-09-01
Also published as: GB0903289D0

Abstract

A gas generator 15, eg for driving an aero or static gas turbine 55, comprises a positive displacement gas motor 30 driving a compressor 20, eg of the axial or centrifugal flow type, which supplies the gas motor 30 with pressurised air. Energy is then added, eg by the combustion of fuel or addition of nuclear heat via a heat exchanger, and the heated air 50 is fed to the turbine 55 via a valve 145, eg a sleeve valve, which is controlled such that the speed of the motor is independent of variation in the supply of energy to the air, allowing the motor and compressor to operate at their most efficient speeds while enabling a variation in gas generator output by varying the supply of energy. The gas motor 30 may be a two-stroke compression ignition piston engine. The turbine 55 may drive contra-rotating open rotors 65 downstream of the compressor 20. Alternatively, a ducted fan may be located at the front of the engine feeding the compressor with air. An exhaust heat recovery unit (220, fig.3) may be provided.

Description

TITLE:. GAS GENERATOR

DESCRIPTION * I

TECHNICAL FIELD

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* 1 The present invention relates generally to gas generators and more particularly, but not exclusively, to * gas generators for use in aero engines.

*:*::* BACKGROUND ART

FLIGHT magazine, February 22, 1945, page 210 suggests the combination of a two-stroke piston engine with a gas turbine. The former is used as a gas generator, its exhaust gases being used to drive a gas turbine.

The piston engine is an example of a positive displacement motor, i.e. a motor in which a volume of fluid (in this case an air/fuel mixture) is trapped and the pressure of the fluid then used to generate an output torque. The gas turbine, in contrast, is an example of a dynamic (also known as "kinetic") motor in which there is no trapping of fluid, the output torque resulting instead from the motion of the fluid.

By their nature, positive displacement devices are capable of much greater pressure ratios than dynamic devices. As noted in the FLIGHT article, the positive displacement piston engine enables gases to be burned at high temperature and pressure before being led to the turbine. This, the article notes, gives better results than either the piston engine or the turbine alone as far as fuel consumption is concerned. No detail that might enable such a combination to be implemented in practice in provided in the FLIGHT article, however. This is understandable given that gas turbine technology was in its suu: 15 infancy at the time it was written.

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* US5692372 discloses an aircraft compound cycle propulsion engine having a fan and a core -gas generator -engine. The core engine comprises three rotary internal combustion engines of the Wankel type. These positive displacement motors are fed with air by a dynamic (also known as "kinetic") axial compressor which is driven by the engines via a first shaft. The combustion products of the engines drive an axial flow power turbine (a dynamic or "kinetic" motor) which in turn drives the fan via a second shaft.

Gas generator assemblies of the free piston, positive displacement, internal combustion type are known, for example, from W01980/000730 and from IEEE Transactions on Control Systems Technology, Volume 10, Issue 2, Narch 2002, pages 177-190, the latter disclosing a free-piston diesel engine.

The present invention has as an objective an improvement in the efficiency of gas generators over a range of operating conditions.

DISCLOSURE OF INVENTION

According to the present invention, there is provided: a gas generator comprising: a positive displacement gas motor configured to allow ingress of gas into the motor, to thereafter supply energy to the gas and thereafter allow egress of gas from the motor, the motor having a valve for controlling said egress; and *.S.

a compressor driven by the motor and configured to 0***** * supply pressurised gas for ingress into the motor; characterised by a valve controller for controlling the valve such that the speed of the motor is independent of variation in the supply of energy to the gas.

By controlling the valve, it is possible to maintain a constant motor speed even when the supply of energy (e.g. fuel flow) to the gas varies. This allows the motor and compressor to continue to operate at their most efficient speed (at which the compressor is properly matched to the positive displacement motor) while enabling a variation in the gas generator output by varying the supply of energy.

For example, if the energy input is increased (e.g. by increasing the amount of fuel added to the air in each cycle), the valve can be adjusted to exhaust the chamber earlier in the cycle such that the energy used in driving the piston and thus the compressor remains the same. As a result, the speed of the compressor remains the same (preferably at its optimum operating speed) while the energy in the exhaust gases increases.

The positive displacement gas motor may comprise a chamber, a supply of energy to the gas in the chamber, a piston configured to move relative to the chamber under the action of the gas; and a valve for controlling the egress of gas from the chamber.

The piston may be configured to reciprocate in the chamber under the action of the gas. Such an arrangement S.....

* enables higher compression than in the Wankel type engine employed in the aforementioned US5692372.

* In particular, both piston and chamber may be of cylindrical form, enabling the kind of high compression known from conventional internal combustion engines.

Where the piston and chamber are cylindrical, the valve may comprise a sleeve moveable relative to a port in the chamber.

The supply of energy may be by way of fuel introduced into the gas in the chamber and then burnt. Combustion may be initiated by compression ignition.

Alternatively, energy may be introduced to the gas by means of a heat exchanger in the chamber.

The gas generator may comprise multiple pistons moving in multiple respective chambers.

The compressor may be a dynamic device (in contrast to a positive displacement compressor), in particular an axial or centrifugal flow compressor.

The present invention also provides an engine comprising a gas generator as set out above together with a turbine configured to be driven by the gas from the gas generator.

The turbine may drive a fan, which may be ducted or unducted.

The fan may be located upstream of the compressor and feed the compressor with air.

Alternatively the fan may be located downstream of the compressor. This may enable a shorter connection between * S. .*I * . the turbine and the fan.

Multiple chambers may be arranged radially about a central point. The resulting circular form may be more *::: easily packaged with a fan and an aerodynamic compressor.

The present invention also provides a method of gas generation comprising: providing a positive displacement gas motor configured to allow ingress of gas into the motor, to thereafter supply energy to the gas and thereafter allow egress of gas from the motor, the motor having a valve for controlling said egress; and providing a compressor driven by the motor and configured to supply pressurised gas for ingress into the motor; and controlling the valve such that the speed of the motor is independent of variation in the supply of energy to the gas.

BRIEF DESCRIPTION OF DRAWINGS

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a block diagram of an aero engine according to a first embodiment of the present invention; Figure 2 is a schematic view of the positive displacement gas motor used in the embodiment of figure 1; Figure 3 is a block diagram of a stationary engine according to a second embodiment of the present invention. *S..

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

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* Figure 1 is a block diagram of an aero engine 10 according to an embodiment of the present invention and * incorporating a gas generator 15 comprising a positive displacement gas motor 30 and a compressor 20.

Air entering the engine (indicated at 15) first undergoes steady flow compression by compressor 20 before being supplied (as indicated at 25) to positive displacement gas motor 30.

Within the gas motor 30, the air undergoes non-flow compression (indicated by process step 35) followed by heat addition at substantially constant volume (indicated by process step 40) followed by non-flow expansion (indicated by process step 45). Work is extracted during the non-flow expansion to drive the compressor via shaft 70.

Gas generated by gas generator 15 (as indicated at 50) is fed to turbine 55 where it undergoes steady flow expansion, exiting the turbine as indicated at 60. The turbine 55 drives a fan 65 which, in the embodiment shown, is of the rear-mounted unducted type.

Figure 2 is a schematic diagram of the positive displacement gas motor 30, which comprises a piston 100 configured to move relative to a chamber 110. In the embodiment shown, piston 100 reciprocates in the chamber and is connected via connecting rod 120 to crankshaft which in turn drives the compressor as indicated at 70 in figure 1. Piston 100 and chamber 110 are both cylindrical and of circular cross-section.

Ingress of air to the chamber (as indicated at 25) S.....

* takes place through an inlet valve 140, which is then closed and the air in the chamber compressed by the upward *..

* (as seen in figure 2) stroke of piston 100. This positive displacement, non-flow compression (process step 35 in figure 1) is followed by energy addition, e.g. by the addition of heat by compression ignition of fuel in the chamber. This occurs towards the end of the upward stroke of the piston and towards the beginning of the downward stroke, i.e. at substantially constant chamber volume (process step 40 in figure 1) . Positive displacement, non-f low expansion of the heated gas then follows on the downward stroke of the piston (process step 45 in figure 1), the work done by the gas being transmitted solely to crankshaft 130 until such time as egress of the heated gas from the chamber is allowed (as indicated at 50) by opening exhaust valve 145.

The operation of exhaust valve 145 is controlled by valve controller 150 such that the speed of the motor 30 is independent of variation in the supply of energy to the gas, the exhaust valve timing being controlled such that sufficient work is extracted during the non-flow expansion of the gas to power the compressor 20 and the motor 30 while retaining the maximum possible energy in the exhaust gases.

Accordingly, if greater exhaust gas energy is required, e.g. to increase the speed of turbine 55 and its associated fan 65, the amount of fuel added to the air in each cycle can be increased (e.g. by a fuel pump feeding a S.....

* fuel injector, not shown) This increase in gas energy results in the work necessary to drive the motor and S..

* compressor at their most efficient ("matched") speed being achieved earlier in the expansion stroke. Accordingly, the valve controller 150 can open the exhaust valve 145 earlier in the expansion stroke, resulting in the exhaust gas having higher energy. It will be appreciated that the rotational speed and power output of the gas generator comprising compressor 20 and motor 30 can be varied independently, which is not the case with a conventional gas turbine. This allows scope for optimisation, and therefore possible further improvement in off-design performance.

The valve controller 150 may be an active electrical or pneumatic system (rather than fixed gearing) and may be linked to the management computer for the engine as a whole.

Valve 145 may be a sleeve valve moveable relative to a port in the chamber, the simplified ductwork enabled by such an arrangement enhancing the overall volumetric efficiency of the gas generator.

Higher power density is achieved by the use of a two-stroke cycle, while the use of compression ignition may allow the use of widely-available fuel such as "Jet A", which is cheaper and more readily available than Avgas.

Jet A has a low octane number, but a reasonable cetane number, and therefore is far better suited to compression *...

ignition. Moreover, spark ignition engines suffer from a..

* limited practical overall pressure ratio due to pre-ignition and/or detonation of the charge, thus limiting *.* * their efficiency. In non-aerospace applications, *:*::* compression ignition engines are inherently better suited to burning alternative fuels.

To facilitate starting of a gas motor using compression ignition, a suitably sized starter may be employed to bring the gas generator up to sufficiently high speed as to ensure successful compression ignition.

Alternatively/in addition, an auxiliary source of ignition may be provided such as an electrically heated glow-plug.

It will be appreciated that limitations on the available starting measures may limit the geometric compression ratio of the gas motor.

Although only a single piston/chamber combination is shown in figure 2, the gas motor may comprise multiple pistons and chambers, in which case a common rail fuel injection system may be used, providing reliability and improved atomisation at high pressure ratios.

Arranging multiple pistons and chambers radially about a central point simplifies the packaging of the engine since the compressor and power turbine are typically circular in cross-section.

As regards the compressor 20, it will be appreciated that this helps reduce the physical size of the motor 30.

Where a two-stroke piston engine is used, the compressor also allows for adequate scavenging. The compressor 20 may be powered by the motor by means of a step-up gearbox S.....

* (not shown) having a ratio between 2 and 3. For steady flow compression, a dynamic rather than positive * displacement compressor is used, which for aeronautical applications may be an axial flow compressor. However, the final stage or stages of the compressor may use centrifugal flow to isolate the compressor from the piston engine downstream. The pressure ratio across the compressor may be around 10.

As regards the turbine 55, this has -unlike the compressor -no mechanical drive connection with the gas motor 30. Rather, it is fed by the exhaust gas (as indicated at 50 in figure 1) . Turbine 30 may have several stages and may drive contra-rotating open rotors 65 at the back of the engine, downstream of the compressor. The ultra-high bypass ratios enabled by such an arrangement are suited to the gas generator machine when used as an aero engine core. They may also enable a shorter connection between the turbine and the fan. The rotational speed of the power turbine may be governed by a control system using variation of the pitch of the open rotors. It may also be advantageous to gear the drive from the turbine to the fan. An alternative, more conventional, arrangement has a ducted fan at the front of the engine, upstream of the compressor and feeding the latter with air.

Typically, for a twin-engined aircraft having a mass of io5 kg and a target Lift:Drag ratio of 20:1, the thrust requirement is around 25 kN per engine. Assuming a cruise S....

* speed of 220 m/s, this equates to 5.5 MW of thrust power.

Figure 3 is a block diagram of a second embodiment of the present invention for stationary applications. As with the first embodiment, it incorporates a gas generator 15 comprising a positive displacement gas motor 30 and a compressor 20 driven by the motor. Without the packaging requirements of an aero engine, the choice of compressor design is less restricted and geometries other than radial may be used for the gas motor.

In the stationary application shown, gas egress 50 is used to drive a gas turbine 200 having an output shaft 210 which may in turn drive, for example, a generator (not shown) Gas exhausting from the turbine (as indicated at 60) can further be passed through an exhaust heat recovery unit 220, as known per se from conventional gas turbines in combined cycle plants.

Moreover, the gas generator according to the present invention is also applicable to plant where there is no combustion, energy generation being instead e.g. by nuclear means, in which case heat transfer to the gas in the chamber may be by means of a heat exchanger. In such circumstances, the gas may be held in a closed circuit, being fed back to the compressor following egress from the turbine or exhaust heat recovery unit. * * S... *

* S.... * S a. .. * . S * . S..

S * *S * . . * .. **.*S S *

Claims

CLAIMS1. A gas generator comprising: a positive displacement gas motor configured to allow ingress of gas into the motor, to thereafter supply energy to the gas and thereafter allow egress of gas from the motor, the motor having a valve for controlling said egress; and a compressor driven by the motor and configured to supply pressurised gas for ingress into the motor; characterised by C a valve controller for controlling the valve such that the speed of the motor is independent of variation in the o supply of energy to the gas.

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2. A gas generator according to claim 1, where in the positive displacement gas motor comprises a chamber, a supply of energy to the gas in the chamber, a piston configured to move relative to the chamber under the action of the gas; and a valve for controlling the egress of gas from the chamber.
3. A gas generator according to claim 2, wherein the piston is configured to reciprocate in the chamber under the action of the gas.
4. A gas generator according to claim 3, wherein both piston and chamber are cylindrical form.
5. A gas generator according to claim 4, wherein the valve comprises a sleeve moveable relative to a port in the chamber.
6. A gas generator according to any preceding claim, wherein the supply of energy is by way of fuel introduced into the gas in the chamber and then burnt.
7. A gas generator according to claim 6, wherein combustion is initiated by compression ignition.

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8. A gas generator according to any one of claims 2 to 5, r wherein energy is introduced to the gas by means of a heat o exchanger in the chamber.

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9. A gas generator according to any one of claims 2 to 8, wherein the gas generator comprises multiple pistons moving in multiple respective chambers.
10. A gas generator according to any preceding claim, wherein the compressor is a dynamic device.
11. An engine comprising a gas generator according to any preceding claim and a turbine configured to be driven by the gas from the gas generator.
12. An engine according to claim 11 and compri.sing a fan driven by the turbine.
13. An engine according to claim 12, wherein the f an is located upstream of the compressor and is configured to feed the compressor with air.S
14. An engine according to claim 12, wherein the fan is located downstream of the compressor.
15. An engine according to any one of claims 11 to 14, wherein multiple chambers are arranged radially about a central point.
16. A method of gas generation comprising: providing a positive displacement gas motor configured to allow ingress of gas into the motor, to thereafter CS*J supply energy to the gas and thereafter allow egress of gas from the motor, the motor having a valve for controlling said egress; and providing a compressor driven by the motor and configured to supply pressurised gas for ingress into the motor; and controlling the valve such that the *speed of the motor is independent of variation in the supply of energy to the gas.