Gas turbine engine with stepped inlet compressor

A gas turbine engine has an inlet compressor with a stepped centrifugal impeller and a double diffuser therefrom controlled by valve means and operative to supply air to combustion apparatus from when motive fluid is directed through a turbine to drive the compressor. The turbine may drive the load or a second power turbine in series with the compressor drive turbine can be included. The discharge from the double diffuser is controlled to vary the mass flow of the engine for operation at idling and under light loads and operable to be completely open for higher power output of the engine.

This invention relates to gas turbine engines and more particularly to gas 
turbine engines for propulsion of automobiles or the like. 
In typical urban automobile operation the engine is operated at idle for a 
substantial part of the time and in typical highway driving, while not at 
idle, the engine is operated far below full capacity. Accordingly, gas 
turbine engines having fixed capacity and a predetermined geometry for 
mass flow of gas therethrough, to be acceptable for automotive use, must 
have considerable reserve power for acceleration and thus at times are 
oversized for power consumption requirements under engine idle and much of 
the other driving cycle. 
Various proposals have been made to vary the configuration of gas turbine 
engines to reduce air flow therethrough for operation under low power 
conditions to improve engine efficiency. For example, U.S. Pat. No. 
2,582,809, issued Jan. 15, 1952, to Weir, discloses a jet engine with two 
centrifugal compressors arranged back-to-back, each supplying a group of 
combustion chambers and each group of combustion chambers supplying one of 
two cascades of blades on the turbine which drives the compressor. A valve 
arrangement is included to close or substantially close the inlet to one 
of the compressors. U.S. Pat. No. 2,662,371 issued Dec. 15, 1953, to 
Rennie et al discloses a turbojet engine with a compressor having an 
arrangement for varying the flow area through the compressor and with a 
number of combustors with valves to close off flow through some of the 
combustors as the compressor flow area is reduced. U.S. Pat. No. 3,124,931 
issued Mar. 17, 1964, to Mock discloses a shaft powered gas turbine for 
automotive use having a variable diffuser at the compressor outlet and a 
sliding throttle plate or other means for varying the turbine flow area. 
A more recent proposal is set forth in U.S. Pat. No. 3,625,003 issued Dec. 
7, 1971, to Liddle et al which has a double entry centrifugal compressor 
and means on one side of the double entry compressor to close its entrance 
under idling and light load conditions to improve gas turbine engine 
efficiency. 
An object of the present invention is to step or change the size of a gas 
turbine engine without altering wheel speed or turbine temperature and 
thereby improve efficiency of conventional automotive gas turbine engines 
by means of a stepped centrifugal compressor discharging radially to a 
pair of diffusers both valve controlled and regulated to control mass flow 
of gas through the gas turbine engine in accordance with power 
requirements. 
Still another object of the present invention is to step or change the size 
of a gas turbine engine without altering wheel speed or turbine 
temperature and thereby improve the efficiency of automotive gas turbine 
engines by the provision therein of an inlet air compressor having an 
impeller with a plurality of blades thereon operative at design speed of 
operation to pump a first predetermined amount of primary inlet air into a 
first diffuser in the gas turbine engine and further including a rotating 
channel within the centrifugal compressor impeller located through the 
interior thereof to exhaust rotating channel secondary air flow into a 
second diffuser in the engine thereby providing a reduced mass flow of 
inlet air to the gas turbine engine and further including means for 
limiting the primary inlet air flow into the engine under idle conditions 
of operation so that the rotating channel secondary air flow constitutes 
the mass flow of gas through the engine for limiting power output 
therefrom under idle engine operation. 
Still another object of the present invention is to provide an improved 
automotive gas turbine engine by the provision therein of a stepped 
compressor for supplying inlet air to a combustor having fuel flow thereto 
combusted with inlet air to produce motive fluid for turbines and wherein 
the centrifugal compressor includes a plurality of radial blades thereon 
configured to produce a primary air flow and further including a rotating 
channel within the centrifugal impeller operative to produce a secondary 
air flow and wherein a double diffuser is located radially outwardly of 
the stepped compressor to receive inlet air therefrom and wherein valve 
controls direct either high capacity air flow into the gas turbine engine 
combustor for acceleration and highway driving conditions or direct a 
reduced air flow into the engine for operation under idle conditions for 
reduced power output from the engine. 
Yet another object of the present invention is to match automotive gas 
turbine engine mass flow to engine power requirements and to avoid 
excessive engine air flow under cruise modes of operation while 
maintaining sufficient engine air flow to meet acceleration requirements 
and to avoid stall torque conditions and to do so by means of a stepped 
centrifugal compressor having a first plurality of radial blades thereon 
operative to produce a primary air flow from the inlet air compressor to a 
downstream primary air flow diffuser to the engine combustor and by the 
further provision in the stepped impeller of a rotating, internal channel 
for discharging a secondary air flow into a secondary downstream diffuser 
for producing a separate, lower air flow to the combustor and wherein 
combustor inlet air flow and turbine gas flow are regulated by control 
means and are suitably variable to maintain controlled gas flow 
requirements through the engine in response to control of air flow from 
both or either one of the primary or secondary diffusers into the 
combustor under idle and cruise modes of automobile operation. 
Further objects and advantages of the present invention will be apparent 
from the following description, reference being had to the accompanying 
drawings wherein a preferred embodiment of the present invention is 
clearly shown.

Referring now to FIG. 1, an automotive type gas turbine engine 10 is 
illustrated diagrammatically. The engine 10 includes an inlet air 
compressor or gasifier 12 that discharges into a pair of conduits 14, 16 
under the control of shut-off valves 18, 20 respectively. An inlet tube 22 
supplies air to a combustor 24. Fuel is directed to the combustor 24 
through a supply conduit 26 under regulation by a fuel control 28 of the 
type set forth in U.S. Pat. No. 3,879,936 issued Apr. 29, 1975, to 
Stoltman. Fuel control 28 is set by a manual operator shown as a foot 
operated accelerator pedal 30. Exhaust from the combustor 24 is directed 
through a conduit 32 to turbine 34 representatively shown as including a 
gasifier turbine 36 connected by a shaft 38 to drive the inlet air 
compressor 12. The automotive gas turbine engine 10 further includes, in 
the representatively shown example, a power turbine 40 connected by a 
shaft 42 to a load 44 which may, for example, be the transmission, 
differential and driving wheels of an automotive type vehicle. The exhaust 
from the power turbine 40 flows through an exhaust line 46 to the second 
pass 47 of a regenerator 48 having the first pass 50 thereof connected to 
the inlet tube 22 upstream of the combustor 24. 
Also, the engine 10 may be of a single shaft type with a single turbine 
drive shaft or a controllable coupling of the type set forth in U.S. Pat. 
No. 3,237,404 issued Mar. 1, 1966, to Flanigan et al. 
In the illustrated arrangement, the conduit 32 splits at 52 to supply both 
the gasifier turbine 36 and the power turbine 40 under the control of 
inlet guide vanes 54, 56, respectively, of the type set forth more 
particularly in FIG. 2 in the aforesaid U.S. Pat. No. 3,625,003 to Liddle 
et al. 
Furthermore, the combustor 24 includes an adjustable controller 58 in 
association therewith to vary the ratio of the primary to secondary air 
flow into the combustor in accordance with air flow demand as preset by 
the accelerator pedal 32 to operate the engine 10 under idle conditions, 
acceleration or cruise modes of operation. Such a controller 58 is better 
disclosed in U.S. Pat. No. 3,930,368 issued Jan. 6, 1976, to Anderson et 
al and is in coupled control relationship to pedal 30. 
Examples of automotive gas turbine engines of the type set forth in Conklin 
et al, U.S. Pat. No. 2,972,230, issued Feb. 21, 1961, and Coleman et al, 
U.S. Pat. No. 3,267,674 issued Aug. 23, 1966, are oversized in order to 
meet acceleration and stall torque requirements. In cruise modes of 
operation the engine air flow in such prior engines is too big and 
consequently the turbine temperature is reduced under idle conditions to 
reduce power and air flow. 
In such engines, when the turbine temperature is reduced and the compressor 
operating line 60 is as shown in FIG. 2, the maximum design operating 
point (shown at 62 on the operating line 60 in the graph of FIG. 2) moves 
to the cruise point as shown by point 64 on operating line 60. The result 
is lower power with an increased fuel consumption since the compressor and 
turbine efficiencies have been lowered. 
In accordance with certain principles of the present invention, the inlet 
air compressor 12 is modified to change the size of the engine without 
altering the spool speeds of the compressor or the turbine temperature of 
operation. Furthermore, it is accomplished without using complicated 
variable geometry in the inlet air compressor. More particularly, this is 
accomplished in the present invention by including a stepped centrifugal 
impeller 66 within the housing 68 of inlet air compressor 12. The stepped 
centrifugal impeller 66 more particularly includes a hub 70 connected to 
the gasifier turbine shaft 38. The hub 70 includes a plurality of radially 
directed centrifugal vanes 72 thereon having an inlet edge 74 located 
immediately downstream of an annular inlet 76 in the inlet air compressor 
12. Each of the radially directed vanes 72 further includes a radial tip 
78 thereon located in close spaced relationship to an annular shroud 80 
which cooperates with pairs of the radial vanes 72 to define a plurality 
of circumferentially located primary centrifugal flow paths 82 through the 
impeller 66 each having a splitter vane 81 therein approximately midpoint 
thereof. Each of the paths 82 includes an axial segment in alignment with 
the annular inlet 76 and a radially directed outlet 84. The outlets 84 are 
in communication with a primary vaned diffuser 86 that receives primary 
air flow from the impeller which is under the control of primary control 
valve 18. 
In order to step or change the size of the engine, the impeller further 
includes a rotating channel 88 incorporated into the interior of each of 
the compressor vanes 72. Each of the rotating channels 88 includes an 
inlet 90 that is axially aligned with the annular inlet 76. Furthermore, 
each of the channels 88 includes an outlet opening 92 that is in 
communication with a secondary vaned diffuser 94 located radially 
outwardly of rear plate 96 on the impeller 66. The rear plate is 
associated with a seal 98 carried on a divider wall 100 formed between the 
primary and secondary vaned diffusers 86, 94 as best seen in FIG. 3. 
The channel 88 is configured to exhaust into the secondary vaned diffuser 
94 at a point behind the primary diffuser 86 and is under the control of 
the secondary control valve 20. 
The illustrated stepped centrifugal impeller 68 can be configured to cause 
the inlet compressor 12 to flow with both primary and secondary inlet mass 
flows at identical ratios or at different pressure ratios as desired. If 
different pressure ratios are used, only one, either the primary or 
secondary flow path may be opened in which case either one of the shut-off 
valves 18 or 20 are closed by coupled control to pedal 30. If the primary 
and secondary flow paths have the same pressure ratios, they can be mixed, 
used separately or one of the primary or secondary flow paths may be shut 
off by closing either one of the valves 18, 20 downstream thereof. 
Alternatively, closure may be effected within the vaned diffusers 86 or 94 
by variable closure vanes therein if desired as shown in U.S. Pat. No. 
3,992,128 issued Nov. 16, 1976, to Lunsford et al. 
The following chart is set forth to representatively illustrate potential 
flow variations within the concept of the present invention. 
______________________________________ 
PRIMARY 
and 
PRIMARY SECONDARY SECONDARY 
______________________________________ 
Air Flow -Wa 
5 1 6 
Pressure Ratio-R.sub.c 
7:1 7:1 7:1 
______________________________________ 
When the primary or secondary shut off valves 18, 20 are closed, the 
compressor component (either paths 82 or channels 88) upstream of such a 
shut off valve is unloaded and the flow tends to recirculate. A modest 
temperature rise can occur but this is not of a magnitude to cause 
mechanical problems. The temperature will finally stabilize at the unused 
component of the impeller 66 at a range in the order of 200.degree. F. to 
300.degree. F. The air flow of the utilized compressor flow path through 
the impeller 66 either the primary centrifugal flow paths 82 or the 
rotating channels 88, or both, will cause the air flow to be significantly 
lowered. The turbine temperature and speed will nevertheless be maintained 
high and component efficiencies will therefore be maximized. Specific fuel 
consumption will be held at design point values and will not suffer usual 
design degradation. Moreover, the illustrated design eliminates the need 
for two separate radial centrifugal inlet air compressors of the type set 
forth in the aforesaid Liddle patent. 
While the embodiments of the present invention, as herein disclosed, 
constitute a preferred form, it is to be understood that other forms might 
be adopted.