A theoretical analysis of the performance of a diesel engine-compressor-turbine combination for aircraft Page: 4 of 25
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NACA AaR No. 'D10
In the first part of the proposed cycle the air is compressed
in a supercharger, interoooled, and introduced into the engine at
high pressure. In the engine it is further compressed to the maxi-
mum cylinder pressure; the fuel is introduced, burned at constant
pressure, and the mixture is then expanded. The compression ratio
of the engine is so computed as to provide the desired maximum oyl-
inder pressure at the end of the compreseeion stroke. At the end of
the expansion stroke the gases are exhausted at a high back pressure
into the turbine and thence to the atmosphere. The amount of fuel
introduced is limited by the condition that the exhaust-gas tempera-
ture does not exceed a value considered safe for turbine operation.
For additional power the exhaust gas could be discharged rearwardly
from the turbine to obtain jet thrust.
When the engine back pressure or the turbine inlet pressure is
made equal to the engine inlet pressure, the turbine power is nor-
mally groater than the compressor power; for this case, the coupled
turbine and compressor are connected by a _eir-reduotion unit to the
engine shaft, and the excuse power is added to the engine power for
driving the propeller. If the engine back pressure is less than the
engine inlet pressure and is of such value that the turbine and the
compressor powers are equal, it would not be advantageous to gear
the turbine and compressor to the engine shaft and these two compon-
ents become a turbosupercharger. The loss in turbine power in this
case Is partly compensated by the increase in engine power that
results from the reduction in back pressure. An optimum back pres-
sure exists at which the net power is a maximum.
With additional engine modifications, the system can be oper-
ated on a two-stroke cycle. With this type of operation, the engine
back pressure should be less than the engine inlet pressure to permit
scavenging of the engine cylinder. Two-stroke-cycle operation,
which is well suited for the Diesel cycle because no loss of fuel
occurs during the scavenging process, further increases the power
output obtainable with this system.
The cycle for the proposed system is shown diagrammatically in
figure 1. The air is first compressed from points 1 6o 2 in a turbo-
supercharger, intercooled from points 2 to 3, and introduced into the
engine under high pressure at point 3. In the engine it is further
compressed to the maximum cylinder pressure at point 4 where fuel Js
injected and combustion is started. The main part of the combustion
process occurs from points 4 to 5 at constant pressure.
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Hall, Eldon W. A theoretical analysis of the performance of a diesel engine-compressor-turbine combination for aircraft, report, April 1945; (digital.library.unt.edu/ark:/67531/metadc62106/m1/4/: accessed June 24, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.