Turbocharged reciprocation engine for power and refrigeration using the modified Ericsson cycle

A Modified Ericsson Turbocharged Reciprocating Engine (METRE), is provided which exhibits a high thermal efficiency for power and refrigeration applications. A Modified Ericsson cycle can include 2, 3, 4, or more stages (number of intercooling and heat/reheat cycles between stages). As stages are added, both cycle efficiency and power density (power/weight flow) increase, therefore, trade-offs between higher performance and number of stages (system complexity, cost, etc.) are necessary to optimize the engine. By combining a turbocompressor for the low pressures of the cycle and a multi-piston reciprocating engine for the high pressures of the cycle, a light weight, highly fuel-efficient, low-polluting, engine can be achieved. The METRE is highly suited for the power range of automobiles and trucks. This engine can use low technology (lower turbine temperatures, efficiencies, etc.) as well as high technology components (higher turbine temperatures, efficiency etc.) and remain competitive with Brayton, Stirling, gas and Diesel engines. The Ericsson cycle, like the Brayton and Stirling, utilizes external combustion or heating and thus can use readily available optional fuels such as natural gas, kerosene, propane, butane and gases derived from coal. Solar and nuclear energy are also useable heat source candidates.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to machinery designs and supporting component 
integration (intercoolers, regenerator, combustor or heater and reheaters) 
for achieving a high thermal efficiency engine. 
The engine is based on the Modified Ericsson cycle, capable of using low 
technology as well as advanced technology components, that are combined 
into various optional systems for power, efficiency, and ease of 
development considerations. 
The above and other features of this invention will be more fully 
understood from the following detailed description of the engine, a 
discussion of various design options and the accompanying drawings. 
2. Description of Related Art 
The subject invention pertains to the selection of rotating and 
reciprocating machinery along with the integration of this machinery with 
intercoolers, a regenerator and a high temperature combustor or heater and 
reheaters to achieve a very high efficiency engine based on the Modified 
Ericsson cycle. This engine has the size and operating charateristics that 
are comparable to or better than current internal combustion automobile 
and truck engines. These include: (1) higher efficiency potential; (2) 
lower working fluid operating temperatures and pressures and thus lower 
exhaust gas pollutants; (3) external combustion that can use optional 
fuels such as natural gas, lower grade fuels other than high octane gas 
(kerosene, propane, butane) and gases derived from coal. 
The Ericsson cycle, although not currently used for reasons to be 
discussed, remains an attractive cycle because it, like a Stirling, 
ideally achieves Carnot efficiencies when operated between given upper and 
lower temperature limits. Ericsson engines have been used in the past to a 
limited extent, however, the mean effective pressure was too low for it to 
compete with internal combustion or steam engines. In a non-flow cycle 
such as hot gas in a cylinder, the work is obtained through the action of 
a moving piston being acted upon by a variable pressure. The net average 
pressure, called mean effective pressure (m.e.p.), times the displacement 
volume of the cylinder represents the work produced in one stroke. Low 
m.e.p. results in a large engine for a given power and thus a heavier 
design. 
A practical way to overcome the low m.e.p., in order to take advantage of 
this high efficiency cycle, is the incorporation of a supercharger using a 
high speed turbocompressor for the first stage of the cycle. This addition 
allows a compressor of much smaller size than a comparable reciprocating 
design to perform the gas compression and expansion at the low ambient 
pressures. 
By combining a turbocompressor for the low pressures of the cycle and a 
multi-piston reciprocating engine for the high pressures of the cycle 
along with intercoolers, a regenerator, a combustor or heater and 
reheaters, various versions (stages) of the Modified Ericsson cycle can be 
achieved. The Modified Ericsson approximates the Ideal Ericsson isothermal 
compression by using multiple stages of compression, with intercooling 
between stages, and the isothermal expansion by using multi-power 
expansion (turbine) stages, with reheat between stages. The regenerator is 
used to recover the exhaust heat from the last turbine stage and deliver 
it to the final stage compressor discharge gas prior to entering the 
combustor or heater. A high efficiency (also called effectiveness) 
regenerator is a key component in a regenerative thermal cycle. However, 
as stages are added to a Modified Ericsson cycle, the regenerator 
effectiveness becomes less critical to the overall cycle efficiency. This 
significant factor makes a multi-stage Modified Ericsson engine very 
attractive for a regenerative cycle and the benefits will be discussed in 
more detail in the following section. 
SUMMARY OF THE INVENTION 
The present invention provides a means for achieving the high thermal cycle 
efficiencies of the Modified Ericsson cycle using a combination of: (1) 
high speed turbocompressor for the low pressure high flow rate initial 
stage, and (2) reciprocating machinery for the high pressure low flow rate 
later stages of the cycle. 
Using this combination, the Modified Ericsson Turbosupercharged 
Reciprocating Engine (METRE), achieves thermal efficiencies in the 50% to 
60% range, as compared to 30% for current internal combustion gas engines 
and 40% for Diesels. 
The METRE high efficiency thermodynamic cycle has many applications 
including: (1) power generation for space and earth, (2) drive motors for 
sea and land transportation, and (3) refrigeration application; such as 
helium liquefication for superconductivity, cryogenic fluid production, 
cooling of high speed computers and electronic equipment, and 
air-conditioning. Current cycles being used today are less efficient, 
except for the Stirling cycle. However, the Stirling cycle, operates at 
much higher pressure levels (3 to 5 times), than the METRE. 
Since METRE uses both turbo, also referred to as dynamic, and reciprocating 
machinery in its power and refrigeration cycle, advanced technology can be 
used which is currently being developed by the gas turbine industry, the 
automotive industry, NASA and the Department of Energy (DOE). 
This technology includes: (1) ceramic turbines, combustors, heaters, 
regenerators, etc., (2) electronic fuel metering sensors and controls, (3) 
light weight aluminum blocks, (4) ceramic pistons, liners and valves, and 
(5) high strength, light weight carbon-carbon composites for lines and 
ducting. By combining the high efficiency power cycle of METRE with this 
advanced technology, a highly fuel-efficient, low-polluting, engine is 
possible.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
According to one embodiment of the present invention, a Modified Ericsson 
Turbocharged Reciprocating Engine (METRE) shown in FIG. 1, consists of an 
independent turbine driven centrifugal type compressor assembly 10 
operating in series with a multi-piston reciprocating engine 20 and 
gearbox 30. 
Engine operation begins as gas flow enters the centrifugal compressor 2, 
through inlet duct 1 and is raised to design discharge pressure; it exits 
through duct 3 into intercooler 4 where the heat of compression is removed 
by external cooling means (i.e. air, water, Freon etc.). After the gas 
exits intercooler 4 through ducts 5A 5B at a temperature equal to the 
compressor gas-flow at the inlet; it enters the reciprocating compressors 
6A 6B and is raised to the design pressure. The gas then exits through 
ducts 7A 7B into intercooler 8 and is again cooled to the inlet 
temperature of the compressors 26A-26B. This compression/cooling cycle is 
repeated as the gas flows through inlet ducts 9A 9B, compressors 11A 11B, 
exit ducts 12A 12B, and intercooler 13, to complete the pressurizing and 
cooling phase of the cycle. This phase can include 2, 3, 4, or more 
stages, depending upon the design over-all pressure ratio, the pressure 
rise per stage considered optimum for high cycle efficiency, and other 
considerations including structural limits. 
Note, the last intercooler 13 could be located at inlet duct 1 for a 
"closed cycle" (helium, nitrogen, argon, etc.), however, the size and 
weight would increase because the lower pressure gas requires larger flow 
areas to maintain constant velocities and larger heat transfer surface 
area due to lower heat transfer coefficients on the gas side. For "open 
cycles" (air/fuel) intercooler 13 can be eliminated. 
After the gas is cooled by intercooler 13 to the inlet temperature of 
compressors 11A 11B, it exits through duct 14 and enters the regenerator 
15 where heat is absorbed from the exhaust gas exiting the turbocompressor 
turbine 16 through duct 29, discussed below. The as then exits through 
duct 17 into the combustor 18 "open cycle", or heater 18 "closed cycle" 
where additional heat is added until the maximum allowable operating 
temperature is reached. The high pressure hot gas exits through ducts 19A 
19B and enters pistons 21A 21B, functioning as reciprocating expanders, 
where the hot gas expands and exhausts through ducts 22A 22B. The hot gas 
then enters reheater 23, where the gas is again reheated to maximum 
allowable operating temperature and exits through ducts 24A 24B, enters 
pistons 25A 25B, expands and exits through ducts 26A 26B. The gas then 
enters reheater 27 where it is again reheated to maximum allowable 
operating temperature. It then exits through duct 28 and drives the 
turbocompressor turbine 16 of the assembly 10. The turbine exhaust gas 
exits through duct 29 and enters the regenerator 15 where it gives up 
heat, as noted above, to the high pressure gas exiting intercooler 13 and 
duct 14. The gas exiting through duct 31 can either discharge to the 
atmosphere through duct 32 to complete an "open-cycle" system, or it can 
return to the compressor inlet through duct 33, where it begins a new 
cycle for a "closed cycle" system. The net output power produced by the 
cycle is extracted through the gearbox 30 connected to the reciprocating 
engine drive shaft 34. 
In general various types of compressors and turbines can be used with a 
Modified Ericsson cycle. At lower power levels, positive displacement, 
including reciprocating machinery, are more efficient up to approximately 
500 horsepower. As power increases beyond this range, centrifugal and 
axial flow compressors and turbines, also called dynamic compressors and 
dynamic turbines, become more efficient and have higher power to weight 
ratios. 
The basic characteristic of compression and expansion for a reciprocating 
engine is shown in FIG. 2 for one cycle (one complete revolution of the 
piston). It should be noted that, unlike an internal combustion engine, 
the compression and expansion phase of a Modified Ericsson engine are 
performed by separate pistons with a compression and expansion occurring 
during each revolution of the pistons. Both the ideal 40 and actual cycles 
41 are shown along with the valve sequencing 42. 
A comparison of typical pressures, temperatures and specific volumes for an 
internal combustion engine and a typical Modified Ericsson engine is shown 
in FIG. 3. The METRE solves a major deficiency, of a reciprocating engine 
operating with a Modified Ericsson cycle, of low mean effective pressure 
(m.e.p.) 45, as illustrated in FIG. 3. The turbocompressor increases the 
m.e.p. from 41 psia 46 to 109 psia 47. Therefore METRE becomes 
more-competitive, in terms of size, with the internal combustion engine 
m.e.p. of 217 psia 48. In addition, METRE efficiencies are higher (55% 
versus 30%) and these will be discussed later. 
Alternate concepts of METRE are illustrated in FIG. 4. The 2-cylinder METRE 
50 shows the simplest type design and may be used for either an "open" or 
"closed" cycle. The 8-cylinder METRE concept 51 is an attractive concept 
for a helium system where many low pressure ratio stages (P.sub.r =2 to 3) 
are required to achieve high efficiency cycles. 
Another feature of METRE is that the turbocharger and reciprocating engine 
can each operate at or near optimum speed to achieve maximum efficiency. 
This speed corresponds to the optimum specific speed of the units and is 
defined as: 
EQU N.sub.S =N*Q**1/2/H**3/4 
WHERE: (compressor/turbine) 
N-speed 
Q-volume flow rate (inlet/exit) 
H-head (rise/drop) 
Specific speed as defined above is an aerodynamic flow parameter of 
rotating and positive displacement machinery and the corresponding 
efficiency is presented in FIG. 5. FIG. 5 illustrates that the maximum 
efficiency (.about.90%) for rotating machinery 55 56 has an optimum 
specific speed (N.sub.s .about.200) while that for reciprocating piston 
type 57 remains constant (80%) over a specified range (N.sub.s .about.0.2 
to 0.3). Thus, the speed of multi-stage rotating machinery should increase 
with increasing pressure (since volume flow rate decreases) while that for 
reciprocating machinery can remain constant over a wide range of pressures 
for maximum cycle efficiency. 
The basic characteristic of a Modified Ericsson cycle for four stages of 
compression 60 is shown in FIG. 6 on a temperature-entropy diagram. The 
number of compressions may vary from 2 to greater than 4 stages, however, 
the gain in efficiency becomes incrementally smaller as the number of 
stages increase. When only a single stage is used, the cycle is called a 
Brayton cycle; that may or may not have regeneration. A universal 
efficiency equation 61 for all these cycles is included in FIG. 6. A close 
examination of the input power equation 62 shows that as the number of 
stages increases, the regenerator effectiveness becomes less critical to 
the over-all cycle efficiency. 
Performance characteristics of the Modified Ericsson engine using 
state-of-the-art technology 62 (turbine temperature of 2600.degree. R), is 
presented in FIG. 7. These efficiencies 63 64 65 (0.50 to 0.58) are 
approximately 50% higher than those achievable by current internal 
combustion gas (0.30) and Diesel (0.40) engines. 
Performance characteristics of the engine using lower technology machinery 
(turbine temperature of 1900.degree. R), would have efficiencies in the 30 
to 40 percent range and still remain competitive. Advanced technology 
machinery, (turbine temperature of 3000.degree. R), increases the 
efficiency to the 0.55 to 0.65 range; nearly twice current internal 
combustion gas engine efficiencies. 
Another embodiment of this invention applies to refrigeration applications. 
For illustrative purposes, a four (4) stage METRE 74, FIG. 8, is used for 
helium liquefication 75. For this application power is not generated and 
the excess helium flow, not required as drive turbine gas, is tapped-off 
at the last stage of intercooler output 76. The amount that may be 
tapped-off 77 is a function of the cycle efficiency 78. 
The cycle efficiencies and specific power coefficient (SPC) for helium, 
using advanced technology 80, is presented in FIG. 9. Based on these 
predicted efficiencies, the amount of helium tap-off flow 77 is 
approximately 60% of the total system flow rate. 
Having described the preferred embodiments of the invention, it should now 
be apparent that numerous modifications could be made thereto without 
departing from the scope and fair meaning of this invention as described 
hereinabove and as claimed.