Power plants – Fluid motor means driven by waste heat or by exhaust energy... – Having fluid motor motive fluid treating – controlling or...
Reexamination Certificate
2002-09-25
2004-01-06
Nguyen, Hoang (Department: 3748)
Power plants
Fluid motor means driven by waste heat or by exhaust energy...
Having fluid motor motive fluid treating, controlling or...
C060S646000, C060S660000
Reexamination Certificate
active
06672063
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates in general to bottom cycle engines for utilizing the waste exhaust heat of an engine to produce mechanical work. In particular it relates to an improved reciprocating hot air engine that provides a simple, low cost to manufacture, means for recovering energy otherwise lost in the exhaust of an internal combustion, gas turbine or similar heat engine.
2. Description of Prior Art
The rising cost of fuels and concern about the environmental effects of burning those fuels have increased the need for developing novel methods for obtaining the maximum amount of heat and mechanical power from the combustion process. One very effective measure is to recover as much of the heat as possible from the exhaust gases. There are two general types of combustion processes, low pressure and high pressure, and each type has optimal methods for recovering the exhaust heat.
Low pressure combustion processes are used by steam boilers and industrial heating processes. In both cases, combustion occurs in a furnace that is essentially at atmospheric pressure. The most direct method for reducing the fuel requirements in these low-pressure processes is to use a counterflow heat exchanger to heat the incoming air with the hot exhaust gases. Alternatively, mechanical work can be obtained from a bottom cycle engine that is heated by the exhaust. The most effective method of recovering heat from a low-pressure combustion process is to combine both the air preheating and mechanical work recovery methods with an Afterburning Ericsson Cycle Engine as described in my U.S. Pat. No. 5,894,729 (1999). This engine can integrate the furnace process into the engine process by simply using the furnace as the afterburner. With this engine, the furnace receives a forced draft of hot air from the engine expander exhaust and the furnace then provides the hot exhaust back to the engine to produce mechanical work.
High-pressure combustion processes are the most common type of engine combustion process and are found in spark-ignition, Diesel, and gas turbine engines throughout the world. Their wide use makes them an ideal market for devices to effectively recover exhaust heat. Because their combustion process is high pressure, it cannot be integrated as the low-pressure afterburner of an Afterburning Ericsson Cycle engine. Instead, gas turbines can use a heat exchanger (generally termed a recuperator) to use the hot exhaust from the expander turbine to preheat the air from the compressor turbine to greatly improve the gas turbine engine efficiency. Nevertheless, adding a recuperator greatly alters the engine because of the need to insert a large heat exchanger into the otherwise compact engine. For this reason it is very difficult, and often impossible, to modify an existing, aircraft type, gas turbine engine for a recuperator.
Spark-ignition and Diesel engines are not able to use a recuperator and instead frequently use some form of “bottom cycle” where another engine is attached to the exhaust to use the exhaust heat and/or pressure to drive another engine. Such a system of two engines is called a combined cycle engine and consists of the spark-ignition, Diesel, or gas turbine engine (the top cycle engine) and a bottom cycle engine that is attached to the top cycle engine's exhaust.
Gas turbine top engines are commonly joined with Rankine cycle bottom engines to make a very effective combined cycle engine that is widely used in large powerplants. The Rankine cycle makes a very effective bottom cycle that can effectively use much of the top cycle exhaust heat. However, the complexity and potential safety issues of the Rankine cycle engine are not justified for smaller, “micro-generation” applications of less than 100-kilowatt output that are now entering the distributed power market.
Turbochargers have become the most common form of bottom cycle engine for spark-ignition and Diesel top cycles. Turbochargers use the hot, high pressure, exhaust from the top cycle to spin a turbine that is connected to a compressor that boosts the pressure of the air entering the top cycle engine. The boost pressure increases the mean effective pressure of the top cycle engine and increases its power. Turbochargers work very well with Diesel engines because they have no combustion limits from the increased boost pressure. There is a penalty in reliability and durability however.
Although a turbocharger can increase the power of a spark-ignition engine, there is usually very little gain in efficiency. The increased boost pressure leads to increased risk of detonation. Consequently, turbocharged engines are “detuned” from their normally aspirated versions by reducing the compression ratio and adjusting the ignition timing. As a result, although capable of increased power, a turbocharged spark-ignition engine frequently “ . . . lowers fuel economy in comparison to the same engine naturally aspirated. The decision to use supercharging in this way is more one of marketing than one of utility.” [Taylor, Charles Fayette: “The Internal Combustion Engine in Theory and Practice, Vol II”, The M.I.T. Press (1995) p. 367].
It would seem that a bottom cycle based on Stirling or Ericsson Cycle engines would be ideal. The theoretical efficiency of both these engines is the same as a Carnot engine—the maximum efficiency possible with a supply of heat at one temperature and a reservoir at a lower temperature for receiving the exhaust. However, a bottom cycle engine meets only half the Carnot engine requirements; although the surroundings provide the necessary constant temperature reservoir, the heat from a top cycle exhaust is not available at a constant temperature.
FIG. 1
shows a temperature-entropy diagram of a typical top cycle and resulting exhaust heat loss. Top cycle engines operate by taking in air at state
1
, compressing it to state
2
, then heating in either an approximately constant pressure or constant volume process from state
2
to state
3
, and finally expanding it from state
3
to the exhaust at state
4
. Because FIG.
1
. is a temperature entropy diagram, the potential for generating mechanical work from the heat wasted in the top cycle exhaust is defined by the shaded area, A-
1
-
4
-A. A-
1
-
4
-A describes the difference between the heat available to the bottom cycle from the top cycle exhaust and the potential heat rejection to the surroundings by the bottom cycle. A bottom cycle engine must then be one that is capable of operating at maximum efficiency with a supply of heat of heat obtained by cooling the top cycle exhaust from state
4
to state
1
while rejecting its own heat at nearly the temperature of the surrounding environment, temperature
1
-A. The ideal engine is one that best fills that area; A-
1
-
4
-A.
FIG. 2
shows an attempt to use an ideal Stirling engine cycle as a bottom engine. The Stirling engine is a closed cycle engine that starts with a low pressure gas at state A, the temperature of the environment. The gas is compressed at constant temperature to a higher pressure at state B, heated in a regenerator from state B to state C, and expanded in an expander from state C to state D. It is then cooled back to the surrounding temperature, state D to state A, in the regenerator (by giving up the same heat used to warm it from state B to state C). The Stirling engine receives heat from the top cycle exhaust during the expansion process, state C to state D; and rejects it to the environment in the compression process of state A to state B.
Although it efficiently uses what heat it can extract from the top cycle, the Stirling engine is not efficient in obtaining that heat. First, the upper temperature of the cycle (temperature of states C and D) is limited by a heat balance across the expander heat exchanger. The enthalpy change in the top cycle exhaust in going from its state
4
to the temperature of state C, (H
4
−H
C
)
top
, is equal to the Stirling engine's upper temperature multiplied by the entropy change betw
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