14-12-2012, 04:03 PM
A Six-Stroke, High-Efficiency Quasiturbine Concept Engine With Distinct, Thermally-Insulated Compression and Expansion Components
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Abstract:
One of the most difficult challenges in engine technology today is the urgent
need to increase engine thermal efficiency. This paper presents a Quasiturbine thermal
management strategy in the development of high-efficiency engines for the 21st century.
In the concept engine, high-octane fuels are preferred because higher engine
efficiencies can be attained with these fuels. Higher efficiencies mean less fuel
consumption and lower atmospheric emissions per unit of work produced by the engine.
While the concept engine only takes a step closer to the efficiency principles of Beau de
Rochas (Otto), it is readily feasible and constitutes the most efficient alternative to the
ideal efficiencies awaiting the development of the Quasiturbine photo-detonation engine,
in which compression pressure and rapidity of ignition are maximized.
INTRODUCTION
One of the most difficult challenges in engine technology today is the urgent need
to increase engine thermal efficiency. Thermal management strategies and the choice of
fuels will play crucial roles in the development of high-efficiency engines for the 21st
century. However, it was during the 19th century that the fundamental principles
governing the efficiency of internal combustion engines were first posited.
In 1862, Alphonse Beau de Rochas published his theory regarding the ideal
operating cycle of the internal combustion engine. He stated that the conditions necessary
for maximum efficiency were: (1) maximum cylinder volume with minimum cooling
surface; (2) maximum rapidity of expansion; (3) maximum pressure of the ignited charge
and (4) maximum ratio of expansion. Beau de Rochas' engine theory was first applied by
Nikolaus Otto in 1876 to a four-stroke engine of Otto's own design. The four-stroke
combustion cycle later became known as the "Otto cycle". In the Otto cycle, the piston
descends on the intake stroke, during which the inlet valve is held open. The valves in the
cylinder head are usually of the poppet type. The fresh fuel/air charge is inducted into the
cylinder by the partial vacuum created by the descent of the piston. The piston then
ascends on the compression stroke with both valves closed and the charge is ignited by an
electric spark as the end of the stroke is approached. The power stroke follows, with both
valves still closed and gas pressure acting on the piston crown because of the expansion
of the burned charge. The exhaust stroke then completes the cycle with the ascending
piston forcing the spent products of combustion past the open exhaust valve. The cycle
then repeats itself. Each Otto cycle thereby requires four strokes of the piston- intake,
compression, power and exhaust- and two revolutions of the crankshaft.
Maximum Volume / Minimum Cooling Surface
The first Beau de Rochas principle teaches that the engine should have a
minimum cooling surface area while still allowing for maximum charge volume during
intake ("volumetric charge efficiency"). Otto cycle engines generally have cooling
systems.1 The cooling system represents an engineering compromise. Without a cooling
system, the pre-mixed fuel/air charge could prematurely ignite (or "knock") during the
compression stroke, especially with low-octane fuels like gasoline. Knock reduces the
engine’s power because the pressure of the combustion event is not properly
synchronized with the engine’s power stroke. Knock can also seriously damage engine
parts. A cooling system also serves to maximize volumetric charge efficiency by
reducing the temperature of the charge during intake.
Targeting high-efficiency, the proposed concept engine eliminates the engine
cooling system. Instead, cooling of the inducted fuel/air charge is achieved through the
use of methanol, a liquid with a high latent heat of vaporization, which is injected into the
intake port (port fuel injection or "PFI") during the intake stroke.2 The compressor can
then be thermally insulated in order to minimize the cooling surface, while still
maintaining volumetric charge efficiency. Similarly, the expander can be thermally
insulated to minimize the cooling surface and to maximize the pressure of the combusted
gases during the power stroke, as discussed below.
Maximum Rapidity of Expansion
Rapidity of expansion in a spark-ignition engine can be achieved by increasing
the engine's compression ratio. A higher compression ratio brings the fuel and oxygen
molecules in closer proximity during ignition and facilitates rapid expansion. In order to
increase engine compression ratio, a high-octane fuel is used. A high-octane fuel is a fuel
that has a high autoignition temperature in air. Because the fuel/air mixture is heated
during the engine's compression stroke (especially in the thermally insulated compressor
cylinder of the concept engine), it is critical to avoid premature ignition or knock during
that stroke. With high-octane fuels, such as methanol, premature ignition can be
prevented while still increasing the engine's compression ratio.
Maximum Pressure of the Ignited Charge
The pressure of the ignited charge is subject to several conditions: the
compression pressure of the fuel/air charge prior to ignition, the ratio of fuel to air in the
charge itself and the temperature of the combusted gases after ignition. While ideal,
maximum pressure cannot be achieved in the concept engine4, the concept engine does
improve on the Otto cycle engine by eliminating the cooling system and by allowing high
compression pressures with high-octane fuels. The Otto cycle cooling system reduces
pressure both during the compression stroke and during the expansion stroke. By using
thermal insulation for both the compression function and for the expansion function and
by using a near stoichiometric ratio of high-octane fuel and air, the concept engine takes a
significant step closer to Beau de Rochas ideal cycle efficiency.
Maximum Expansion
The fourth Beau de Rochas efficiency principle teaches that the expansion volume
of the combusted fuel/air charge should be maximized. In Otto cycle engines, the
compression volume and the expansion volume are equal because the cylinder
volume swept by the piston is the same for both the compression stroke and for the power
stroke. For maximum efficiency, the expansion volume should always exceed the
compression volume. The constant-volume Atkinson cycle has this characteristic.
The Atkinson cycle engine is a type of internal combustion engine invented by
James Atkinson in 1882. The Atkinson cycle is designed to provide efficiency at the
expense of power. The Atkinson cycle allows the intake, compression, power and exhaust
strokes of the four-stroke cycle to occur in a single turn of the crankshaft. Because of the
engine’s novel linkage, the expansion ratio is greater than the compression ratio, which
results in greater efficiency than a comparable engine operating in the Otto cycle.5
Engine Components
There are four principal engine components necessary to perform the engine's
three functions. The first component is a thermally-insulated, piston-type air compressor
The air compressor shares a common shaft (or is linked by a belt drive or chain
drive) with the Quasiturbine expander. The expander provides the necessary power for
compression work. The second component is a Holzwarth combustion chamber, which is
described in more detail below. The third component is the Quasiturbine expander, which
is also comprised of thermally insulating materials. The fourth component is a
compressed fuel/air line, which delivers the fuel/air charge under pressure to the
combustion chambers. The engine is a six-stroke engine. The six strokes occur during
each 90 degrees of shaft rotation. The six strokes are: one intake stroke, one compression
stroke, two power strokes and two exhaust strokes. A special linkage, not unlike the
Atkinson engine linkage, allows the compressor to complete eight strokes (four intake
strokes and four compression strokes) during each 360 degrees of shaft rotation, which
results in one complete compression cycle over each 90 degrees.
Piston-type Compressor
The concept engine's compressor is a thermally-insulated, positive-displacement,
piston-type compressor. The piston crown is a "pancake" or "flat aspect" crown. The
compressor shares a common shaft with the Quasiturbine expander. See, Figure 1. The
maximum temperature in the compressor is limited by the temperature tolerance of the
oil-free piston ring lubricant and by the autoignition temperature of the fuel. The
compressor temperature, however, can be moderated by port injection of a liquid with a
high latent heat of vaporization. The liquid in this case is the methanol fuel itself. The
operation of the compressor must be considered over 90 degrees of shaft rotation, which
represents one complete compression cycle.
Compressed Fuel/Air Line
The compressed fuel/air line interconnects the compressor with the combustion
chambers of the concept engine. The compressed fuel/air line is thermally insulated. At
one end of the compressed fuel/air line, there is a connection with the compressor's outlet
port. The compressed fuel/air charge enters the line at that point. See, Figure 2. At the
other end of the compressed fuel/air line, the line splits into four separate "feeder" lines.
Each feeder line connects with the compressed fuel/air inlet valve of one of the four
combustion chambers. The purpose of the compressed fuel/air line is to convey the
compressed fuel/air charge from the compressor to the combustion chambers with a
minimum of heat and pressure loss.
Holzwarth Combustion Chambers
There are a total of four combustion chambers in the concept engine design. Each
combustion chamber has two valves: a compressed fuel/air inlet valve and a combusted
gas outlet valve. The fuel/air charge is ignited by spark ignition. The combustion
chambers are "Holzwarth-type" combustion chambers.
In 1905 Hans Holzwarth of Germany began a long series of experiments with
respect to the "explosion" turbine. His turbine consisted of a constant-volume combustion
chamber into which a charge of fuel and air was introduced under pressure. Following
ignition, the pressure was increased to about 4 1/2 times the original value. The pressure
increase caused a spring-loaded valve to open, admitting gases to a nozzle directed
against the blades of the turbine. The engine was arranged so that the valve remained
open until the combustion chamber pressure equalized with atmospheric pressure, after
which the valve would close and a new fuel/air charge was introduced. Although an air
compressor was employed in the Holzwarth turbine, the efficiency of the compressor was
not extremely important because the air could be supplied at a pressure only about onefourth
that ultimately achieved during combustion and also because only enough air was
supplied to furnish the oxygen necessary for combustion. The concept engine follows the
Holzwarth combustion chamber model.