Proe Power Afterburningtm Solid Biofuel Engine
US Patent
7,028,476 B2 and Pending
How it Works
Background
All
successful combustion engines operate on the same basic 3 step process. 1) Air intake and compression,
2 ) Air heating,
and 3) Air expansion.
In
the familiar automobile engine, those 3 steps take place in "4
stokes" of the piston: 1) The piston goes down and draws in
air through the intake valve. The intake
valve closes and the air is compressed as the piston goes up, 2) the air is heated at the top of the compression stroke by
internal combustion of gasoline drawn in with the air. The combustion further raises the pressure in
the cylinder and 3)
that pressure pushes on the piston to drive it down to turn the crank. Lastly, the exhaust valve opens and the
piston goes up to force out the air and combustion gases.
The 4 stroke automobile engine, and its close relative
the Diesel engine, are by far the most common engines in the world today. However, they have the major disadvantage of
needing either a gaseous or a liquid fuel.
Unfortunately, biofuels and agricultural waste mostly come in solid
form. The solids must first be converted
to a more conventional automotive type fuel by chemical conversion to a gas or
liquid. That requires the expense and
complexity of devices such as gasifiers, digesters, fermenters, etc.
Today we take liquid and gaseous fuels for granted. However, economic, political and
environmental factors now make them much less desirable. The late 19th century was a similar
time. Liquid fuels were rare, expensive
and were exclusively used for lighting.
Solid fuels such as coal and wood were used to boil water to make steam
for steam engines. Those steam engines
provided the power for ships, locomotives, machine shops, pumping water, and
eventually to make electric power. Those
engines were successful but required huge amounts of maintenance and attention
while also being very inefficient. At
the end of the steam era, and just before the internal combustion era, another
form of engine emerged that addressed the limitations of steam.
The new form of engine was the "air engine", an
engine that used the air itself, rather than steam, to make power. George Brayton and John Ericsson (Brayton
and Ericsson page ) were
two of the most well known pioneers of these engines and made successful
inroads into the steam market. The air
engines were simpler and more efficient.
However, the 4 stroke internal combustion engine was a more compact
engine for the newly created automobiles and the "air engine" soon
became a backwater of development.
Basic
Principles
Today,
the advantages of the "air engine" can no longer be ignored. Proe Power Systems has revived and improved
the Brayton/Ericsson engine to provide an extremely simple and straightforward
method for extracting mechanical and/or electric power from the most basic form
of fuels.
Just
like an automobile engine, the Proe Afterburningtm Engine operates
on the basic 3 steps: . 1) Air intake and compression, 2
) Air heating, and 3) Air expansion.
The Proe Afterburningtm Engine has two types
of piston/cylinders: Compressors and Expanders.
As the names suggest, each is dedicated to the sole function of
accomplishing the process steps of 1) Air intake and compression
and 3) Air expansion respectively. The volumes (displacement) of the cylinders
are different: the expanders have roughly twice the displacement of the
compressors. That volume ratio is what allows the engine to run.
The air cycles through the engine as follows: 1) the compressor piston goes down and draws air through the compressor
intake check valve. The compressor
piston then drives upwards, raising the pressure in the compressor and closing
the intake check valve. When the
compressor piston is about 2/3 through the upward stroke the pressure is
sufficient to force open the compressor exhaust check valve to force the compressed
air out. 2) The compressed air passes through an
Air Heater where it is indirectly heated by heat transfer from the Combustor
flue gas. 3) The
heating expands the air so that the amount of air that fit in the cold
compressor is now capable of filling the expander volume (twice the size). A cam opens the expander inlet valve and
allows the hot pressurized air to fill the first 1/3 of the expander
volume. The inlet valve closes and the
air expands to fill the remaining expander volume as the piston descends to
bottom dead center. Because the
work in compressing or expanding air is proportional to pressure x volume,
heating the air allowed it to produce twice as much work in expanding as was
needed to compress it. That difference
is what allows the engine to perform external work.
When the expander piston reaches bottom dead center, the
expander exhaust valve opens so the air in the expander can be expelled. The air is still at a fairly high temperature
(~900oF/480oC) and is used to provide a high temperature
air blast to the Combustor furnace (just like a blacksmith's bellows only at a
higher temperature and therefore more efficient). The high temperature air blast "fans the
flames" while also recovering a significant portion of the heat from the
engine cycle. The hot flue gases then
pass through the Air Heater to heat the incoming compressed air charge.
The flue gases leave the Air Heater at a temperature of
~500oF/260oC and can be used in a fuel dryer, to heat
water, to heat air, or to run an absorption cycle chiller for air
conditioning. In the latter case, the
chilled air can further increase the engine efficiency by cooling the incoming
air stream.
Component
Principles
The engine is extremely simple, reflecting its 19th
century rootstock. All the pieces are
low tech and based on existing compressor, automotive, and heat exchanger
technology. The following highlights
each component.
Compressor:
The Compressor
operates exactly like a mechanically driven bicycle pump or a standard shop air
compressor. Air is drawn in through a
check valve as the piston descends and is then expelled through an exhaust
check valve as the piston rises. There
are no cams and no special timing needed.
When the engine is at operating temperature the pressure is determined
by the ratio of compressor and expander volumes and the temperature at the
inlet to the expander. The compressor
pressurizes the air to ~3.3 atmospheres gage pressure while heating the air,
due to compression heating, to 330oF/166oC.
A butterfly valve in the compressor inlet can be used to
throttle the engine so that the RPM is maintained over a range of engine loads.
Air
Heater:
The
Air Heater is a bank of simple tubular, counterflow heat exchangers in
parallel. In each tube assembly,
compressed air from the compressor travels through an annulus bounded by a tube
on the outside and a tube containing hot flue gas from the combustor on the
inside. As the compressed air from the
compressor passes through, it is heated to about 1450oF/790oC
while the flue gas is cooled from about 1500oF/816oC to
500oF/260oC.
Expander:
Hot,
compressed air from the Air Heater is put to work in the Expander. The expander works much like a steam engine
cylinder except it works with clean air instead of steam. The hot compressed air is admitted through a
cam operated intake valve and pushes the piston down about 1/3 of the way. The intake valve then closes, and the air
continues to expand, and continues to push down the piston, until the air in
the cylinder is reduced to near atmospheric pressure. A cam opens the exhaust valve, and the piston
rises to exhaust the air.
The
expander construction is almost
identical to an automotive cylinder. The
major difference is that the expander is a "two stroke" cylinder and
the valves operate on every crank revolution, rather than every other
revolution as in a 4 stroke automotive engine.
Combustor:
When
the piston in the expander reaches the bottom of its stroke, the exhaust valve
opens and the expanded air is forced out as the piston rises. The air is still at about 900oF
and it makes a very effective blast when it is pushed into the Combustor. The blast works just like a blacksmith's
bellows, but being 900oF instead of room temperature, is much more
efficient.
Chipped
fuel is deposited in the Fuel Hopper where it is metered into the Combustor
through airlocks and augers. Ash is
similarly removed by an ash auger that conveys the ash to an Ash Bin.
The
solid fuel combustor that we will use is essentially the same, including the
fuel and ash handling equipment, that is in increasing use as a biofuel heat
source for air and water heating and for steam generation. Our major changes will be to provide it with
the hot air blast, instead of room temperature air, and to alter the air flow
control to match the engine requirements.
Acknowledgement:
Thank you to Matt Keveney of Animated Engines: http://www.animatedengines.com/index.shtml
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