Proe Power Afterburningtm Solid Biofuel Engine
US Patent 7,028,476 B2 and Pending
How it Works
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.
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.
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.
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.
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.
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.
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.
Thank you to Matt Keveney of Animated Engines: http://www.animatedengines.com/index.shtml