1. Field of the Invention
This invention relates to internal combustion engines, and more specifically to thermally regenerated internal combustion engines.
2. Description of Prior Art
Conventional Otto and Diesel cycle piston engines operate by intaking fresh air, compressing the air, burning fuel with the air to produce high temperature, high pressure combustion products, expanding the combustion products to convert part of the heat into work, and then exhausting the spent gases. These engine cycles are relatively inefficient in converting heat into work because of the practical limitations on the extent of the compression and expansion processes. In order to operate at the highest efficiency which is theoretically possible, the hot product gases would need to be expanded until they had cooled to room temperature. However, with the thermal properties of ordinary combustion products, this would require an expansion volume ratio in excess of one thousand. Those skilled in the art will be aware that an engine with a volume expansion ratio of one thousand and a final gas pressure at or near atmospheric would require a peak pressure so high as to threaten destruction of cylinder, piston and other moving parts, and would aggravate losses from friction, heat transfer, and gas leakage.
In the past, attempts have been made to recover the residual heat energy in the exhaust gas from internal combustion engines by passing the gas through a gas turbine, but such turbines have pressure ratios far too low to convert much of the exhaust heat into work. Internal combustion engine exhaust gases have also been used as the heat source for a Rankine (steam engine) bottoming cycle, but this introduces undesirable complexity, cost and size into the engine design.
A very promising technique for substantially increasing the thermal efficiency of an internal combustion piston engine is through thermal regeneration. Thermal regeneration, as used herein, implies the capture of exhaust gas heat from one engine cycle and the transfer of this heat to the working fluid of the subsequent cycle following its compression, but prior to the combustion of the fuel, so as to reduce the required quantity of fuel to be burned. A number of attempts have been made to devise means by which regenerative features similar to those employed in a Stirling or Ericsson type engine could be used to accomplish this. Most notable of these techniques are those of Hirsch (1874, U.S. Pat. No. 155,087), Martinka (1937, U.S. Pat. No. 2,239,922), Pattas (1973, U.S. Pat. No. 3,777,718), Bland (1975, U.S. Pat. No. 3,871,179), Pfefferle (1975, U.S. Pat. No. 3,923,011), Cowans (1977, U.S. Pat. No. 4,004,421), and Stockton (1978, U.S. Pat. No. 4,074,533).
The Hirsch, Martinka, Pattas and Stockton engines all employ multiple cylinders and pistons for each working unit to accomplish the regenerative engine cycle. The Cowans engine uses a single cylinder shared by the working piston and a displacer, but uses an external regenerator. All of these prior art techniques involve great mechanical complexity in the form of additional valves, pistons, heat exchangers or regenerators, flow passages, mechanical linkages, etc. In the cases of Hirsch, Pattas, and Cowans, the mechanical features of the engines preclude a compression ratio high enough to develop adequate specific power output from a normally aspirated engine. For this reason, the engines of Hirsch, Martinka, and Cowans were designed to be run at an elevated mean pressure, which requires external compressors to compress the inlet air. The Pattas engine, and the two-piston version of the Stockton engine require external blowers to scavenge the exhaust and provide fresh air for combustion.
Analyses of regenerative engine cycles show that the idealized thermodynamic thermal efficiency increases when the compression ratio is decreased, however the cycle mean effective pressure and specific power output decreases as the compression ratio is decreased. Since an engine with low cycle effective pressure must be larger for a given power output, and since heat conduction losses and friction losses increase with engine size, an optimum design must have an intermediate value for compression ratio, i.e. it must be low enough to give acceptable thermodynamic efficiency, but high enough to give low heat conduction and friction losses. For most applications a compression ratio between four and eight appears to be the best compromise. The Hirsch, Pattas, and Cowans engines are incapable of attaining a compression ratio high enough to fall within the optimum range. The reason that these prior art engines cannot attain sufficiently high compression ratios is that non-optimum, near-sinusoidal motions were employed for the displacers (or for second pistons in two-piston type engines) and in all cases the regenerator was in a fixed position relative to the power piston. In engines of the Hirsch and Pattas types, for instance, the pistons undergo sinusoidal motions with approximately a ninety degree phase difference, and the piston motions cannot overlap since they are separated by a fixed regenerator. In this situation, simple geometric arguments show that the maximum volume ratio cannot be greater than about six, even with a regenerator of zero volume, and if real hardware limitations are taken into account, the resulting compression ratio is far below the optimum value.
The engine disclosed herein uses a basic working unit which consists of a single cylinder and piston and a thin, movable, permeable regenerator which can be swept axially through the working fluid in the space above the piston, and whose motion can overlap the motion of the piston. This is much simpler than any of the prior art engines. The thinness of the regenerator, and the possibility of having the regenerator motions overlap the piston motions makes it possible to have a higher compression ratio than the prior art engines. The regenerator of the disclosed engine is moved by a mechanism such as a cam which can produce the non-sinusoidal regenerator motions which are required to approach the thermodynamically optimum sequence of events. The use of optimum regenerator motions can increase the specific power output of the engine by as much as a factor of five compared to an engine which uses only sinusoidal piston or displacer motions. In contrast to the prior art engines, certain embodiments of the engine herein disclosed can intake air and exhaust spent gases without requiring any assistance from a blower or supercharger. Supercharging may be used with the disclosed engine in order to increase power output, or to provide a lower regenerator cold end temperature, or for other reasons, but it is not necessarily required. The features of the disclosed engine result in a specific power output which is much higher than prior art regenerated internal combustion engines, this in turn permits the use of a smaller engine for a given power output, which results in friction and conduction heat losses which are much lower than in prior art engines.
To summarize, the advantages of the disclosed engine relative to prior art engines include its mechanical simplicity, its high thermal efficiency and power output, its ability to operate without auxiliary superchargers or blowers, and the close similarity of many of the disclosed engine components to those of existing diesel or gasoline engine technology.