High-efficiency two-stroke internal combustion engine

Abstract

A two-stroke internal combustion engine achieves high performance levels by using an innovatively timed sequence of injecting and igniting fuel and oxidant. The operating cycle of the engine does not utilize a compression process. This permits the injection of fuel and oxidant to be coordinated with the initiation of the combustion process in such a way that the engine achieves high efficiency and provides high torque, while at the same time producing low thermal loading of engine components and low levels of engine noise and vibration.

Claims

1. A two-stroke internal combustion engine comprising: a) a multiplicity of hollow circular cylinders, with each of said cylinders having a central longitudinal axis, and with each of said cylinders having one closed end and one open end, and with each of said cylinders having, in its closed end, fuel and oxidant inlet valves and an exhaust valve, and with each of said valves being completely recessed relative to the inner surface of the closed end of said cylinder for open and closed configurations of said valves; b) a multiplicity of moveable circular pistons, with each of said pistons being situated coaxially within one of said cylinders, and with each of said pistons being mechanically constrained to move in a reciprocating manner within the associated cylinder, and with the mechanical contour of the front surface of each of said pistons matching exactly the mechanical contour of the inner surface of the closed end of the associated cylinder; c) a crankshaft, with said crankshaft having a central longitudinal axis which is intersected by, and is perpendicular to, the central longitudinal axes of said cylinders, and with said crankshaft being capable of rotational motion about its central longitudinal axis, and with said crankshaft having a multiplicity of crank arms, and with each of said crank arms being mechanically linked to one of said pistons by a connecting rod, and with the axial length of each of said pistons being such that for any set of engine operating conditions, when said piston is at its top-dead-center position, a small gap exists between the front surface of said piston and the inner surface of the closed end of the associated cylinder, and with the thickness of said gap being such that for any set of engine operating conditions, when said piston is at its top dead center position, the internal volume of the associated cylinder is less than 2 percent of the internal volume of said cylinder when said piston is at its bottom-dead-center position; d) a control system, with said control system effecting a sequenced series of actions for each of said cylinders, with said series of actions comprising: i) opening said cylinder's exhaust valve when said cylinder's associated piston reaches its bottom-dead-center position, thereby initiating said piston's exhaust stroke; and then ii) closing said cylinder's exhaust valve when said cylinder's associated piston reaches its top-dead-center position, thereby terminating said piston's exhaust stroke; and then iii) opening said cylinder's fuel and oxidant inlet valves at a time corresponding to the complete closure of said cylinder's exhaust valve, thereby initiating said piston's power stroke and enabling said cylinder to accept high pressure fuel and oxidant gases as said cylinder's internal volume is increasing; and then iv) closing said cylinder's fuel and oxidant inlet valves as soon as required load-specific quantities of fuel and oxidant have entered said cylinder; and then v) initiating the combustion of fuel and oxidant gases within said cylinder when said crankshaft reaches a specific, pre-determined crank angle; and then vi) opening said cylinder's exhaust valve when said cylinder's associated piston reaches its bottom-dead-center position, thereby terminating said piston's power stroke and initiating its next exhaust stroke.

2. An engine as described in claim 1 wherein, for each of said cylinders, said cylinder's exhaust valve remains open for the entire duration of the associated piston's exhaust stroke, thereby allowing gases within said cylinder to pass into an exhaust manifold for the entire duration of the associated piston's exhaust stroke.

3. An engine as described in claim 2 wherein each of said engine's cylinders accepts oxidant in the form of high-pressure oxygen gas and fuel in the form of high-pressure hydrogen gas while said cylinder's internal volume is increasing.

4. An engine as described in claim 2 wherein each of said engine's cylinders accepts oxidant in the form of high-pressure air and fuel in the form of high-pressure hydrogen gas while said cylinder's internal volume is increasing.

5. An engine as described in claim 2 wherein each of said engine's cylinders accepts oxidant in the form of high-pressure oxygen gas and fuel in the form of high-pressure natural gas while said cylinder's internal volume is increasing.

6. An engine as described in claim 2 wherein each of said engine's cylinders accepts oxidant in the form of high-pressure air and fuel in the form of high-pressure natural gas while said cylinder's internal volume is increasing.

7. An engine as described in claim 2 wherein each of said engine's cylinders accepts oxidant in the form of high-pressure oxygen gas and fuel in the form of ammonia vapor while said cylinder's internal volume is increasing.

8. An engine as described in claim 2 wherein each of said engine's cylinders accepts oxidant in the form of high-pressure air and fuel in the form of ammonia vapor while said cylinder's internal volume is increasing.

9. An engine as described in claim 2 wherein each of said engine's cylinders accepts a mixture of high-pressure gaseous oxidant and high-pressure gaseous fuel while said cylinder's internal volume is increasing.

10. An engine as described in claim 2 wherein said engine's exhaust manifold is maintained at sub-atmospheric pressure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a piston situated coaxially within a cylinder, with the piston linked to a crankshaft by a connecting rod.

(2) FIG. 2 is a plot, for the engine revealed herein, of expansion ratio of the combustion products as a function of crank ignition angle for two values of rod ratio.

(3) FIG. 3 shows the thermodynamic operating cycle of the engine revealed herein, with the operating cycle presented in terms of the pressure of gasses within a cylinder and the internal volume of the cylinder.

(4) FIG. 4 shows the thermodynamic operating cycle typical of a two-stroke diesel engine, with the operating cycle presented in terms of the pressure of gasses within a cylinder and the internal volume of the cylinder.

DETAILED DISCUSSION OF THE INVENTION

(5) This specification uses terms which have a technical meaning that may differ from the meaning assumed in everyday usage. The following paragraphs contain definitions and explanations of various terms and concepts with regard to the meaning intended herein.

(6) The term cylinder is used herein to refer to a hollow circular cylinder having a central longitudinal axis of symmetry, and also having one open end and one closed end which is immovable, and also having inlet and exhaust ports in its closed end. The engine disclosed herein has a multiplicity of such cylinders which serve as combustion chambers for fuel and oxidant gasses. The term valve is used herein to refer to a mechanical or electro-mechanical device which controls the flow of gasses through an inlet or an exhaust port. Inlet valves control the flow of fuel and oxidant passing into a cylinder and exhaust valves control the flow of combustion products passing into an exhaust manifold. It is to be understood that modern engine valves may have multiple orifices that open and close in different combinations to accurately regulate gas flows.

(7) The term piston is used herein to refer to one of a multiplicity of circularly cylindrical objects, each of which is coaxially situated within one of the aforementioned cylinders. Each piston is constrained to move in a reciprocating fashion within its associated cylinder and as it moves, a piston provides a movable closed end for its associated cylinder. The term front surface is used herein to refer to the surface of a piston which faces towards, and is closest to, the immovable closed end of said piston's associated cylinder. The term back surface is used herein to refer to the surface of a piston which faces away from, and is farthest from, the immovable closed end of said piston's associated cylinder.

(8) The terms internal volume and internal volume of a cylinder are used interchangeably herein to refer to the volume which is enclosed by the following three surfaces: the inner surface of the closed end of a cylinder, the inner surface of the circular wall of that same cylinder, and the front surface of the associated piston. The internal volume of a cylinder varies continuously and cyclically from a maximum value to a minimum value and then back to a maximum value as its associated piston executes its reciprocating motion. The time-varying internal volume of a cylinder is represented herein by the symbol V.

(9) The term wrist pin joint is used herein to designate a smoothly rotating joint which is attached, either directly or indirectly, to the back surface of a piston for the purpose of joining one end of a connecting rod (explained later) to said piston. The term crank pin joint is used herein to refer to a smoothly rotating joint which joins one end of a connecting rod to a crankshaft (explained in the following paragraph).

(10) The term crankshaft is used herein to designate an elongated shaft which is free to rotate about its central longitudinal axis. A crankshaft has a multiplicity of crank arms (one for each cylinder/piston assembly), with each crank arm having a crank pin joint that allows for the eccentric attachment of a connecting rod to the crankshaft.

(11) The term connecting rod is used herein to designate a rigid mechanical member which is attached at one end to a piston's wrist pin joint, and is attached at the other end to the crank pin joint of one of the crank arms on the crankshaft. A connecting rod provides an eccentric mechanical linkage between a piston and the crankshaft. A connecting rod provides a means of transferring forces from a piston to the crankshaft and vice versa.

(12) The terms useful work and useful work done by an engine are used interchangeably herein to refer to the work done by forces transferred from a piston to the crankshaft while the internal volume of the piston's associated cylinder is increasing. The terms internal work and internal work done by an engine are used interchangeably herein to refer to the work done by forces transferred from the crankshaft to a piston while the internal volume of the piston's associated cylinder is decreasing. The term net useful work is used herein to refer to the difference between the useful work done by an engine and the internal work done by that same engine. The term load is used herein to refer to the amount of net useful work which an engine must do to accomplish a specific task. An engine responds to an increased or decreased load by allowing a greater or lesser amount of fuel and oxidant into its cylinders. The amount of fuel and oxidant required to accomplish a specific amount of net useful work is referred to herein as a load-specific quantity of fuel and oxidant.

(13) The term operating cycle is used herein to designate an ordered sequence of thermodynamic processes which occur repetitively within an engine. For reciprocating internal combustion engines in general, and for the presently proposed engine in particular, the operating cycle is carried out repetitively within each of the engine's individual cylinders.

(14) In this specification, combustion processes and compression processes are of importance in discussing operating cycles. The term combustion process is used herein to refer to the chemical reaction of a fuel with an oxidant, with the reaction producing heat and new chemical compounds (combustion products). All internal combustion engines, by definition, utilize a combustion process. In general, the term compression process refers to the mechanical application of force to a confined fluid, with a resultant increase in pressure and a decrease in the volume of the confined fluid. The term compression energy is used herein to refer to the energy used to carry out a compression process. In this specification, when discussing engine operating cycles, the term compression process refers to the application of force exerted by a piston on fuel and/or oxidant gasses that are confined within a cylinder, with a resultant increase in the pressure and a decrease in the volume of the confined gasses. The compression processes referred to herein are approximately adiabatic, so there is also an increase in temperature of the confined gasses as the compression process is carried out. For an internal combustion engine, internal work is done by the engine during a compression process. All internal combustion engines currently in service do a significant amount of internal work because their operating cycles include a compression process.

(15) This invention reveals a high-efficiency two-stroke internal combustion engine whose operating cycle does not include a compression process. Instead, the engine operates with supplies of high pressure fuel and oxidant gasses that are provided from external reservoirs. The engine revealed herein efficiently converts a portion of the compression energy of high-pressure fuel and oxidant gasses, as well a portion of their chemical energy, into useful work.

(16) The disclosed engine is comprised of a multiplicity of cylinders, each with an associated piston which is constrained to move in a reciprocating fashion within its associated cylinder. Each cylinder has, in its closed end, inlet valves which regulate the flow of fuel and oxidant gasses coming into the cylinder and an exhaust valve which regulates the flow of combustion products passing out of the cylinder and into an exhaust manifold.

(17) Each of the engine's pistons is joined to a common crankshaft by a connecting rod. It should be noted that the angular orientation of the crankshaft continuously determines position of each piston within its associated cylinder. The crankshaft converts the reciprocating motion of the pistons into circular motion by means of the eccentric attachment of the connecting rods to the crankshaft. The length of each of the engine's connecting rods, as measured between the centers of its wrist pin joint and its crank pin joint attachments, is the same. That common length is designated herein by the symbol L. Also, the length of each of the crankshaft's crank arms, as measured between center of its crank pin joint and the crankshaft's central longitudinal axis, is the same. That common length is designated herein by the symbol R. The stroke length of any piston (one-way extent of the piston's reciprocating movement) is equal to 2R. The ratio of L to R is referred to herein as the rod ratio of the engine. It is designated herein by the symbol . The rod ratio is an important parameter in determining the amount of friction-induced wear of pistons and/or crosshead bearings. As will be seen later, the value of the rod ratio also affects the expansion ratio of combustion products and thus the achievable efficiency of an engine.

(18) For the purposes of this document, the term top-dead-center refers to the position of a piston which minimizes the internal volume of its associated cylinder. The internal volume of the cylinder at the top-dead-center position of the piston is designated herein by the symbol V.sub.T. For the presently revealed engine, V.sub.T is, by design, very close to zero.

(19) For the purposes of this document, the term bottom-dead-center refers to the position of a piston which maximizes the internal volume of its associated cylinder. The internal volume of the cylinder at the bottom-dead-center position of the piston is designated herein by the symbol V.sub.B. In general, V.sub.B is equal to V.sub.T plus the product of the cross-sectional area of the cylinder and 2R. For the engine revealed herein, since V.sub.T is approximately zero, V.sub.B is just equal to the product of the cross-sectional area of the cylinder and 2R.

(20) The term crank angle, represented herein by the symbol , is used to designate the angle between a crank arm of the crankshaft and the axis of the associated cylinder/piston assembly, with being referenced to zero when the piston is at its top-dead-center position. (It should be noted that, even though all of the pistons move together in a temporally synchronized manner, they do not all achieve a top-dead-center position simultaneously. Therefore, for any given physical orientation of the crankshaft, the value of is different for each individual piston, and is just equal to the angle through which the crankshaft has rotated since that particular piston was at its top-dead-center position.) The term rod angle, designated herein by the symbol , is used to designate the angle between a connecting rod and the axis of its associated piston/cylinder assembly, with referenced to zero degrees when the piston is at its top-dead-center position. The instantaneous torque provided by a piston is proportional to the sine of the sum of and . In particular, very little torque is provided by a piston acting on the crankshaft when said piston is near its top-dead-center position, that is, when both and are close to zero.

(21) The term power stroke is used herein to designate the movement of a piston from its top-dead-center position to its bottom-dead-center position. During a piston's power stroke, the crankshaft rotates through an angle of 180 degrees, that is, from =0 degrees to =180 degrees for a particular piston. Similarly, the term exhaust stroke is used herein to refer to the movement of a piston from its bottom-dead-center position to its top-dead-center position. During a piston's exhaust stroke, the crankshaft rotates through an angle of 180 degrees, that is, from =180 degrees to =360 degrees for that particular piston. (The crankshaft orientation corresponding to =360 degrees for a particular piston is identical to the orientation designated as =0 degrees for that same piston.)

(22) FIG. 1 is a line drawing which illustrates the interconnection of the engine components discussed in the previous paragraphs. FIG. 1 shows piston 101 positioned coaxially within cylinder 102. Piston 101 is joined to the crank arm of crankshaft 103 by connecting rod 104. FIG. 1 also shows the length L of the connecting rod, the length R of the crankshaft's crank arm, the measure of crank angle , the measure of rod angle , and the internal volume of the cylinder, V. In addition, FIG. 1 shows high-pressure fuel inlet valve 105, high-pressure oxidant inlet valve 106, exhaust valve 107 and exhaust manifold 108. The crank angle and the rod angle are both referenced to zero degrees when the piston is at its top-dead-center position, that is, when the connecting rod and the crank arm are aligned with the central longitudinal axis of the cylinder/piston assembly, which is designated by dotted line 109 in FIG. 1.

(23) The operating cycle of the presently revealed engine is now described in terms of the operation of a single cylinder/piston assembly. The starting point of the cycle is chosen as the time at which the piston approaches its bottom-dead-center position. At this point, the piston has just completed a power stroke and the cylinder contains expansion-cooled combustion products at relatively low pressure. As the piston approaches its bottom-dead-center position, the exhaust valve in the closed end of the associated cylinder opens and the pressure in the cylinder drops rapidly to near atmospheric pressure. The subsequent motion of the piston towards its top-dead-center position (exhaust stroke) forces the combustion products out of the cylinder and into the exhaust manifold. As was mentioned earlier, an important feature of this engine's design is that the internal volume of the cylinder is vanishingly small when the piston is at its top-dead-center position. This assures that, when the piston reaches its top-dead-center position, all of the combustion products have been expelled from the cylinder. When the piston reaches its top-dead-center position, the exhaust valve closes and the exhaust stroke is complete. It should be noted that, for the engine disclosed herein, negligible internal work is done by the engine during the exhaust stroke.

(24) After the exhaust valve is completely closed, the oxidant and fuel inlet valves open, creating an overpressure (pressure above atmospheric pressure) in the cylinder. This marks the beginning of the engine's power stroke. Forces produced by the overpressure drive the piston away from its top-dead-center position and a portion of the compression energy of the incoming high-pressure fuel and oxidant gasses is converted to useful work. After the required load-specific quantities of fuel and oxidant have entered the cylinder, the inlet valves close. The high-pressure gasses in the cylinder continue to expand and continue to do useful work until the crankshaft reaches a specific, predetermined crank angle at which ignition of the fuel/oxidant mix is initiated by an electric spark. The term crank ignition angle, represented by the symbol .sub.1, is used herein to refer to the crank angle at which ignition occurs. The internal volume of the cylinder at the time of ignition is referred to herein by the term ignition volume and it is designated by the symbol V.sub.i. After the ignition event has occurred, the hot combustion products expand and force the piston towards its bottom-dead-center position, where the internal volume of the cylinder is V.sub.B. As the piston approaches its bottom-dead-center position, the exhaust valve begins to open, marking the end of the power stroke and the beginning of another exhaust stroke.

(25) The expansion ratio for the combustion products, designated herein by the symbol , is given by
=V.sub.B(V.sub.i).sup.1
For the engine disclosed herein, V.sub.T, the internal volume of the cylinder at the top-dead-center position of the piston, is very small in relation to either V.sub.i or V.sub.B. V.sub.T may therefore be neglected in deriving a relationship between and engine design variables and .sub.i. The derivation uses geometrical relationships that are evident from FIG. 1. The result is
=2[1+cos .sub.i(.sup.2sin.sup.2 .sub.i).sup.1/2].sup.1
FIG. 2 shows a plot of versus .sub.i for two values of . For a given value of .sub.i, increases as increases. At small values of .sub.i, increases rapidly as .sub.i decreases. The plot shows that, once certain mechanical dimensions of the engine have been chosen, namely those dimensions which define , it is possible to obtain a desired expansion ratio for the combustion products merely by choosing an appropriate value for .sub.i. This is an extremely important design option that is only possible in engines of the type revealed herein, that is, in engines that do not utilize a compression process.

(26) FIG. 3 shows the operating cycle of the presently revealed engine in terms of the pressure, P, of gasses within one of the engine's cylinders, and the internal volume, V, of that cylinder. Segment 301 of the cycle begins with the opening of the exhaust valve as the piston approaches its bottom-dead-center position. The segment represents a nearly isochoric process wherein the pressure of the combustion products drops to near one atmosphere. Isobaric segment 302 represents the exhaust stroke for the engine, wherein the piston moves from its bottom-dead-center position to its top-dead-center position. During this stroke, negligible internal work is done by the engine because the pressure inside the cylinder is approximately equal to atmospheric pressure. The mechanical design of the engine is such that the piston's top-dead-center position produces a vanishingly small internal volume of the cylinder, as is shown in FIG. 3. Segment 303 represents the expansion of fuel and oxidant gasses into the cylinder after the piston has passed its top-dead-center position. During this segment, the pressure in the cylinder rises, the internal volume of the cylinder increases, and useful work is done by the engine as the fuel and oxidant gasses expand against the piston. Segment 304 represents the brief period of time between the closing of the inlet valves and ignition of the fuel/oxidant mix. The pressure drops slightly during this segment as the internal volume of the cylinder continues to increase. Useful work is done by the engine during this segment as the high-pressure fuel and oxidant gasses continue to expand against the piston. Segment 305 begins when the fuel/oxidant gasses are ignited and it continues until the combustion process is complete. During this segment, the pressure and temperature of the gasses in the cylinder increase dramatically and the engine does useful work as the hot combustion products expand against the piston. Segment 306 begins when the combustion process is complete. During this segment, the pressure and temperature of the combustion products decrease as they undergo adiabatic expansion. The majority of the useful work done by the engine is done during this segment. Segment 306 ends as the piston approaches bottom-dead-center and the exhaust valve begins to open. The area enclosed by the six segments is representative of the useful work performed by the engine during one complete operating cycle.

(27) FIG. 4 shows the operating cycle of a conventional two-stroke diesel engine, with the cycle again shown in terms of the pressure, P, of gasses in one of the engines cylinders, and the internal volume, V, of that cylinder. Segment 401, which begins as the piston approaches its bottom-dead-center position, shows the beginning of the piston's combined exhaust/compression stroke, when both the exhaust valve and an air inlet valve open. Pressure in the cylinder decreases slightly as high-pressure incoming air (supplied by a turbocharger or supercharger) forces combustion products out of the cylinder. After a pre-determined period of time, the exhaust and air inlet valves close. This marks the beginning of segment 402, wherein the pressure within the cylinder increases rapidly as the piston moves towards its top-dead-center position, compressing and heating the air trapped in the cylinder. It should be noted that, during this segment of the cycle, the engine does a considerable amount of internal work as the trapped air is compressed. The isobaric segment 403 in the diagram begins with the ignition of the diesel fuel and continues until combustion is complete. (In an ideal diesel engine, the fuel burns slowly and the internal pressure of the cylinder stays relatively constant as the fuel burns.) Segment 404 represents the post-combustion adiabatic expansion of the combustion products. Segment 404 is terminated by the opening of the exhaust and air inlet valves, which occurs as the piston approaches its bottom-dead-center position. The area enclosed by the four segments is representative of the net useful work done by the engine during one complete operating cycle.

(28) It is clear from FIG. 3 and FIG. 4 that the engine revealed herein is capable of higher efficiency than its diesel counterpart because of the greater expansion ratio of the combustion products. In addition, the engine provides high torque because ignition of the fuel/oxidant gasses occurs well after the pistons have passed their respective top-dead-center positions. Also, the engine has a low overall operating temperature and low peak cylinder pressures because the fuel and oxidant gasses are relatively cool prior to their ignition. Finally, the engine has reduced noise and vibration levels because ignition of fuel/oxidant gasses occurs after the pistons have achieved non-zero velocities, that is, after they have moved away from their top-dead-center positions. The engine has these improved performance characteristics because (1) it does not utilize a compression process as part of its operating cycle, and (2) its exhaust stroke ends, and its power stroke begins, when the cylinder's internal volume is very close to zero. The engine does useful work during the pistons' power strokes by utilizing both the chemical energy of the fuel and the compression energy of the high-pressure fuel and oxidant gasses.