US 3024591 A
Description (OCR text may contain errors)
March 13, 1962 A. J. EHRAT ETAL BOUNCE COMPENSATOR FOR FREE PISTON ENGINES Filed Deo. 25, 1958 United States Patent() 3,024,591 BOUNCE COMPENSATOR FOR FREE PISTON ENGINES Alfred J. Ehrat, Norwalk, and John B. Williams, Greenwich, Conn., assignors to American Machine & Foundry Company, a corporation of New Jersey Filed Dec. 23, 1958, Ser. No. 182,519 4 Claims. (Cl. 60-19) The invention relates to free piston engines and more particularly rto free piston engines in which the power output of the engine is delivered in the form of a hydraulic tluid under pressure to one or more hydraulic motors.
In engine systems of the above type, hydraulic pump plungers are customarily operatively connected to the power pistons and serve to impel fluid under high pressure to one or more hydraulic motors.
For the purpose, primarily, of smoothing out the pressure lluctuations due to the intermittent action of the reciprocating pump plungors, a high pressure accumulator is interposed between .the pump and the motor or motors and a low pressure accumulator is interposed in the return line between the motor or motors and the pump plungers.
In tree piston engines, means are provided for returning the pistons. This means, as a rule, consists of a low pressure accumulator whose fluid is conveyed during the compression strokes to the pump pistons for returning the pump and the power pistons to their inner position. -The low pressure accumulator thus provides the so-called' bounce pressure for the hydraulic pump plungers and lfor this purpose a closed hydraulic system has been used in which the pressure in the low pressure accumulator is utilized to provide the bounce force, the pressure in the low pressure accumulator being regulated to maintain a value suicicnt for producing the compression stroke.
The maintenance of the required bounce pressure does not present any serious ditiiculty when the engine operates always at constant load and when the pressures in the system are maintained at generally constant values. These conditions are readily attained when the engine is controlled in such a way that it starts and stops in response to accumulator pressure, thus `maintaining nearly uniform pressure regardless of motor output fluctuation.
In orderto utilize constant hydraulic pressure conditions, however, it is necessary to control motor output by changing the volumetric displacement of the motors. Formany installations this presents great diculties.
Lf it were attempted to use constant displacement motors, or motors having inadequate displacement variability, in a system having varyingv accumulator pressure,
. the bounce pressure supplied by the low pressure accumulator would be found to be unsuitable 'for providing the bounce forec requirements. Thus, for example, if the `motor torque and the power impulse lare simultaneously increased to'mect av demand for increased power output, the pressure in the low pressure accumulator will decrease on account of the greater pressure drop across the motors. Consequently, the available bounce pressure is reduced, inadequate compression is attained andy power output is seriously impaired.
The present invention overcomes the inadequacy of hydraulic bounce by introducing a resilient element, such as a auxiliary accumulator, in the hydraulic line between the pump plungers and the check valves which control the flow to and from the high and low pressure accumulators. 4The auxiliary accumulator may take any one of several forms such as arelatively small hydropneumatic 3,024,591 Patented Mar. 13, 1962 ICC pressure uctuation incident to the reciprocation of the plungers. Of these instrumentalitcs the last named, which depends for its functioning only on the elastic compressibility of the hydraulic fluid and, to a lesser degree,
the elastic expansion of the container, is preferred on account of simplicity and freedom from the fatigue failure properties of rapidly oscillated membranes or springs.
1 Since the volume fluctuation required is relatively small,
auxiliary accumulator, a spring-loaded piston in'a cylthis safe and uncomplicated expedient may be applied without objectionable bulk and weight.
Since the maximum pressure in the compensating vessel, or the auxiliary accumulator, increases with increased motor torque and piston impulse, it provides a bounce impulse which varies oppositely to the adverse Variation of the pressure in the low pressure accumulator. Thus, with suitable proportioning of pump displacement and the auxiliary accumulator volume, a corrective impulse may be obtained on every stroke capable of substantially neutralizing the adverse response to load iluctuation of the lowprcssure accumulator bounce action, Operation of the free piston engine with a closed hydraulic transmission system is thus made practicable with load uctuations over the widest range that may be required. In order .to obtain optimum compensation, the volumes and the elastic properties of the two accumulators should be l properly proportioned to provide the desired compensation of the bounce pressure.
It is, therefore, an object of the invention to provide a free piston engine with hydraulic power transmission capable of satisfactory operation with variable load.
Another object of the invention is to provide a hydraulic .transmission system for a free piston engine which accommodates itself to varying load without recourse to complicated regulatory instrumentalities.
Still another object is the provision of a resilient compensatorv of simplicity and durability for use in the hydraulic transmission of a free piston engine.
It is also an object of the invention to provide a free piston engine having two high pressure accumulators and a low pressure accumulator, the power stroke of the engine subjecting the working tluid in the two high pressure accumulators to a high pressure, one of the high pressure accumulators supplying the hydraulic motive power to a hydraulic motor for producing useful work, while the second high pressure accumulator and the low pressure accumulator, in combination, supply an approximately constant bounce pressure for producing a compression stroke in the engine, this stroke being substantially independent of the variable load conditions imposed upon lthe hydraulic motor.
It is an additional object of this invention to provide a free piston engine utilizing a high bounce pressure accumulator and a low bounce pressure accumulator for producing a compression stroke in the engine which is reasonably independent of the variation in an external load connected to the engine.
These and other objects will become more apparent from the following description and from the accompanying drawing illustrating a simple embodiment thereof, in y which:
'Ihe only FIGURE illustrates a diagrammatic longitudinal cross-section of the engine.
Referring'to the only FIGURE in the drawing, it illustrates, in diagrammatic longitudinal cross-section, a free piston engine with hydraulic transmission arranged in accordance with one embodiment of the invention.
The engine, generally designated by 10, is of the twocycle diesel type.v The power pistons 22 are arranged to slide in the cylinder 20 in which ports 34 and 36 are provided for the admission of air and for the discharge of exhaust gas, respectively. The scavenging pistons 48, sliding in the enlarged portions of the cylinder 20, in
conjunction with the check valves 45 and 46 supply air to the inlet ports 34 through ducts 11 and 12. lntegral with the pistons 22 and 48 are the pump plungers 32 slidable in cylinders 28.
The fluid-carrying duct 58 connects the two pump cylinders 28. Duct 58 is connected through check valve 60 and duct 61, accumulator 50 and duct 65 to a hydraulic motor 78 of any suitable known construction.
Duct 64 connects motor 78 to a low pressure accumulator 98 and to duct 62. Duct 62 is connected through a check valve 76 to duct 58.
A resilient compensator 63 is connected to duct 58. Compensator, or the auxiliary accumulator, 63 may take the form of a simple receptacle, of spherical or other convenient form, lled with the working liquid.
A tube 38 is connected to a fuel injection device (not shown) which serves to spray fuel into the combustion chamber 13 when the pistons 22 are near the inner ends of their strokes, in accordance with the usual manner of diesel engine operation. A hydraulic motor 78 is connected to a shaft 79 which drives a vehicle, machine or other external load 99. Motor 78 is connected through a duct 65 to the high pressure accumulator 50 on the high pressure side and to the low pressure accumulator on the low side through a duct 64. Motor 78 may be a constant displacement motor which is never shut off from lines 65 and 64. Shaft 79, therefore, may rotate continuously at a speed corresponding to the operating frequency of the free piston engine 10.
In the position shown, pistons 22 have reached the outer limits of their travel at the end of the power stroke and have started back on their compression strokes under the influence of the bounce pressure exerted on the pump plungers 32 by the working uid contained in the auxiliary accumulator 63 and the low pressure accumulator 98.
Pressure in the passage 58 and accumulator 63 has been raised to that of the high pressure accumulator 50 by the outward strokes of the plungers 32 and has begun to diminish because of the inward travel of the pistons and cylinders 28. Since, during the power stroke, the accumulators 50 and 63 are connected in parallel, with the check valve 60 wide open, the working Huid attains the same equal maximum pressure in both accumulators at the end of the power stroke. When the outward travel of the pistons ceases at the end of the power stroke, check valve 60 closes immediately upon the beginning of the compression stroke and the compression stroke takes place because of the pressure exerted by the liquid tirst supplied by the auxiliary accumulator 63 and then by the low pressure accumulator 98, which completes the compression stroke.
The high pressure remaining in passage 58 due to the resilient action of accumulator 63 produces a rapid acceleration of pistons .22, the pressure in compensator 63 falling olf as they move inward. When pistons 22 have moved through a portion of their inward strokes, as determined by the elastic capacity of accumulator 63 and the area of the plungers 32, the pressure in passage 58 falls ot until it is no longer greater than that in low pressure accumulator 98. Valve 76 then opens and hydraulic uid flows from low pressure accumulator 98 to till the remainder of the cylinders 28. Thus, it will be seen, the rst portion of the compression stroke is activated by the high pressure from compensator 63 and the latter portion by the lower pressure from accumulator 98, the mean pressure being substantially greater than that of accumulator 98 alone.
Resistance to turning a shaft 79 by an external load tends to raise the pressure in accumulator 50 and the maximum pressure in compensator 63, while lowering the pressure in accumulator 98. An increased load, therefore, produces an increased bounce pressure, and force, on pistons 32 during the initial part of their inward, or compression, strokes and a reduced force during the balance of the compression stroke. With suitable proportioning of the relative lengths of the two portions of the strokes and the volumetric capacities of the accumulators 63 and 98, it is possible to obtain any desired bounce characteristics in response to a wide variation in the external load. Thus, for example, a constant mean bounce effect may approximately be provided regardless of load, whereby uniformity of compression in the combustion space 13 is assured regardless of the fuel charge injected through tube 38 at each stroke. Or, if desired, any variation from constant compression ratio may also be attained by changing the volumes of the accumulators 63 and 98 and the area parameters of the pumping pistons 32 to imbalance the high and low pressure portions of the bounce in either direction.
Control of fuel injection, and thus of the power irnpulse, may be coordinated with the torque demands on motor shaft 79 either manually or by an automatic regulator responsive to the pressure in some part of the system or to the pressure drop over the motor 78. Control systems of this type are known and need no additional description.
The operation of the engine is as follows: Fuel is injected through injector 38. The injector ymay be a manually or automatically controlled injectorj its control being determined in response to the magnitude'of external power demanded of the engine. Accordingly, the fuel control corresponds to the well known control of conventional diesel engines. When the engine is idling, the pistons reciprocate at low speed with the fuel injection being reduced to the point at which the above low speed is obtained. Fuel injection is increased in response to the increase in the external load. The engine is stopped by shutting olf fuel, at which time the pistons return to their inner position due to the pressure exerted upon them by the uid in the auxiliary and the low pressure accumulators; the pistons remain in their inner position as long as the engine is shut down. The engine is started by withdrawing the pistons to their outward positions by means of an external mechanical starter, which may be similar to the automobile engine starter, connected to the piston linkages, not shown in the drawing. Such starting means and linkages are known to the art and need no illustration or description. The pistons are then released, the pressure in accumulator 98 driving them inward to produce a compression stroke and ignition of combustible mixture of compressed air and dispersed fuel. Since the engine depends on the hydraulic bounce for producing compression stroke, it is necessary to provide some auxiliary means for restoring the pressure in accumulator 98 after long shut down periods. This means (not illustrated) may be a pressure-controlled pump, well known in the art. As mentioned previously, control of load and speed is obtained by manually or automatically controlling the amount of fuel injected at each stroke of the engine.
It has been stated in the introductory part of the specification that the maximum pressure in the auxiliary accumulator increases with the increase in the external load. It follows that the compensating action of the auxiliary accumulator during the compression stroke increases with the increase in the external load. This is the type of relationship which is desired since the pressure in the low pressure accumulator decreases with the increase in load and such decrease must be compensated for by the inverse increase in Apressure obtainable in the auxiliary accumulator.
What is claimed as new is:
l. In combination with a two stroke free piston engine having a mutually actuating free power piston and pumpmg piston, a hydraulic motor, and working uid for ope'rating said motor, a closed hydraulic power transmission system for said working fluid connecting said engine and said motor, comprising a high pressure accumulator connected to the pumping piston to receive fluid under pressure therefrom during the power stroke of said e11- gine and to the input of said motor to deliver fluid under pressure thereto, a Alow pressure accumulator connected to the output of the motor to receive uid therefrom and to said pumping piston to return fluid thereto during the compression stroke of said engine, means preventingthe ow of uid from said engine to said low pressure accumulator during the power stroke of said engine, and an auxiliary accumulator connected directly to said pumping piston to receive uid under pressure during the power stroke of said engine and return the same without external regulation to said pumping piston for the compression stroke of said engine.
2Q In combination with a two stroke free piston engine having a mutually actuating free power piston and pumping piston, a hydraulic motor, and working fluid for operating said motor, a closed hydraulic power transmission system for said working tluid connecting said engine and said motor, comprising a high pressure conduit connecting said pumping piston to the input of said motor to deliver said uid during the power stroke of saidvengine, a low pressure conduit connecting the output of the motor to said pumpingpiston to return said fluid during the compression stroke of said engine, valve means for preventing the owA of uid in said low pressure conduit from said engine during the power stroke thereof, a high pressure accumulator interposed in said high pressure conduit, a low pressure accumuator interposed in said low pressure conduit andan axuiliary ac cumulator, interposed directly between said high and low pressure accumulators and said pumping piston, to receive fluid under pressure during the power stroke of said engine and return the same without external regulation to said piston for the compression stroke of said engine.
3. In combination with a two stroke free piston engine having a mutually actuating free power piston and pump- 6 ing piston, a hydraulic motor, and working fluid for operating said hydraulic motor, a closed hydraulic power transmission system for said working uid connecting said engine and said motor, comprising a high pressure accumulator, a 'conduit hydraulically connecting said accumulator to said pumping piston and the input of said motor, valve means in said conduit permitting the ow of said lluid from said pumping piston to said accumulator only during the power stroke of said engine, a low pressure accumulator. a low pressure conduit hydraulically connecting said low pressure accumulator to said pumping piston and the output of said motor, valve means in said low pressure conduit preventing the ow of said Huid from said pumping piston to said low pressure accumulator during the power stroke of said engine, and an auxiliary high pressure accumulator having a direct hydraulic connection with said pumping piston to receive uid under pressure during the power stroke of said engine and return the same, without external regulation, together with fluid from said low pressure accumulator to produce pressure on said pumping piston for the compression stroke of said engine.
4. The system according to claim 3 wherein said auxiliary accumulator is pre-proportioned with respect to said high and low pressure accumulators to accommodate variations in output of said engine.
References Cited in the tile of this patent UNITED STATES PATENTS 2,178,310 Pescara Oct. 3l, 1939 2,230,760 Pateras Pescara Feb. 4, 1941 2,434,280 Morain Jan. 13, 1948 2,581,600 Pescara Jan. 8, 1952 2,746,670 Huber May 22, 1956 2,849,995 Lewis Sept. 2, 1958