Relevance. Goals of the Free-Piston Engine Research Prototype. – Study the effects of continuous operation (i.e. gas exchange) on indicated thermal efficiency. Goals of the Free-Piston Engine Research. Prototype. • Study the effects of continuous operation (i.e. gas exchange) on indicated thermal. This document reviews the history of free-piston internal combustion engines, from Unique features of the free-piston engine are presented and their effects on.
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Instead, pistons are free to move back and forth under the action of combustion and bounce chamber pressure. A linear electrical machine is coupled directly to. 4. Components of a free piston engine . fviropluravor.cf fviropluravor.cf fviropluravor.cf This document reviews the history of free-piston internal combustion engines, from the air compressors and gas generators used in the midth century through.
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No notes for slide. Seminar Report on free piston engine 1. Of Mechanical Engg. Also thanks for support by Mr. We are very grateful for the encouragement, guidance and assistance that he accorded us from the beginning of the Presentation to its successful completion. Shivam Kushwah 4. The main challenge with such engines is the control of the piston motion, and this has not yet been fully resolved for all types of free- piston engines.
This Report discusses the basic features of a single piston free-piston engine generator under development at Newcastle University and investigates engine control issues using a full-cycle simulation model. Control variables and disturbances are identified, and a control strategy is proposed. It is found that the control of the free-piston engine is a challenge, but that the proposed control strategy is feasible.
Engine speed control does, however, represent a challenge in the current design. Introduction 1 2. Principal 2 3. Features 4. Piston Configurations 3 4. Applications 6 5. Introduction The development of industry and technology had led to a massive energy crisis, environmental pollution and consequently, high levels of fuel prices present a great challenge for internal combustion engine designers.
More research efforts are put into engine technology to explore and study more efficient unconventional engines, aiming at reducing engine emissions and improving efficiency. The free piston engine is a kind of unconventional engine with the characteristics of simplicity and operational flexibility, which draw a great amount of attention from engine researchers. The advanced microprocessor-based control systems and modern engine technologies significantly promote the development of this research as a result of improved operational control of the free piston engine, along with enhanced optimization possibilities for various operating conditions.
However, as conventional engine and gas turbine technology matured, the free-piston engine concept was abandoned in the early s. After being abandoned, free-piston engines are being investigated by a number of research groups worldwide as an alternative to conventional engine- generator sets or for generating hydraulic power in off- highway vehicles.
The experimental analysis on hydraulic free piston engine. By coupling with a suitable 7. This can be written as: Free piston engines usually works on two-stroke compression ignition or spark ignition operating principle, as a power stroke is required on every cycle.
Features The free-piston engine has a number of unique features, some give it potential advantages and some represent challenges that must be overcome for the free-piston engine to be a realistic alternative to conventional technology.
As the piston motion between the endpoints is not mechanically restricted by a crank mechanism, the free-piston engine has the valuable feature of variable compression ratio, which may provide extensive operation optimization, higher part Fig. These are enhanced by variable fuel injection timing and valve timing through proper control methods. Variable stroke length is achieved by a proper frequency control scheme such as PPM Pulse Pause Modulation control , in which piston motion is paused at BDC using a controllable hydraulic cylinder as rebound device.
The frequency can therefore be controlled by applying a pause between the time the piston reaches BDC and the release of compression energy for the next stroke.
Since there is few number of moving parts frictional losses and manufacturing cost reduces. The simple and compact design thus requires less maintenance and this increases lifetime.
The purely linear motion leads to very low side loads on the piston, hence lesser lubrication requirements for the piston. The combustion process of free piston engine is well suited for Homogeneous Charge Compression Ignition HCCI mode, in which the premixed charge is compressed and self-ignited, resulting in very rapid combustion, along with lower requirements for accurate ignition timing control.
Also, high efficiencies are obtained due to nearly constant volume combustion and the possibility to burn lean mixtures to reduce gas temperatures and thereby some types of emissions . By running multiple engines in parallel, vibrations due to balancing issues may be reduced, but this requires accurate control of engine speed. Another possibility is to apply counterweights, which results in more complex design, increased engine size and weight and additional friction losses.
With the absence of an energy storage device, like flywheel in conventional engines, it will not be capable of driving the engine for several revolutions. This result in misfiring and the need for accurate speed control. A simple design with high controllability is the main strength of this design compared to others. This allows a simple and compact device with higher power to weight ratio.
The control of piston motion has proved difficult since small variations in the combustion in either of the two cylinders will have high influence on the next compression. Each piston requires a re coupled to one or both of the pistons ensure symmetric piston motion.
The main advantages are Fig. Each piston requires a re-bound device, and a load device may be coupled to one or both of the pistons. Mechanical linkages connecting the two pistons ensure symmetric piston motion.
The main advantages are perfectly But, it is seen that the need for a piston synchronization mechanism together with the dual set of the main components makes the engine complicated and bulky. Applications Since the free-piston engine was first developed around , a number of different designs have been proposed using the free-piston concept.
The majority of these were, however, not commercially successful. This section gives an overview of known free-piston engine developments, with an emphasis on engines where experimental results or operational performance data have been reported. These engines were of the opposed piston type, making them vibration- free. In these engines, air compressor cylinders were coupled to the moving pistons, often in a multi-stage configuration.
Some of these engines utilized the air remaining in the clearance of compressor cylinders to return the piston, thereby eliminating the need for a rebound device. In the figure shown, during the expansion stroke, the air in the compressor cylinder is compressed along with the sc combustion chamber by the scavenging pump. In the compression stroke, which results from the bounce chamber, the charge is compressed and ignited One of the earliest successful free described by Toutant, developed by German company Junkers and was used by the German Navy during World War II to provide compressed torpedoes.
It had the advantages of high efficiency, compactness and low noise and vibration. In the compression stroke, which results from the bounce chamber, the charge is compressed and ignited One of the earliest successful free-piston engine air compressors was escribed by Toutant, developed by German company Junkers and was used by the German Navy during World War II to provide compressed air for launching.
The lack of success of the free-piston air compressor may be due to some possible factors, including Fig. In the compression stroke, which results from the bounce chamber, the charge is compressed and ignited . These engines were typically diesel powered, opposed piston engines with The most important advantages were: These engines were highly supercharged and operated on higher mean effective pressures than conventional diesel engines.
The working of a free piston gas turbine starts with the influx air being compressed by the piston that rebounds from the cushion cylinder. The pressurized air then flows though the delivery valves to the combustion chamber, where combustion occurs, producing the power stroke. The exhaust gases will be scavenged by the further flow of fresh air and is passed to the turbine. Still, free-piston gas generator never became a real competitor to either the diesel engine or the gas turbine.
Some of the reasons for its limited success are: The energy released from combustion process is converted directly into hydraulic energy.
They may apply a hydraulically rebound device, using part of the produced hydraulic energy to return the piston, or chamber. Experimental tests on the prototypes show generally good fuel economy and high part load efficiency by using the PPM Pulse Pause Modulation control .
Many of the modern approaches in free engines. In the following sections, the components and working principle of middle spring-rebounded FPEG are introduced. In Section 3 , the one-dimensional flow simulation model of the FPEG is built, which is validated by means of a four-stroke experiment. Then, the two-stroke thermodynamic cycle of FPEG is simulated under different influential factors, and the simulation results are compared and analyzed in detail.
The optimized results will help us to understand how the two-stroke thermodynamic cycle of FPEG affects the indicated power and system efficiency. The main parts of the FPEG are a gasoline engine, back spring, and linear electric generator.
The system has only one combustion chamber, a rebounding device, and a reciprocating moving component. The combustion chamber is a single-cylinder free-piston engine, which is equipped with electromagnetic valves, injector, and spark plug. A back spring is assembled between the combustion chamber and linear electric generator. The single piston and the moving coil of the linear generator are connected into one compact component, as a whole mover of FPEG.
The free-piston will move freely between the top dead center TDC and the bottom dead center BDC , and its reciprocating motion is determined by the imbalance of all forces acting on the mover [ 11 , 13 ]. The free-piston engine will operate with trapped fuel mixture and spark plug ignition.
Because the generating efficiency of a linear electrical generator reduced significantly at low-speed conditions, the back spring pushes the piston upward to achieve continuous operation.
A supercapacitor is used to incorporate the electricity output by the generator. The power converter is used for matching the linear generator and storing the electric energy [ 14 , 15 ].
The electronic controller unit ECU could control the system to adjust engine performance after acquiring the signals of cylinder pressure, piston displacement, armature current, and others.
Besides, scavenging is implemented by the electromagnetic valves, which are fixed on the cylinder head. In a complete working cycle, the linear generator works in the motoring mode only in the intake stroke, while other strokes work in the generating mode.
In the FPEG system, there is a great deal of freedom in defining the piston motion. The FPEG working cycle can be switched by changing the movement law of the piston.
Thus, the four-stroke thermodynamic cycle and two-stroke thermodynamic cycle can be used for different working cycles of the FPEG. Thermodynamic Cycle of the FPEG Four-stroke free-piston engines have relatively more energy saving and higher efficiency than two-stroke free-piston engines, but two-stroke has the advantages of power density.
At the same working frequency, the two-stroke working cycle number is twice that of the four-stroke, and the time of gas exchange is shorter than the four-stroke [ 16 ]. The four-stroke and two-stroke thermodynamic cycles of FPEG are presented to optimize the thermodynamic performance.
As can be seen from Figure 2 , the remarkable characteristics of the four-stroke thermodynamic cycle are the short intake and compression stroke, which are supplemented by pressurized the intake air [ 17 ]. During the intake stroke, the linear generator works as an electric machine to drive the piston assembly move downward from point to point to absorb the fuel mixture. It can adjust the intake pressure or air temperature to increase the mixture flow and improve the combustion process.
When the piston moves to TDC and approaches the point , the fuel mixture is compressed in the compression stroke. During the expansion stroke, the ignition of the spark plug is the start point for the combustion process and it will end at point. After that, the piston moves from bottom to top and reaches the point to expel the burned gas.
Thus, the expansion and exhaust strokes are longer than intake and compression strokes, and it can achieve the full combustion to increase the power density. As shown in Figure 3 , the two-stroke thermodynamic cycle is characterized by short compression and expansion stroke, which is supplemented by adjusting the spark advance angle to realize more full combustion.
The longer valve overlap can increase the valve opening duration of the intake and exhaust strokes. Before the piston reaches the point , the spark plug ignites the fuel mixture and the piston moves upward to accomplish the compression stroke. During the exhaust stroke, the piston moves from point to point. Then, the piston moves from point to point in the intake stroke. When the piston moves from point to point , the valve overlap realizes the intake and exhaust valves open simultaneously to absorb the fuel mixture and expel the residual gas.
It can increase the volumetric efficiency and improve the process of gas exchange. Besides, the advance ignition can achieve the sufficient combustion to release more energy. The prototype is a single-piston, four-stroke, gasoline engine, which is equipped with four electromagnetic valves.
It employs the water-cooled cooling method, closed-loop control of intake port fuel injection, and electronically controlled spark-ignition system.
Compared with the design requirements of the FPEG, the prototype performances are very consistent and facilitate to refit. Table 1 lists the main structure parameters of prototype. Table 1: Specification of prototype. The overall structure of the electromagnetic valve is shown in Figure 4.
The tubular structure consists of iron core, coil skeleton, coil, permanent magnet layer, and outer wall of actuator. In the electromagnetic valve system, the coil and valve are connected rigidly, and the back spring is assembled between the coil skeleton and the cylinder head. The electromagnetic valve is used for providing the scavenging air and realizing effective control of the gas exchange process. Under the control of electronic controller unit ECU , it can change valve lift, valve opening time, and valve opening duration, so it can achieve flexible control of the valve mechanism.
The linear generator consists of a permanent magnet PM , core, moving coil, and end cover. An air-gap reserved between outer core and inner core. The nonmagnetic coil skeleton is wound two coils, which is the whole mover of MCLG.