Experimental study on the operating characteristics of a reciprocating free-piston linear engine

Accepted Manuscript Experimental study on the operating characteristics of a reciprocating free-piston linear engine Fujun Huang, Wenjun Kong PII: DOI: Article Number: Reference:

S1359-4311(18)37951-1 https://doi.org/10.1016/j.applthermaleng.2019.114131 114131 ATE 114131

To appear in:

Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

2 January 2019 8 July 2019 13 July 2019

Please cite this article as: F. Huang, W. Kong, Experimental study on the operating characteristics of a reciprocating free-piston linear engine, Applied Thermal Engineering (2019), doi: https://doi.org/10.1016/j.applthermaleng. 2019.114131

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Experimental study on the operating characteristics of a reciprocating free-piston linear engine Fujun Huanga,b, Wenjun Konga,b,c aInstitute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China bSchool of Engineering Sciences, University of Chinese Academy of Sciences, Beijing 100049, China c School of Astronautics, Beihang University, Beijing 100083, China Abstract: A miniature reciprocating free-piston linear engine (FPLE1) with glow plug ignition (GPI2) was designed and manufactured. The steady-state operation of the prototype is successfully achieved with and without load. The operating characteristics of the FPLE were investigated using a premixed methane/air mixture. The experimental results showed that the in-cylinder mixture was successfully ignited by the glow plug within five cycles and the FPLE could be successfully started under the single-cylinder self-starting strategy. Based on the comparison of the FPLE operating characteristics in the GPI and spark-ignition (SI3), it is found that the performance and operating stability of the FPLE were significantly improved in the GPI due to the flexible ignition timing. Besides, the effects of key parameters such as glow plug heating power (GPHP4), piston assembly mass and fuel/air flow rate on the performance of the FPLE were studied. The electric power and work conversion efficiency increase first and then decrease with increasing external load, and both of them reached maximum values of 31.6 W and 42.8%, respectively, with an external resistance of 6.3 Ω. Keywords: free-piston linear engine, glow plug ignition, single cylinder self-startup strategy, electric power. Graphical abstract:

Abbreviations: 1FPLE: free-piston linear engine. 2GPI: glow plug ignition. 3SI: spark-ignition. 4GPHP: glow plug heating power.  Corresponding author. E-mail: [email protected].

Piston displacement In-cylinder pressure

20

24

10

(7)

(9)

(4) (5) (6) (2)

(8)

(3)

(11)

1 2 3 4

0 a1 Vcc1 b1 a2 a3

b2 b3

a4 GND b4

5

15

0

12 9

-5

(13)

(15)

18

5

6

-10

(14)

3

6 7 8

0

-15

(17) (16)

0

-20 0.0

0.3

0.6

0.9

1.2 Time/s

1.5

1.8

2.1

205 0.61 0.60 0.59 0.58

0.61 6.3

32.0

10.5 10.2 4.5

5.0

5.5

6.0

6.5

Glow plug heating power/W

7.0

7.5

2.4

49.2

30.4 28.8

220

3

0.60

2

0.56 0.52

11.0

ea

n

/m

.s

po

ed

d

e sp

w er /W

-1

M

dic ate

250 240 230

11.2 ITE/%

ITE/%

10.8

13.6

33.6

In

Indicated power/W

220 215 210

1.88

nc y/H z

28.0

10.1

204

1.92

ue

28.2

1.96

Fr eq

Frequency/Hz

1.90 28.4

1.9

30.4

Mixture flow rate=36L/min Glow plug heating power=7.2W

m pr ean es su pea re k /a tm

1.95

22.2

2.00

Piston velocity/m.s-1

Mean piston speed/m.s-1

2.00

IMEP/MPa

Mean piston speed/m.s-1 Frequency/Hz Indicated power/W IMEP/MPa

Mixture flow rate=36L/min Piston assembly mass=1.1Kg

2.4

Glow plug ignition

Spark plug ignition

2.05

4.0

In-cylinder pressure/atm

21 (12)

(10) (1)

27

Mixture flow rate=36L/min Glow plug heating power=4.16W

15 BDC

IT E/%

-X

0

Piston displacemen/mm

+X

TDC

Mixture flow rate=36L/min Glow plug heating power=6.1W

1 0 -1 -2

10.8

-3

0.6

0.7

0.8

0.9

1.0

Piston assembly mass/Kg

1.1

-20

-15

-10

-5

0

5

Piston displacement/mm

10

15

20

a

1. Introduction The conventional internal combustion engines (ICEs) have been around for over a century now and are widely used in various vehicles due to the high power density, compact design and mature production technology. However, the increased usage of conventional ICEs also causes the crises of energy shortage and produces substantial emission of exhaust gas, including carbon dioxide, carbon monoxide, hydrocarbons, particulate matter and nitric oxides, which can contaminate the environment and pose a potential threat to people's health [1,2]. Therefore, various attempts have been performed by researchers to address these challenges. Such as the usage of clean energy (biogas [3], hydrogen [4], etc.,), renewable energy [5, 6] and the development of new engine technologies, e.g. fuel additives [7], supercharging or pressurizing the fresh charge [8], exhaust gas recirculation (EGR) [9] etc.. These efforts improved engine performance and emissions to a certain extent, but the progresses are limited. Because these attempts were carried out on existing ICE, and there is no major modification to the engine infrastructure. Thus much effort has been devoted to developing efficient, energy-saving and environmentally friendly energy conversion device by scientists and institutes. In recent years, a novel type of energy conversion device, free-piston linear engine, attracted people interest [10,11]. Unlike conventional ICEs, FPLE eliminates the crankshaft mechanism, and the piston can move freely in the cylinder, thus allowing the engine to change the compression ratio and optimize the combustion process to operate with various kinds of fuels and homogeneous charge compression ignition (HCCI) combustion [12], leading to higher part load efficiency with virtually no NOx and particulate emissions. In addition, FPLE is also mechanically simple, and the integrated linear generator allows for a compact design, which reduces manufacturing costs and frictional loss [13]. As a promising energy conversion device with the potential advantages over traditional ICEs in efficiency and emissions, the FPLE has received increasing attention recently [14-17]. Tian [18] at the University of Minnesota designed and constructed a single-cylinder free-piston engine/compressor with bore of 12.5 mm, and the prototype

employed hobby-style glow plug ignition. Experimental results demonstrated that the prototype can run continuously at a frequency of over 120 Hz. Moreover, the effect of spring stiffness on the operating characteristics was investigated. Results of the investigation showed that a stiffer spring can achieve higher compression ratio and running frequency. However, the author stated that some technical issues must be addressed prior to the practical application of the free-piston engine/compressor. First, a better lubrication and sealing system should be designed to improve the device efficiency because engine overall fuel to cold compressed air efficiency was low. Second, a practical starting method must be devised. Third, a reliable fuel delivery system must be designed. Aerodyne Research, Inc. [19] developed a single-cylinder free-piston engine based on two strokes. This design introduced the integration of a linearly oscillating piston, a double-helix spring, and linear electric alternator for generating electrical energy. The engine could run over a wide range of conditions on propane, butane, and Jet-A fuels with glow plug-assisted combustion. The engine successfully produced 10-50 W power. Toyota Central R&D Labs, Inc. focused on gasoline and diesel free-piston engines [20, 21]. They developed a two-stroke freepiston engine (i.e., single piston and integrated linear generator). A premixed charge compression ignition and a spark ignition (SI) were applied on the prototype. Cooling and lubricating systems were developed to ensure continuous engine operation. The researchers discussed the effect of ignition timing on engine performance. An experimental analysis showed that the precise control of ignition position was essential for stable operation of the FPLE. The main combustion occurred in an expansion stroke, which slowed down the flame propagation and caused unstable combustion when the ignition timing was close to or after top dead center (TDC). In their experiment, the thermal efficiency was estimated to be up to 46% at the power of 23 kW. Nandkumar et al. at West Virginia University (WVU) established a SI double-cylinder FPLE prototype using gasoline as fuel with a bore of 36.5 mm and a maximum possible stroke of 50 mm [22-24]. The engine had a peak output electric power of 316 W with a frequency of 23.1 Hz. This is the most successful prototype that has been found. They found that with a change in the moving assembly mass, the peak in-cylinder pressure and the maximum stroke varied in a directly proportional manner; i.e., the in-cylinder pressure and the maximum stroke increased with moving assembly mass while the frequency of the FPLE varied inversely with the moving assembly mass. Jia. et al. [25] presented an experimental investigation on the starting process of a diesel-fuel dualpiston FPLE with a bore of 52.5 mm under the effects of motoring force. Results showed that the in-cylinder mixture was successfully ignited at the fourth cycle with a compression ratio of over 9:1 with a fixed starting force of 125 N. The peak in-cylinder pressure exceeded 4 MPa, and the peak piston velocity increased significantly to approximately 4.0 m/s. However, the cycle-to-cycle variation of the piston movement was significant, and the engine misfired frequently. Yuan et al. [26] established a compression ignition diesel FPLE and performed experiments to analyze the effects of starting force and injection position on the ignition performance. Results revealed that a larger starting force could increase the possibility for the engine to gain a higher compression ratio from its resonant reciprocation. However, it failed to bring about

higher ignition efficiency for FPLE initialization. A large but inappropriate starting force might lead to a serious incomplete combustion for the ignition. Their test pointed out that the level of combustion completion of ignition was influenced by the injection triggering advance position (ITAP). Properly enlarging the ITAP could enhance combustion efficiency of the ignition and was beneficial for enlarging the compression ratio of opposite cylinders, but an excessive ITAP might advance the injection and ignition cycle and further induce a misfire for the FPLE. Feng et al. designed two types of FPLE: SI FPLE and compression ignition FPLE [27-29]. The operating characteristics and the control strategies of the FPLE were researched by simulations and experiments. Earlier results showed that the prototype failed to run continuously during the generating process because of poor ventilation quality. However, the FPLG could achieve stable operation by an improvement in the prototype, and a reasonable staring control strategy and a switching control strategy were designed to enable the prototype to run smoothly in each working process. Woo et al. [30] presented experimental investigations on the performance of a two-stroke SI double-cylinder FPLE by using compressed natural gas and hydrogen fuels. Results showed that the FPLE could be operated at a frequency of 17 Hz. The hydrogen fuel had higher burn rate and thus showed improvements in power output and emission. The FPLE had irregular piston movement at TDC, thereby significantly affecting the compression process and the subsequent combustion of the other cylinder of the engine. In addition, the in-cylinder pressure and piston displacement fluctuations were strong because of the ignition timing delay. Kim et al. [31, 32] presented an experimental investigation to examine the performance characteristics of a two-stroke SI double-cylinder FPLE with a bore of 25 mm and a stroke of 22 mm. The effects of ignition timing, mixture flow rate, and alternator load on the operating characteristics of FPLE with liquefied petroleum gas were investigated. The engine was able to operate steadily. Results showed that the ignition timing conditions of 4 mm (from the cylinder head) presented the widest operation range in terms of conductance, longer stroke, and higher frequency. The piston stroke and frequency decreased as the mixture flow rate decreased due to the reduced amount of delivered fuel. The cyclic variation of the piston work was higher at lower alternator load conditions, and the high induced current could deteriorate the piston kinetic energy due to the application of high magnetic resistive force on the piston. Despite having many advantages over existing engines [33, 34], the free-piston engine also faces many technical challenges (e.g., complex ignition control strategy, difficulties in starting, incomplete combustion, and collision between the cylinder head and piston due to the leakage). These unsolved problems were accountable for the unstable and noncontinuous running of certain prototypes, let alone commercial applications. Among the identified challenges, ignition timing is an extremely important parameter that directly affects engine performance and steady operation. Ignition timing is determined by the crank angle in conventional ICEs. However, the FPLE has no crankshaft or rotational motion, and the ignition timing is determined by the piston’s position in front of the cylinder head. In fact, the proper ignition location for the FPLE cannot be captured accurately due to the lack of crankshaft mechanism,

especially when the FPLE is running for high frequency. Therefore, in some previous studies [14, 23, 31], to reduce the design difficulty of control systems, the fixed ignition timing is preferred. However, this ignition strategy also heavily inhibits engine performance. Mikalsen and Roskilly [35] pointed out that FPLE is well-suited for HCCI operation because the piston motion is not controlled by a crankshaft, thus having few ignition timing control requirements, in addition, the ideal Otto cycle performance could be closely approached. In this study, the FPLE works with GPI, which like HCCI has naturally adjusted timing but allows an easier start of the engine. This GPI model applied to the FPLE can greatly simplify the ignition control system and significantly improve operation stability. In the literature, the linear motor coupled with the FPLE was operated as a starting device as regards the starting strategy of the FPLE [22, 28, 30]. Different from the existing starting method mentioned above, a single-cylinder self-starting strategy was proposed and applied to start the FPLE in this paper. More detailed descriptions of the glow plug ignition and starting strategy will be introduced in the following sections. A double-cylinder methane-fueled FPLE and the test setup were designed and established based on the GPI mode. This study presented an experimental study to investigate the starting process and the FPLE performance comparisons under two ignition modes. Meanwhile, the effects of key parameters such as glow plug heating power, piston assembly mass and fuel/air flow rate on the operating characteristics of the FPLE were studied, and eventually the electric power generation of the FPLE was presented. This paper is outlined as follows. Section 2 introduces the FPLE system, including the description of the FPLE, the ignition system, the starting method, test facilities, and date processing methods. Section 3 presents the self-starting process; comparison of engine performance in two ignition modes; and effects of GPHP, piston assembly mass, and mixture flow rate on the operating characteristics of the FPLE without load, and the electric power with load. Finally, Section 4 presents the main conclusions and a discussion of technical difficulties to be addressed in future work.

2. FPLE system description 2.1. Brief description of the engine A diagram and the FPLE prototype are demonstrated in Fig. 1. This prototype engine contains two two-stroke combustion cylinders using the loop scavenging type, a dual piston, two ignition systems, a linear generator, and two engine mounts. The pistons of each cylinder are connected with a connecting rod and can reciprocate freely between the TDC and the bottom dead center (BDC). An electric current is generated according to the electromagnetic theory when the connecting rod combined with the linear generator reciprocates [29]. Table 1 presents a list of the prototype design parameters.

Table 1 Parameters of the FPLE

TDC

Parameter

Value

Unite

Number of cylinder Cylinder bore Maximum possible stroke Effective displacement Exhaust port opening Transfer port connecting rod diameter

2 22.4 32 6.3 15 (from cylinder head) 16 (from cylinder head) 16

mm mm cm3 mm mm mm

-X

+X

0

BDC

(12)

Mass flow controller Premixer

(10) (1)

(7)

(9)

(4) (5) (6) (2)

(8)

(3)

(11) (13)

(15) 1 2 3 4

0 a1 Vcc1 b1 a2 a3

b2 b3

a4 GND b4

5

(14)

6 7 8

0

(17)

Computer External resistance

FPLE Displacement sensor

Date acquisition board ECU

Pressure sensor DC supply power

(16)

(1) Glow plug (2) Pressure sensor(3) Spark plug (4) Cylinder (5) Exhaust port (6) Piston (7) Scavenging chamber (8) Lubricating oil (9) connecting rod (10) Transfer port (11) Displacement sensor (12) Pressure balancing vessel (13) Linear generator (14) ECU (15) Data acquisition board (16) computer (17) DC supply power Fig. 1 The schematic diagram and the FPLE prototype.

A stoichiometric premixed methane/air mixture is used as fuel. High-purity methane (>99.999%) is stored in a high-pressure tank, and air is supplied by an air compressor. The mass flow rates of air and methane are controlled using the mass flow controllers. Fuel and air are premixed homogeneously by a premixer. The mixture drawn into the cylinders is unsteady because of pressure fluctuations within the intake manifold caused by the opening and closing of transfer ports. Thus, a 20 L pressure balancing vessel is connected between the premixer and transfer port of the FPLE to damp out the pressure fluctuations. Free-piston engine lubrication is acknowledged as a challenge, especially for the miniature two-cylinder free-piston engine. In this experiment, an appropriate amount of lubricating oil is injected into the scavenging chamber as shown in Fig. 1 to reduce friction between the cylinder and piston and therefore enhance the FPLE durability. The liquid level of the lubricant oil is approximately 2 mm higher than that of the bottom wall of the cylinders. The lubricating oil is carried to the gap between the piston and the cylinder when the piston reciprocates. However, this lubricating strategy has the following flaws. (1) The piston surface is not perfectly lubricated, and (2) part of the lubricating oil may also be brought into the combustion cylinder during the scavenging. However, no evidence of lubricant oil burning is observed during the FPLE operation. A single-phase, six-slot/nine-pole tubular linear generator is integrated with the FPLE. The stator is composed of the generator casing, stator laminations, and windings. Additionally, the translator consists of axially magnetized permanent magnets arranged with opposite polarities, the mover laminations, a titanium support tube, and a chromium-plated steel shaft (i.e., the connecting rod). Fig. 2 shows a

schematic of the linear generator and Table 2 presents the corresponding design parameters. Titanium support tube Stator lamination Winding Connecting rod

Generator casing

Mover lamination

Permanent magnet

Fig. 2 Schematic of single-phase, six-slot/nine-pole, axially magnetized tubular linear generator. Table 2 Parameters of the linear generator. Parameter Titanium support tube inner diameter Titanium support tube outer diameter Magnet inner diameter Magnet outer diameter Magnet thickness Poles Poles pitch Air gap Stator inner diameter Stator outer diameter Slots Tooth width Slot width Slot depth Number of turns per slot

Value

Unit

16 20 20 30 10 9 15 1.5 33 58 6 7 8 8 200

mm mm mm mm mm poles mm mm mm mm Slots mm mm mm turns

In this work, a couple of “hobby-style” glow plugs (briefly referred to as glow plug in this work) are used, which are widely applied in miniature airplane model engines. The glow plugs are arranged eccentrically at each cylinder head. Fig. 3 shows a typical glow plug used in this work, which consists of a metal body, a central conductor mandrel, an insulating layer, and a resistive heating platinum wire of a catalytic material. The conductor mandrel and the platinum wire are inlaid in the metal body, and one end side of the platinum wire is connected to the conductor mandrel and the other side to the body. The conductor and the metal body are insulated from each other. Resistive heating makes the platinum wire temperature rise quickly when current is applied to the conductor mandrel. The mixture cannot be ignited by the glow plug at atmospheric pressure. Therefore, the fast compression of the mixture is critical for enabling the mixture to combust. Specifically speaking, the compression ratio is gradually increased with the piston approaching the TDC during the compression stroke. The temperature of the compressed mixture is raised, and the catalytic reaction of fuel is enhanced. Eventually, the mixture temperature reaches a state that can support flame propagation

with the heating effect of the platinum wire, and a flame initiates near the hot platinum wire and propagates through the combustion cylinder. The glow plug in this method performs the same function as the spark plug widely used in the conventional SI engine, except that the ignition timing is controlled electronically for the spark plug, whereas the ignition timing is related to the state of the mixture for the glow plug. The platinum wire temperature for the glow plug is increased with the working current [36]. In this experiment, the voltage supplied to the glow plug is kept constant at 2 V. Table 3 shows the corresponding relationship between the platinum wire temperature and the heating power. Metal body Conductor mandrel

Insulating layer

Platinum wire

Fig. 3 A schematic configuration of the glow plug for the FPLE. Table 3 The relationship between the heating power and the platinum wire temperature. Current/A

Heating power/W

Platinum wire temperature/℃

2.08

4.16

440

2.5

5

521

3.05

6.1

606

3.6

7.2

620

2.2. Ignition system and starting methods The FPLE consists of two ignition systems: SI device and glow plug ignition device, which are commercially available products. The SI device contains a 12 V battery, ignition coil, and spark plug. The glow plug device contains two glow plugs and a dualchannel adjustable DC power supply with a voltage range of 0-36 V and a current of 0-5 A. Table 4 shows the energy consumption of ignition devices in one cycle for two ignition modes [37]. Table 4 Released energy in one cycle under different ignition mode. Ignition mode

Energy consumption /(mJ/cycle)

Spark plug

50～100

Glow plug

150～250

In this experiment, the starting method is different from that mentioned in previous studies. Specifically, the ignition systems and the fuel/air supply system are activated during the starting process. Normally, the transfer ports of the FPLE are closed by pistons, as shown in Fig.1. Moving the connecting rod back and forth several times by hand to open the transfer port and then release the connecting rod is necessary to ensure

that sufficient fuel/air mixture in the right cylinder is available before starting. Subsequently, the spark plug installed only in the right cylinder is electrically triggered to ignite the compressed gas mixture. The gas mixture releases energy rapidly and raises the pressure and temperature in the cylinder volume. The hot, high-pressure gases in the right cylinder drive the piston to compress the left cylinder, and the left cylinder merely acts as a bounce chamber. Thereafter, the rebounded piston continues to compress the right cylinder, and the ignition is triggered again when the piston reaches the ignition position (The ignition timing in this process is fixed and set at 6 mm from the cylinder head.). The prototype can then run as a single-cylinder free-piston engine. The glow plug starts to work after a few cycles, and the displacement amplitude and the peak in-cylinder gas pressure are expected to increase and finally reach the required value for igniting the mixture with the heating effect of the glow plug. Immediately, the speed and displacement signals obtained from the laser displacement sensor are fed back into the engine control unit, and the SI device is disconnected. The ignition mode starts to switch smoothly from the SI mode to the GPI mode. This starting method is the single-cylinder self-starting strategy. 2.3. Test facilities The piston displacement is measured through a customized laser displacement sensor (Panasonic Corp: HG-C1100) with a maximum sampling rate of 20 kHz with a measurement accuracy of ± 0.06 mm. The combustion cylinder volume is calculated from the piston displacement data. A micro pressure sensor (HELM sensor, type: HM91) installed into the cylinder head is used to record the instantaneous in-cylinder pressure. The pressure sensor has a maximum sampling rate of 100 kHz and a measurement accuracy of ±4 kPa respectively. A micro stainless steel tube with a diameter of 0.6 mm and a length of 5 mm is connected between the pressure sensor and cylinder head (Fig. 4) to protect the pressure sensor from being damaged by the high-temperature flame. Controlled experiments are conducted by running a 6.3 cc two-stroke crankshaft engine without combustion to estimate the influence of the micro stainless steel tube on pressure measurement. The crankshaft engine is driven by an electric motor using a synchronous belt. The pressure sensor with and without micro stainless steel tube is successively installed into the cylinder head during the experiments to record the incylinder pressure. Fig. 5 presents the comparison of pressure profiles for two cases at the same running frequency. The pressure curves are found to display a very good agreement, and the difference of mean peak pressure obtained from more than 100 consecutive cycles is rather small (less than 2%). Therefore, the influence of the micro stainless steel tube on pressure measurement is negligible. High-speed data are acquired by an Advantech DAQ card (PCI-1710HG). This DAQ card has a maximum sample frequency of 100 kHz and each channel is given 20 kHz in this experiment. The data are acquired for more than 600 cycles (more than 20 s). Electrical power generated from the linear alternator is measured through the output voltage and current. Various external resistance loads are used as the input.

Micro stainless steel tube

Pressure sensor

Glow plug

Fig. 4 The cylinder head. 20

12

Pressure/atm

18 16

15

No-tube tube Frequency=26.4Hz

Pressure/atm

14

9 6 3 0

0.01

0.02

Time/s

0.03

12 10

Ptube=14.69atm

8

Pno-tube=14.97atm

6 4 2 0 0.00

0.05

0.20

0.15

0.10

0.25

0.30

0.35

0.40

Time/s

Fig. 5 Comparison of pressure with and without micro stainless steel tube.

2.4. Data processing methods In this paper some engine parameters mentioned are calculated using the following expression: 𝑣 = 2·

(𝑋𝑇𝐷𝐶 ‒ 𝑋𝐵𝐷𝐶) 𝑇

= 2·(𝑋𝑇𝐷𝐶 ‒ 𝑋𝐵𝐷𝐶)·𝑓

(1)

1

𝑁

(2)

1

𝑁

(3)

𝑋𝑇𝐷𝐶 = 𝑁∑𝑖 = 1𝑋𝑇𝐷𝐶𝑖 𝑋𝐵𝐷𝐶 = 𝑁∑𝑖 = 1𝑋𝐵𝐷𝐶𝑖 1

𝑁

𝑃𝑝𝑒𝑎𝑘 = 𝑁∑𝑖 = 1𝑃𝑝𝑒𝑎𝑘𝑖 1 𝑁 2 σ = N ∑𝑖 = 1(𝑃𝑝𝑒𝑎𝑘 ‒ 𝑃𝑝𝑒𝑎𝑘)

COV = 𝑃

σ 𝑝𝑒𝑎𝑘

𝑊 = ∮(𝑃·𝐴)𝑑𝑥

(4) (5) (6) (7)

𝑃𝑒𝑛𝑔𝑖𝑛𝑒 = 2·𝑓·𝑊

(8)

𝑊 𝑉

(9)

IMEP =

𝜂𝑖𝑒𝑡 = 𝑄

𝑃𝑒𝑛𝑔𝑖𝑛𝑒

(10)

𝑓𝑢𝑒𝑙.𝑄𝐿𝐻𝑉

1 𝑇

(11)

𝑃𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 = 𝑇∫0 𝑢𝑖𝑑𝑡 𝜂𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 =

𝑃𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐

(12)

𝑃𝑒𝑛𝑔𝑖𝑛𝑒 1

(13)

𝑄𝑐𝑜𝑛𝑠𝑢𝑚 = 𝑈𝐼𝑓

where 𝑋𝑇𝐷𝐶𝑖(>0) and 𝑋𝐵𝐷𝐶𝑖(<0) are the TDC and BDC positions, respectively. With the left cylinder as an example in Fig. 1, the limit positions near and away from the cylinder head that the piston can reach are called TDC and BDC, respectively. The direction from the coordinate origin (0) to the TDC of the left cylinder is defined as the positive direction (+x), whereas the opposed direction is the negative direction (-x). 𝑣 is the mean piston speed; 𝑋𝑇𝐷𝐶， 𝑋𝐵𝐷𝐶, and 𝑃𝑝𝑒𝑎𝑘 are averages of the TDC, BDC, and peak pressure over 𝑁 cycles; N is the cycle number; 𝑁＞600; σ is the standard deviation; COV is the coefficient of variation of the maximum in-cylinder pressure; 𝑊 is the indicated work of a cylinder in one cycle; 𝐴 is the piston top surface area; 𝑃𝑒𝑛𝑔𝑖𝑛𝑒 is the indicated power of the FPLE; IMEP is the indicate mean effective pressure; 𝑉 is the effective compression volume of cylinder (displacement volume of cylinder) and 𝑉 = 6.3 cm3; 𝑓 is the operation frequency of the FPLE; 𝜂𝑖𝑒𝑡 is the indicated thermal efficiency; 𝑃𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 is the electric power; 𝑢 and 𝑖 are the induced 1

voltage and current by the linear generator; 𝑇 is the cycle and 𝑇 = 𝑓; 𝜂𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛is the engine work conversion efficiency; 𝑄𝑓𝑢𝑒𝑙 is the fuel flow rate which can be obtained through the mass flow controller; 𝑄𝐿𝐻𝑉 is the low heat value of the fuel; 𝑄𝑐𝑜𝑛𝑠𝑢𝑚 is the energy consumption of the glow plug in one cycle; 𝑈 and 𝐼 are the voltage and current supplied to the glow plug.

3. Prototype testing results and analysis 3.1. Self-starting process

Figure 6 demonstrates the piston displacement and the in-cylinder pressure during the single-cylinder self-starting process of the FPLE. Figs. 7 and 8 show the enlarged areas (indicated by the red and blue dotted lines in Fig. 6). The mixture flow rate is set to 36 L/min, and the GPHP is 4.16 W. The ignition timing is set to 6 mm from the cylinder head. Fig. 7 shows the displacement and pressure rise, which indicate that the combustion is triggered by the spark plug. The in-cylinder peak pressure grows sharply from 2.2 atm at the first cycle to 19.5 atm at the fifth cycle because the high volumetric efficiency promotes the in-cylinder combustion pressure during the engine starting. Thereafter, the engine runs stably as a single-cylinder free-piston engine, and the incylinder peak pressure starts to drop and is kept at approximately 6.5 ± 1.5 atm. Fig. 8 presents that the glow plugs installed into the cylinder heads start to work at around t = 1.5 s. However, the fuel/air mixture is not immediately ignited until the temperature of the platinum wire reaches a state that can support flame propagation at the fifth cycle. A sharp rise of the in-cylinder peak pressure is evident from 7.8 atm to 22.5 atm. Thereafter, the in-cylinder peak pressure is maintained at approximately 22 atm. And the FPLE could run stably and continuously for around 20 minutes, and then it was turned off to complete the test. The engine can be successfully started, demonstrating that the self-starting strategy proposed in this method is feasible for the FPLE. Piston displacement In-cylinder pressure

20

27

Mixture flow rate=36L/min Glow plug heating power=4.16W

24

15

Piston displacemen/mm

18

5

15

0

12 9

-5

6

-10

3 -15

0

-20 0.0

0.3

0.6

0.9

1.2 Time/s

1.5

1.8

2.1

Fig. 6 The self-starting process of FPLE.

2.4

In-cylinder pressure/atm

21 10

Piston displacement In-cylinder pressure

Piston displacemen/mm

15

Mixture flow rate=36L/min Ignition timing=6mm from cylinder head

Spark plug ignition

27 24 21

10

18

5

15

0

12

-5

9 6

-10

In-cylinder pressure/atm

20

3 -15

0

-20 0.0

0.1

0.2

0.3

0.4 Time/s

0.5

0.6

0.7

Fig. 7 Piston displacement and in-cylinder pressure with the spark plug ignition. Piston displacement In-cylinder pressure

20 15

Mixture flow rate=36L/min Glow plug heating power=4.16W

Glow plug working

27 24

18

5

15

0

12 9

-5

6

-10

In-cylinder pressure/atm

Piston displacemen/mm

21 10

3 -15

0

-20 1.55

1.60

1.65

1.70

1.75

1.80

1.85

1.90

1.95

Time/s

Fig. 8 Piston displacement and in-cylinder pressure with the glow plug ignition.

3.2. Comparison of engine performance under two ignition modes First, a comparison of the FPLE performance under two ignition modes is demonstrated. The piston assembly mass in this experiment is fixed at 1.1 kg, and the mixture flow rate is retained at Q = 36L/min. The mean peak pressure and COV of peak pressure is the main parameter that represents engine performance and the engine stability [38]. Table 5 shows the variations of two parameters with different spark timings. The mean peak pressure has a maximum value with the lowest COV of peak pressure when the spark timing is 4 mm from the cylinder head. Therefore, the optimized spark timing of 4 mm is selected in the following tests in SI mode. In addition, the GPHP is 7.2 W in the GPI mode. Fig. 9 (a) implies that the pressure and displacement extremely fluctuate when the engine is operated in SI mode with severe cycle-to-cycle variations. These fluctuations can seriously affect the stability of engine operation and deteriorate thermal efficiency. In particular, two pressure peaks are observed, as indicated by the red dotted lines. The first pressure peak is caused by the in-cylinder gas compression as the piston reaches the TDC, and then, a second pressure

peak appears due to the combustion heat released from the mixture. Kim et. al [32] also found this phenomenon in their study. They attributed the phenomenon to the ignition timing delay, which is the time difference between the start of ignition trigger and the start of combustion. Meanwhile, Fig. 9 (b) shows the in-cylinder pressure and piston displacement in GPI mode. The pressure’s double peaks are not observed, and only a slight fluctuation in the pressure and displacement is evident. The engine runs in a very stable state. Fig. 10 presents the statistical pressure results in the two ignition modes. Fig. 10 (a) shows that the peak pressure distributions in SI mode concentrate from 4.5 atm to 7 atm and are considerably scattered, indicating that the engine runs under poor conditions. By contrast, the peak pressure is primarily concentrated from 23 atm to 25 atm and accounts for 76.3% in 300 cycles in GPI mode, as shown in Fig. 10 (b).

Table 5 The COV of peak pressure with different spark timings in the SI mode. 7

6

5

4

3

Mean Peak pressure/ atm

4.99

5.52

5.87

6.30

5.54

COVpeak pressure/%

21.97

20.22

19.53

18.13

20.19

Pressure Displacment

9

30

Ignition timing=4mm piston assembly mass=1Kg

15

7

6

6

3

5

0

4

-3

-12

3

0

-15

0

250

300

350

400

450

500

0

550

-4

9

1

200

4

12

-9

150

8

15

2

100

12

18

-6

50

16

21

3

0

20

24

In-cylinder pressure/atm

9

Piston displacement/mm

In-cylinder pressure/atm

8

Pressure Glow plug heating power=7W Displacement Piston assembly mass=1Kg

27

12

-8

6

Piston displacement/mm

10

Spark timing/mm

-12 -16 0

20

40

60

80

100

120

140

160

180

-20 200

Time/ms

Time/ms

(a) (b) Fig. 9 The in-cylinder pressure and piston displacement: SI mode (a) and GPI mode (b). 120

Ignition timing=4mm piston assembly mass=1.1Kg

34.15%

240

80

Cycles of peak pressure

Cycles of peak pressure

100

21.95%

60

14.63% 40

14.63%

12.19%

20

160 120 80 40

4.5～ 5

5～ 5.5 5.5～ 6 6～ 6.5 6.5～ 7 Peak pressure range/atm

7～ 7.5

76.3%

200

2.44% 0

Glow plug heating power=7.2W Piston assembly mass=1.1Kg

1.31% 0

13～ 15

5.26%

6.58%

5.26%

5.26%

15～ 17 17～ 19 19～ 21 21～ 23 Peak pressure range/atm

23～ 25

(a) (b) Fig. 10 Cycles of peak pressure in different pressure ranges (during a totally of 300 cycles: SI mode (a) and GPI mode (b).

Figure 11 demonstrates the detailed comparisons of the FPLE operating characteristics in the two ignition modes. The mean peak pressure in SI mode is only 6.3 atm, which is far less than the mean peak pressure of 22.2 atm in GPI mode. The lower the mean in-cylinder peak pressure, the worse the engine performance. The engine running frequency, mean piston speed, indicated power, and indicated thermal efficiency working in GPI mode are greater than those in SI mode by approximately 2.2, 3.1, 4.1, and 4.2 times, respectively. The significant performance differences can be explained as follows. In SI mode, the spark timing of the FPLE is fixed, and the working principle of the spark plug is similar to that in the conventional ICEs. However, the TDC of the FPLE is varied from cycle to cycle due to the lack of crankshaft, and it is practically impossible to trigger the ignition signal at the proper location; thus, The ignition strategy of the conventional engine is evidently not suitable for a free-piston

engine. On the contrary, the ignition timing of FPLE is coupled to the state of the compressed mixture and cylinder properties in GPI mode. Moreover, the ignition time is not fixed and can be controlled automatically and flexibly by itself. Thus, the engine cycle is closer to the ideal and high-thermal-efficiency Otto cycle [10], which promotes the engine’s overall performance and improves FPLE’s operating state. Spark plug ignition 22.2

Glow plug ignition

1.9

30.4

10.1

204

13.6 0.61 6.3

49.2

-1 z ak m /W /H .s pe at er cy /m n re/ n d ow a e e p e su u e m es eq ed sp at n Fr pr ic ea d M In

2.4

E/ IT

%

Fig. 11 FPLE performance comparison under different ignition mode.

3.3. The effects of GPHP on the engine operating characteristics GPHP is an important parameter that directly affects engine performance because it controls the time of combustion starting. In the experiments, the GPHP should not be too low to realize the stable operation of the FPLE. It also should not be too high because it can shorten the life of the glow plug. Therefore, the glow plug heating power is eventually determined in a reasonable range (4.16W-7.2W) after repeated tests, which can ensure the stable operation of the engine and the life of the glow plug. Fig. 12 shows the piston velocity versus piston displacement of approximately 100 consecutive cycles randomly selected from a total measurement of 600 cycles. The piston assembly mass is fixed at 1.1 kg, and the mixture flow rate is maintained at 36 L/min. The piston motion trajectory has a “fillet rectangle” shape, and the displacement cycles are in a central symmetry structure when the displacement is zero and the piston is in the middle stroke. This condition illustrates that the fuel/air mixture in both cylinders can be successfully ignited to realize initial combustion. The piston velocity for a certain cycle is nearly constant and kept at approximately 2 m/s in the middle of the stroke. However, a slight fluctuation between the different cycles is observed when the piston moves to the middle of the stroke because the fuel/air mixture drawn to the cylinders is slightly varied from cycle to cycle, resulting in the pressure variation.

4

4

Mixture flow rate=36L/min Glow plug heating power=4.16W

3 2

Piston velocity/m.s-1

Piston velocity/m.s-1

Mixture flow rate=36L/min Glow plug heating power=5W

3

1 0 -1 -2

2 1 0 -1 -2

-3 -3

-20

3

-15

-10

-5

0

5

10

15

-15

0

-5

-10

5

Piston displacement/mm

Piston displacement/mm

(A)

(B)

Mixture flow rate=36L/min Glow plug heating power=6.1W

Piston velocity/m.s-1

1 0

10

15

20

Mixture flow rate=36L/min Glow plug heating power=7.2W

3

2

Piston velocity/m.s-1

-20

20

2 1 0

-1

-1

-2

-2

-3

-3 -20

-15

-10

-5

0

5

Piston displacement/mm

(C)

10

15

20

-20

-15

-10

-5

0

5

10

15

20

Piston displacement/mm

(D)

Fig. 12 Piston speed and displacement profiles under different glow plug heating power.

Figure 13 demonstrates the FPLE performance under different GPHP. It is observed the mean piston speed, FPLE running frequency, indicated power IMEP, and indicated thermal efficiency show a similar changing trend. They are decreased when the GPHP is increased by an adjustment in the range from 4.16 W to 7.2 W. This condition can be explained as follows. The temperature of the platinum wire is higher when the GPHP is increased. The higher GPHP can result in over-advanced combustion phasing, which depresses the compression ratio and slow down the flame propagation during the combustion, thereby producing weaker engine performance. The variations of these parameters are small (within 10% under different GPHP). Taking the indicated power as an example, the value is 206 W when the GPHP is 7.2 W, and it reaches 219 W when GPHP is 4.17 W, which increases by 6.4%. Fig. 13 shows that the maximum indicated power is 219 W and the maximum indicated thermal efficiency is 10.85% when the GPHP is 4.16 W.

Mean piston speed/m.s-1 Frequency/Hz Indicated power/W IMEP/MPa

2.05

Mixture flow rate=36L/min Piston assembly mass=1.1Kg

2.00 1.95 1.90 28.4 28.2 28.0 220 215 210 205 0.61 0.60 0.59 0.58

ITE/%

10.8 10.5 10.2 4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

Glow plug heating power/W

Fig.13 FPLE performance map under different glow plug heating power.

3.4. Effect of piston assembly mass The piston assembly (namely mover) mainly consists of a dual piston, connecting rod, and mass block. The piston assembly mass is modified by an adjustment in the mass block fixed on the connecting rod. This experiment has varied assembly mass (0.63, 0.91, and 1.1 kg), and the GPHP and the mixture flow rate are kept at 7.2 W and 36 L/min, respectively. Fig. 14 describes the maximum in-cylinder pressure and the corresponding COV under different piston assembly masses. The mean peak pressure increases linearly from 21 atm to 23 atm with increasing piston assembly mass because greater piston assembly mass means larger inertial force, which contributes greatly to the rise of the compression ratio at the end of the compression stroke. Furthermore, greater piston assembly mass means slower engine speed, leading to having additional time available for scavenging and thus added fuel/air mixture drawn into the cylinders. The coefficient of variation is often used to show the engine operating stability. The increase of piston assembly mass results in a decrease in the COV of maximum incylinder pressure of 600 cycles from 11.7% to 7.5%, indicating that the FPLE runs at an increasingly steady state.

23.2

12

Mean peak pressure COV

11

22.4 10

COV/%

Mean peak pressure/atm

22.8

22.0 9 21.6 8

21.2

7

20.8 0.6

0.7

0.8

0.9

1.0

1.1

Piston assembly mass/Kg

Fig. 14 The maximum in-cylinder pressure and the corresponding COV of pressure.

Indicated power/W

Frequency/Hz

Mean piston speed/m.s-1

The FPLE can be considered a damped spring-mass system, and the piston assembly mass is the main parameter that determines the FPLE operation speed. A smaller assembly mass is expected to have higher piston speed and engine running frequency. Fig. 15 shows the FPLE performance under different piston assembly masses. The trends of piston speed and frequency are consistent with the above analyses. The running frequency and maximum mean piston speed are 33.4 Hz and 2 m/s, respectively, with a minimum piston assembly mass of 0.63 kg. The larger piston assembly mass can result in higher in-cylinder pressure and IMEP. However, the indicated power and thermal efficiency are decreased with piston assembly mass because of the reduced running frequency. Their maximum values are 248 W and 11.24%, respectively. 2.00

Mixture flow rate=36L/min Glow plug heating power=7.2W

1.96 1.92 1.88 33.6 32.0 30.4 28.8 250 240 230 220

IMEP/MPa

0.60 0.56 0.52

ITE/%

11.2 11.0 10.8 0.6

0.7

0.8

0.9

1.0

Piston assembly mass/Kg

1.1

Fig. 15 The FPLE performance map under different piston assembly mass.

3.5. Effect of mixture flow rate The effects of mixture flow rate on the operating characteristics of FPLE are investigated. The mixture flow rates are adjusted from 32 L/min to 44 L/min at 4 L/min intervals. The GPHP and the piston assembly mass are retained at 4.16 W and 1.1 kg, respectively. Table shows the corresponding relationship between gas flow rate and pressure in the balancing vessel. Table 6 The relationship between mixture flow rate and the pressure in the balancing vessel. Mixture flow rate(L/min)

32

36

40

44

pressure in the balancing vessel (kPa)

102.34

103.05

104.03

104.91

2.1 2.0

Glow plug heating power=7.2W Piston assembly mass=1.1Kg

1.9

Frequency/Hz

1.8 30

Indicated power/W

Mean piston speed/m.s-1

Figure 16 presents that the mean piston speed, the running frequency, the indicated power, and the IMEP increase with the mixture flow rate. A higher charging efficiency is achieved with a larger mixture flow rate. Therefore, the absolute amount of fuel/air mixture drawn into the cylinder is increased, thereby improving the performance of the FPLE. The indicated power and peak frequency are 251 W and 30.3 Hz, respectively, at a mixture flow rate of 44 L/min. However, the increase of the mixture flow rate results in a decrease of the indicated thermal efficiency due to the increased severity of the mixture loss through the exhaust port. The peak indicated thermal efficiency is 11.65% when the mixture flow rate is 32 L/min.

250 240 230 220 210

29 28

IMEP/MPa

0.64 0.62 0.60

ITE/%

11.5 11.0 10.5 10.0 32

34

36

38

40

42

Mixture flow rate/L.min-1

44

Fig. 16 The FPLE performance map under different mixture flow rate.

3.6. Electric power For our study of the electrical characteristics, the linear generator is coupled with the connecting rod with load. The electrical power characteristics of the FPLE system are explored with different resistance loads. Mixture flow rate, GPHP, and the piston assembly mass are retained at 44 L/min, 4.16 W, and 1.1 kg, respectively. The engine running frequency with increased external resistance is increased from 13.9 Hz to 15.3 Hz and then starts to decrease, as shown in Fig.17 (A), but the running frequency is markedly lower than that of 30.3 Hz without load, as can be seen in Fig. 16. Two major factors can explain this phenomenon. First, the induced current generated in the coil increases the magnetic resistive force on the mover, thus restraining the increase of the running speed of the engine. Second, the cogging force from the magnetic attraction between permanent magnets and stator teeth attempts to maintain the alignment. The electric power first increases and then starts to drop as resistance loads increase. The peak electric power of 31.6 W is obtained at the resistance of 6.3 Ω. Fig. 17 (C) shows that the changing trend of the work conversion efficiency is similar to that of the electric power. The maximum work conversion efficiency is 42.8% with an external resistance of 6.3 Ω. 15.4 15.2

Mixture flow rate=44L/min

32

A

30

Mixture flow rate=44L/min

B

Electric power/W

Frequency/Hz

15.0 14.8 14.6 14.4 14.2

28 26 24 22

14.0

20 13.8 4

5

6

7

Work conversion efficiency/%

44 42

8

Electric resistance/Ω

9

10

11

4

5

6

7

8

9

10

Electric resistance/Ω

Mixture flow rate=44L/min

C

40 38 36 34 32 30 28 26 4

5

6

7

8

9

10

Electric resistance/Ω

Fig. 17 The electric characteristics: (A) running frequency, (B) electric power, (C) work conversion efficiency.

4. Conclusions

11

A miniature reciprocating free-piston linear engine with glow plug ignition was designed and manufactured. This study investigated the FPLE starting process, performance comparison between two ignition modes, and the operating characteristics under various parameters. The main conclusions can be summarized as follows: 1. The in-cylinder fuel/air mixture could be ignited successfully by the glow plug within the fifth cycle under the single-cylinder self-starting strategy, and the steady FPLE operation was implemented, demonstrating that the starting strategy proposed was feasible for the FPLE. 2. The performance and operating stability of the FPLE were significantly improved in GPI mode due to the flexible ignition timing in comparison with that of SI mode, and the GPI mode was more suitable for the FPLE. 3. The effects of key parameters such as glow plug heating power, piston assembly mass, fuel/air flow rate on the operating characteristics as well as performance of the FPLE were investigated. It was indicated that by increasing the GPHP, the engine showed a weaker performance because of the over-advanced ignition timing, which depressed the compression ratio and slowed down the flame propagation during combustion. The larger piston assembly mass contributed to the operating stability of the FPLE while the engine performance was decreases due to the lower running frequency. By increasing the fuel/air flow rate, the running frequency, indicated power as well as IMEP of the engine could be enhanced resulting from greater scavenging efficiency. However, the serious mixture loss through exhaust ports also caused a decrease in the indicated thermal efficiency. 4. During the electric power generation, the FPLE could achieve steady operating with varied external loads. The running frequency, electric power and work conversion efficiency firstly went up with the increased external load and then decreased. The maximum electric power of 31.6 W and the maximum work conversion efficiency of 42.8% were obtained with an external resistance of 6.3 Ω. Although the stable operation of the FPLE was realized with and without load, some significant technical difficulties remain and require resolution. These obstacles present barriers to practical use of the FPLE as an energy supply system for small-scale vehicles and emergency power supply system for some electronic devices. And the FPLE still suffers from poorer performance compared to those in either four-stroke diesel or sparkignition engines. The future work will focus on the improvement of scavenging type, attenuating of heat transfer loss and the optimization of linear alternator to increase the work conversion efficiency.

Acknowledgements Funding: This work was supported by the National Basic Research Program of China (No. 2014CB239603) and the National Natural Science Foundation of China (No. U1738113).

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