Hanzheng Wang, Changlu Zhao, Fujun Zhang, Zhe Zuo and Yi Lu(School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China)

With the increase of requirements for fuel economy, emission performance and compactness of vehicle power system, new technologies and principles applied to new power units are being explored[1-3].

Opposed-piston two-stroke engine (OP2S) is a unique power plant, which possesses the characteristics of high thermal efficiency and high power density, which made once widely utilized[4]. The most important feature for these kind of engines is that there are two pistons moving towards each other in opposite directions in a single cylinder[5-9]. However, the development of OP2S engines has been limited because of dynamic problems such as the major lateral force from the multi-linkage mechanism[10-11].

A hydraulic free piston engine (HFPE) is a combination of both the reciprocating piston internal combustion engine and the axial piston pump system, which has the characteristics of a variable compression ratio, high isopycnal and flexible layout[12]. However, most synchronous chambers in the HFPE engines are controlled by check valves. Without a fast response speed of the valve or a sufficient fuel supply pressure, there would be serious cavitation erosion phenomena in the pump chamber[13].

On this basis, a new type of opposed-piston hydraulic-output (OPHO) engine has been designed, which combines the concepts of the OP2S engine with the HFPE engine.

1 Integral Structure of OPHO Engine

The configuration of the OPHO engine is shown in Fig.1, which consists of a combustion mechanism, synchronous drive mechanisms and output mechanisms. The two synchronous drive mechanisms and output mechanisms are symmetrical relative to the top dead center(TDC) position.

Since working processes are basically the same for opposed four cylinders, only one of them needs to be discussed. Fig.2 shows a schematic of opposed two pistons sharing one cylinder.

While the power piston is moving between the top dead center(TDC) and the bottom dead center(BDC), both the air inlet and outlet would be opened and closed continually.

Fig.1 Configuration of the OPHO engine (4 cylinders)

Fig.2 Schematic depiction of a single cylinder of the OPHO engine

At the beginning of the compression stroke, both the power piston and the outer drive plunger are located at the BDC. Since the eccentric wheel on both sides of the inner and outer sides have 180°phase difference, the inner drive plunger stays at the TDC. In this process, the low pressure oil in the low pressure energy accumulator (not shown) flows into the inner connection oil groove inside the distribution plate. Thereafter, the flywheel (not shown) drives the eccentric shaft to rotate so that the outer drive plunger moves upwardly. The hydraulic pressure on the outer face of the two synchronous plungers urges the power piston to move inwardly at the same time, and the oil in the inner driving oil chamber is urged by the hydraulic pressure on the end face of the left side ring of the synchronous plunger so that the inner drive plunger body would move to the BDC.

In the expansion stroke, due to high temperature and high pressure mixture combustion and rapid expansion,the power piston would move to the BDC, thereby to drive the common movement of the piston components. The outer drive plunger moves downwardly and drives the outer eccentric wheel and the eccentric shaft. At the same time, under the action of the eccentric shaft, the inner drive plunger moves upward, and the oil pressure in the inner synchronous oil chamber acts on the end face of the left side of the synchronous plunger due to the incompressible volume of the oil. In this stroke, the output plunger moves together with the power piston to push the high pressure oil. Finally, the oil flows into the high pressure accumulator(not shown) and the hydraulic energy would be output.

2 Composition of the Synchronous Drive Mechanism

This engine incorporates a novel synchronous drive mechanism as shown in Fig.3, which consists of a synchronous part and a drive part. The mechanism contains a power piston, synchronous plunger, drive plunger, oil chamber, connecting rod with slipper pair, eccentric wheel and eccentric shaft. It adopts an eccentric wheel to drive the plunger reciprocating motion. The design concept comes from the structure of the crank-type low-speed high-torque hydraulic motor[14-15]. In order to facilitate the study, only the unilateral single side of the driver mechanism is utilized to carry out the modeling analysis.

Fig.3 Schematic depiction of the drive mechanism

In the compression stroke, the flywheel drives the eccentric shaft to rotate continuously, while the outer eccentric wheel rotates and urges the outer drive plunger to move upward. In the expansion stroke, the high temperature and high pressure mixture burns vigorously and expands rapidly, which promotes the common movement of the piston components. In this process, the hydraulic pressure on the outer side face of the plunger is synchronized, the outer drive plunger moves downwardly and drives the eccentric shaft. At the same time, under the action of the eccentric shaft, the inner plunger body moves to the TDC position.

3 Mathematical Theory Analysis

In the kinematics analysis of the synchronous drive mechanism, the eccentric shaft is usually regarded as a uniform rotational motion and the various kinematic parameters of the mechanism are expressed as a function of the crank angle. When the rotation of the eccentric shaft is uniform, any point on the eccentric wheel is rotating towards the center of the eccentric shaft. Power piston moves along the cylinder center line, and the synchronous plunger and drive plunger move in the synchronous drive oil chamber.

Fig.4 Kinematic analysis of the drive mechanism

The traditional crank-link mechanism of the engine has been quite mature, and the analysis of its motion is shown in Fig.4a. In this paper, the motion of the drive plunger is analyzed and it is converted into the rule of the power piston movement to compare with the movement rule of the traditional crank-link engine. The advantages and disadvantages of the scheme are analyzed from the kinematics point of view. In particular, the synchronous chamber oil leakage caused by the mechanical structure is supplied continuously, which ensures that the hydraulic oil mass in the synchronous chamber is the same in each cycle, and thus will not affect the kinetic rule of the piston components. Fig.4b is a schematic diagram of the motion analysis of the drive mechanism. For the conventional engine shown in Fig.4a, whereAis the current moving position of the piston,A1andA2are the TDC and BDC positions of the piston movement, and the piston displacement could be obtained as

(1)

whereλis the crank link ratio.

For the drive mechanism of the present invention shown in Fig.4b, it is assumed that the TDC of the drive plunger is the starting point ofthe movement. When the eccentric shaft rotation angle isφ, the displacement of the drive plunger iss, the distance between the ball centerBof the drive plunger and the centerOof the eccentric shaft issOB, then

(2)

This formula can be simplified as

(3)

where

(4)

Kis a dimensionless parameter which purpose is to facilitate the design of the parameter selection. For the designed synchronous drive mechanism,e=21.5 mm,R=45 mm,l=40 mm.

So the displacement of the drive plunger is

Xplg=e+R+l-sOB

(5)

whereeis the eccentricityOQ,Ris the eccentric radiusAQ,lis the connecting rod lengthABas noted in Fig.4.

By finding the derivative of the drive plunger displacement Eq.(5), the drive plunger velocity is obtained as

(6)

whereω1is the eccentric shaft angular velocity,ω1=2πn, andnis the eccentric shaft revolving speed.

By finding the derivative of Eq.(6), the drive plunger acceleration is obtained

(7)

In this program, because the synchronous mechanism has to complete the work process by hydraulic oil, it is necessary to transfer the plunger movement rule into the power piston movement rule. When the outer drive plunger moves to the TDC, the hydraulic oil is compressed, and the entire piston components move inwardly with the two synchronous plungers. In this process, the volume of the oil discharged by the outer drive plunger movement is equal to the volume of the oil corresponding to the movement of the synchronized plunger, and the displacement of the synchronous plunger is equal to the displacement of the power piston, that is

S5Xplg=2S2Xpst

(8)

whereS5is the cross-sectional area of the outer drive plunger andS2is the lateral cross-sectional area of the synchronous plunger.

4 Kinematic Characteristics Ana-lysis

In order to facilitate the comparative study, a mathematical model of the OPHO drive mechanism and the conventional two-stroke crank-link mechanism have been established, and the motion of the plunger movement is converted into the power piston motion according to the corresponding relationship, so that the stroke of the two engines are the same stroke and both are working under the same speed. Main parameter values are entered into the model which are shown in Tab.1, and the comparison of kinematic characteristics is achieved.

The comparison of the piston displacement curves between the OPHO engine and the conventional crank-link engine is shown in Fig.5a. As can be seen, there are significant differences between the displacement of the two engines. The peak value of the conventional engine displacement curve is near 180°CA with a good symmetry. Meanwhile, the peak of the displacement curve of the OPHO engine is advanced, and the crankshaft of the expansion stroke is larger. To achieve the same actual stroke, the OPHO drive mechanism equivalent link length is less than that of the conventional engine due to the corresponding transformation relationship between the power piston and the drive plunger. At the same crank angle, swing angle of the connecting rod changes more slowly near the TDC.

Tab.1 Main parameters of drive mechanisms of two different engines

Under the same speed, the compression stroke time is shortened, which could increase the flow rate and turbulence kinetic energy of the mixture, increase the turbulence intensity near the compression TDC,and speed up the fuel and air mixing rate of the engine. Besides, it is conducive to the internal combustion engine to improve the combustion efficiency and improve the combustion process.

Fig.5b shows the piston velocity comparison. It could be seen that the piston of the OPHO engine moves faster in the vicinity of the BDC,so that the quality of scavenging deteriorated, which is not conducive to the scavenging process. Movement is slower near the TDC, which lengthen the isometric process of combustion, contribute to the cycle of thermal work, and is conducive to the combustion process. During the compression process, the piston moves fast, piston ring leakage loss and heat loss are relatively reduced, the compression temperature and pressure increases, which could shorten the combustion delay period.

Fig.5c shows the piston acceleration comparison. Due to the asymmetry of kinematics, compared with the conventional two-stroke crank-link piston acceleration, the power piston acceleration difference is large, the engine is forced by the reciprocating inertia. On the other hand, because of the unique structure of the synchronous drive mechanism, the speed change of the OPHO engine before and after the TDC is close to constant, so the piston acceleration is basically stable near the TDC, its acceleration extreme value is relatively large, the maximum acceleration point appears near the BDC, and the conventional crank-link two-stroke engine piston maximum acceleration in the vicinity of the phenomenon of the TDC is just the opposite. The piston acceleration of the OPHO is 2 500 m/s2near the BDC, which is larger than 1 400 m/s2while near the TDC. It means that the inertia force of the piston at the BDC would be greater than the inertial force experienced by the piston at the TDC, thus exhibiting a different kinetic characteristic from the conventional internal combustion engine.

Fig.5 Kinematic characteristics of the piston

Fig.6 shows a comparison of the velocity vs. displacement curves of the two engine pistons. It can be seen that the OPHO engine demonstrates significant differences of the speed change rule. The OPHO engine moves slower around the TDC and takes a short compression time, which can improve the flow rate of the mixture and increase the turbulence energy, and make the turbulence intensity of the mixture near the TDC increase. Unlike the existing HFPE engine structure, the OPHO engine utilizes a hydraulic motor-based drive mechanism. So theoretically, power piston motion is not completely “free”, thus showing the corresponding characteristics in Fig.6. In addition, the OPHO engine is not influenced by lateral forces during engine operation, because the end faces of the piston components are only forced by the mixture pressure and hydraulic pressure, which are all perpendicular to the surface.

Fig.6 Velocity vs. displacement of the engine piston

5 Conclusions

①Compared to the conventional two-stroke crank-link engine, the OPHO engine piston movement is significantly different, mainly reflected in the asymmetry of the piston kinematic curve relative to the TDC. When the engine is held at a fixed speed, the piston obtains a slower motion and a longer isometric process around the TDC, which is beneficial for the enhancement of the combustion efficiency.

②Compared to the hydraulic free piston (HFPE) engine, the kinematic rules of the OPHO engine are similar, that is, the piston displacement, velocity and acceleration are asymmetrical with respect to the TDC, and the piston velocity changes faster near the BDC.

[1] Martti L, Sten I, Seppo T, et al. Performance simulation of a compression ignition free piston engine[J] .SAE Technical Paper, 2001, 110(1): 0280.

[2] Zhang Shuanlu, Zhao Changlu, Zhong Ke, et al. Analysis of indicated work effect factors of hydraulic free-piston diesel engine[J]. Acta Armamentarii, 2016, 37(3): 394-399. (in Chinese)

[3] Peter A, Johan P, Jeroen P, et al. Horsepower with brains: the design of the chiron free piston engine[J]. SAE Technical Paper, 2001, 109(1): 2545.

[4] Regner G, Johnson D, Koszewnik J, et al. Modernizing the opposed piston, two stroke engine for clean, efficient transportation[J]. SAE Technical Paper, 2013, 26(2): 0114.

[5] Ma Fukang, Zhao Changlu, Zhao Zhenfeng et al. Matching analysis of piston motion law of opposed-piston two-stroke gasoline engine[J]. Acta Armamentarii, 2016, 37(10): 1873-1880. (in Chinese)

[6] Ma Fukang, Zhao Changlu, Zhang Shuanlu, et al. Scheme design and performance simulation of opposed-piston two-stroke gasoline direct injection engine[J]. SAE Technical Paper, 2015, 4(1): 1276.

[7] Zhao Zhenfeng, Wang Shan, Zhang Shuanlu, et al. Energy management of the hydraulic free-piston engine[J]. China Science Paper, 2015, 10(23): 2701-2706. (in Chinese)

[8] Wu Wei, Yuan Shihua, Jing Chongbo, et al. Research on working process simulation model of hydraulic free-piston engine[J]. Acta Armamentarii, 2011, 32(9): 1059-1064. (in Chinese)

[9] Zhao Changlu, Zhao Zhenfeng, Zhang Fujun, et al. A kind of mechanical-hydraulic dual power output engine: China, CN102748132A[P]. 2012-10-24. (in Chinese)

[10] Zhao Zhenfeng, Zhang Fujun, Zhao Changlu, et al.A kind of opposed-piston two-stroke engine: China, CN102733947A[P]. 2012-10-17. (in Chinese)

[11] Roberts M. Benefits and challenges of variable compression ratio (VCR)[J]. Emissions Control, 2003, 3(1): 398.

[12] Yang Huayong, Xia Bizhong, Fu Xin. Hydraulic free piston engine evolution process and recent studies[J]. Journal of Mechanical Engineering, 2001, 37(2): 1-7. (in Chinese)

[13] Xiong Siren. Performance analysis and optimization of key pair in camshaft and connecting-rod type hydraulic motor[D]. Wuhan: Wuhan University of Technology, 2011. (in Chinese)

[14] Xia Bizhong, Zhang Hui, Duan Guanghong, et al. Simulation study of dynamic characteristics for a hydraulic free piston engine[J]. Mechanical Science & Technology, 2005, 24(11):1331-1333. (in Chinese)

[15] Li Yong. Research on camshaft connecting-rod type low speed high torque hydraulic motors with high-pressure rank[D]. Shanghai: Shanghai Jiaotong University, 2007. (in Chinese)