Basic study on high precision positioning in electromagnetic valve drive system for improving internal combustion engine using linear actuator

Abstract

In recent years, further improvements in the performance of internal combustion engines to reduce environmental burden have been demanded. The combustion state of an engine is affected by the parameters of the dynamic valve system, such as the opening/closing timing and the lift of the intake and exhaust valves. Therefore, we investigate an electromagnetic valve-drive system in this study. Two models are created to improve the magnetic circuit and increase thrust. To investigate the thrust characteristics of these actuators, an electromagnetic field analysis is conducted using the finite element method. Meanwhile, to investigate the effect of the yoke material on the thrust, an electromagnetic field analysis is performed based on materials with different saturation magnetic flux densities and magnetic permeabilities.

Keywords

Linear motor linear actuator voice coil motor electromagnetic valve drive system EVDS electromagnetic

1. Introduction

In recent years, further performance improvements in internal combustion engines to reduce environmental burden have been demanded. Improved combustion conditions are necessary to improve the performance. The combustion state of an engine is governed by the parameters of the valve train, such as the timing of opening and closing the intake and exhaust valves, and the lift amount. The valves are operated by a cam mechanism; however, the valve motion is constant because it depends on the cam shape, which affects the fuel consumption and power output depending on the engine operating conditions. Variable valve mechanisms have been used to solve these problems [1–5]. The variable valve mechanism changes the cam profiles for each engine speed range to optimize the valve opening/closing timing and lift, thereby increasing the engine output. Conventional variable valve mechanisms cannot be used to operate the valves at the optimum timing and lift for all engine rotation speeds. Therefore, an electromagnetic valve drive system (EVDS) was proposed. The linear motor operates the valve, enabling stepless changes in valve lift and timing, and optimal valve operation at each engine rotation speed. This is expected to further increase the engine output. Other research groups have proposed intake and exhaust systems using actuators; however, these systems feature a large actuator size, a large mass owing to the complex structure, and inability to obtain high thrust [6–10]. Hence, we propose a linear actuator that utilizes the Lorentz force [11]. This actuator presents a simple structure and can operate at low alternating current frequencies, thereby enabling high responsiveness and precise motions via direct operations. Images of a conventional engine cylinder head are shown in Fig. 1. On the cylinder head, timing chains and sprockets transmit engine power to the camshafts, and valve springs to actuate the valves. If the valves can be operated by the EVDS, then all of these components can be eliminated, and power loss can be reduced because the engine power need not be allocated to operate the valves. In addition, as the variable valve lift can replace the throttle valve operation in spark ignition engines, their pumping losses at the low and medium engine loads are reduced. Figure 2 shows a block diagram of the control of the EVDS as envisioned by the author. An angle sensor is attached to the crank and a displacement sensor to the mover of the actuator. The angle and displacement sensors are connected to a digital signal processor (DSP). A control signal u is sent from the DSP based on the crank angle θ and displacement z of the mover, and the signal is amplified by an amplifier (AMP) to control the actuator. The valve is controlled by changing the voltage by feed-forward control according to the crank angle. Furthermore, a servo system is constructed to correct the deviation from the command by feedback control based on the displacement of the mover. In this study, three finite element models were constructed for the proposed linear actuator and their thrust characteristics were evaluated via electromagnetic field analysis. In addition, the effect of the stator material on the thrust characteristics was investigated.

Fig. 1.

Conventional engine intake and exhaust system mounted on cylinder head.

Table 1

Dimensions of each model

Parameters		Model A	Model B	Model C
	Outer diameter	𝜙 83.5 mm	𝜙 120.9 mm	𝜙 168.5 mm
VCM case	Inside diameter	𝜙 80.5 mm	𝜙 80.5 mm	𝜙 108.5 mm
	Height	76.5 mm	100 mm	140 mm
	Outer diameter	𝜙 80.1 mm	𝜙 80.1 mm	𝜙 108.1 mm
Permanent magnet	Inside diameter	𝜙 48.4 mm	𝜙 48.4 mm	𝜙 76.2 mm
	Height	64 mm	64 mm	72.5 mm
	Outer diameter	𝜙 48 mm	𝜙 48 mm	𝜙 75.8 mm
	Inside diameter	𝜙 12 mm	𝜙 12 mm	𝜙 40.2 mm
Coil	Height	50 mm	50 mm	50 mm
	Turns	200	200	200
	Coil space factor	69.8%	69.8%	70.6%

Fig. 2.

EVDS control system.

2. Electromagnetic valve drive system

As shown in Fig. 3, the electromagnetic valve drive system operates the intake and exhaust valves of an engine using the Lorentz force via a solenoid comprising a coil, a magnet, and an iron core. The case and magnet are the stators, and the coil and valve are the movers. In previous studies, an electromagnetic field analysis was performed on a prototype model created using three-dimensional CAD software to investigate the magnetic field and thrust. The magnetic flux generated by the permanent magnet was not sufficiently orthogonal to the coil, and magnetic saturation occurred outside the yoke. Furthermore, the thrust was only approximately 35.3 N and was operable only in low revolutions per minute ranges. Therefore, a model that can provide thrust more efficiently is required to accommodate higher RPM ranges. In this study, we created two models to improve the magnetic circuit and increase thrust using the 3D CAD software SolidWorks (Dassault Systèmes SolidWorks Corporation). Figure 4 shows the model created, along with previous models. Figure 4(a) shows the model from a previous study. Figure 4(b) shows the newly created model, i.e., Model B, which eliminated the use of bobbins. Furthermore, this model features a solid axis at the center of the yoke, which facilitates the passage of magnetic flux. Figure 4(c) shows Model C, which is an improved version of Model B, with a larger outer yoke and shaft diameter. The dimensions of the analyzed model are listed in Table 1. For Model B, the thickness of the yoke was 20 mm, but the coil shape and shaft diameter were identical to those of Model A. Model C had a yoke thickness of 30 mm and a shaft diameter of 40 mm. The cross-sectional area of the coil was the same for all Models A, B, and C. However, the larger shaft of Model C resulted in a coil outer diameter of 75.8 mm, which was approximately 1.6 times larger than that of Model A. Transient response analysis was performed on the model shown in Fig. 4, with a whole model and a valve lift of 10 m. The electromagnetic field analysis software JMAG (JSOL Corporation) was used to perform the transient response analysis using the finite element method. An air area five times larger than the model was set, and the number of steps was set to 21, with a displacement of 1 mm per step every 0.1 second. The mover was lifted from 0 mm to 10 mm, the current was reversed at 10 mm, and the mover was descended from 10 mm to 0 mm. The mesh size was set to 2.8 mm, The number of coil turns was set to 200, the coil resistance to 1 Ω, and the supply voltage to 20 V. Based on these conditions, the thrust characteristics of the valve in the reciprocating motion were investigated. The conditions for the electromagnetic field analysis are listed in Table 2.

Table 2
Parameters of each model

Case YEP-B(Permalloy)

Bobbin YEP-B(Permalloy)

Permanent magnet NEOMAX-P11

Coil Copper

Copper wire diameter 𝜙 2.0 mm

Coil resistance 1 Ω

Voltage 20 V

Fig. 3.

Electromagnetic valve drive system.

Fig. 4.

Models from previous studies (Model A) and newly created models (Model B and C) for analysis.

Fig. 5.

Vector plot diagram of saturation magnetic flux density.

Fig. 6.

Relationship between Lorentz force and displacement.

3. Effect of actuator geometry on magnetic circuit and thrust

A vector plot of the magnetic flux density obtained from the analysis of each model is shown in Fig. 5. In Models B and C, the increased thickness of the outer yoke did not cause magnetic saturation. However, the center of the yoke, where the magnetic flux was concentrated, became saturated. This might have caused thrust loss. Thus, magnetic saturation occurred in all models, and the magnetic flux emitted from the permanent magnet was not efficiently converted into the Lorentz force. These magnetic saturations can be eliminated to improve the thrust. Figure 6 shows a graph of the thrust obtained from analyzing each model. The Lorentz force generated in each element from the obtained magnetic flux density and current density is expressed by the following equation. $\begin{eqnarray}\boldsymbol{f}_{n}=\boldsymbol{i}_{n}\times \boldsymbol{B}_{n}.\end{eqnarray}$ (1) The Lorentz force in each element is denoted by f _n, i _n is the current, and B _n is the magnetic field. The Lorentz force generated in each element is summed up to the thrust generated by the coil. Figure 6 plots only the axial component of the vector field obtained from the analysis. The vertical axis represents the thrust and Lorentz force, and the horizontal axis represents the displacement of the mover. The single dotted line with triangular markers represents Model A; the dashed line with square markers, Model B; and the solid line with round markers, Model C. The valve began to move upward from 0 mm, turned back at 10 mm, and then descended toward 0 mm. In all the models, the thrust did not change significantly with the valve motion and remained approximately constant. Mover lifts of 10 mm resulted in stable actuation with a constant thrust at any point. The required thrust varies with the lift, engine rotation, and open/close time, so it varies from engine to engine, but under conditions similar to those of a typical automobile gasoline engine, Models A and B can operate at approximately 3000 rotation per minute (rpm), and Model C at approximately 4000 rpm. The thrust increased significantly in Model C, which is attributable to the following three factors: a thicker yoke center section, an increased magnet volume, and an improved magnetic circuit. However, magnetic saturation continued to occur in some regions. These are expected to be improved by the material properties of the yoke.

Table 3

Materials used in current analysis

Name of material	Permeability μ [H/m]	Relative permeability μ∕μ₀ [−]	Saturation magnetic flux density B [T]
Phytherm230-1150_160deg	0.491	390992	0.57
Phytherm260-1150_160deg	0.264	210312	0.64
Supermimphy_LSS-1170	0.229	181891	0.66
Phytherm120-1150	0.031	24757	0.72
Phytherm230-1150_20deg	0.050	39789	0.74
Cryophy-1170_20deg	0.425	338204	0.74
Permimphy_SP-1170	0.810	644199	0.74
Mumetal-1170	0.463	368046	0.74
Supermimphy_L-1170	0.800	636620	0.75
Phytherm260-1150_20deg	0.056	44563	0.80
Cryophy-1170_-269deg	0.350	278521	0.80
Supra36-1050	0.038	30411	1.25
Supra50-1150	0.279	222093	1.47
Satimphy-1170	0.460	366056	1.48

Fig. 7.

Relationship between average thrust and saturation magnetic flux density.

4. Relationship between yoke material and actuator thrust

4.1. Consideration of yoke material

To investigate the effect of the yoke material on thrust, electromagnetic field analysis was performed for 14 materials with different saturation magnetic flux densities and magnetic permeabilities. The materials used in this study are listed in Table 3. The model used in the analysis was Model B, which had a sufficiently thick yoke and a required relatively short analysis time. The saturation flux density was varied from 0.57 to 1.48 T, and the magnetic permeability was varied from 0.031 to 0.81 H/m. These 14 materials were applied to the yoke of Model B for transient response analysis.

4.2. Analysis results

A graph of the saturation flux density vs. average thrust is shown in Fig. 7. The vertical axis represents the average thrust and the horizontal axis represents the saturation flux density. As shown in Fig. 7, the average thrust improved as the saturation magnetic flux density increased. A difference of approximately 26 N was observed between the lowest and highest saturation flux densities. This indicates that the increase in the thrust is related to the saturation magnetic flux density. Furthermore, the thrust can be increased by changing the yoke material to one with a higher saturation magnetic flux density.

5. Conclusion

In this study, a new model was created, and an electromagnetic field analysis was performed to improve the magnetic circuit of a linear actuator. Furthermore, the effects of changing the yoke material on the magnetic flux flow and thrust were examined. The results confirmed that the thrust was stable and almost constant regardless of the mover position in a reciprocating motion with a mover lift of 10 mm. However, in all models, magnetic saturation occurred in the center of the yoke, where the magnetic flux is concentrated. Consequently, the thrust improved as the saturation magnetic flux density increased. Therefore, the actuator for the electromagnetic valve drive system proposed herein should be fabricated using a material with a high saturation magnetic flux density, which is expected to improve thrust and operate optimally depending on the rotation speed and combustion state of the engine. Currently, it is difficult for any of the models to fit into a cylinder head, but with further optimization of the geometry and materials, it is possible to design a more efficient and compact actuator that can be installed in an actual engine. In actual engines, mechanical friction and aerodynamic loads caused by injected fuel are added to the valve, so these effects must also be considered. In the future, we intend to improve the efficiency of actuators by analyzing the mechanical friction and aerodynamic load generated by valves and by optimizing their shapes and materials.

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Case	YEP-B(Permalloy)
Bobbin	YEP-B(Permalloy)
Permanent magnet	NEOMAX-P11
Coil	Copper
Copper wire diameter	𝜙 2.0 mm
Coil resistance	1 Ω
Voltage	20 V