Key Theoretical Research On Decoupling Of Engine Mounting System And Design Of Novel Semi-active Hydraulic Engine Mount

Engine mounting system is elastic equipment which connects powertrain and thechassis of vehicle to isolate vibration of engine. It is composed of somecomponents, such as rubber mount and hydraulic engine mount, and plays animportant role in vibration isolation, vibration reduction and noise reductionperformance for modern passenger vehicle. On the other hand, better enginemounting system can also reduce the stress of structure to increase the durability ofthe components. The optimization is mainly carried out to improve relative indexesof engine mounting system’s performance, where the decoupling of the system isimportant performance. There are two kinds of decoupling principles: one is thedecoupling of the system associating with Center of Rigidity (CR), the other is toachieve that only one model shape is excited by the exciting force. Theoptimization of engine mounting system by the decoupling principle associatedwith CR is to make CR of engine mounting system coincide with center of mass ofthe engine, but there are no articles that study CR completely. For the otherdecoupling principle, the method called TRA (Torque Roll Axis) in current articlesonly considers the influence of torque without force, so it is limited in theory.Traditional rubber mount has nearly frequency-independent dynamic stiffness,and it cannot fulfill the complex work conditions of engine. Ideal engine mountshould have large stiffness at low frequency and small dynamic stiffness at highfrequency, and hydraulic engine mount has better frequency-dependent dynamicstiffness than rubber mount. Resonant curves which are used to judge the vibrationisolation performance are suitable for traditional frequency-independent systemwith single degree of freedom or they cannot be used to judge the performance offrequency-dependent isolator rapidly, so new method should be developed. The research on hydraulic engine mount focuses on: studying the performance ofcurrent hydraulic mount and developing novel hydraulic mount.The paper theoretically studies the two decoupling principles of enginemounting system, researches the dynamic characteristic of hydraulic engine mountwith frequency-dependent dynamic stiffness and designs a novel semi-activehydraulic engine mount. The main contents and some creative conclusions in thepaper are as follows:(1) Derive decoupling criteria for a general exciting force to excite onlyone model shape.Dynamic equations are set up at the center of mass around equilibriumposition of the rigid engine which is supported by some mounts. Considering or notconsidering the influence of disproportional damping respectively, the decouplingcriteria that the system should meet are derived through Fourier transformation sothat a general exciting force (including force and torque) excites only one modelshape. Traditional TRA method is derived through Laplace transformation in statespace and only considers the influence of torque without force. Then the featuresand physical meaning of the decoupling criteria are introduced. Throughoptimization of engine mounting system, the system can be fully decoupledconsidering the influence of disproportional damping, whereas the system cannotbe fully decoupled without considering the influence of damping.(2) Derive the sufficient and necessary condition for the existence of CRand the method to calculate the location of CR.Center of rigidity is a point: arbitrary static force along CR can make the rigidbody have only translational displacement, whereas the torque along CR can makethe rigid body have only rotational angles at CR. This kind of CR is called CR withstrict definition in the paper.Using equations of static equilibrium at center of mass of engine, theequations of static equilibrium are set up at the point where force and torque is applied. From the definition of CR and the characteristic of anti-symmetric matrix,the conditions that the system should meet are derived under static force conditionand torque condition respectively. A symmetric judging matrix B is defined in thepaper, and CR with strict definition exists only when matrix B is null. Themethod to calculate its position is given. It is found that CR with strict definitionfor the isolation system with the symmetric layout may not exist sometimes.(3) Derive the sufficient and necessary conditions for the existence of CRwith weak definition and the method to calculate its location, and research thecharacteristic of displacement response when CR with weak definition exists.Generally, CR with strict definition does not exist, but a static force alongcertain point may have only translational displacement of the system, and this kindof point is called CR with weak definition. From this definition, the conditions thatthe system should satisfy for the existence of CR with weak definition aredescribed by the eigenvalue of judging matrix B. According to the characteristicof eigenvalue of judging matrix B, when CR with weak definition exists, thebehaviors of displacement response are classified into some types and the methodto get the location of CR with weak definition is introduced. A sample is given tocalculate CR with weak definition of the system and validates the proposedmethod.(4) Give a graphical presentation method to rapidly judge the vibrationisolation performance for the isolator with frequency-dependent dynamicstiffness.Using vibration isolation model with single degree of freedom, the definitionsof transfer dynamic stiffness and drive-point dynamic stiffness of isolator withfrequency-dependent dynamic stiffness are introduced. The force transmissibility ofthe vibration isolation system is derived by using transfer dynamic stiffness anddrive-point dynamic stiffness. Two curves are defined in frequency domain:resonant curve and vibration isolation curve. The vibration isolation performance can be concluded rapidly by judging relationships between the dynamic stiffnessin-phrase of the isolator and the two defined curves in various frequency bands. Bythis graphical presentation method, we find that hydraulic engine mount can havebetter vibration isolation performance in middle frequency band whereas thevibration isolation performance for hydraulic mount is worse than that for rubbermount in low and high frequency band, and this conclusion is validated.(5) Design a novel semi-active hydraulic mount with high performance.The expression of dynamic stiffness of hydraulic is derived by making use oflumped parameter model. The differences and comparisons between passive TMD(Tuned Mass Damper) system and hydraulic engine mount are obtained bycomparing the two concepts of dynamic stiffness between them. The relationshipsbetween the three key frequency points (notch frequency, resonant frequency andpeak frequency) of dynamic stiffness for hydraulic mount and physical parameters(such as length or cross section of inertia track) that influence their locations areresearched, and it is concluded that tuning only one parameter can only tune thesaid frequency points in narrow frequency band. A novel configuration ofsemi-active hydraulic mount is given, and this mount can tune both cross sectionand length of inertia track with the help of screw thread at the same time. Thehydraulic mount can tune the three frequency points as wide as [0,). Two energyconservations (friction self-lock and optimal tuning time) are applied to decreaseenergy consumed. Finally, the optimal length of inertial track and correspondingdynamic stiffness are gotten by optimal algorithm. The result shows that: withoptimal length of inertial track, the ideal dynamic stiffness which is large at lowfrequency and small at high frequency can be gotten.