Design of engine mounts to optimize the vibrations

Vibrations in the two-wheeler is most unwanted but inevitable issue. The most important source of vibrations are road profile and engine vibrations. The vibrations from the road gets transferred to the frame via wheels and shock absorber. However the vibrations from the engine are transferred to the frame via engine mounts. So there have been extensive research on this topic to reduce the transmissiblity of engine mounts to avoid transfer of undesirable vibrations from engine to frame. 

Design of Engine Mounts: The design task includes determining the locations of the engine mounts on the frame and the stiffness of the engine mounts. There has been rigorous research in car industry to solve this problem. The car engine mounts were designed appropriately to reduce the transfer of vibrations from engine to frame. The engine mounts can be of various types on these criteria. But when it comes to 2 wheeler industry, due to various design constraints, the complexity of the problem increases several folds.

In order to reduce unwanted vibration of the flexible structure system, three different types of mounts are normally employed: passive, semi-active and active.  The ideal engine mount system should isolate engine vibration caused by engine disturbance force in engine speed range and prevent engine bounce from shock excitation. This implies that the dynamic stiffness and damping of the engine mount should be frequency and amplitude dependent. The development of engine mounting systems has mostly concentrated on improvement of frequency- and amplitude-dependent properties.

                                     The conventional elastomeric mounts do not meet all the requirements and can only offer a trade-off between static deflection and vibration isolation. Passive hydraulic mounts can provide a better performance than elastomeric mounts especially in the low frequency range. Semi-active techniques are usually used to further improve performance of hydraulic mounts by making them more tunable. The active engine mounting system can be very stiff at low frequency and be tuned to be very soft at the higher frequency range to isolate the vibration. The active engine mounts have been considered as the next generation of engine mounts.

• The passive rubber mount, which has low damping, shows efficient vibration performance at the non-resonant and high frequency excitation. Thus, the rubber mount is the most popular method applied for various vibrating systems. However, it cannot have a favourable performance due to small damping effect at the resonant frequency excitation. On the other hand, the passive hydraulic mount has been developed to utilize dynamic absorber effect or meet large damping requirement in the resonance of low frequency domain. However, the high dynamic stiffness property of the hydraulic mount may deteriorate isolation performance in the non-resonant excitation domain. Thus, the damping and stiffness of the passive mounts are not simultaneously controllable to meet imposed performance criteria in a wide frequency range. Passive hydro mounts have asserted themselves as one response to dampening noise levels.

• The active engine mounts: Current developments in the automotive industry such as three cylinder engines and the use of cylinder deactivation are leading to an increase in vibration levels that passive systems can no longer compensate for. The implementation of active systems show more promise in improving vibration comfort and the acoustic impression. These kinds of mounts use piezo electric and MEMS systems to curb the vibrations. Companies like Porsche, Audi, uses such kind of system extensively in their models. 

• Semi-Active mounts: It has properties lying between passive and active mounts. Based on technology of Magnetorheological fluid and electrorheological fluids, these mounts have an ability to improve vibration comfort in the vibration ranges in which passive mounts are ineffective. These fluids change their yield stress in response to an externally applied magnetic and electric field, respectively. As a result, these fluids allow one to transform a passive hydraulic vibration isolator into a semi-active device.

One way of reducing N.V.H. transmissions to the body structure is to adopt softer engine mountings. Unfortunately this increases engine movement under drive torque reaction and so requires greater space in the engine bay and more flexible hose and cable connections. A further disadvantage of soft mountings, especially in small vehicles, is that they promote an undesirable phenomenon termed shunt where movement of the engine is noticeable to the vehicle driver and can adversely affect drivability of the vehicle. Thus engine mountings are presently called upon to perform two tasks, reaction of final drive torque which requires relatively hard mountings and reduction of N.V.H. transmissions which requires relatively soft mountings. The present mounting is for vibrating system and in particular to a mounting capable of eliminating or substantially reducing the transmission of vibration from a vibrating system to its support base.

Resilient mountings: In order to reduce the transmission of vibration from a vibrating system to its support base it is known to interpose a resilient mounting between the vibrating system & its base. The resilient mounting is alternately compressed & expanded as the system vibrates and some of the motion of the system is thus absorbed. However, even if the resilient mounting has a low elastic resistance its deflection by the vibration system requires a force which in turn causes vibratory motion of the base.

Hydro-Mechanical Mounts: An engine mount must satisfy two essential but conflicting criteria. First, it should be stiff and highly damped to control the idle shake and engine mounting resonance over 5±30 Hz. Also, it must be able to control, like a shock absorber, the motion resulting from quasistatic load conditions such as travel on bumpy roads, abrupt vehicle acceleration or deceleration, and braking and cornering. Second, for a small amplitude excitation over the higher frequency range, a compliant but lightly damped mount is required for vibration isolation and acoustic comfort, like a conventional rubber mount.

 These issues make optimization of engine mounting systems in two-wheeler industry, quite desirable.

Transmissibility is the ratio of vibration of the isolated surface to that of the source. Vibrations are never completely eliminated, but they can be greatly reduced. The best isolation system for a given situation depends on the frequency, direction, and magnitude of vibrations present and the desired level of attenuation of those frequencies.

Methodology:

Desired properties of the engine mounts:

1. Variable transmissibility

2. The stiffness properties of the elastic mounts should exhibit non-linear variations with the excitation frequencies, the static load and the deflection. 

3. The dynamic stiffness at lower frequencies should be higher than its stiffness at higher frequencies. 

The project proceeded with the modelling of engine as the rigid mass and mounting systems as the spring-damper system. The forces generated by the engine were calculated from the anatomy of the engine, considering the piston motion, crankshaft motion, etc.

                                                                                A simple car engine model with mountings is shown below:- (courtesy Google)

The knowledge of engine forces was necessary, which were calculated by the various motions included in the engine. These forces can be calculated by using commercial software like ADAMS. A simple piston and crankshaft model is shown below, used to calculate the forces generated of the engine. It was checked with the actual data obtained experimentally, to correlate the analytical and experimental results.

After the forces generated by the engine was calculated, the optimization study was carried out. The variables chosen were the mounting locations which included design constraints, and the stiffness of the engine mounts. The task was to reduce the transmissibility of the engine mounts. It should be stiff at the start, i.e. at lower frequencies, however it should be soft at higher frequencies. 

                                                                                                  A picture shown below shows the effect of damping on the transmissibility. 

Choosing the type of engine mounts is as important as choosing the mounting position. Mounting position dominates the amount and type of vibrations transmitted to the frame. The forces generated by engine, the natural frequencies of the engine were then used to isolate the the frequencies at the mount locations. In simple language it means, by changing the location of the engine mounts, the direction and amplitude of forces and torque transferred to the engine mounts was monitored and natural frequencies of the engine were isolated. This methodology leads to the determination of locations of the engine. After finding the locations, next task was to find the stiffness of the engine mounts. The stiffness was determined to reduce the engine vibrations.  The resilient mounting were considered keeping the cost constraints in mind. The standard products available in the markets were taken into consideration.

Nature of vibrations to be isolated :

1. Frequencies: It is important to know the frequencies of ambient vibrations. This can be determined with a site survey or accelerometer data processed through FFT analysis.

2. Amplitudes: Isolators are designed for ranges of vibration amplitudes. 

3. Direction: Knowing whether vibrations are horizontal or vertical can help to target isolation where it is needed

This work on the optimization of the engine mount systems however has its own limitations. The engine vibrations found analytically and the experimental data varies substantially due to modelling constraints. The external factors also affect the technique.

However keeping the above limitations in mind, this

Input required:

1. Engine vibration data

2. Transmissibility allowable (depends of frame stiffness)

Factor determined: 

1. Position of engine mounts

2. Structure of engine mounts

3. Stiffness and damping properties