The powerplant or engine of a car is arguably the single most vital piece of equipment in an automobile. Its positioning and drive-train connection are obviously subjects of a great deal of research and analysis. Of note, and of particular importance to our project, is the NVH analysis of the Engine block while it is running, and how modifying the orientation and positioning of the mounts can help reduce the overall vibrations and noise generated by the engine. Engine mounts are used to support the engine and isolate the base frame of an automobile from the vibrations. In most cars, the engine and transmission are bolted in place by three, or four mounts. In this project, we will work on a three mount system. Of the two major challenges that engineers face when it comes to vibration isolation the first challenge is force isolation, due to rotating or reciprocating machinery with unbalanced masses. As the engine has pistons which undergo linear translatory motion, and a shaft that rotates, there are very complicated interactions that are a result of the reciprocating motion and the torque generated. The objective of analysing this is to minimize the force transmitted from the engine block to the supporting foundation. The second challenge is motion isolation where we are concerned with minimizing the transmitted vibration amplitude into the structure itself such that the engine and the vehicle are protected from each others’ inertial reactions. This is achieved by mounting the engine on a resilient support, or an isolator such that the natural frequency of the system is lower than the frequency of the incoming vibrations that are to be isolated.
Noise, Vibration and Harshness (NVH) analysis is the study and modification of the noise and vibration aspects of a vehicle, with a particular focus on noise and vibration generation mechanisms, and how to change their properties. While noise and vibration can be easily calibrated by changing parameters of secondary parts in the system, harshness is a subjective quality and is measured either via board evaluations or with analytical tools that can provide results showing subjective human impressions that are quite often in the psychoacoustic realm. The field of NVH analysis is mostly dominated by engineering analysis and research, but more often than not, the objective measurements fail to predict the subjective impression on human observers, and end-users. The field of psychoacoustics is partly concerned with this correlation. In some situations NVH engineers are requested to change sound quality by adding or removing particular harmonics, rather than making the vehicle less noisy, rather counterintuitively. An excellent example is fake noise generation in electric vehicles, because they are usually virtually noiseless, and are thus sometimes dangerous on roads as pedestrians don’t pay attention. The predominant sources of noise in vehicle are:- ● Aerodynamic (HVAC inlets, spoilers, open windows, etc.) ● Mechanical (Brake squeal, engine/shaft vibrations, structural vibrations)
● Electrical (Electromagnetically induced acoustic noise and vibration due to oscillating circuits (in electric cars))
The Powertrain Mounting System (PMS) is the link between the powertrain and the chassis of the car and plays an important role in supporting the powertrain, limiting the displacement, as well as isolating any vibration excitation. Optimal design of the PMS plays a vital role in improving the vibration performance of any vehicle.
In this project, we will work on a multi-body dynamics model. The PMS has three mountings, wherein the upper mounting is located above the transmission, the down mounting is placed close to the joint surface of the engine and the transmission, the left mounting is placed on the left side of the engine. In the Cartesian coordinate system, the model simulates the actual powertrain by setting the corresponding mass and inertia properties.
Engine mounts are the rigid clamps or brackets by the help of which an engine is mounted on the chassis of a vehicle. They are made in such a way that they isolate the transfer of vibration from engine to the frame or vice versa. There are various types of mounts, which work based on different techniques. The two main categories are usually active mounts and passive mounts. Active mounts usually have some form of digital control logic and a feedback loop that actively, through the use of some algorithm similar to a PID control, keeps track of the excitation forces being exerted, and counteract them live using actuators in the mounts. We will be focussing on passive mounts in this project. Passive mounts too have various types, including the current best industry-standard being the passive hydraulic mount. We will, however, be working with a regular elastoviscous “rubber” mount. The engine mount keeps the engine secured, prevents it from vibrating excessively and ensures that it stays properly connected. It also keeps vibrations from travelling through the frame and other parts of the vehicle. The mounts that are made of rubber can withstand large amounts of deformation under loads with the ability to almost retain their original shape when the load is removed. This is due to the viscoelastic property of the used rubber, allowing it to be used as an isolator and as a damper. The rubber absorbs vibrations while allowing a small amount of movement.
A typical mounting system consists of a powertrain and a number of mounts that connect the powertrain to the supporting frame. The major objective of the engine mount is to prevent the engine vibrations from being transferred to the supporting frame. Most rear-wheel drive cars have almost the same engine mount system. The conventional arrangement has developed over time, almost in a manner similar to natural evolution, and the industry has settled on the current model to be a reliable system and not much major variation is seen from one car to another. However, the same cannot be said for front-wheel-drive cars. Designing the mounting system of a front-wheel-drive is much more demanding, owing to factors such as engine compartment space restrictions, greater equivalent torque acting on the powerplant, fewer cylinders and less symmetry, which make the conventional design methods rather challenging to employ. To make matters worse, the direction and positioning of the engine are wildly different, which means that torque is usually acting in an entirely different direction altogether. In our project, using optimization methods, the mount attributes are varied to minimize the force transmission, subject to constraints such as the maximum allowable motion of the body at any of the mount attachment points. Previously, this approach has been applied to engine mount optimization using