Internal combustion engines generate vibrations during operation. The first class of excitation is due to the design parameters: inertial forces and torques from reciprocating masses, variations in gas pressure in the cylinders, centrifugal forces. A second class of excitation is generated by variations in the production process: combustion problems, crank mechanism unbalanced components, unbalanced ancillaries. A powertrain suspension is used to shield the car body from these vibrations.
Fig.1 shows a typical engine suspension for a front-mounted East/ West engine: two mountings in the vertical plan of the center of gravity and a torque rod.
In the case of a three-cylinder engine without countershafts, the engine excitation (Fig.2) can be described as follows:
- Order 0.5: Combustion and cylinder-to-cylinder irregularities
- Order 1: Unbalanced yaw irregularities
- Order 1.5: Combustion (3 burns during 2 revolutions)
During a car journey, road irregularities are transmitted to the powertrain, which has at least 6 degrees of freedom as a rigid body within its bay – due to the engine’s suspension which allows it to move under both static and dynamic loads. The energy of these engine vibrations is transmitted back to the car body, which can cause vibrational discomfort for passengers. This is called “engine shake”.
Fig.3 provides an example of a modal analysis obtained by linearizing the system at amplitudes of ride comfort, while
Fig. 3 - Example of one modal analysis of engine shake
Fig.4 shows the power spectral densities (PSD) of the vertical accelerations on the wheel hub, car body and engine during a ride comfort mission.
Fig. 4 - PSDs of a ride comfort mission and engine shake