Abstract

The paper presents the results of comparisons tests carried out on one model (Fiat Bravo model 198 produced in 2010) of new cars equipped with three types of engine: Version 54A with the 1.4BZ 90CV CD spark-ignition engine, Version 54G with the 1.4BZ 120CV CD spark-ignition engine equipped with a supercharging system and Version 54W with the 1,6D 105CV diesel engine. The car body vibrations experimentally determined in several specific repeatable points, i.e. behind the front side indicator and behind the passenger handle, were compared. A PSV-400 laser Doppler vibrometer made by Polytec was used to measure vibration velocities. The vibrometer directly measures two quantities: displacement and velocity. In the investigated case, vibration velocity turned out to be the variable supplying better diagnostic information. Vibrations were measured for the car standing on its wheels and for the car jacked up to reduce the influence of the car vibration damping systems on the measurement results. The latter are presented in the form of comparative diagrams. Moreover, the fast Fourier transform was used to determine the frequency distribution. Prior to that the signal was subjected to conditioning operations in time domain, such as parametric windowing and filtering. On the basis of the results the effect of the drive unit on the behaviour of the car body can be assessed for different engine types and rotational speeds. It is shown that the way in which the engine is mounted affects the vibrations of the car.