INNOVA Research Journal 2019, Vol 4, No. 1, pp. 9-20
Introduction
The study of alternative and ecological fuel blends, mainly aims to reduce the pollutant
emissions compared with the use of neat diesel. At the same time, for sustainable use of a fuel
blend as an energy source for internal combustion engines, it must have properties and
characteristics that keep or improve the engine parameters such as performance, fuel
consumption, durability and stability. As the quality of the combustion is conditioned by the
cetane number and flash point of the fuel, the present research focuses on determining of the
effect of these properties on the engine vibration level.
The vibrations on the internal combustion engine are the result of unbalanced forces of
the rotational and reciprocating parts; as well as cyclic variations of pressure inside of the engine
cylinders, which depend on the rotation velocity, fuel supply and quality of the combustion
process [1]. There are identified two types of engines vibrations: torsional and longitudinal. The
torsional vibrations have an effect over the engine crankshaft, because of the pressure fluctuation
coming from the fuel combustion and engine loads; while the longitudinal ones, have an effect
over the engine block and mountings, because of the rotary and alternating components [2]. The
present research considers the effect of the fuel properties and the quality of the combustion on
the total end engine vibration resulting from the torsional and longitudinal engine vibrations.
The vibrations are mainly understood as mechanical effects, which can be controlled by
attenuators [3]; for this reason, another methods and researches are carried out, in order to
analyze this phenomenon in a more detailed way. One of the investigations in special determined
that the irregular torque on the engine cylinders are produced by unequal fuel injection in each
cylinder. The control of the fuel injection improves the exactitude of the delivered fuel quantity,
allowed to reduce the level of vibration up to 92% [4].
One of the applied techniques is non-intrusive, which allows measuring of the engine
vibrations without interrupting functionalities of either process [5]. With this technique, the data
of the vibrations measurements, can be collected during the combustion process. It allows also
distinguishing and analyzing of the effects of different fuel types.
The method to construct the frequencies spectrum and evaluate the vibration level of the
engine was performed by means of the fast Fourier Transforms (FFT) [6]. This method consists
of discomposing the resulting signal of the acceleration spectrum in individual components of a
sinusoidal wave in order to get a frequency spectrum [7]. In this way it is possible to visualize
the value of the dominant frequencies corresponding to rotation and ignition process of the
engine. The relationship between the frequencies of these two processes is expressed as a
function of the engine rotation speed and the number of cylinders [8, 9]. It means that in case of
a four-cylinder and four-stroke engine the frequency values corresponding to the ignition are
equal to the double of the rotation frequency of the engine. Any additional excitation source, for
example the effect of the concentration of the fuel blend, is evidenced with the apparition of
addition dominant frequencies in the spectrum.
The studies of engine vibrations with different types and concentrations of biodiesel,
beside of valuation of engine stability, show also the capacity of a determined type of biodiesel
to be burned, and its ability to keep or improve the engine performance [6, 10]. Engine
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